diff --git a/Android.bp b/Android.bp
index c7fe6ce..c8e7c29 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1,8 +1,8 @@
 cc_library_headers {
     name: "libabsl_headers",
-    export_include_dirs: ["."],
-    host_supported: true,
     device_supported: false,
+    host_supported: true,
+    export_include_dirs: ["."],
 }
 
 cc_library_host_static {
@@ -10,7 +10,9 @@
     srcs: [
         "absl/base/dynamic_annotations.cc",
         "absl/base/internal/cycleclock.cc",
+        "absl/base/internal/exponential_biased.cc",
         "absl/base/internal/low_level_alloc.cc",
+        "absl/base/internal/periodic_sampler.cc",
         "absl/base/internal/raw_logging.cc",
         "absl/base/internal/spinlock.cc",
         "absl/base/internal/spinlock_wait.cc",
@@ -56,11 +58,11 @@
         "absl/flags/usage.cc",
         "absl/flags/flag.cc",
         "absl/flags/parse.cc",
+        "absl/flags/internal/flag.cc",
         "absl/flags/internal/program_name.cc",
         "absl/flags/internal/registry.cc",
         "absl/flags/internal/usage.cc",
         "absl/flags/internal/type_erased.cc",
-        "absl/flags/internal/commandlineflag.cc",
         "absl/flags/flag_test_defs.cc",
     ],
 }
@@ -79,13 +81,23 @@
 }
 
 cc_library_host_static {
+    name: "libabsl_status",
+    srcs: [
+        "absl/status/status.cc",
+        "absl/status/status_payload_printer.cc",
+    ],
+}
+
+cc_library_host_static {
     name: "libabsl_strings",
     srcs: [
         "absl/strings/ascii.cc",
         "absl/strings/charconv.cc",
+        "absl/strings/cord.cc",
         "absl/strings/escaping.cc",
         "absl/strings/internal/charconv_bigint.cc",
         "absl/strings/internal/charconv_parse.cc",
+        "absl/strings/internal/escaping.cc",
         "absl/strings/internal/memutil.cc",
         "absl/strings/internal/ostringstream.cc",
         "absl/strings/internal/str_format/arg.cc",
diff --git a/CMake/AbseilDll.cmake b/CMake/AbseilDll.cmake
new file mode 100644
index 0000000..90c9f1f
--- /dev/null
+++ b/CMake/AbseilDll.cmake
@@ -0,0 +1,504 @@
+include(CMakeParseArguments)
+
+set(ABSL_INTERNAL_DLL_FILES
+  "algorithm/algorithm.h"
+  "algorithm/container.h"
+  "base/attributes.h"
+  "base/call_once.h"
+  "base/casts.h"
+  "base/config.h"
+  "base/const_init.h"
+  "base/dynamic_annotations.cc"
+  "base/dynamic_annotations.h"
+  "base/internal/atomic_hook.h"
+  "base/internal/bits.h"
+  "base/internal/cycleclock.cc"
+  "base/internal/cycleclock.h"
+  "base/internal/direct_mmap.h"
+  "base/internal/endian.h"
+  "base/internal/errno_saver.h"
+  "base/internal/exponential_biased.cc"
+  "base/internal/exponential_biased.h"
+  "base/internal/hide_ptr.h"
+  "base/internal/identity.h"
+  "base/internal/invoke.h"
+  "base/internal/inline_variable.h"
+  "base/internal/low_level_alloc.cc"
+  "base/internal/low_level_alloc.h"
+  "base/internal/low_level_scheduling.h"
+  "base/internal/per_thread_tls.h"
+  "base/internal/periodic_sampler.cc"
+  "base/internal/periodic_sampler.h"
+  "base/internal/pretty_function.h"
+  "base/internal/raw_logging.cc"
+  "base/internal/raw_logging.h"
+  "base/internal/scheduling_mode.h"
+  "base/internal/scoped_set_env.cc"
+  "base/internal/scoped_set_env.h"
+  "base/internal/spinlock.cc"
+  "base/internal/spinlock.h"
+  "base/internal/spinlock_wait.cc"
+  "base/internal/spinlock_wait.h"
+  "base/internal/sysinfo.cc"
+  "base/internal/sysinfo.h"
+  "base/internal/thread_annotations.h"
+  "base/internal/thread_identity.cc"
+  "base/internal/thread_identity.h"
+  "base/internal/throw_delegate.cc"
+  "base/internal/throw_delegate.h"
+  "base/internal/tsan_mutex_interface.h"
+  "base/internal/unaligned_access.h"
+  "base/internal/unscaledcycleclock.cc"
+  "base/internal/unscaledcycleclock.h"
+  "base/log_severity.cc"
+  "base/log_severity.h"
+  "base/macros.h"
+  "base/optimization.h"
+  "base/options.h"
+  "base/policy_checks.h"
+  "base/port.h"
+  "base/thread_annotations.h"
+  "container/btree_map.h"
+  "container/btree_set.h"
+  "container/fixed_array.h"
+  "container/flat_hash_map.h"
+  "container/flat_hash_set.h"
+  "container/inlined_vector.h"
+  "container/internal/btree.h"
+  "container/internal/btree_container.h"
+  "container/internal/common.h"
+  "container/internal/compressed_tuple.h"
+  "container/internal/container_memory.h"
+  "container/internal/counting_allocator.h"
+  "container/internal/hash_function_defaults.h"
+  "container/internal/hash_policy_traits.h"
+  "container/internal/hashtable_debug.h"
+  "container/internal/hashtable_debug_hooks.h"
+  "container/internal/hashtablez_sampler.cc"
+  "container/internal/hashtablez_sampler.h"
+  "container/internal/hashtablez_sampler_force_weak_definition.cc"
+  "container/internal/have_sse.h"
+  "container/internal/inlined_vector.h"
+  "container/internal/layout.h"
+  "container/internal/node_hash_policy.h"
+  "container/internal/raw_hash_map.h"
+  "container/internal/raw_hash_set.cc"
+  "container/internal/raw_hash_set.h"
+  "container/internal/tracked.h"
+  "container/node_hash_map.h"
+  "container/node_hash_set.h"
+  "debugging/failure_signal_handler.cc"
+  "debugging/failure_signal_handler.h"
+  "debugging/leak_check.h"
+  "debugging/leak_check_disable.cc"
+  "debugging/stacktrace.cc"
+  "debugging/stacktrace.h"
+  "debugging/symbolize.cc"
+  "debugging/symbolize.h"
+  "debugging/internal/address_is_readable.cc"
+  "debugging/internal/address_is_readable.h"
+  "debugging/internal/demangle.cc"
+  "debugging/internal/demangle.h"
+  "debugging/internal/elf_mem_image.cc"
+  "debugging/internal/elf_mem_image.h"
+  "debugging/internal/examine_stack.cc"
+  "debugging/internal/examine_stack.h"
+  "debugging/internal/stack_consumption.cc"
+  "debugging/internal/stack_consumption.h"
+  "debugging/internal/stacktrace_config.h"
+  "debugging/internal/symbolize.h"
+  "debugging/internal/vdso_support.cc"
+  "debugging/internal/vdso_support.h"
+  "functional/internal/front_binder.h"
+  "functional/bind_front.h"
+  "functional/function_ref.h"
+  "functional/internal/function_ref.h"
+  "hash/hash.h"
+  "hash/internal/city.h"
+  "hash/internal/city.cc"
+  "hash/internal/hash.h"
+  "hash/internal/hash.cc"
+  "hash/internal/spy_hash_state.h"
+  "memory/memory.h"
+  "meta/type_traits.h"
+  "numeric/int128.cc"
+  "numeric/int128.h"
+  "random/bernoulli_distribution.h"
+  "random/beta_distribution.h"
+  "random/bit_gen_ref.h"
+  "random/discrete_distribution.cc"
+  "random/discrete_distribution.h"
+  "random/distribution_format_traits.h"
+  "random/distributions.h"
+  "random/exponential_distribution.h"
+  "random/gaussian_distribution.cc"
+  "random/gaussian_distribution.h"
+  "random/internal/distributions.h"
+  "random/internal/distribution_caller.h"
+  "random/internal/fast_uniform_bits.h"
+  "random/internal/fastmath.h"
+  "random/internal/gaussian_distribution_gentables.cc"
+  "random/internal/generate_real.h"
+  "random/internal/iostream_state_saver.h"
+  "random/internal/nonsecure_base.h"
+  "random/internal/pcg_engine.h"
+  "random/internal/platform.h"
+  "random/internal/pool_urbg.cc"
+  "random/internal/pool_urbg.h"
+  "random/internal/randen.cc"
+  "random/internal/randen.h"
+  "random/internal/randen_detect.cc"
+  "random/internal/randen_detect.h"
+  "random/internal/randen_engine.h"
+  "random/internal/randen_hwaes.cc"
+  "random/internal/randen_hwaes.h"
+  "random/internal/randen_slow.cc"
+  "random/internal/randen_slow.h"
+  "random/internal/randen_traits.h"
+  "random/internal/salted_seed_seq.h"
+  "random/internal/seed_material.cc"
+  "random/internal/seed_material.h"
+  "random/internal/sequence_urbg.h"
+  "random/internal/traits.h"
+  "random/internal/uniform_helper.h"
+  "random/internal/wide_multiply.h"
+  "random/log_uniform_int_distribution.h"
+  "random/poisson_distribution.h"
+  "random/random.h"
+  "random/seed_gen_exception.cc"
+  "random/seed_gen_exception.h"
+  "random/seed_sequences.cc"
+  "random/seed_sequences.h"
+  "random/uniform_int_distribution.h"
+  "random/uniform_real_distribution.h"
+  "random/zipf_distribution.h"
+  "status/status.h"
+  "status/status.cc"
+  "status/status_payload_printer.h"
+  "status/status_payload_printer.cc"
+  "strings/ascii.cc"
+  "strings/ascii.h"
+  "strings/charconv.cc"
+  "strings/charconv.h"
+  "strings/cord.cc"
+  "strings/cord.h"
+  "strings/escaping.cc"
+  "strings/escaping.h"
+  "strings/internal/cord_internal.h"
+  "strings/internal/charconv_bigint.cc"
+  "strings/internal/charconv_bigint.h"
+  "strings/internal/charconv_parse.cc"
+  "strings/internal/charconv_parse.h"
+  "strings/internal/stl_type_traits.h"
+  "strings/match.cc"
+  "strings/match.h"
+  "strings/numbers.cc"
+  "strings/numbers.h"
+  "strings/str_format.h"
+  "strings/str_cat.cc"
+  "strings/str_cat.h"
+  "strings/str_join.h"
+  "strings/str_replace.cc"
+  "strings/str_replace.h"
+  "strings/str_split.cc"
+  "strings/str_split.h"
+  "strings/string_view.cc"
+  "strings/string_view.h"
+  "strings/strip.h"
+  "strings/substitute.cc"
+  "strings/substitute.h"
+  "strings/internal/char_map.h"
+  "strings/internal/escaping.h"
+  "strings/internal/escaping.cc"
+  "strings/internal/memutil.cc"
+  "strings/internal/memutil.h"
+  "strings/internal/ostringstream.cc"
+  "strings/internal/ostringstream.h"
+  "strings/internal/pow10_helper.cc"
+  "strings/internal/pow10_helper.h"
+  "strings/internal/resize_uninitialized.h"
+  "strings/internal/str_format/arg.cc"
+  "strings/internal/str_format/arg.h"
+  "strings/internal/str_format/bind.cc"
+  "strings/internal/str_format/bind.h"
+  "strings/internal/str_format/checker.h"
+  "strings/internal/str_format/extension.cc"
+  "strings/internal/str_format/extension.h"
+  "strings/internal/str_format/float_conversion.cc"
+  "strings/internal/str_format/float_conversion.h"
+  "strings/internal/str_format/output.cc"
+  "strings/internal/str_format/output.h"
+  "strings/internal/str_format/parser.cc"
+  "strings/internal/str_format/parser.h"
+  "strings/internal/str_join_internal.h"
+  "strings/internal/str_split_internal.h"
+  "strings/internal/utf8.cc"
+  "strings/internal/utf8.h"
+  "synchronization/barrier.cc"
+  "synchronization/barrier.h"
+  "synchronization/blocking_counter.cc"
+  "synchronization/blocking_counter.h"
+  "synchronization/mutex.cc"
+  "synchronization/mutex.h"
+  "synchronization/notification.cc"
+  "synchronization/notification.h"
+  "synchronization/internal/create_thread_identity.cc"
+  "synchronization/internal/create_thread_identity.h"
+  "synchronization/internal/graphcycles.cc"
+  "synchronization/internal/graphcycles.h"
+  "synchronization/internal/kernel_timeout.h"
+  "synchronization/internal/per_thread_sem.cc"
+  "synchronization/internal/per_thread_sem.h"
+  "synchronization/internal/thread_pool.h"
+  "synchronization/internal/waiter.cc"
+  "synchronization/internal/waiter.h"
+  "time/civil_time.cc"
+  "time/civil_time.h"
+  "time/clock.cc"
+  "time/clock.h"
+  "time/duration.cc"
+  "time/format.cc"
+  "time/time.cc"
+  "time/time.h"
+  "time/internal/cctz/include/cctz/civil_time.h"
+  "time/internal/cctz/include/cctz/civil_time_detail.h"
+  "time/internal/cctz/include/cctz/time_zone.h"
+  "time/internal/cctz/include/cctz/zone_info_source.h"
+  "time/internal/cctz/src/civil_time_detail.cc"
+  "time/internal/cctz/src/time_zone_fixed.cc"
+  "time/internal/cctz/src/time_zone_fixed.h"
+  "time/internal/cctz/src/time_zone_format.cc"
+  "time/internal/cctz/src/time_zone_if.cc"
+  "time/internal/cctz/src/time_zone_if.h"
+  "time/internal/cctz/src/time_zone_impl.cc"
+  "time/internal/cctz/src/time_zone_impl.h"
+  "time/internal/cctz/src/time_zone_info.cc"
+  "time/internal/cctz/src/time_zone_info.h"
+  "time/internal/cctz/src/time_zone_libc.cc"
+  "time/internal/cctz/src/time_zone_libc.h"
+  "time/internal/cctz/src/time_zone_lookup.cc"
+  "time/internal/cctz/src/time_zone_posix.cc"
+  "time/internal/cctz/src/time_zone_posix.h"
+  "time/internal/cctz/src/tzfile.h"
+  "time/internal/cctz/src/zone_info_source.cc"
+  "types/any.h"
+  "types/bad_any_cast.cc"
+  "types/bad_any_cast.h"
+  "types/bad_optional_access.cc"
+  "types/bad_optional_access.h"
+  "types/bad_variant_access.cc"
+  "types/bad_variant_access.h"
+  "types/compare.h"
+  "types/internal/conformance_aliases.h"
+  "types/internal/conformance_archetype.h"
+  "types/internal/conformance_profile.h"
+  "types/internal/variant.h"
+  "types/optional.h"
+  "types/internal/optional.h"
+  "types/span.h"
+  "types/internal/span.h"
+  "types/variant.h"
+  "utility/utility.h"
+)
+
+set(ABSL_INTERNAL_DLL_TARGETS
+  "stacktrace"
+  "symbolize"
+  "examine_stack"
+  "failure_signal_handler"
+  "debugging_internal"
+  "demangle_internal"
+  "leak_check"
+  "leak_check_disable"
+  "stack_consumption"
+  "debugging"
+  "hash"
+  "spy_hash_state"
+  "city"
+  "memory"
+  "strings"
+  "strings_internal"
+  "cord"
+  "str_format"
+  "str_format_internal"
+  "pow10_helper"
+  "int128"
+  "numeric"
+  "utility"
+  "any"
+  "bad_any_cast"
+  "bad_any_cast_impl"
+  "span"
+  "optional"
+  "bad_optional_access"
+  "bad_variant_access"
+  "variant"
+  "compare"
+  "algorithm"
+  "algorithm_container"
+  "graphcycles_internal"
+  "kernel_timeout_internal"
+  "synchronization"
+  "thread_pool"
+  "bind_front"
+  "function_ref"
+  "atomic_hook"
+  "log_severity"
+  "raw_logging_internal"
+  "spinlock_wait"
+  "config"
+  "dynamic_annotations"
+  "core_headers"
+  "malloc_internal"
+  "base_internal"
+  "base"
+  "throw_delegate"
+  "pretty_function"
+  "endian"
+  "bits"
+  "exponential_biased"
+  "periodic_sampler"
+  "scoped_set_env"
+  "type_traits"
+  "meta"
+  "random_random"
+  "random_bit_gen_ref"
+  "random_distributions"
+  "random_seed_gen_exception"
+  "random_seed_sequences"
+  "random_internal_traits"
+  "random_internal_distribution_caller"
+  "random_internal_distributions"
+  "random_internal_fast_uniform_bits"
+  "random_internal_seed_material"
+  "random_internal_pool_urbg"
+  "random_internal_explicit_seed_seq"
+  "random_internal_sequence_urbg"
+  "random_internal_salted_seed_seq"
+  "random_internal_iostream_state_saver"
+  "random_internal_generate_real"
+  "random_internal_wide_multiply"
+  "random_internal_fastmath"
+  "random_internal_nonsecure_base"
+  "random_internal_pcg_engine"
+  "random_internal_randen_engine"
+  "random_internal_platform"
+  "random_internal_randen"
+  "random_internal_randen_slow"
+  "random_internal_randen_hwaes"
+  "random_internal_randen_hwaes_impl"
+  "random_internal_uniform_helper"
+  "status"
+  "time"
+  "civil_time"
+  "time_zone"
+  "container"
+  "btree"
+  "compressed_tuple"
+  "fixed_array"
+  "inlined_vector_internal"
+  "inlined_vector"
+  "counting_allocator"
+  "flat_hash_map"
+  "flat_hash_set"
+  "node_hash_map"
+  "node_hash_set"
+  "container_memory"
+  "hash_function_defaults"
+  "hash_policy_traits"
+  "hashtablez_sampler"
+  "hashtable_debug"
+  "hashtable_debug_hooks"
+  "have_sse"
+  "node_hash_policy"
+  "raw_hash_map"
+  "container_common"
+  "raw_hash_set"
+  "layout"
+  "tracked"
+)
+
+function(absl_internal_dll_contains)
+  cmake_parse_arguments(ABSL_INTERNAL_DLL
+    ""
+    "OUTPUT;TARGET"
+    ""
+    ${ARGN}
+  )
+
+  STRING(REGEX REPLACE "^absl::" "" _target ${ABSL_INTERNAL_DLL_TARGET})
+
+  list(FIND
+    ABSL_INTERNAL_DLL_TARGETS
+    "${_target}"
+    _index)
+
+  if (${_index} GREATER -1)
+    set(${ABSL_INTERNAL_DLL_OUTPUT} 1 PARENT_SCOPE)
+  else()
+    set(${ABSL_INTERNAL_DLL_OUTPUT} 0 PARENT_SCOPE)
+  endif()
+endfunction()
+
+function(absl_internal_dll_targets)
+  cmake_parse_arguments(ABSL_INTERNAL_DLL
+  ""
+  "OUTPUT"
+  "DEPS"
+  ${ARGN}
+  )
+
+  set(_deps "")
+  foreach(dep IN LISTS ABSL_INTERNAL_DLL_DEPS)
+    absl_internal_dll_contains(TARGET ${dep} OUTPUT _contains)
+    if (_contains)
+      list(APPEND _deps abseil_dll)
+    else()
+      list(APPEND _deps ${dep})
+    endif()
+  endforeach()
+
+  # Because we may have added the DLL multiple times
+  list(REMOVE_DUPLICATES _deps)
+  set(${ABSL_INTERNAL_DLL_OUTPUT} "${_deps}" PARENT_SCOPE)
+endfunction()
+
+function(absl_make_dll)
+  add_library(
+    abseil_dll
+    SHARED
+      "${ABSL_INTERNAL_DLL_FILES}"
+  )
+  target_link_libraries(
+    abseil_dll
+    PRIVATE
+      ${ABSL_DEFAULT_LINKOPTS}
+  )
+  set_property(TARGET abseil_dll PROPERTY LINKER_LANGUAGE "CXX")
+  target_include_directories(
+    abseil_dll
+    PUBLIC
+      "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+      $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
+  )
+
+  target_compile_options(
+    abseil_dll
+    PRIVATE
+      ${ABSL_DEFAULT_COPTS}
+  )
+
+  target_compile_definitions(
+    abseil_dll
+    PRIVATE
+      ABSL_BUILD_DLL
+      NOMINMAX
+    INTERFACE
+      ${ABSL_CC_LIB_DEFINES}
+  )
+  install(TARGETS abseil_dll EXPORT ${PROJECT_NAME}Targets
+        RUNTIME DESTINATION ${ABSL_INSTALL_BINDIR}
+        LIBRARY DESTINATION ${ABSL_INSTALL_LIBDIR}
+        ARCHIVE DESTINATION ${ABSL_INSTALL_LIBDIR}
+  )
+endfunction()
diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 58f98c8..86ff9eb 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -16,6 +16,7 @@
 
 include(CMakeParseArguments)
 include(AbseilConfigureCopts)
+include(AbseilDll)
 include(AbseilInstallDirs)
 
 # The IDE folder for Abseil that will be used if Abseil is included in a CMake
@@ -80,95 +81,173 @@
     ${ARGN}
   )
 
-  if(NOT ABSL_CC_LIB_TESTONLY OR ABSL_RUN_TESTS)
-    if(ABSL_ENABLE_INSTALL)
-      set(_NAME "${ABSL_CC_LIB_NAME}")
-    else()
-      set(_NAME "absl_${ABSL_CC_LIB_NAME}")
-    endif()
+  if(ABSL_CC_LIB_TESTONLY AND NOT ABSL_RUN_TESTS)
+    return()
+  endif()
 
-    # Check if this is a header-only library
-    # Note that as of February 2019, many popular OS's (for example, Ubuntu
-    # 16.04 LTS) only come with cmake 3.5 by default.  For this reason, we can't
-    # use list(FILTER...)
-    set(ABSL_CC_SRCS "${ABSL_CC_LIB_SRCS}")
-    foreach(src_file IN LISTS ABSL_CC_SRCS)
-      if(${src_file} MATCHES ".*\\.(h|inc)")
-        list(REMOVE_ITEM ABSL_CC_SRCS "${src_file}")
-      endif()
-    endforeach()
-    if("${ABSL_CC_SRCS}" STREQUAL "")
+  if(ABSL_ENABLE_INSTALL)
+    set(_NAME "${ABSL_CC_LIB_NAME}")
+  else()
+    set(_NAME "absl_${ABSL_CC_LIB_NAME}")
+  endif()
+
+  # Check if this is a header-only library
+  # Note that as of February 2019, many popular OS's (for example, Ubuntu
+  # 16.04 LTS) only come with cmake 3.5 by default.  For this reason, we can't
+  # use list(FILTER...)
+  set(ABSL_CC_SRCS "${ABSL_CC_LIB_SRCS}")
+  foreach(src_file IN LISTS ABSL_CC_SRCS)
+    if(${src_file} MATCHES ".*\\.(h|inc)")
+      list(REMOVE_ITEM ABSL_CC_SRCS "${src_file}")
+    endif()
+  endforeach()
+
+  if("${ABSL_CC_SRCS}" STREQUAL "")
+    set(ABSL_CC_LIB_IS_INTERFACE 1)
+  else()
+    set(ABSL_CC_LIB_IS_INTERFACE 0)
+  endif()
+
+  # Determine this build target's relationship to the DLL. It's one of four things:
+  # 1. "dll"     -- This target is part of the DLL
+  # 2. "dll_dep" -- This target is not part of the DLL, but depends on the DLL.
+  #                 Note that we assume any target not in the DLL depends on the
+  #                 DLL. This is not a technical necessity but a convenience
+  #                 which happens to be true, because nearly every target is
+  #                 part of the DLL.
+  # 3. "shared"  -- This is a shared library, perhaps on a non-windows platform
+  #                 where DLL doesn't make sense.
+  # 4. "static"  -- This target does not depend on the DLL and should be built
+  #                 statically.
+  if (${ABSL_BUILD_DLL})
+    absl_internal_dll_contains(TARGET ${_NAME} OUTPUT _in_dll)
+    if (${_in_dll})
+      # This target should be replaced by the DLL
+      set(_build_type "dll")
       set(ABSL_CC_LIB_IS_INTERFACE 1)
     else()
-      set(ABSL_CC_LIB_IS_INTERFACE 0)
+      # Building a DLL, but this target is not part of the DLL
+      set(_build_type "dll_dep")
     endif()
+  elseif(BUILD_SHARED_LIBS)
+    set(_build_type "shared")
+  else()
+    set(_build_type "static")
+  endif()
 
-    if(NOT ABSL_CC_LIB_IS_INTERFACE)
+  if(NOT ABSL_CC_LIB_IS_INTERFACE)
+    if(${_build_type} STREQUAL "dll_dep")
+      # This target depends on the DLL. When adding dependencies to this target,
+      # any depended-on-target which is contained inside the DLL is replaced
+      # with a dependency on the DLL.
       add_library(${_NAME} STATIC "")
       target_sources(${_NAME} PRIVATE ${ABSL_CC_LIB_SRCS} ${ABSL_CC_LIB_HDRS})
-      target_include_directories(${_NAME}
-        PUBLIC
-          $<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>
-          $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
+      absl_internal_dll_targets(
+        DEPS ${ABSL_CC_LIB_DEPS}
+        OUTPUT _dll_deps
       )
-      target_compile_options(${_NAME}
-        PRIVATE ${ABSL_CC_LIB_COPTS})
       target_link_libraries(${_NAME}
-        PUBLIC ${ABSL_CC_LIB_DEPS}
+        PUBLIC ${_dll_deps}
         PRIVATE
           ${ABSL_CC_LIB_LINKOPTS}
           ${ABSL_DEFAULT_LINKOPTS}
       )
-      target_compile_definitions(${_NAME} PUBLIC ${ABSL_CC_LIB_DEFINES})
 
-      # Add all Abseil targets to a a folder in the IDE for organization.
-      if(ABSL_CC_LIB_PUBLIC)
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER})
-      elseif(ABSL_CC_LIB_TESTONLY)
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
+      if (ABSL_CC_LIB_TESTONLY)
+        set(_gtest_link_define "GTEST_LINKED_AS_SHARED_LIBRARY=1")
       else()
-        set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal)
+        set(_gtest_link_define)
       endif()
 
-      # INTERFACE libraries can't have the CXX_STANDARD property set
-      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
-      set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+      target_compile_definitions(${_NAME}
+        PUBLIC
+          ABSL_CONSUME_DLL
+          "${_gtest_link_define}"
+      )
 
-      # When being installed, we lose the absl_ prefix.  We want to put it back
-      # to have properly named lib files.  This is a no-op when we are not being
-      # installed.
+    elseif(${_build_type} STREQUAL "static" OR ${_build_type} STREQUAL "shared")
+      add_library(${_NAME} "")
+      target_sources(${_NAME} PRIVATE ${ABSL_CC_LIB_SRCS} ${ABSL_CC_LIB_HDRS})
+      target_link_libraries(${_NAME}
+      PUBLIC ${ABSL_CC_LIB_DEPS}
+      PRIVATE
+        ${ABSL_CC_LIB_LINKOPTS}
+        ${ABSL_DEFAULT_LINKOPTS}
+      )
+    else()
+      message(FATAL_ERROR "Invalid build type: ${_build_type}")
+    endif()
+
+    # Linker language can be inferred from sources, but in the case of DLLs we
+    # don't have any .cc files so it would be ambiguous. We could set it
+    # explicitly only in the case of DLLs but, because "CXX" is always the
+    # correct linker language for static or for shared libraries, we set it
+    # unconditionally.
+    set_property(TARGET ${_NAME} PROPERTY LINKER_LANGUAGE "CXX")
+
+    target_include_directories(${_NAME}
+      PUBLIC
+        "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+        $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
+    )
+    target_compile_options(${_NAME}
+      PRIVATE ${ABSL_CC_LIB_COPTS})
+    target_compile_definitions(${_NAME} PUBLIC ${ABSL_CC_LIB_DEFINES})
+
+    # Add all Abseil targets to a a folder in the IDE for organization.
+    if(ABSL_CC_LIB_PUBLIC)
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER})
+    elseif(ABSL_CC_LIB_TESTONLY)
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
+    else()
+      set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/internal)
+    endif()
+
+    # INTERFACE libraries can't have the CXX_STANDARD property set
+    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
+    set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD_REQUIRED ON)
+
+    # When being installed, we lose the absl_ prefix.  We want to put it back
+    # to have properly named lib files.  This is a no-op when we are not being
+    # installed.
+    if(ABSL_ENABLE_INSTALL)
       set_target_properties(${_NAME} PROPERTIES
         OUTPUT_NAME "absl_${_NAME}"
       )
-    else()
-      # Generating header-only library
-      add_library(${_NAME} INTERFACE)
-      target_include_directories(${_NAME}
-        INTERFACE
-          $<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>
-          $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
-        )
-      target_link_libraries(${_NAME}
-        INTERFACE
-          ${ABSL_CC_LIB_DEPS}
-          ${ABSL_CC_LIB_LINKOPTS}
-          ${ABSL_DEFAULT_LINKOPTS}
+    endif()
+  else()
+    # Generating header-only library
+    add_library(${_NAME} INTERFACE)
+    target_include_directories(${_NAME}
+      INTERFACE
+        "$<BUILD_INTERFACE:${ABSL_COMMON_INCLUDE_DIRS}>"
+        $<INSTALL_INTERFACE:${ABSL_INSTALL_INCLUDEDIR}>
       )
-      target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES})
+
+    if (${_build_type} STREQUAL "dll")
+        set(ABSL_CC_LIB_DEPS abseil_dll)
     endif()
 
-    # TODO currently we don't install googletest alongside abseil sources, so
-    # installed abseil can't be tested.
-    if(NOT ABSL_CC_LIB_TESTONLY AND ABSL_ENABLE_INSTALL)
-      install(TARGETS ${_NAME} EXPORT ${PROJECT_NAME}Targets
-            RUNTIME DESTINATION ${ABSL_INSTALL_BINDIR}
-            LIBRARY DESTINATION ${ABSL_INSTALL_LIBDIR}
-            ARCHIVE DESTINATION ${ABSL_INSTALL_LIBDIR}
-      )
-    endif()
+    target_link_libraries(${_NAME}
+      INTERFACE
+        ${ABSL_CC_LIB_DEPS}
+        ${ABSL_CC_LIB_LINKOPTS}
+        ${ABSL_DEFAULT_LINKOPTS}
+    )
+    target_compile_definitions(${_NAME} INTERFACE ${ABSL_CC_LIB_DEFINES})
+  endif()
+
+  # TODO currently we don't install googletest alongside abseil sources, so
+  # installed abseil can't be tested.
+  if(NOT ABSL_CC_LIB_TESTONLY AND ABSL_ENABLE_INSTALL)
+    install(TARGETS ${_NAME} EXPORT ${PROJECT_NAME}Targets
+          RUNTIME DESTINATION ${ABSL_INSTALL_BINDIR}
+          LIBRARY DESTINATION ${ABSL_INSTALL_LIBDIR}
+          ARCHIVE DESTINATION ${ABSL_INSTALL_LIBDIR}
+    )
+  endif()
 
     add_library(absl::${ABSL_CC_LIB_NAME} ALIAS ${_NAME})
-  endif()
 endfunction()
 
 # absl_cc_test()
@@ -221,23 +300,42 @@
   )
 
   set(_NAME "absl_${ABSL_CC_TEST_NAME}")
+
   add_executable(${_NAME} "")
   target_sources(${_NAME} PRIVATE ${ABSL_CC_TEST_SRCS})
   target_include_directories(${_NAME}
     PUBLIC ${ABSL_COMMON_INCLUDE_DIRS}
     PRIVATE ${GMOCK_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS}
   )
-  target_compile_definitions(${_NAME}
-    PUBLIC ${ABSL_CC_TEST_DEFINES}
-  )
+
+  if (${ABSL_BUILD_DLL})
+    target_compile_definitions(${_NAME}
+      PUBLIC
+        ${ABSL_CC_TEST_DEFINES}
+        ABSL_CONSUME_DLL
+        GTEST_LINKED_AS_SHARED_LIBRARY=1
+    )
+
+    # Replace dependencies on targets inside the DLL with abseil_dll itself.
+    absl_internal_dll_targets(
+      DEPS ${ABSL_CC_TEST_DEPS}
+      OUTPUT ABSL_CC_TEST_DEPS
+    )
+  else()
+    target_compile_definitions(${_NAME}
+      PUBLIC
+        ${ABSL_CC_TEST_DEFINES}
+    )
+  endif()
   target_compile_options(${_NAME}
     PRIVATE ${ABSL_CC_TEST_COPTS}
   )
+
   target_link_libraries(${_NAME}
     PUBLIC ${ABSL_CC_TEST_DEPS}
     PRIVATE ${ABSL_CC_TEST_LINKOPTS}
   )
-  # Add all Abseil targets to a a folder in the IDE for organization.
+  # Add all Abseil targets to a folder in the IDE for organization.
   set_property(TARGET ${_NAME} PROPERTY FOLDER ${ABSL_IDE_FOLDER}/test)
 
   set_property(TARGET ${_NAME} PROPERTY CXX_STANDARD ${ABSL_CXX_STANDARD})
diff --git a/CMake/Googletest/DownloadGTest.cmake b/CMake/Googletest/DownloadGTest.cmake
index 3c682ae..8a00b45 100644
--- a/CMake/Googletest/DownloadGTest.cmake
+++ b/CMake/Googletest/DownloadGTest.cmake
@@ -7,6 +7,13 @@
   ${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt
 )
 
+set(ABSL_SAVE_CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
+set(ABSL_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+if (BUILD_SHARED_LIBS)
+  set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DGTEST_CREATE_SHARED_LIBRARY=1")
+endif()
+
 # Configure and build the downloaded googletest source
 execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .
   RESULT_VARIABLE result
@@ -22,6 +29,9 @@
   message(FATAL_ERROR "Build step for googletest failed: ${result}")
 endif()
 
+set(CMAKE_CXX_FLAGS ${ABSL_SAVE_CMAKE_CXX_FLAGS})
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${ABSL_SAVE_CMAKE_RUNTIME_OUTPUT_DIRECTORY})
+
 # Prevent overriding the parent project's compiler/linker settings on Windows
 set(gtest_force_shared_crt ON CACHE BOOL "" FORCE)
 
diff --git a/CMake/README.md b/CMake/README.md
index 469dfef..04d5df3 100644
--- a/CMake/README.md
+++ b/CMake/README.md
@@ -85,13 +85,15 @@
 Here's a non-exhaustive list of Abseil CMake public targets:
 
 ```cmake
-absl::base
 absl::algorithm
+absl::base
 absl::debugging
 absl::flat_hash_map
+absl::flags
 absl::memory
 absl::meta
 absl::numeric
+absl::random
 absl::strings
 absl::synchronization
 absl::time
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 74a3a4c..48cb6eb 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,6 +32,12 @@
 
 project(absl CXX)
 
+# Output directory is correct by default for most build setups. However, when
+# building Abseil as a DLL, it is important to have the DLL in the same
+# directory as the executable using it. Thus, we put all executables in a single
+# /bin directory.
+set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+
 # when absl is included as subproject (i.e. using add_subdirectory(abseil-cpp))
 # in the source tree of a project that uses it, install rules are disabled.
 if(NOT "^${CMAKE_SOURCE_DIR}$" STREQUAL "^${PROJECT_SOURCE_DIR}$")
@@ -47,6 +53,7 @@
 
 include(AbseilInstallDirs)
 include(CMakePackageConfigHelpers)
+include(AbseilDll)
 include(AbseilHelpers)
 
 
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f4cb4a2..9dadae9 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -123,10 +123,13 @@
 
 ## Running Tests
 
-Use "bazel test <>" functionality to run the unit tests.
+If you have [Bazel](https://bazel.build/) installed, use `bazel test
+--test_tag_filters="-benchmark" ...` to run the unit tests.
 
-Prerequisites for building and running tests are listed in
-[README.md](README.md)
+If you are running the Linux operating system and have
+[Docker](https://www.docker.com/) installed, you can also run the `linux_*.sh`
+scripts under the `ci/`(https://github.com/abseil/abseil-cpp/tree/master/ci)
+directory to test Abseil under a variety of conditions.
 
 ## Abseil Committers
 
diff --git a/FAQ.md b/FAQ.md
new file mode 100644
index 0000000..78028fc
--- /dev/null
+++ b/FAQ.md
@@ -0,0 +1,164 @@
+# Abseil FAQ
+
+## Is Abseil the right home for my utility library?
+
+Most often the answer to the question is "no." As both the [About
+Abseil](https://abseil.io/about/) page and our [contributing
+guidelines](https://github.com/abseil/abseil-cpp/blob/master/CONTRIBUTING.md#contribution-guidelines)
+explain, Abseil contains a variety of core C++ library code that is widely used
+at [Google](https://www.google.com/). As such, Abseil's primary purpose is to be
+used as a dependency by Google's open source C++ projects. While we do hope that
+Abseil is also useful to the C++ community at large, this added constraint also
+means that we are unlikely to accept a contribution of utility code that isn't
+already widely used by Google.
+
+## How to I set the C++ dialect used to build Abseil?
+
+The short answer is that whatever mechanism you choose, you need to make sure
+that you set this option consistently at the global level for your entire
+project. If, for example, you want to set the C++ dialect to C++17, with
+[Bazel](https://bazel/build/) as the build system and `gcc` or `clang` as the
+compiler, there several ways to do this:
+* Pass `--cxxopt=-std=c++17` on the command line (for example, `bazel build
+  --cxxopt=-std=c++17 ...`)
+* Set the environment variable `BAZEL_CXXOPTS` (for example,
+  `BAZEL_CXXOPTS=-std=c++17`)
+* Add `build --cxxopt=-std=c++17` to your [`.bazelrc`
+  file](https://docs.bazel.build/versions/master/guide.html#bazelrc)
+
+If you are using CMake as the build system, you'll need to add a line like
+`set(CMAKE_CXX_STANDARD 17)` to your top level `CMakeLists.txt` file. See the
+[CMake build
+instructions](https://github.com/abseil/abseil-cpp/blob/master/CMake/README.md)
+for more information.
+
+For a longer answer to this question and to understand why some other approaches
+don't work, see the answer to ["What is ABI and why don't you recommend using a
+pre-compiled version of
+Abseil?"](#what-is-abi-and-why-dont-you-recommend-using-a-pre-compiled-version-of-abseil)
+
+## What is ABI and why don't you recommend using a pre-compiled version of Abseil?
+
+For the purposes of this discussion, you can think of
+[ABI](https://en.wikipedia.org/wiki/Application_binary_interface) as the
+compiled representation of the interfaces in code. This is in contrast to
+[API](https://en.wikipedia.org/wiki/Application_programming_interface), which
+you can think of as the interfaces as defined by the code itself. [Abseil has a
+strong promise of API compatibility, but does not make any promise of ABI
+compatibility](https://abseil.io/about/compatibility). Let's take a look at what
+this means in practice.
+
+You might be tempted to do something like this in a
+[Bazel](https://bazel.build/) `BUILD` file:
+
+```
+# DON'T DO THIS!!!
+cc_library(
+    name = "my_library",
+    srcs = ["my_library.cc"],
+    copts = ["-std=c++17"],  # May create a mixed-mode compile!
+    deps = ["@com_google_absl//absl/strings"],
+)
+```
+
+Applying `-std=c++17` to an individual target in your `BUILD` file is going to
+compile that specific target in C++17 mode, but it isn't going to ensure the
+Abseil library is built in C++17 mode, since the Abseil library itself is a
+different build target. If your code includes an Abseil header, then your
+program may contain conflicting definitions of the same
+class/function/variable/enum, etc. As a rule, all compile options that affect
+the ABI of a program need to be applied to the entire build on a global basis.
+
+C++ has something called the [One Definition
+Rule](https://en.wikipedia.org/wiki/One_Definition_Rule) (ODR). C++ doesn't
+allow multiple definitions of the same class/function/variable/enum, etc. ODR
+violations sometimes result in linker errors, but linkers do not always catch
+violations. Uncaught ODR violations can result in strange runtime behaviors or
+crashes that can be hard to debug.
+
+If you build the Abseil library and your code using different compile options
+that affect ABI, there is a good chance you will run afoul of the One Definition
+Rule. Examples of GCC compile options that affect ABI include (but aren't
+limited to) language dialect (e.g. `-std=`), optimization level (e.g. `-O2`),
+code generation flags (e.g. `-fexceptions`), and preprocessor defines
+(e.g. `-DNDEBUG`).
+
+If you use a pre-compiled version of Abseil, (for example, from your Linux
+distribution package manager or from something like
+[vcpkg](https://github.com/microsoft/vcpkg)) you have to be very careful to
+ensure ABI compatibility across the components of your program. The only way you
+can be sure your program is going to be correct regarding ABI is to ensure
+you've used the exact same compile options as were used to build the
+pre-compiled library. This does not mean that Abseil cannot work as part of a
+Linux distribution since a knowledgeable binary packager will have ensured that
+all packages have been built with consistent compile options. This is one of the
+reasons we warn against - though do not outright reject - using Abseil as a
+pre-compiled library.
+
+Another possible way that you might afoul of ABI issues is if you accidentally
+include two versions of Abseil in your program. Multiple versions of Abseil can
+end up within the same binary if your program uses the Abseil library and
+another library also transitively depends on Abseil (resulting in what is
+sometimes called the diamond dependency problem). In cases such as this you must
+structure your build so that all libraries use the same version of Abseil.
+[Abseil's strong promise of API compatibility between
+releases](https://abseil.io/about/compatibility) means the latest "HEAD" release
+of Abseil is almost certainly the right choice if you are doing as we recommend
+and building all of your code from source.
+
+For these reasons we recommend you avoid pre-compiled code and build the Abseil
+library yourself in a consistent manner with the rest of your code.
+
+## What is "live at head" and how do I do it?
+
+From Abseil's point-of-view, "live at head" means that every Abseil source
+release (which happens on an almost daily basis) is either API compatible with
+the previous release, or comes with an automated tool that you can run over code
+to make it compatible. In practice, the need to use an automated tool is
+extremely rare. This means that upgrading from one source release to another
+should be a routine practice that can and should be performed often.
+
+We recommend you update to the [latest commit in the `master` branch of
+Abseil](https://github.com/abseil/abseil-cpp/commits/master) as often as
+possible. Not only will you pick up bug fixes more quickly, but if you have good
+automated testing, you will catch and be able to fix any [Hyrum's
+Law](https://www.hyrumslaw.com/) dependency problems on an incremental basis
+instead of being overwhelmed by them and having difficulty isolating them if you
+wait longer between updates.
+
+If you are using the [Bazel](https://bazel.build/) build system and its
+[external dependencies](https://docs.bazel.build/versions/master/external.html)
+feature, updating the
+[`http_archive`](https://docs.bazel.build/versions/master/repo/http.html#http_archive)
+rule in your
+[`WORKSPACE`](https://docs.bazel.build/versions/master/be/workspace.html) for
+`com_google_abseil` to point to the [latest commit in the `master` branch of
+Abseil](https://github.com/abseil/abseil-cpp/commits/master) is all you need to
+do. For example, on February 11, 2020, the latest commit to the master branch
+was `98eb410c93ad059f9bba1bf43f5bb916fc92a5ea`. To update to this commit, you
+would add the following snippet to your `WORKSPACE` file:
+
+```
+http_archive(
+  name = "com_google_absl",
+  urls = ["https://github.com/abseil/abseil-cpp/archive/98eb410c93ad059f9bba1bf43f5bb916fc92a5ea.zip"],  # 2020-02-11T18:50:53Z
+  strip_prefix = "abseil-cpp-98eb410c93ad059f9bba1bf43f5bb916fc92a5ea",
+  sha256 = "aabf6c57e3834f8dc3873a927f37eaf69975d4b28117fc7427dfb1c661542a87",
+)
+```
+
+To get the `sha256` of this URL, run `curl -sL --output -
+https://github.com/abseil/abseil-cpp/archive/98eb410c93ad059f9bba1bf43f5bb916fc92a5ea.zip
+| sha256sum -`.
+
+You can commit the updated `WORKSPACE` file to your source control every time
+you update, and if you have good automated testing, you might even consider
+automating this.
+
+One thing we don't recommend is using GitHub's `master.zip` files (for example
+[https://github.com/abseil/abseil-cpp/archive/master.zip](https://github.com/abseil/abseil-cpp/archive/master.zip)),
+which are always the latest commit in the `master` branch, to implement live at
+head. Since these `master.zip` URLs are not versioned, you will lose build
+reproducibility. In addition, some build systems, including Bazel, will simply
+cache this file, which means you won't actually be updating to the latest
+release until your cache is cleared or invalidated.
diff --git a/LTS.md b/LTS.md
index 08606f1..94363b6 100644
--- a/LTS.md
+++ b/LTS.md
@@ -12,3 +12,4 @@
 
 * [LTS Branch December 18, 2018](https://github.com/abseil/abseil-cpp/tree/lts_2018_12_18/)
 * [LTS Branch June 20, 2018](https://github.com/abseil/abseil-cpp/tree/lts_2018_06_20/)
+* [LTS Branch August 8, 2019](https://github.com/abseil/abseil-cpp/tree/lts_2019_08_08/)
diff --git a/METADATA b/METADATA
index 1dcdc0a..7ce40ff 100644
--- a/METADATA
+++ b/METADATA
@@ -10,9 +10,9 @@
   }
   url {
     type: GIT
-    value: "https://github.googlesource.com/abseil/abseil-cpp/+/refs/heads/lts_2018_12_18"
+    value: "https://github.googlesource.com/abseil/abseil-cpp/+/refs/heads/lts_2020_02_25"
   }
   version: "fcb104594b0bb4b8ac306cb2f55ecdad40974683"
-  last_upgrade_date { year: 2018 month: 12 day: 18 }
+  last_upgrade_date { year: 2020 month: 04 day: 27 }
 }
 
diff --git a/WORKSPACE b/WORKSPACE
index 49e2d3c..f2b1046 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,17 +1,22 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
 workspace(name = "com_google_absl")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
-# Bazel toolchains
-http_archive(
-  name = "bazel_toolchains",
-  urls = [
-    "https://mirror.bazel.build/github.com/bazelbuild/bazel-toolchains/archive/bc09b995c137df042bb80a395b73d7ce6f26afbe.tar.gz",
-    "https://github.com/bazelbuild/bazel-toolchains/archive/bc09b995c137df042bb80a395b73d7ce6f26afbe.tar.gz",
-  ],
-  strip_prefix = "bazel-toolchains-bc09b995c137df042bb80a395b73d7ce6f26afbe",
-  sha256 = "4329663fe6c523425ad4d3c989a8ac026b04e1acedeceb56aa4b190fa7f3973c",
-)
-
 # GoogleTest/GoogleMock framework. Used by most unit-tests.
 http_archive(
      name = "com_google_googletest",
@@ -27,3 +32,14 @@
     strip_prefix = "benchmark-16703ff83c1ae6d53e5155df3bb3ab0bc96083be",
     sha256 = "59f918c8ccd4d74b6ac43484467b500f1d64b40cc1010daa055375b322a43ba3",
 )
+
+# C++ rules for Bazel.
+http_archive(
+    name = "rules_cc",
+    sha256 = "9a446e9dd9c1bb180c86977a8dc1e9e659550ae732ae58bd2e8fd51e15b2c91d",
+    strip_prefix = "rules_cc-262ebec3c2296296526740db4aefce68c80de7fa",
+    urls = [
+        "https://mirror.bazel.build/github.com/bazelbuild/rules_cc/archive/262ebec3c2296296526740db4aefce68c80de7fa.zip",
+        "https://github.com/bazelbuild/rules_cc/archive/262ebec3c2296296526740db4aefce68c80de7fa.zip",
+    ],
+)
diff --git a/absl/BUILD.bazel b/absl/BUILD.bazel
index 853330d..5a03acf 100644
--- a/absl/BUILD.bazel
+++ b/absl/BUILD.bazel
@@ -14,12 +14,15 @@
 # limitations under the License.
 #
 
+load(
+    ":compiler_config_setting.bzl",
+    "create_llvm_config",
+)
+
 package(default_visibility = ["//visibility:public"])
 
 licenses(["notice"])  # Apache 2.0
 
-load(":compiler_config_setting.bzl", "create_llvm_config")
-
 create_llvm_config(
     name = "llvm_compiler",
     visibility = [":__subpackages__"],
diff --git a/absl/CMakeLists.txt b/absl/CMakeLists.txt
index 3e78397..fbfa782 100644
--- a/absl/CMakeLists.txt
+++ b/absl/CMakeLists.txt
@@ -14,20 +14,24 @@
 # limitations under the License.
 #
 
-
-
 add_subdirectory(base)
 add_subdirectory(algorithm)
 add_subdirectory(container)
 add_subdirectory(debugging)
 add_subdirectory(flags)
+add_subdirectory(functional)
 add_subdirectory(hash)
 add_subdirectory(memory)
 add_subdirectory(meta)
 add_subdirectory(numeric)
 add_subdirectory(random)
+add_subdirectory(status)
 add_subdirectory(strings)
 add_subdirectory(synchronization)
 add_subdirectory(time)
 add_subdirectory(types)
 add_subdirectory(utility)
+
+if (${ABSL_BUILD_DLL})
+  absl_make_dll()
+endif()
diff --git a/absl/abseil.podspec.gen.py b/absl/abseil.podspec.gen.py
new file mode 100755
index 0000000..6aefb79
--- /dev/null
+++ b/absl/abseil.podspec.gen.py
@@ -0,0 +1,229 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+"""This script generates abseil.podspec from all BUILD.bazel files.
+
+This is expected to run on abseil git repository with Bazel 1.0 on Linux.
+It recursively analyzes BUILD.bazel files using query command of Bazel to
+dump its build rules in XML format. From these rules, it constructs podspec
+structure.
+"""
+
+import argparse
+import collections
+import os
+import re
+import subprocess
+import xml.etree.ElementTree
+
+# Template of root podspec.
+SPEC_TEMPLATE = """
+# This file has been automatically generated from a script.
+# Please make modifications to `abseil.podspec.gen.py` instead.
+Pod::Spec.new do |s|
+  s.name     = 'abseil'
+  s.version  = '${version}'
+  s.summary  = 'Abseil Common Libraries (C++) from Google'
+  s.homepage = 'https://abseil.io'
+  s.license  = 'Apache License, Version 2.0'
+  s.authors  = { 'Abseil Team' => 'abseil-io@googlegroups.com' }
+  s.source = {
+    :git => 'https://github.com/abseil/abseil-cpp.git',
+    :tag => '${tag}',
+  }
+  s.module_name = 'absl'
+  s.header_mappings_dir = 'absl'
+  s.header_dir = 'absl'
+  s.libraries = 'c++'
+  s.compiler_flags = '-Wno-everything'
+  s.pod_target_xcconfig = {
+    'USER_HEADER_SEARCH_PATHS' => '$(inherited) "$(PODS_TARGET_SRCROOT)"',
+    'USE_HEADERMAP' => 'NO',
+    'ALWAYS_SEARCH_USER_PATHS' => 'NO',
+  }
+  s.ios.deployment_target = '7.0'
+  s.osx.deployment_target = '10.9'
+  s.tvos.deployment_target = '9.0'
+  s.watchos.deployment_target = '2.0'
+"""
+
+# Rule object representing the rule of Bazel BUILD.
+Rule = collections.namedtuple(
+    "Rule", "type name package srcs hdrs textual_hdrs deps visibility testonly")
+
+
+def get_elem_value(elem, name):
+  """Returns the value of XML element with the given name."""
+  for child in elem:
+    if child.attrib.get("name") != name:
+      continue
+    if child.tag == "string":
+      return child.attrib.get("value")
+    if child.tag == "boolean":
+      return child.attrib.get("value") == "true"
+    if child.tag == "list":
+      return [nested_child.attrib.get("value") for nested_child in child]
+    raise "Cannot recognize tag: " + child.tag
+  return None
+
+
+def normalize_paths(paths):
+  """Returns the list of normalized path."""
+  # e.g. ["//absl/strings:dir/header.h"] -> ["absl/strings/dir/header.h"]
+  return [path.lstrip("/").replace(":", "/") for path in paths]
+
+
+def parse_rule(elem, package):
+  """Returns a rule from bazel XML rule."""
+  return Rule(
+      type=elem.attrib["class"],
+      name=get_elem_value(elem, "name"),
+      package=package,
+      srcs=normalize_paths(get_elem_value(elem, "srcs") or []),
+      hdrs=normalize_paths(get_elem_value(elem, "hdrs") or []),
+      textual_hdrs=normalize_paths(get_elem_value(elem, "textual_hdrs") or []),
+      deps=get_elem_value(elem, "deps") or [],
+      visibility=get_elem_value(elem, "visibility") or [],
+      testonly=get_elem_value(elem, "testonly") or False)
+
+
+def read_build(package):
+  """Runs bazel query on given package file and returns all cc rules."""
+  result = subprocess.check_output(
+      ["bazel", "query", package + ":all", "--output", "xml"])
+  root = xml.etree.ElementTree.fromstring(result)
+  return [
+      parse_rule(elem, package)
+      for elem in root
+      if elem.tag == "rule" and elem.attrib["class"].startswith("cc_")
+  ]
+
+
+def collect_rules(root_path):
+  """Collects and returns all rules from root path recursively."""
+  rules = []
+  for cur, _, _ in os.walk(root_path):
+    build_path = os.path.join(cur, "BUILD.bazel")
+    if os.path.exists(build_path):
+      rules.extend(read_build("//" + cur))
+  return rules
+
+
+def relevant_rule(rule):
+  """Returns true if a given rule is relevant when generating a podspec."""
+  return (
+      # cc_library only (ignore cc_test, cc_binary)
+      rule.type == "cc_library" and
+      # ignore empty rule
+      (rule.hdrs + rule.textual_hdrs + rule.srcs) and
+      # ignore test-only rule
+      not rule.testonly)
+
+
+def get_spec_var(depth):
+  """Returns the name of variable for spec with given depth."""
+  return "s" if depth == 0 else "s{}".format(depth)
+
+
+def get_spec_name(label):
+  """Converts the label of bazel rule to the name of podspec."""
+  assert label.startswith("//absl/"), "{} doesn't start with //absl/".format(
+      label)
+  # e.g. //absl/apple/banana -> abseil/apple/banana
+  return "abseil/" + label[7:]
+
+
+def write_podspec(f, rules, args):
+  """Writes a podspec from given rules and args."""
+  rule_dir = build_rule_directory(rules)["abseil"]
+  # Write root part with given arguments
+  spec = re.sub(r"\$\{(\w+)\}", lambda x: args[x.group(1)],
+                SPEC_TEMPLATE).lstrip()
+  f.write(spec)
+  # Write all target rules
+  write_podspec_map(f, rule_dir, 0)
+  f.write("end\n")
+
+
+def build_rule_directory(rules):
+  """Builds a tree-style rule directory from given rules."""
+  rule_dir = {}
+  for rule in rules:
+    cur = rule_dir
+    for frag in get_spec_name(rule.package).split("/"):
+      cur = cur.setdefault(frag, {})
+    cur[rule.name] = rule
+  return rule_dir
+
+
+def write_podspec_map(f, cur_map, depth):
+  """Writes podspec from rule map recursively."""
+  for key, value in sorted(cur_map.items()):
+    indent = "  " * (depth + 1)
+    f.write("{indent}{var0}.subspec '{key}' do |{var1}|\n".format(
+        indent=indent,
+        key=key,
+        var0=get_spec_var(depth),
+        var1=get_spec_var(depth + 1)))
+    if isinstance(value, dict):
+      write_podspec_map(f, value, depth + 1)
+    else:
+      write_podspec_rule(f, value, depth + 1)
+    f.write("{indent}end\n".format(indent=indent))
+
+
+def write_podspec_rule(f, rule, depth):
+  """Writes podspec from given rule."""
+  indent = "  " * (depth + 1)
+  spec_var = get_spec_var(depth)
+  # Puts all files in hdrs, textual_hdrs, and srcs into source_files.
+  # Since CocoaPods treats header_files a bit differently from bazel,
+  # this won't generate a header_files field so that all source_files
+  # are considered as header files.
+  srcs = sorted(set(rule.hdrs + rule.textual_hdrs + rule.srcs))
+  write_indented_list(
+      f, "{indent}{var}.source_files = ".format(indent=indent, var=spec_var),
+      srcs)
+  # Writes dependencies of this rule.
+  for dep in sorted(rule.deps):
+    name = get_spec_name(dep.replace(":", "/"))
+    f.write("{indent}{var}.dependency '{dep}'\n".format(
+        indent=indent, var=spec_var, dep=name))
+
+
+def write_indented_list(f, leading, values):
+  """Writes leading values in an indented style."""
+  f.write(leading)
+  f.write((",\n" + " " * len(leading)).join("'{}'".format(v) for v in values))
+  f.write("\n")
+
+
+def generate(args):
+  """Generates a podspec file from all BUILD files under absl directory."""
+  rules = filter(relevant_rule, collect_rules("absl"))
+  with open(args.output, "wt") as f:
+    write_podspec(f, rules, vars(args))
+
+
+def main():
+  parser = argparse.ArgumentParser(
+      description="Generates abseil.podspec from BUILD.bazel")
+  parser.add_argument(
+      "-v", "--version", help="The version of podspec", required=True)
+  parser.add_argument(
+      "-t",
+      "--tag",
+      default=None,
+      help="The name of git tag (default: version)")
+  parser.add_argument(
+      "-o",
+      "--output",
+      default="abseil.podspec",
+      help="The name of output file (default: abseil.podspec)")
+  args = parser.parse_args()
+  if args.tag is None:
+    args.tag = args.version
+  generate(args)
+
+
+if __name__ == "__main__":
+  main()
diff --git a/absl/algorithm/BUILD.bazel b/absl/algorithm/BUILD.bazel
index c506f3b..6a96420 100644
--- a/absl/algorithm/BUILD.bazel
+++ b/absl/algorithm/BUILD.bazel
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -30,6 +31,7 @@
     hdrs = ["algorithm.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
diff --git a/absl/algorithm/CMakeLists.txt b/absl/algorithm/CMakeLists.txt
index 9fbe36f..56cd0fb 100644
--- a/absl/algorithm/CMakeLists.txt
+++ b/absl/algorithm/CMakeLists.txt
@@ -21,6 +21,8 @@
     "algorithm.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   PUBLIC
 )
 
diff --git a/absl/algorithm/algorithm.h b/absl/algorithm/algorithm.h
index 7c2b787..e9b4733 100644
--- a/absl/algorithm/algorithm.h
+++ b/absl/algorithm/algorithm.h
@@ -26,8 +26,10 @@
 #include <iterator>
 #include <type_traits>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace algorithm_internal {
 
@@ -85,6 +87,8 @@
 
 }  // namespace algorithm_internal
 
+// equal()
+//
 // Compares the equality of two ranges specified by pairs of iterators, using
 // the given predicate, returning true iff for each corresponding iterator i1
 // and i2 in the first and second range respectively, pred(*i1, *i2) == true
@@ -105,8 +109,8 @@
       typename std::iterator_traits<InputIter2>::iterator_category{});
 }
 
-// Performs comparison of two ranges specified by pairs of iterators using
-// operator==.
+// Overload of equal() that performs comparison of two ranges specified by pairs
+// of iterators using operator==.
 template <typename InputIter1, typename InputIter2>
 bool equal(InputIter1 first1, InputIter1 last1, InputIter2 first2,
            InputIter2 last2) {
@@ -114,6 +118,8 @@
                      algorithm_internal::EqualTo{});
 }
 
+// linear_search()
+//
 // Performs a linear search for `value` using the iterator `first` up to
 // but not including `last`, returning true if [`first`, `last`) contains an
 // element equal to `value`.
@@ -127,6 +133,8 @@
   return std::find(first, last, value) != last;
 }
 
+// rotate()
+//
 // Performs a left rotation on a range of elements (`first`, `last`) such that
 // `middle` is now the first element. `rotate()` returns an iterator pointing to
 // the first element before rotation. This function is exactly the same as
@@ -136,7 +144,6 @@
 // The complexity of this algorithm is the same as that of `std::rotate`, but if
 // `ForwardIterator` is not a random-access iterator, then `absl::rotate`
 // performs an additional pass over the range to construct the return value.
-
 template <typename ForwardIterator>
 ForwardIterator rotate(ForwardIterator first, ForwardIterator middle,
                        ForwardIterator last) {
@@ -146,7 +153,7 @@
                    ForwardIterator>());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_ALGORITHM_ALGORITHM_H_
diff --git a/absl/algorithm/container.h b/absl/algorithm/container.h
index 16389be..d72532d 100644
--- a/absl/algorithm/container.h
+++ b/absl/algorithm/container.h
@@ -55,7 +55,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_algorithm_internal {
 
 // NOTE: it is important to defer to ADL lookup for building with C++ modules,
@@ -113,6 +113,18 @@
 struct IsUnorderedContainer<std::unordered_set<Key, Hash, KeyEqual, Allocator>>
     : std::true_type {};
 
+// container_algorithm_internal::c_size. It is meant for internal use only.
+
+template <class C>
+auto c_size(C& c) -> decltype(c.size()) {
+  return c.size();
+}
+
+template <class T, std::size_t N>
+constexpr std::size_t c_size(T (&)[N]) {
+  return N;
+}
+
 }  // namespace container_algorithm_internal
 
 // PUBLIC API
@@ -257,7 +269,8 @@
 // c_find_first_of()
 //
 // Container-based version of the <algorithm> `std::find_first_of()` function to
-// find the first elements in an ordered set within a container.
+// find the first element within the container that is also within the options
+// container.
 template <typename C1, typename C2>
 container_algorithm_internal::ContainerIter<C1> c_find_first_of(C1& container,
                                                                 C2& options) {
@@ -366,7 +379,8 @@
 
 template <typename C1, typename C2>
 bool c_equal(const C1& c1, const C2& c2) {
-  return ((c1.size() == c2.size()) &&
+  return ((container_algorithm_internal::c_size(c1) ==
+           container_algorithm_internal::c_size(c2)) &&
           std::equal(container_algorithm_internal::c_begin(c1),
                      container_algorithm_internal::c_end(c1),
                      container_algorithm_internal::c_begin(c2)));
@@ -376,7 +390,8 @@
 // the function's test condition.
 template <typename C1, typename C2, typename BinaryPredicate>
 bool c_equal(const C1& c1, const C2& c2, BinaryPredicate&& pred) {
-  return ((c1.size() == c2.size()) &&
+  return ((container_algorithm_internal::c_size(c1) ==
+           container_algorithm_internal::c_size(c2)) &&
           std::equal(container_algorithm_internal::c_begin(c1),
                      container_algorithm_internal::c_end(c1),
                      container_algorithm_internal::c_begin(c2),
@@ -1706,7 +1721,7 @@
                           output_first, std::forward<BinaryOp>(op));
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_ALGORITHM_CONTAINER_H_
diff --git a/absl/algorithm/container_test.cc b/absl/algorithm/container_test.cc
index 86bf9d3..0a4abe9 100644
--- a/absl/algorithm/container_test.cc
+++ b/absl/algorithm/container_test.cc
@@ -163,23 +163,29 @@
 TEST_F(NonMutatingTest, Equal) {
   EXPECT_TRUE(absl::c_equal(vector_, sequence_));
   EXPECT_TRUE(absl::c_equal(sequence_, vector_));
+  EXPECT_TRUE(absl::c_equal(sequence_, array_));
+  EXPECT_TRUE(absl::c_equal(array_, vector_));
 
   // Test that behavior appropriately differs from that of equal().
   std::vector<int> vector_plus = {1, 2, 3};
   vector_plus.push_back(4);
   EXPECT_FALSE(absl::c_equal(vector_plus, sequence_));
   EXPECT_FALSE(absl::c_equal(sequence_, vector_plus));
+  EXPECT_FALSE(absl::c_equal(array_, vector_plus));
 }
 
 TEST_F(NonMutatingTest, EqualWithPredicate) {
   EXPECT_TRUE(absl::c_equal(vector_, sequence_, Equals));
   EXPECT_TRUE(absl::c_equal(sequence_, vector_, Equals));
+  EXPECT_TRUE(absl::c_equal(array_, sequence_, Equals));
+  EXPECT_TRUE(absl::c_equal(vector_, array_, Equals));
 
   // Test that behavior appropriately differs from that of equal().
   std::vector<int> vector_plus = {1, 2, 3};
   vector_plus.push_back(4);
   EXPECT_FALSE(absl::c_equal(vector_plus, sequence_, Equals));
   EXPECT_FALSE(absl::c_equal(sequence_, vector_plus, Equals));
+  EXPECT_FALSE(absl::c_equal(vector_plus, array_, Equals));
 }
 
 TEST_F(NonMutatingTest, IsPermutation) {
diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index a512272..bae7942 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -14,12 +14,11 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
@@ -35,6 +34,21 @@
     visibility = [
         "//absl:__subpackages__",
     ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_library(
+    name = "errno_saver",
+    hdrs = ["internal/errno_saver.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [":config"],
 )
 
 cc_library(
@@ -43,16 +57,27 @@
     hdrs = ["log_severity.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [":core_headers"],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
 )
 
 cc_library(
     name = "raw_logging_internal",
+    srcs = ["internal/raw_logging.cc"],
+    hdrs = ["internal/raw_logging.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
     ],
+    deps = [
+        ":atomic_hook",
+        ":config",
+        ":core_headers",
+        ":log_severity",
+    ],
 )
 
 cc_library(
@@ -64,22 +89,24 @@
         "internal/spinlock_wait.cc",
         "internal/spinlock_win32.inc",
     ],
-    hdrs = [
-        "internal/scheduling_mode.h",
-        "internal/spinlock_wait.h",
-    ],
+    hdrs = ["internal/spinlock_wait.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl/base:__pkg__",
     ],
-    deps = [":core_headers"],
+    deps = [
+        ":base_internal",
+        ":core_headers",
+        ":errno_saver",
+    ],
 )
 
 cc_library(
     name = "config",
     hdrs = [
         "config.h",
+        "options.h",
         "policy_checks.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
@@ -130,14 +157,15 @@
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
     visibility = [
-        "//absl:__subpackages__",
+        "//visibility:public",
     ],
     deps = [
         ":base",
+        ":base_internal",
         ":config",
         ":core_headers",
         ":dynamic_annotations",
-        ":spinlock_wait",
+        ":raw_logging_internal",
     ],
 )
 
@@ -156,6 +184,7 @@
         "//absl:__subpackages__",
     ],
     deps = [
+        ":config",
         "//absl/meta:type_traits",
     ],
 )
@@ -164,7 +193,6 @@
     name = "base",
     srcs = [
         "internal/cycleclock.cc",
-        "internal/raw_logging.cc",
         "internal/spinlock.cc",
         "internal/sysinfo.cc",
         "internal/thread_identity.cc",
@@ -176,7 +204,6 @@
         "internal/cycleclock.h",
         "internal/low_level_scheduling.h",
         "internal/per_thread_tls.h",
-        "internal/raw_logging.h",
         "internal/spinlock.h",
         "internal/sysinfo.h",
         "internal/thread_identity.h",
@@ -185,7 +212,9 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = select({
-        "//absl:windows": [],
+        "//absl:windows": [
+            "-DEFAULTLIB:advapi32.lib",
+        ],
         "//conditions:default": ["-pthread"],
     }) + ABSL_DEFAULT_LINKOPTS,
     deps = [
@@ -195,11 +224,25 @@
         ":core_headers",
         ":dynamic_annotations",
         ":log_severity",
+        ":raw_logging_internal",
         ":spinlock_wait",
         "//absl/meta:type_traits",
     ],
 )
 
+cc_library(
+    name = "atomic_hook_test_helper",
+    testonly = 1,
+    srcs = ["internal/atomic_hook_test_helper.cc"],
+    hdrs = ["internal/atomic_hook_test_helper.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":atomic_hook",
+        ":core_headers",
+    ],
+)
+
 cc_test(
     name = "atomic_hook_test",
     size = "small",
@@ -208,6 +251,7 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":atomic_hook",
+        ":atomic_hook_test_helper",
         ":core_headers",
         "@com_google_googletest//:gtest_main",
     ],
@@ -232,28 +276,41 @@
     name = "throw_delegate",
     srcs = ["internal/throw_delegate.cc"],
     hdrs = ["internal/throw_delegate.h"],
-    copts = ABSL_DEFAULT_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = [
         "//absl:__subpackages__",
     ],
     deps = [
-        ":base",
         ":config",
+        ":raw_logging_internal",
     ],
 )
 
 cc_test(
     name = "throw_delegate_test",
     srcs = ["throw_delegate_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":config",
         ":throw_delegate",
         "@com_google_googletest//:gtest_main",
     ],
 )
 
+cc_test(
+    name = "errno_saver_test",
+    size = "small",
+    srcs = ["internal/errno_saver_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":errno_saver",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "exception_testing",
     testonly = 1,
@@ -281,8 +338,8 @@
     testonly = 1,
     srcs = ["internal/exception_safety_testing.cc"],
     hdrs = ["internal/exception_safety_testing.h"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":config",
         ":pretty_function",
@@ -297,8 +354,8 @@
 cc_test(
     name = "exception_safety_testing_test",
     srcs = ["exception_safety_testing_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":exception_safety_testing",
         "//absl/memory",
@@ -347,8 +404,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":base",
+        ":base_internal",
         ":core_headers",
-        ":spinlock_wait",
         "//absl/synchronization",
         "@com_google_googletest//:gtest",
     ],
@@ -363,8 +420,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":base",
+        ":base_internal",
         ":core_headers",
-        ":spinlock_wait",
         "//absl/synchronization",
         "@com_google_googletest//:gtest_main",
     ],
@@ -382,6 +439,7 @@
     deps = [
         ":base",
         ":base_internal",
+        ":raw_logging_internal",
         "//absl/synchronization",
         "@com_github_google_benchmark//:benchmark_main",
     ],
@@ -455,7 +513,7 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":base",
+        ":raw_logging_internal",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
@@ -519,7 +577,10 @@
     visibility = [
         "//absl:__subpackages__",
     ],
-    deps = [":core_headers"],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
 )
 
 cc_test(
@@ -535,6 +596,75 @@
 )
 
 cc_library(
+    name = "exponential_biased",
+    srcs = ["internal/exponential_biased.cc"],
+    hdrs = ["internal/exponential_biased.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl:__subpackages__",
+    ],
+    deps = [
+        ":config",
+        ":core_headers",
+    ],
+)
+
+cc_test(
+    name = "exponential_biased_test",
+    size = "small",
+    srcs = ["internal/exponential_biased_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":exponential_biased",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "periodic_sampler",
+    srcs = ["internal/periodic_sampler.cc"],
+    hdrs = ["internal/periodic_sampler.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":core_headers",
+        ":exponential_biased",
+    ],
+)
+
+cc_test(
+    name = "periodic_sampler_test",
+    size = "small",
+    srcs = ["internal/periodic_sampler_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":core_headers",
+        ":periodic_sampler",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "periodic_sampler_benchmark",
+    testonly = 1,
+    srcs = ["internal/periodic_sampler_benchmark.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":core_headers",
+        ":periodic_sampler",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
+
+cc_library(
     name = "scoped_set_env",
     testonly = 1,
     srcs = ["internal/scoped_set_env.cc"],
@@ -543,7 +673,10 @@
     visibility = [
         "//absl:__subpackages__",
     ],
-    deps = [":base"],
+    deps = [
+        ":config",
+        ":raw_logging_internal",
+    ],
 )
 
 cc_test(
@@ -565,8 +698,10 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":base",
         ":log_severity",
+        "//absl/flags:flag_internal",
+        "//absl/flags:marshalling",
+        "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
 )
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index cc7960e..14c52ea 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -14,11 +14,27 @@
 # limitations under the License.
 #
 
+find_library(LIBRT rt)
+
 absl_cc_library(
   NAME
     atomic_hook
   HDRS
     "internal/atomic_hook.h"
+  DEPS
+    absl::config
+    absl::core_headers
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+)
+
+absl_cc_library(
+  NAME
+    errno_saver
+  HDRS
+    "internal/errno_saver.h"
+  DEPS
+    absl::config
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
@@ -39,6 +55,15 @@
 absl_cc_library(
   NAME
     raw_logging_internal
+  HDRS
+    "internal/raw_logging.h"
+  SRCS
+    "internal/raw_logging.cc"
+  DEPS
+    absl::atomic_hook
+    absl::config
+    absl::core_headers
+    absl::log_severity
   COPTS
     ${ABSL_DEFAULT_COPTS}
 )
@@ -47,7 +72,6 @@
   NAME
     spinlock_wait
   HDRS
-    "internal/scheduling_mode.h"
     "internal/spinlock_wait.h"
   SRCS
     "internal/spinlock_akaros.inc"
@@ -58,7 +82,9 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::base_internal
     absl::core_headers
+    absl::errno_saver
 )
 
 absl_cc_library(
@@ -66,6 +92,7 @@
     config
   HDRS
     "config.h"
+    "options.h"
     "policy_checks.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
@@ -116,10 +143,11 @@
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
+    absl::base_internal
     absl::config
     absl::core_headers
     absl::dynamic_annotations
-    absl::spinlock_wait
+    absl::raw_logging_internal
     Threads::Threads
 )
 
@@ -131,9 +159,11 @@
     "internal/identity.h"
     "internal/inline_variable.h"
     "internal/invoke.h"
+    "internal/scheduling_mode.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::type_traits
 )
 
@@ -146,23 +176,23 @@
     "internal/cycleclock.h"
     "internal/low_level_scheduling.h"
     "internal/per_thread_tls.h"
-    "internal/raw_logging.h"
     "internal/spinlock.h"
     "internal/sysinfo.h"
     "internal/thread_identity.h"
     "internal/tsan_mutex_interface.h"
     "internal/unscaledcycleclock.h"
-    "log_severity.h"
   SRCS
     "internal/cycleclock.cc"
-    "internal/raw_logging.cc"
     "internal/spinlock.cc"
     "internal/sysinfo.cc"
     "internal/thread_identity.cc"
     "internal/unscaledcycleclock.cc"
-    "log_severity.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${LIBRT}>:${LIBRT}>
+    $<$<BOOL:${MINGW}>:"advapi32">
   DEPS
     absl::atomic_hook
     absl::base_internal
@@ -170,6 +200,7 @@
     absl::core_headers
     absl::dynamic_annotations
     absl::log_severity
+    absl::raw_logging_internal
     absl::spinlock_wait
     absl::type_traits
     Threads::Threads
@@ -185,9 +216,9 @@
     "internal/throw_delegate.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
   DEPS
-    absl::base
+    absl::config
+    absl::raw_logging_internal
 )
 
 absl_cc_library(
@@ -221,9 +252,6 @@
     "internal/exception_safety_testing.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::config
     absl::pretty_function
@@ -242,15 +270,25 @@
     "exception_safety_testing_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::exception_safety_testing
     absl::memory
     gtest_main
 )
 
+absl_cc_library(
+  NAME
+    atomic_hook_test_helper
+  SRCS
+    "internal/atomic_hook_test_helper.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::atomic_hook
+    absl::core_headers
+  TESTONLY
+)
+
 absl_cc_test(
   NAME
     atomic_hook_test
@@ -259,8 +297,10 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::atomic_hook_test_helper
     absl::atomic_hook
     absl::core_headers
+    gmock
     gtest_main
 )
 
@@ -279,6 +319,19 @@
 
 absl_cc_test(
   NAME
+    errno_saver_test
+  SRCS
+    "internal/errno_saver_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::errno_saver
+    gmock
+    gtest_main
+)
+
+absl_cc_test(
+  NAME
     throw_delegate_test
   SRCS
     "throw_delegate_test.cc"
@@ -286,6 +339,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::config
     absl::throw_delegate
     gtest_main
 )
@@ -329,8 +383,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::base_internal
     absl::core_headers
-    absl::spinlock_wait
     absl::synchronization
     gtest
   TESTONLY
@@ -346,8 +400,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::base
+    absl::base_internal
     absl::core_headers
-    absl::spinlock_wait
     absl::synchronization
     gtest_main
 )
@@ -415,7 +469,7 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::base
+    absl::raw_logging_internal
     absl::strings
     gtest_main
 )
@@ -468,6 +522,7 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
 )
 
@@ -485,6 +540,60 @@
 
 absl_cc_library(
   NAME
+    exponential_biased
+  SRCS
+    "internal/exponential_biased.cc"
+  HDRS
+    "internal/exponential_biased.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+)
+
+absl_cc_test(
+  NAME
+    exponential_biased_test
+  SRCS
+    "internal/exponential_biased_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::exponential_biased
+    absl::strings
+    gmock_main
+)
+
+absl_cc_library(
+  NAME
+    periodic_sampler
+  SRCS
+    "internal/periodic_sampler.cc"
+  HDRS
+    "internal/periodic_sampler.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::core_headers
+    absl::exponential_biased
+)
+
+absl_cc_test(
+  NAME
+    periodic_sampler_test
+  SRCS
+    "internal/periodic_sampler_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::core_headers
+    absl::periodic_sampler
+    gmock_main
+)
+
+absl_cc_library(
+  NAME
     scoped_set_env
   SRCS
     "internal/scoped_set_env.cc"
@@ -493,7 +602,8 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::base
+    absl::config
+    absl::raw_logging_internal
 )
 
 absl_cc_test(
@@ -525,8 +635,10 @@
   SRCS
     "log_severity_test.cc"
   DEPS
-    absl::base
+    absl::flags_internal
+    absl::flags_marshalling
     absl::log_severity
+    absl::strings
     gmock
     gtest_main
 )
diff --git a/absl/base/attributes.h b/absl/base/attributes.h
index a7da62a..ff13862 100644
--- a/absl/base/attributes.h
+++ b/absl/base/attributes.h
@@ -157,10 +157,12 @@
 // Tags a function as weak for the purposes of compilation and linking.
 // Weak attributes currently do not work properly in LLVM's Windows backend,
 // so disable them there. See https://bugs.llvm.org/show_bug.cgi?id=37598
-// for futher information.
-#if (ABSL_HAVE_ATTRIBUTE(weak) || \
+// for further information.
+// The MinGW compiler doesn't complain about the weak attribute until the link
+// step, presumably because Windows doesn't use ELF binaries.
+#if (ABSL_HAVE_ATTRIBUTE(weak) ||                   \
      (defined(__GNUC__) && !defined(__clang__))) && \
-    !(defined(__llvm__) && defined(_WIN32))
+    !(defined(__llvm__) && defined(_WIN32)) && !defined(__MINGW32__)
 #undef ABSL_ATTRIBUTE_WEAK
 #define ABSL_ATTRIBUTE_WEAK __attribute__((weak))
 #define ABSL_HAVE_ATTRIBUTE_WEAK 1
@@ -561,7 +563,19 @@
 
 // ABSL_ATTRIBUTE_PACKED
 //
-// Prevents the compiler from padding a structure to natural alignment
+// Instructs the compiler not to use natural alignment for a tagged data
+// structure, but instead to reduce its alignment to 1. This attribute can
+// either be applied to members of a structure or to a structure in its
+// entirety. Applying this attribute (judiciously) to a structure in its
+// entirety to optimize the memory footprint of very commonly-used structs is
+// fine. Do not apply this attribute to a structure in its entirety if the
+// purpose is to control the offsets of the members in the structure. Instead,
+// apply this attribute only to structure members that need it.
+//
+// When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
+// natural alignment of structure members not annotated is preserved. Aligned
+// member accesses are faster than non-aligned member accesses even if the
+// targeted microprocessor supports non-aligned accesses.
 #if ABSL_HAVE_ATTRIBUTE(packed) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_PACKED __attribute__((__packed__))
 #else
@@ -599,7 +613,6 @@
 //
 // Note that this attribute is redundant if the variable is declared constexpr.
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
-// NOLINTNEXTLINE(whitespace/braces)
 #define ABSL_CONST_INIT [[clang::require_constant_initialization]]
 #else
 #define ABSL_CONST_INIT
diff --git a/absl/base/bit_cast_test.cc b/absl/base/bit_cast_test.cc
index be20185..8a3a41e 100644
--- a/absl/base/bit_cast_test.cc
+++ b/absl/base/bit_cast_test.cc
@@ -22,7 +22,7 @@
 #include "absl/base/macros.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 template <int N>
@@ -105,5 +105,5 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/call_once.h b/absl/base/call_once.h
index 373f015..bc5ec93 100644
--- a/absl/base/call_once.h
+++ b/absl/base/call_once.h
@@ -41,7 +41,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 class once_flag;
 
@@ -143,12 +143,13 @@
 };
 
 template <typename Callable, typename... Args>
+ABSL_ATTRIBUTE_NOINLINE
 void CallOnceImpl(std::atomic<uint32_t>* control,
                   base_internal::SchedulingMode scheduling_mode, Callable&& fn,
                   Args&&... args) {
 #ifndef NDEBUG
   {
-    uint32_t old_control = control->load(std::memory_order_acquire);
+    uint32_t old_control = control->load(std::memory_order_relaxed);
     if (old_control != kOnceInit &&
         old_control != kOnceRunning &&
         old_control != kOnceWaiter &&
@@ -166,14 +167,23 @@
   // Must do this before potentially modifying control word's state.
   base_internal::SchedulingHelper maybe_disable_scheduling(scheduling_mode);
   // Short circuit the simplest case to avoid procedure call overhead.
+  // The base_internal::SpinLockWait() call returns either kOnceInit or
+  // kOnceDone. If it returns kOnceDone, it must have loaded the control word
+  // with std::memory_order_acquire and seen a value of kOnceDone.
   uint32_t old_control = kOnceInit;
   if (control->compare_exchange_strong(old_control, kOnceRunning,
-                                       std::memory_order_acquire,
                                        std::memory_order_relaxed) ||
       base_internal::SpinLockWait(control, ABSL_ARRAYSIZE(trans), trans,
                                   scheduling_mode) == kOnceInit) {
     base_internal::Invoke(std::forward<Callable>(fn),
                           std::forward<Args>(args)...);
+    // The call to SpinLockWake below is an optimization, because the waiter
+    // in SpinLockWait is waiting with a short timeout. The atomic load/store
+    // sequence is slightly faster than an atomic exchange:
+    //   old_control = control->exchange(base_internal::kOnceDone,
+    //                                   std::memory_order_release);
+    // We opt for a slightly faster case when there are no waiters, in spite
+    // of longer tail latency when there are waiters.
     old_control = control->load(std::memory_order_relaxed);
     control->store(base_internal::kOnceDone, std::memory_order_release);
     if (old_control == base_internal::kOnceWaiter) {
@@ -210,7 +220,7 @@
   }
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CALL_ONCE_H_
diff --git a/absl/base/call_once_test.cc b/absl/base/call_once_test.cc
index 0e89bd2..11d26c4 100644
--- a/absl/base/call_once_test.cc
+++ b/absl/base/call_once_test.cc
@@ -24,18 +24,18 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 absl::once_flag once;
 
 ABSL_CONST_INIT Mutex counters_mu(absl::kConstInit);
 
-int running_thread_count GUARDED_BY(counters_mu) = 0;
-int call_once_invoke_count GUARDED_BY(counters_mu) = 0;
-int call_once_finished_count GUARDED_BY(counters_mu) = 0;
-int call_once_return_count GUARDED_BY(counters_mu) = 0;
-bool done_blocking GUARDED_BY(counters_mu) = false;
+int running_thread_count ABSL_GUARDED_BY(counters_mu) = 0;
+int call_once_invoke_count ABSL_GUARDED_BY(counters_mu) = 0;
+int call_once_finished_count ABSL_GUARDED_BY(counters_mu) = 0;
+int call_once_return_count ABSL_GUARDED_BY(counters_mu) = 0;
+bool done_blocking ABSL_GUARDED_BY(counters_mu) = false;
 
 // Function to be called from absl::call_once.  Waits for a notification.
 void WaitAndIncrement() {
@@ -61,7 +61,7 @@
 }
 
 // Returns true if all threads are set up for the test.
-bool ThreadsAreSetup(void*) EXCLUSIVE_LOCKS_REQUIRED(counters_mu) {
+bool ThreadsAreSetup(void*) ABSL_EXCLUSIVE_LOCKS_REQUIRED(counters_mu) {
   // All ten threads must be running, and WaitAndIncrement should be blocked.
   return running_thread_count == 10 && call_once_invoke_count == 1;
 }
@@ -103,5 +103,5 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/casts.h b/absl/base/casts.h
index b67b047..322cc1d 100644
--- a/absl/base/casts.h
+++ b/absl/base/casts.h
@@ -34,7 +34,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace internal_casts {
 
@@ -178,7 +178,7 @@
   return dest;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CASTS_H_
diff --git a/absl/base/config.h b/absl/base/config.h
index 1c3cb08..ee99f94 100644
--- a/absl/base/config.h
+++ b/absl/base/config.h
@@ -63,8 +63,72 @@
 #include <TargetConditionals.h>
 #endif
 
+#include "absl/base/options.h"
 #include "absl/base/policy_checks.h"
 
+// Helper macro to convert a CPP variable to a string literal.
+#define ABSL_INTERNAL_DO_TOKEN_STR(x) #x
+#define ABSL_INTERNAL_TOKEN_STR(x) ABSL_INTERNAL_DO_TOKEN_STR(x)
+
+// -----------------------------------------------------------------------------
+// Abseil namespace annotations
+// -----------------------------------------------------------------------------
+
+// ABSL_NAMESPACE_BEGIN/ABSL_NAMESPACE_END
+//
+// An annotation placed at the beginning/end of each `namespace absl` scope.
+// This is used to inject an inline namespace.
+//
+// The proper way to write Abseil code in the `absl` namespace is:
+//
+// namespace absl {
+// ABSL_NAMESPACE_BEGIN
+//
+// void Foo();  // absl::Foo().
+//
+// ABSL_NAMESPACE_END
+// }  // namespace absl
+//
+// Users of Abseil should not use these macros, because users of Abseil should
+// not write `namespace absl {` in their own code for any reason.  (Abseil does
+// not support forward declarations of its own types, nor does it support
+// user-provided specialization of Abseil templates.  Code that violates these
+// rules may be broken without warning.)
+#if !defined(ABSL_OPTION_USE_INLINE_NAMESPACE) || \
+    !defined(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+#error options.h is misconfigured.
+#endif
+
+// Check that ABSL_OPTION_INLINE_NAMESPACE_NAME is neither "head" nor ""
+#if defined(__cplusplus) && ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+
+#define ABSL_INTERNAL_INLINE_NAMESPACE_STR \
+  ABSL_INTERNAL_TOKEN_STR(ABSL_OPTION_INLINE_NAMESPACE_NAME)
+
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "not be empty.");
+static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[1] != 'e' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[2] != 'a' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[3] != 'd' ||
+                  ABSL_INTERNAL_INLINE_NAMESPACE_STR[4] != '\0',
+              "options.h misconfigured: ABSL_OPTION_INLINE_NAMESPACE_NAME must "
+              "be changed to a new, unique identifier name.");
+
+#endif
+
+#if ABSL_OPTION_USE_INLINE_NAMESPACE == 0
+#define ABSL_NAMESPACE_BEGIN
+#define ABSL_NAMESPACE_END
+#elif ABSL_OPTION_USE_INLINE_NAMESPACE == 1
+#define ABSL_NAMESPACE_BEGIN \
+  inline namespace ABSL_OPTION_INLINE_NAMESPACE_NAME {
+#define ABSL_NAMESPACE_END }
+#else
+#error options.h is misconfigured.
+#endif
+
 // -----------------------------------------------------------------------------
 // Compiler Feature Checks
 // -----------------------------------------------------------------------------
@@ -84,6 +148,12 @@
 #define ABSL_HAVE_BUILTIN(x) 0
 #endif
 
+#if defined(__is_identifier)
+#define ABSL_INTERNAL_HAS_KEYWORD(x) !(__is_identifier(x))
+#else
+#define ABSL_INTERNAL_HAS_KEYWORD(x) 0
+#endif
+
 // ABSL_HAVE_TLS is defined to 1 when __thread should be supported.
 // We assume __thread is supported on Linux when compiled with Clang or compiled
 // against libstdc++ with _GLIBCXX_HAVE_TLS defined.
@@ -125,13 +195,24 @@
 #error ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE cannot directly set
 #elif (defined(__clang__) && defined(_LIBCPP_VERSION)) ||        \
     (!defined(__clang__) && defined(__GNUC__) &&                 \
-     (__GNUC__ > 5 || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1)) && \
+     (__GNUC__ > 7 || (__GNUC__ == 7 && __GNUC_MINOR__ >= 4)) && \
      (defined(_LIBCPP_VERSION) || defined(__GLIBCXX__))) ||      \
     (defined(_MSC_VER) && !defined(__NVCC__))
 #define ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE 1
 #define ABSL_HAVE_STD_IS_TRIVIALLY_ASSIGNABLE 1
 #endif
 
+// ABSL_HAVE_SOURCE_LOCATION_CURRENT
+//
+// Indicates whether `absl::SourceLocation::current()` will return useful
+// information in some contexts.
+#ifndef ABSL_HAVE_SOURCE_LOCATION_CURRENT
+#if ABSL_INTERNAL_HAS_KEYWORD(__builtin_LINE) && \
+    ABSL_INTERNAL_HAS_KEYWORD(__builtin_FILE)
+#define ABSL_HAVE_SOURCE_LOCATION_CURRENT 1
+#endif
+#endif
+
 // ABSL_HAVE_THREAD_LOCAL
 //
 // Checks whether C++11's `thread_local` storage duration specifier is
@@ -235,13 +316,19 @@
 #error ABSL_HAVE_EXCEPTIONS cannot be directly set.
 
 #elif defined(__clang__)
-// TODO(calabrese)
-// Switch to using __cpp_exceptions when we no longer support versions < 3.6.
-// For details on this check, see:
-//   http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
+
+#if __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 6)
+// Clang >= 3.6
+#if __has_feature(cxx_exceptions)
+#define ABSL_HAVE_EXCEPTIONS 1
+#endif  // __has_feature(cxx_exceptions)
+#else
+// Clang < 3.6
+// http://releases.llvm.org/3.6.0/tools/clang/docs/ReleaseNotes.html#the-exceptions-macro
 #if defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
 #define ABSL_HAVE_EXCEPTIONS 1
 #endif  // defined(__EXCEPTIONS) && __has_feature(cxx_exceptions)
+#endif  // __clang_major__ > 3 || (__clang_major__ == 3 && __clang_minor__ >= 6)
 
 // Handle remaining special cases and default to exceptions being supported.
 #elif !(defined(__GNUC__) && (__GNUC__ < 5) && !defined(__EXCEPTIONS)) &&    \
@@ -307,7 +394,7 @@
 
 // ABSL_HAVE_SEMAPHORE_H
 //
-// Checks whether the platform supports the <semaphore.h> header and sem_open(3)
+// Checks whether the platform supports the <semaphore.h> header and sem_init(3)
 // family of functions as standardized in POSIX.1-2001.
 //
 // Note: While Apple provides <semaphore.h> for both iOS and macOS, it is
@@ -334,8 +421,15 @@
 #define ABSL_HAVE_ALARM 1
 #elif defined(_MSC_VER)
 // feature tests for Microsoft's library
+#elif defined(__MINGW32__)
+// mingw32 doesn't provide alarm(2):
+// https://osdn.net/projects/mingw/scm/git/mingw-org-wsl/blobs/5.2-trunk/mingwrt/include/unistd.h
+// mingw-w64 provides a no-op implementation:
+// https://sourceforge.net/p/mingw-w64/mingw-w64/ci/master/tree/mingw-w64-crt/misc/alarm.c
 #elif defined(__EMSCRIPTEN__)
 // emscripten doesn't support signals
+#elif defined(__Fuchsia__)
+// Signals don't exist on fuchsia.
 #elif defined(__native_client__)
 #else
 // other standard libraries
@@ -461,6 +555,68 @@
 #define ABSL_HAVE_STD_STRING_VIEW 1
 #endif
 
+// ABSL_USES_STD_ANY
+//
+// Indicates whether absl::any is an alias for std::any.
+#if !defined(ABSL_OPTION_USE_STD_ANY)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_ANY == 0 || \
+    (ABSL_OPTION_USE_STD_ANY == 2 && !defined(ABSL_HAVE_STD_ANY))
+#undef ABSL_USES_STD_ANY
+#elif ABSL_OPTION_USE_STD_ANY == 1 || \
+    (ABSL_OPTION_USE_STD_ANY == 2 && defined(ABSL_HAVE_STD_ANY))
+#define ABSL_USES_STD_ANY 1
+#else
+#error options.h is misconfigured.
+#endif
+
+// ABSL_USES_STD_OPTIONAL
+//
+// Indicates whether absl::optional is an alias for std::optional.
+#if !defined(ABSL_OPTION_USE_STD_OPTIONAL)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_OPTIONAL == 0 || \
+    (ABSL_OPTION_USE_STD_OPTIONAL == 2 && !defined(ABSL_HAVE_STD_OPTIONAL))
+#undef ABSL_USES_STD_OPTIONAL
+#elif ABSL_OPTION_USE_STD_OPTIONAL == 1 || \
+    (ABSL_OPTION_USE_STD_OPTIONAL == 2 && defined(ABSL_HAVE_STD_OPTIONAL))
+#define ABSL_USES_STD_OPTIONAL 1
+#else
+#error options.h is misconfigured.
+#endif
+
+// ABSL_USES_STD_VARIANT
+//
+// Indicates whether absl::variant is an alias for std::variant.
+#if !defined(ABSL_OPTION_USE_STD_VARIANT)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_VARIANT == 0 || \
+    (ABSL_OPTION_USE_STD_VARIANT == 2 && !defined(ABSL_HAVE_STD_VARIANT))
+#undef ABSL_USES_STD_VARIANT
+#elif ABSL_OPTION_USE_STD_VARIANT == 1 || \
+    (ABSL_OPTION_USE_STD_VARIANT == 2 && defined(ABSL_HAVE_STD_VARIANT))
+#define ABSL_USES_STD_VARIANT 1
+#else
+#error options.h is misconfigured.
+#endif
+
+// ABSL_USES_STD_STRING_VIEW
+//
+// Indicates whether absl::string_view is an alias for std::string_view.
+#if !defined(ABSL_OPTION_USE_STD_STRING_VIEW)
+#error options.h is misconfigured.
+#elif ABSL_OPTION_USE_STD_STRING_VIEW == 0 || \
+    (ABSL_OPTION_USE_STD_STRING_VIEW == 2 &&  \
+     !defined(ABSL_HAVE_STD_STRING_VIEW))
+#undef ABSL_USES_STD_STRING_VIEW
+#elif ABSL_OPTION_USE_STD_STRING_VIEW == 1 || \
+    (ABSL_OPTION_USE_STD_STRING_VIEW == 2 &&  \
+     defined(ABSL_HAVE_STD_STRING_VIEW))
+#define ABSL_USES_STD_STRING_VIEW 1
+#else
+#error options.h is misconfigured.
+#endif
+
 // In debug mode, MSVC 2017's std::variant throws a EXCEPTION_ACCESS_VIOLATION
 // SEH exception from emplace for variant<SomeStruct> when constructing the
 // struct can throw. This defeats some of variant_test and
@@ -469,4 +625,47 @@
 #define ABSL_INTERNAL_MSVC_2017_DBG_MODE
 #endif
 
+// ABSL_INTERNAL_MANGLED_NS
+// ABSL_INTERNAL_MANGLED_BACKREFERENCE
+//
+// Internal macros for building up mangled names in our internal fork of CCTZ.
+// This implementation detail is only needed and provided for the MSVC build.
+//
+// These macros both expand to string literals.  ABSL_INTERNAL_MANGLED_NS is
+// the mangled spelling of the `absl` namespace, and
+// ABSL_INTERNAL_MANGLED_BACKREFERENCE is a back-reference integer representing
+// the proper count to skip past the CCTZ fork namespace names.  (This number
+// is one larger when there is an inline namespace name to skip.)
+#if defined(_MSC_VER)
+#if ABSL_OPTION_USE_INLINE_NAMESPACE == 0
+#define ABSL_INTERNAL_MANGLED_NS "absl"
+#define ABSL_INTERNAL_MANGLED_BACKREFERENCE "5"
+#else
+#define ABSL_INTERNAL_MANGLED_NS \
+  ABSL_INTERNAL_TOKEN_STR(ABSL_OPTION_INLINE_NAMESPACE_NAME) "@absl"
+#define ABSL_INTERNAL_MANGLED_BACKREFERENCE "6"
+#endif
+#endif
+
+#undef ABSL_INTERNAL_HAS_KEYWORD
+
+// ABSL_DLL
+//
+// When building Abseil as a DLL, this macro expands to `__declspec(dllexport)`
+// so we can annotate symbols appropriately as being exported. When used in
+// headers consuming a DLL, this macro expands to `__declspec(dllimport)` so
+// that consumers know the symbol is defined inside the DLL. In all other cases,
+// the macro expands to nothing.
+#if defined(_MSC_VER)
+#if defined(ABSL_BUILD_DLL)
+#define ABSL_DLL __declspec(dllexport)
+#elif defined(ABSL_CONSUME_DLL)
+#define ABSL_DLL __declspec(dllimport)
+#else
+#define ABSL_DLL
+#endif
+#else
+#define ABSL_DLL
+#endif  // defined(_MSC_VER)
+
 #endif  // ABSL_BASE_CONFIG_H_
diff --git a/absl/base/const_init.h b/absl/base/const_init.h
index 30d6a3c..16520b6 100644
--- a/absl/base/const_init.h
+++ b/absl/base/const_init.h
@@ -22,6 +22,8 @@
 #ifndef ABSL_BASE_CONST_INIT_H_
 #define ABSL_BASE_CONST_INIT_H_
 
+#include "absl/base/config.h"
+
 // In general, objects with static storage duration (such as global variables)
 // can trigger tricky object lifetime situations.  Attempting to access them
 // from the constructors or destructors of other global objects can result in
@@ -62,13 +64,13 @@
 // or thread_local storage duration.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 enum ConstInitType {
   kConstInit,
 };
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_CONST_INIT_H_
diff --git a/absl/base/exception_safety_testing_test.cc b/absl/base/exception_safety_testing_test.cc
index 2ed3860..a59be29 100644
--- a/absl/base/exception_safety_testing_test.cc
+++ b/absl/base/exception_safety_testing_test.cc
@@ -14,6 +14,8 @@
 
 #include "absl/base/internal/exception_safety_testing.h"
 
+#ifdef ABSL_HAVE_EXCEPTIONS
+
 #include <cstddef>
 #include <exception>
 #include <iostream>
@@ -326,17 +328,15 @@
   UnsetCountdown();
 }
 
-using Storage =
-    absl::aligned_storage_t<sizeof(ThrowingValue<>), alignof(ThrowingValue<>)>;
-
 TEST(ThrowingValueTest, NonThrowingPlacementDelete) {
   constexpr int kArrayLen = 2;
   // We intentionally create extra space to store the tag allocated by placement
   // new[].
   constexpr int kStorageLen = 4;
 
-  Storage buf;
-  Storage array_buf[kStorageLen];
+  alignas(ThrowingValue<>) unsigned char buf[sizeof(ThrowingValue<>)];
+  alignas(ThrowingValue<>) unsigned char
+      array_buf[sizeof(ThrowingValue<>[kStorageLen])];
   auto* placed = new (&buf) ThrowingValue<>(1);
   auto placed_array = new (&array_buf) ThrowingValue<>[kArrayLen];
 
@@ -900,12 +900,12 @@
 }
 
 TEST(ConstructorTrackerTest, NotDestroyedAfter) {
-  absl::aligned_storage_t<sizeof(Tracked), alignof(Tracked)> storage;
+  alignas(Tracked) unsigned char storage[sizeof(Tracked)];
   EXPECT_NONFATAL_FAILURE(
       {
         exceptions_internal::ConstructorTracker ct(
             exceptions_internal::countdown);
-        new (&storage) Tracked;
+        new (&storage) Tracked();
       },
       "not destroyed");
 }
@@ -922,11 +922,11 @@
 
 TEST(ConstructorTrackerTest, ConstructedTwice) {
   exceptions_internal::ConstructorTracker ct(exceptions_internal::countdown);
-  absl::aligned_storage_t<sizeof(Tracked), alignof(Tracked)> storage;
+  alignas(Tracked) unsigned char storage[sizeof(Tracked)];
   EXPECT_NONFATAL_FAILURE(
       {
-        new (&storage) Tracked;
-        new (&storage) Tracked;
+        new (&storage) Tracked();
+        new (&storage) Tracked();
         reinterpret_cast<Tracked*>(&storage)->~Tracked();
       },
       "re-constructed");
@@ -952,3 +952,5 @@
 }  // namespace
 
 }  // namespace testing
+
+#endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/base/inline_variable_test.cc b/absl/base/inline_variable_test.cc
index 40e8b57..37a40e1 100644
--- a/absl/base/inline_variable_test.cc
+++ b/absl/base/inline_variable_test.cc
@@ -20,7 +20,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace inline_variable_testing_internal {
 namespace {
 
@@ -60,5 +60,5 @@
 
 }  // namespace
 }  // namespace inline_variable_testing_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/inline_variable_test_a.cc b/absl/base/inline_variable_test_a.cc
index c19a6e5..f96a58d 100644
--- a/absl/base/inline_variable_test_a.cc
+++ b/absl/base/inline_variable_test_a.cc
@@ -15,7 +15,7 @@
 #include "absl/base/internal/inline_variable_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace inline_variable_testing_internal {
 
 const Foo& get_foo_a() { return inline_variable_foo; }
@@ -23,5 +23,5 @@
 const int& get_int_a() { return inline_variable_int; }
 
 }  // namespace inline_variable_testing_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/inline_variable_test_b.cc b/absl/base/inline_variable_test_b.cc
index ae8da10..038adc3 100644
--- a/absl/base/inline_variable_test_b.cc
+++ b/absl/base/inline_variable_test_b.cc
@@ -15,7 +15,7 @@
 #include "absl/base/internal/inline_variable_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace inline_variable_testing_internal {
 
 const Foo& get_foo_b() { return inline_variable_foo; }
@@ -23,5 +23,5 @@
 const int& get_int_b() { return inline_variable_int; }
 
 }  // namespace inline_variable_testing_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/atomic_hook.h b/absl/base/internal/atomic_hook.h
index 6757e75..ae21cd7 100644
--- a/absl/base/internal/atomic_hook.h
+++ b/absl/base/internal/atomic_hook.h
@@ -11,7 +11,6 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
-//
 
 #ifndef ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
 #define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
@@ -21,29 +20,51 @@
 #include <cstdint>
 #include <utility>
 
-#ifdef _MSC_FULL_VER
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+
+#if defined(_MSC_VER) && !defined(__clang__)
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 0
+#else
+#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 1
+#endif
+
+#if defined(_MSC_VER)
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
 #else
 #define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 template <typename T>
 class AtomicHook;
 
-// AtomicHook is a helper class, templatized on a raw function pointer type, for
-// implementing Abseil customization hooks.  It is a callable object that
-// dispatches to the registered hook.
+// To workaround AtomicHook not being constant-initializable on some platforms,
+// prefer to annotate instances with `ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES`
+// instead of `ABSL_CONST_INIT`.
+#if ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
+#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES ABSL_CONST_INIT
+#else
+#define ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+#endif
+
+// `AtomicHook` is a helper class, templatized on a raw function pointer type,
+// for implementing Abseil customization hooks.  It is a callable object that
+// dispatches to the registered hook.  Objects of type `AtomicHook` must have
+// static or thread storage duration.
 //
 // A default constructed object performs a no-op (and returns a default
 // constructed object) if no hook has been registered.
 //
-// Hooks can be pre-registered via constant initialization, for example,
-// ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);
-// and then changed at runtime via a call to Store().
+// Hooks can be pre-registered via constant initialization, for example:
+//
+// ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static AtomicHook<void(*)()>
+//     my_hook(DefaultAction);
+//
+// and then changed at runtime via a call to `Store()`.
 //
 // Reads and writes guarantee memory_order_acquire/memory_order_release
 // semantics.
@@ -58,12 +79,23 @@
 
   // Constructs an object that by default dispatches to/returns the
   // pre-registered default_fn when no hook has been registered at runtime.
-#if ABSL_HAVE_WORKING_ATOMIC_POINTER
+#if ABSL_HAVE_WORKING_ATOMIC_POINTER && ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
   explicit constexpr AtomicHook(FnPtr default_fn)
       : hook_(default_fn), default_fn_(default_fn) {}
-#else
+#elif ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
   explicit constexpr AtomicHook(FnPtr default_fn)
       : hook_(kUninitialized), default_fn_(default_fn) {}
+#else
+  // As of January 2020, on all known versions of MSVC this constructor runs in
+  // the global constructor sequence.  If `Store()` is called by a dynamic
+  // initializer, we want to preserve the value, even if this constructor runs
+  // after the call to `Store()`.  If not, `hook_` will be
+  // zero-initialized by the linker and we have no need to set it.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+  explicit constexpr AtomicHook(FnPtr default_fn)
+      : /* hook_(deliberately omitted), */ default_fn_(default_fn) {
+    static_assert(kUninitialized == 0, "here we rely on zero-initialization");
+  }
 #endif
 
   // Stores the provided function pointer as the value for this hook.
@@ -159,9 +191,10 @@
 };
 
 #undef ABSL_HAVE_WORKING_ATOMIC_POINTER
+#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
diff --git a/absl/base/internal/atomic_hook_test.cc b/absl/base/internal/atomic_hook_test.cc
index ecc8040..e577a8f 100644
--- a/absl/base/internal/atomic_hook_test.cc
+++ b/absl/base/internal/atomic_hook_test.cc
@@ -14,16 +14,22 @@
 
 #include "absl/base/internal/atomic_hook.h"
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook_test_helper.h"
 
 namespace {
 
+using ::testing::Eq;
+
 int value = 0;
 void TestHook(int x) { value = x; }
 
 TEST(AtomicHookTest, NoDefaultFunction) {
-  ABSL_CONST_INIT static absl::base_internal::AtomicHook<void(*)(int)> hook;
+  ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+      void (*)(int)>
+      hook;
   value = 0;
 
   // Test the default DummyFunction.
@@ -49,8 +55,9 @@
 
 TEST(AtomicHookTest, WithDefaultFunction) {
   // Set the default value to TestHook at compile-time.
-  ABSL_CONST_INIT static absl::base_internal::AtomicHook<void (*)(int)> hook(
-      TestHook);
+  ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+      void (*)(int)>
+      hook(TestHook);
   value = 0;
 
   // Test the default value is TestHook.
@@ -67,4 +74,24 @@
   EXPECT_EQ(value, 2);
 }
 
+ABSL_CONST_INIT int override_func_calls = 0;
+void OverrideFunc() { override_func_calls++; }
+static struct OverrideInstaller {
+  OverrideInstaller() { absl::atomic_hook_internal::func.Store(OverrideFunc); }
+} override_installer;
+
+TEST(AtomicHookTest, DynamicInitFromAnotherTU) {
+  // MSVC 14.2 doesn't do constexpr static init correctly; in particular it
+  // tends to sequence static init (i.e. defaults) of `AtomicHook` objects
+  // after their dynamic init (i.e. overrides), overwriting whatever value was
+  // written during dynamic init.  This regression test validates the fix.
+  // https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(0));
+  absl::atomic_hook_internal::func();
+  EXPECT_THAT(absl::atomic_hook_internal::default_func_calls, Eq(0));
+  EXPECT_THAT(override_func_calls, Eq(1));
+  EXPECT_THAT(absl::atomic_hook_internal::func.Load(), Eq(OverrideFunc));
+}
+
 }  // namespace
diff --git a/absl/base/internal/atomic_hook_test_helper.cc b/absl/base/internal/atomic_hook_test_helper.cc
new file mode 100644
index 0000000..537d47c
--- /dev/null
+++ b/absl/base/internal/atomic_hook_test_helper.cc
@@ -0,0 +1,32 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/atomic_hook_test_helper.h"
+
+#include "absl/base/attributes.h"
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace atomic_hook_internal {
+
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<VoidF>
+    func(DefaultFunc);
+ABSL_CONST_INIT int default_func_calls = 0;
+void DefaultFunc() { default_func_calls++; }
+void RegisterFunc(VoidF f) { func.Store(f); }
+
+}  // namespace atomic_hook_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/atomic_hook_test_helper.h b/absl/base/internal/atomic_hook_test_helper.h
new file mode 100644
index 0000000..3e72b49
--- /dev/null
+++ b/absl/base/internal/atomic_hook_test_helper.h
@@ -0,0 +1,34 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
+#define ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
+
+#include "absl/base/internal/atomic_hook.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace atomic_hook_internal {
+
+using VoidF = void (*)();
+extern absl::base_internal::AtomicHook<VoidF> func;
+extern int default_func_calls;
+void DefaultFunc();
+void RegisterFunc(VoidF func);
+
+}  // namespace atomic_hook_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_ATOMIC_HOOK_TEST_HELPER_H_
diff --git a/absl/base/internal/bits.h b/absl/base/internal/bits.h
index ef978e9..8b03453 100644
--- a/absl/base/internal/bits.h
+++ b/absl/base/internal/bits.h
@@ -20,6 +20,8 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 // Clang on Windows has __builtin_clzll; otherwise we need to use the
 // windows intrinsic functions.
 #if defined(_MSC_VER)
@@ -46,15 +48,27 @@
 
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) {
   int zeroes = 60;
-  if (n >> 32) zeroes -= 32, n >>= 32;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
+  if (n >> 32) {
+    zeroes -= 32;
+    n >>= 32;
+  }
+  if (n >> 16) {
+    zeroes -= 16;
+    n >>= 16;
+  }
+  if (n >> 8) {
+    zeroes -= 8;
+    n >>= 8;
+  }
+  if (n >> 4) {
+    zeroes -= 4;
+    n >>= 4;
+  }
   return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
 }
 
@@ -96,9 +110,18 @@
 
 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) {
   int zeroes = 28;
-  if (n >> 16) zeroes -= 16, n >>= 16;
-  if (n >> 8) zeroes -= 8, n >>= 8;
-  if (n >> 4) zeroes -= 4, n >>= 4;
+  if (n >> 16) {
+    zeroes -= 16;
+    n >>= 16;
+  }
+  if (n >> 8) {
+    zeroes -= 8;
+    n >>= 8;
+  }
+  if (n >> 4) {
+    zeroes -= 4;
+    n >>= 4;
+  }
   return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
 }
 
@@ -189,7 +212,7 @@
 #undef ABSL_BASE_INTERNAL_FORCEINLINE
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_BITS_H_
diff --git a/absl/base/internal/cycleclock.cc b/absl/base/internal/cycleclock.cc
index 9868e54..0e65005 100644
--- a/absl/base/internal/cycleclock.cc
+++ b/absl/base/internal/cycleclock.cc
@@ -28,7 +28,7 @@
 #include "absl/base/internal/unscaledcycleclock.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if ABSL_USE_UNSCALED_CYCLECLOCK
@@ -103,5 +103,5 @@
 #endif
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/cycleclock.h b/absl/base/internal/cycleclock.h
index 39bce06..a18b584 100644
--- a/absl/base/internal/cycleclock.h
+++ b/absl/base/internal/cycleclock.h
@@ -44,8 +44,10 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // -----------------------------------------------------------------------------
@@ -86,7 +88,7 @@
 };
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_CYCLECLOCK_H_
diff --git a/absl/base/internal/direct_mmap.h b/absl/base/internal/direct_mmap.h
index 2e5e422..5618867 100644
--- a/absl/base/internal/direct_mmap.h
+++ b/absl/base/internal/direct_mmap.h
@@ -62,7 +62,7 @@
 #endif  // __BIONIC__
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Platform specific logic extracted from
@@ -129,7 +129,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else  // !__linux__
@@ -138,7 +138,7 @@
 // actual mmap()/munmap() methods.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 inline void* DirectMmap(void* start, size_t length, int prot, int flags, int fd,
@@ -151,7 +151,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // __linux__
diff --git a/absl/base/internal/endian.h b/absl/base/internal/endian.h
index 8643bff..9677530 100644
--- a/absl/base/internal/endian.h
+++ b/absl/base/internal/endian.h
@@ -19,9 +19,6 @@
 // The following guarantees declaration of the byte swap functions
 #ifdef _MSC_VER
 #include <stdlib.h>  // NOLINT(build/include)
-#elif defined(__APPLE__)
-// macOS / Darwin features
-#include <libkern/OSByteOrder.h>
 #elif defined(__FreeBSD__)
 #include <sys/endian.h>
 #elif defined(__GLIBC__)
@@ -34,7 +31,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Use compiler byte-swapping intrinsics if they are available.  32-bit
 // and 64-bit versions are available in Clang and GCC as of GCC 4.3.0.
@@ -64,11 +61,6 @@
   return _byteswap_ushort(host_int);
 }
 
-#elif defined(__APPLE__)
-inline uint64_t gbswap_64(uint64_t host_int) { return OSSwapInt16(host_int); }
-inline uint32_t gbswap_32(uint32_t host_int) { return OSSwapInt32(host_int); }
-inline uint16_t gbswap_16(uint16_t host_int) { return OSSwapInt64(host_int); }
-
 #else
 inline uint64_t gbswap_64(uint64_t host_int) {
 #if defined(__GNUC__) && defined(__x86_64__) && !defined(__APPLE__)
@@ -114,7 +106,7 @@
 #endif
 }
 
-#endif  // intrinics available
+#endif  // intrinsics available
 
 #ifdef ABSL_IS_LITTLE_ENDIAN
 
@@ -268,7 +260,7 @@
 
 }  // namespace big_endian
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_ENDIAN_H_
diff --git a/absl/base/internal/endian_test.cc b/absl/base/internal/endian_test.cc
index 0c68328..aa6b849 100644
--- a/absl/base/internal/endian_test.cc
+++ b/absl/base/internal/endian_test.cc
@@ -24,7 +24,7 @@
 #include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 const uint64_t kInitialNumber{0x0123456789abcdef};
@@ -261,5 +261,5 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/errno_saver.h b/absl/base/internal/errno_saver.h
new file mode 100644
index 0000000..251de51
--- /dev/null
+++ b/absl/base/internal/errno_saver.h
@@ -0,0 +1,43 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
+#define ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
+
+#include <cerrno>
+
+#include "absl/base/config.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// `ErrnoSaver` captures the value of `errno` upon construction and restores it
+// upon deletion.  It is used in low-level code and must be super fast.  Do not
+// add instrumentation, even in debug modes.
+class ErrnoSaver {
+ public:
+  ErrnoSaver() : saved_errno_(errno) {}
+  ~ErrnoSaver() { errno = saved_errno_; }
+  int operator()() const { return saved_errno_; }
+
+ private:
+  const int saved_errno_;
+};
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_ERRNO_SAVER_H_
diff --git a/absl/base/internal/errno_saver_test.cc b/absl/base/internal/errno_saver_test.cc
new file mode 100644
index 0000000..b845e2d
--- /dev/null
+++ b/absl/base/internal/errno_saver_test.cc
@@ -0,0 +1,44 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/errno_saver.h"
+
+#include <cerrno>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace {
+using ::testing::Eq;
+
+struct ErrnoPrinter {
+  int no;
+};
+std::ostream &operator<<(std::ostream &os, ErrnoPrinter ep) {
+  return os << strerror(ep.no) << " [" << ep.no << "]";
+}
+bool operator==(ErrnoPrinter one, ErrnoPrinter two) { return one.no == two.no; }
+
+TEST(ErrnoSaverTest, Works) {
+  errno = EDOM;
+  {
+    absl::base_internal::ErrnoSaver errno_saver;
+    EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{EDOM}));
+    errno = ERANGE;
+    EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{ERANGE}));
+    EXPECT_THAT(ErrnoPrinter{errno_saver()}, Eq(ErrnoPrinter{EDOM}));
+  }
+  EXPECT_THAT(ErrnoPrinter{errno}, Eq(ErrnoPrinter{EDOM}));
+}
+}  // namespace
diff --git a/absl/base/internal/exception_safety_testing.cc b/absl/base/internal/exception_safety_testing.cc
index 6ef4325..6ccac41 100644
--- a/absl/base/internal/exception_safety_testing.cc
+++ b/absl/base/internal/exception_safety_testing.cc
@@ -14,6 +14,8 @@
 
 #include "absl/base/internal/exception_safety_testing.h"
 
+#ifdef ABSL_HAVE_EXCEPTIONS
+
 #include "gtest/gtest.h"
 #include "absl/meta/type_traits.h"
 
@@ -73,3 +75,5 @@
 }  // namespace exceptions_internal
 
 }  // namespace testing
+
+#endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/base/internal/exception_safety_testing.h b/absl/base/internal/exception_safety_testing.h
index be38ba5..6ba89d0 100644
--- a/absl/base/internal/exception_safety_testing.h
+++ b/absl/base/internal/exception_safety_testing.h
@@ -17,6 +17,10 @@
 #ifndef ABSL_BASE_INTERNAL_EXCEPTION_SAFETY_TESTING_H_
 #define ABSL_BASE_INTERNAL_EXCEPTION_SAFETY_TESTING_H_
 
+#include "absl/base/config.h"
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+
 #include <cstddef>
 #include <cstdint>
 #include <functional>
@@ -27,7 +31,6 @@
 #include <unordered_map>
 
 #include "gtest/gtest.h"
-#include "absl/base/config.h"
 #include "absl/base/internal/pretty_function.h"
 #include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
@@ -1093,4 +1096,6 @@
 
 }  // namespace testing
 
+#endif  // ABSL_HAVE_EXCEPTIONS
+
 #endif  // ABSL_BASE_INTERNAL_EXCEPTION_SAFETY_TESTING_H_
diff --git a/absl/base/internal/exponential_biased.cc b/absl/base/internal/exponential_biased.cc
new file mode 100644
index 0000000..1b30c06
--- /dev/null
+++ b/absl/base/internal/exponential_biased.cc
@@ -0,0 +1,93 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/exponential_biased.h"
+
+#include <stdint.h>
+
+#include <algorithm>
+#include <atomic>
+#include <cmath>
+#include <limits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// The algorithm generates a random number between 0 and 1 and applies the
+// inverse cumulative distribution function for an exponential. Specifically:
+// Let m be the inverse of the sample period, then the probability
+// distribution function is m*exp(-mx) so the CDF is
+// p = 1 - exp(-mx), so
+// q = 1 - p = exp(-mx)
+// log_e(q) = -mx
+// -log_e(q)/m = x
+// log_2(q) * (-log_e(2) * 1/m) = x
+// In the code, q is actually in the range 1 to 2**26, hence the -26 below
+int64_t ExponentialBiased::GetSkipCount(int64_t mean) {
+  if (ABSL_PREDICT_FALSE(!initialized_)) {
+    Initialize();
+  }
+
+  uint64_t rng = NextRandom(rng_);
+  rng_ = rng;
+
+  // Take the top 26 bits as the random number
+  // (This plus the 1<<58 sampling bound give a max possible step of
+  // 5194297183973780480 bytes.)
+  // The uint32_t cast is to prevent a (hard-to-reproduce) NAN
+  // under piii debug for some binaries.
+  double q = static_cast<uint32_t>(rng >> (kPrngNumBits - 26)) + 1.0;
+  // Put the computed p-value through the CDF of a geometric.
+  double interval = bias_ + (std::log2(q) - 26) * (-std::log(2.0) * mean);
+  // Very large values of interval overflow int64_t. To avoid that, we will
+  // cheat and clamp any huge values to (int64_t max)/2. This is a potential
+  // source of bias, but the mean would need to be such a large value that it's
+  // not likely to come up. For example, with a mean of 1e18, the probability of
+  // hitting this condition is about 1/1000. For a mean of 1e17, standard
+  // calculators claim that this event won't happen.
+  if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
+    // Assume huge values are bias neutral, retain bias for next call.
+    return std::numeric_limits<int64_t>::max() / 2;
+  }
+  double value = std::round(interval);
+  bias_ = interval - value;
+  return value;
+}
+
+int64_t ExponentialBiased::GetStride(int64_t mean) {
+  return GetSkipCount(mean - 1) + 1;
+}
+
+void ExponentialBiased::Initialize() {
+  // We don't get well distributed numbers from `this` so we call NextRandom() a
+  // bunch to mush the bits around. We use a global_rand to handle the case
+  // where the same thread (by memory address) gets created and destroyed
+  // repeatedly.
+  ABSL_CONST_INIT static std::atomic<uint32_t> global_rand(0);
+  uint64_t r = reinterpret_cast<uint64_t>(this) +
+               global_rand.fetch_add(1, std::memory_order_relaxed);
+  for (int i = 0; i < 20; ++i) {
+    r = NextRandom(r);
+  }
+  rng_ = r;
+  initialized_ = true;
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/exponential_biased.h b/absl/base/internal/exponential_biased.h
new file mode 100644
index 0000000..94f79a3
--- /dev/null
+++ b/absl/base/internal/exponential_biased.h
@@ -0,0 +1,130 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
+#define ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
+
+#include <stdint.h>
+
+#include "absl/base/config.h"
+#include "absl/base/macros.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// ExponentialBiased provides a small and fast random number generator for a
+// rounded exponential distribution. This generator manages very little state,
+// and imposes no synchronization overhead. This makes it useful in specialized
+// scenarios requiring minimum overhead, such as stride based periodic sampling.
+//
+// ExponentialBiased provides two closely related functions, GetSkipCount() and
+// GetStride(), both returning a rounded integer defining a number of events
+// required before some event with a given mean probability occurs.
+//
+// The distribution is useful to generate a random wait time or some periodic
+// event with a given mean probability. For example, if an action is supposed to
+// happen on average once every 'N' events, then we can get a random 'stride'
+// counting down how long before the event to happen. For example, if we'd want
+// to sample one in every 1000 'Frobber' calls, our code could look like this:
+//
+//   Frobber::Frobber() {
+//     stride_ = exponential_biased_.GetStride(1000);
+//   }
+//
+//   void Frobber::Frob(int arg) {
+//     if (--stride == 0) {
+//       SampleFrob(arg);
+//       stride_ = exponential_biased_.GetStride(1000);
+//     }
+//     ...
+//   }
+//
+// The rounding of the return value creates a bias, especially for smaller means
+// where the distribution of the fraction is not evenly distributed. We correct
+// this bias by tracking the fraction we rounded up or down on each iteration,
+// effectively tracking the distance between the cumulative value, and the
+// rounded cumulative value. For example, given a mean of 2:
+//
+//   raw = 1.63076, cumulative = 1.63076, rounded = 2, bias = -0.36923
+//   raw = 0.14624, cumulative = 1.77701, rounded = 2, bias =  0.14624
+//   raw = 4.93194, cumulative = 6.70895, rounded = 7, bias = -0.06805
+//   raw = 0.24206, cumulative = 6.95101, rounded = 7, bias =  0.24206
+//   etc...
+//
+// Adjusting with rounding bias is relatively trivial:
+//
+//    double value = bias_ + exponential_distribution(mean)();
+//    double rounded_value = std::round(value);
+//    bias_ = value - rounded_value;
+//    return rounded_value;
+//
+// This class is thread-compatible.
+class ExponentialBiased {
+ public:
+  // The number of bits set by NextRandom.
+  static constexpr int kPrngNumBits = 48;
+
+  // `GetSkipCount()` returns the number of events to skip before some chosen
+  // event happens. For example, randomly tossing a coin, we will on average
+  // throw heads once before we get tails. We can simulate random coin tosses
+  // using GetSkipCount() as:
+  //
+  //   ExponentialBiased eb;
+  //   for (...) {
+  //     int number_of_heads_before_tail = eb.GetSkipCount(1);
+  //     for (int flips = 0; flips < number_of_heads_before_tail; ++flips) {
+  //       printf("head...");
+  //     }
+  //     printf("tail\n");
+  //   }
+  //
+  int64_t GetSkipCount(int64_t mean);
+
+  // GetStride() returns the number of events required for a specific event to
+  // happen. See the class comments for a usage example. `GetStride()` is
+  // equivalent to `GetSkipCount(mean - 1) + 1`. When to use `GetStride()` or
+  // `GetSkipCount()` depends mostly on what best fits the use case.
+  int64_t GetStride(int64_t mean);
+
+  // Computes a random number in the range [0, 1<<(kPrngNumBits+1) - 1]
+  //
+  // This is public to enable testing.
+  static uint64_t NextRandom(uint64_t rnd);
+
+ private:
+  void Initialize();
+
+  uint64_t rng_{0};
+  double bias_{0};
+  bool initialized_{false};
+};
+
+// Returns the next prng value.
+// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
+// This is the lrand64 generator.
+inline uint64_t ExponentialBiased::NextRandom(uint64_t rnd) {
+  const uint64_t prng_mult = uint64_t{0x5DEECE66D};
+  const uint64_t prng_add = 0xB;
+  const uint64_t prng_mod_power = 48;
+  const uint64_t prng_mod_mask =
+      ~((~static_cast<uint64_t>(0)) << prng_mod_power);
+  return (prng_mult * rnd + prng_add) & prng_mod_mask;
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_EXPONENTIAL_BIASED_H_
diff --git a/absl/base/internal/exponential_biased_test.cc b/absl/base/internal/exponential_biased_test.cc
new file mode 100644
index 0000000..90a482d
--- /dev/null
+++ b/absl/base/internal/exponential_biased_test.cc
@@ -0,0 +1,199 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/exponential_biased.h"
+
+#include <stddef.h>
+
+#include <cmath>
+#include <cstdint>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/str_cat.h"
+
+using ::testing::Ge;
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+MATCHER_P2(IsBetween, a, b,
+           absl::StrCat(std::string(negation ? "isn't" : "is"), " between ", a,
+                        " and ", b)) {
+  return a <= arg && arg <= b;
+}
+
+// Tests of the quality of the random numbers generated
+// This uses the Anderson Darling test for uniformity.
+// See "Evaluating the Anderson-Darling Distribution" by Marsaglia
+// for details.
+
+// Short cut version of ADinf(z), z>0 (from Marsaglia)
+// This returns the p-value for Anderson Darling statistic in
+// the limit as n-> infinity. For finite n, apply the error fix below.
+double AndersonDarlingInf(double z) {
+  if (z < 2) {
+    return exp(-1.2337141 / z) / sqrt(z) *
+           (2.00012 +
+            (0.247105 -
+             (0.0649821 - (0.0347962 - (0.011672 - 0.00168691 * z) * z) * z) *
+                 z) *
+                z);
+  }
+  return exp(
+      -exp(1.0776 -
+           (2.30695 -
+            (0.43424 - (0.082433 - (0.008056 - 0.0003146 * z) * z) * z) * z) *
+               z));
+}
+
+// Corrects the approximation error in AndersonDarlingInf for small values of n
+// Add this to AndersonDarlingInf to get a better approximation
+// (from Marsaglia)
+double AndersonDarlingErrFix(int n, double x) {
+  if (x > 0.8) {
+    return (-130.2137 +
+            (745.2337 -
+             (1705.091 - (1950.646 - (1116.360 - 255.7844 * x) * x) * x) * x) *
+                x) /
+           n;
+  }
+  double cutoff = 0.01265 + 0.1757 / n;
+  if (x < cutoff) {
+    double t = x / cutoff;
+    t = sqrt(t) * (1 - t) * (49 * t - 102);
+    return t * (0.0037 / (n * n) + 0.00078 / n + 0.00006) / n;
+  } else {
+    double t = (x - cutoff) / (0.8 - cutoff);
+    t = -0.00022633 +
+        (6.54034 - (14.6538 - (14.458 - (8.259 - 1.91864 * t) * t) * t) * t) *
+            t;
+    return t * (0.04213 + 0.01365 / n) / n;
+  }
+}
+
+// Returns the AndersonDarling p-value given n and the value of the statistic
+double AndersonDarlingPValue(int n, double z) {
+  double ad = AndersonDarlingInf(z);
+  double errfix = AndersonDarlingErrFix(n, ad);
+  return ad + errfix;
+}
+
+double AndersonDarlingStatistic(const std::vector<double>& random_sample) {
+  int n = random_sample.size();
+  double ad_sum = 0;
+  for (int i = 0; i < n; i++) {
+    ad_sum += (2 * i + 1) *
+              std::log(random_sample[i] * (1 - random_sample[n - 1 - i]));
+  }
+  double ad_statistic = -n - 1 / static_cast<double>(n) * ad_sum;
+  return ad_statistic;
+}
+
+// Tests if the array of doubles is uniformly distributed.
+// Returns the p-value of the Anderson Darling Statistic
+// for the given set of sorted random doubles
+// See "Evaluating the Anderson-Darling Distribution" by
+// Marsaglia and Marsaglia for details.
+double AndersonDarlingTest(const std::vector<double>& random_sample) {
+  double ad_statistic = AndersonDarlingStatistic(random_sample);
+  double p = AndersonDarlingPValue(random_sample.size(), ad_statistic);
+  return p;
+}
+
+TEST(ExponentialBiasedTest, CoinTossDemoWithGetSkipCount) {
+  ExponentialBiased eb;
+  for (int runs = 0; runs < 10; ++runs) {
+    for (int flips = eb.GetSkipCount(1); flips > 0; --flips) {
+      printf("head...");
+    }
+    printf("tail\n");
+  }
+  int heads = 0;
+  for (int i = 0; i < 10000000; i += 1 + eb.GetSkipCount(1)) {
+    ++heads;
+  }
+  printf("Heads = %d (%f%%)\n", heads, 100.0 * heads / 10000000);
+}
+
+TEST(ExponentialBiasedTest, SampleDemoWithStride) {
+  ExponentialBiased eb;
+  int stride = eb.GetStride(10);
+  int samples = 0;
+  for (int i = 0; i < 10000000; ++i) {
+    if (--stride == 0) {
+      ++samples;
+      stride = eb.GetStride(10);
+    }
+  }
+  printf("Samples = %d (%f%%)\n", samples, 100.0 * samples / 10000000);
+}
+
+
+// Testing that NextRandom generates uniform random numbers. Applies the
+// Anderson-Darling test for uniformity
+TEST(ExponentialBiasedTest, TestNextRandom) {
+  for (auto n : std::vector<int>({
+           10,  // Check short-range correlation
+           100, 1000,
+           10000  // Make sure there's no systemic error
+       })) {
+    uint64_t x = 1;
+    // This assumes that the prng returns 48 bit numbers
+    uint64_t max_prng_value = static_cast<uint64_t>(1) << 48;
+    // Initialize.
+    for (int i = 1; i <= 20; i++) {
+      x = ExponentialBiased::NextRandom(x);
+    }
+    std::vector<uint64_t> int_random_sample(n);
+    // Collect samples
+    for (int i = 0; i < n; i++) {
+      int_random_sample[i] = x;
+      x = ExponentialBiased::NextRandom(x);
+    }
+    // First sort them...
+    std::sort(int_random_sample.begin(), int_random_sample.end());
+    std::vector<double> random_sample(n);
+    // Convert them to uniform randoms (in the range [0,1])
+    for (int i = 0; i < n; i++) {
+      random_sample[i] =
+          static_cast<double>(int_random_sample[i]) / max_prng_value;
+    }
+    // Now compute the Anderson-Darling statistic
+    double ad_pvalue = AndersonDarlingTest(random_sample);
+    EXPECT_GT(std::min(ad_pvalue, 1 - ad_pvalue), 0.0001)
+        << "prng is not uniform: n = " << n << " p = " << ad_pvalue;
+  }
+}
+
+// The generator needs to be available as a thread_local and as a static
+// variable.
+TEST(ExponentialBiasedTest, InitializationModes) {
+  ABSL_CONST_INIT static ExponentialBiased eb_static;
+  EXPECT_THAT(eb_static.GetSkipCount(2), Ge(0));
+
+#if ABSL_HAVE_THREAD_LOCAL
+  thread_local ExponentialBiased eb_thread;
+  EXPECT_THAT(eb_thread.GetSkipCount(2), Ge(0));
+#endif
+
+  ExponentialBiased eb_stack;
+  EXPECT_THAT(eb_stack.GetSkipCount(2), Ge(0));
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/hide_ptr.h b/absl/base/internal/hide_ptr.h
index 3720db1..1dba809 100644
--- a/absl/base/internal/hide_ptr.h
+++ b/absl/base/internal/hide_ptr.h
@@ -17,8 +17,10 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Arbitrary value with high bits set. Xor'ing with it is unlikely
@@ -43,7 +45,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_HIDE_PTR_H_
diff --git a/absl/base/internal/identity.h b/absl/base/internal/identity.h
index b9d0f62..a3154ed 100644
--- a/absl/base/internal/identity.h
+++ b/absl/base/internal/identity.h
@@ -16,8 +16,10 @@
 #ifndef ABSL_BASE_INTERNAL_IDENTITY_H_
 #define ABSL_BASE_INTERNAL_IDENTITY_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace internal {
 
 template <typename T>
@@ -29,7 +31,7 @@
 using identity_t = typename identity<T>::type;
 
 }  // namespace internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_IDENTITY_H_
diff --git a/absl/base/internal/inline_variable_testing.h b/absl/base/internal/inline_variable_testing.h
index 0ebdb9d..3856b9f 100644
--- a/absl/base/internal/inline_variable_testing.h
+++ b/absl/base/internal/inline_variable_testing.h
@@ -18,7 +18,7 @@
 #include "absl/base/internal/inline_variable.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace inline_variable_testing_internal {
 
 struct Foo {
@@ -40,7 +40,7 @@
 const int& get_int_b();
 
 }  // namespace inline_variable_testing_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INLINE_VARIABLE_TESTING_H_
diff --git a/absl/base/internal/invoke.h b/absl/base/internal/invoke.h
index 030b98d..c4eceeb 100644
--- a/absl/base/internal/invoke.h
+++ b/absl/base/internal/invoke.h
@@ -45,7 +45,7 @@
 // top of this file for the API documentation.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // The five classes below each implement one of the clauses from the definition
@@ -181,7 +181,7 @@
                                            std::forward<Args>(args)...);
 }
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_INVOKE_H_
diff --git a/absl/base/internal/low_level_alloc.cc b/absl/base/internal/low_level_alloc.cc
index 419c0e4..1bf9443 100644
--- a/absl/base/internal/low_level_alloc.cc
+++ b/absl/base/internal/low_level_alloc.cc
@@ -63,7 +63,7 @@
 #endif  // __APPLE__
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // A first-fit allocator with amortized logarithmic free() time.
@@ -204,32 +204,33 @@
 
   base_internal::SpinLock mu;
   // Head of free list, sorted by address
-  AllocList freelist GUARDED_BY(mu);
+  AllocList freelist ABSL_GUARDED_BY(mu);
   // Count of allocated blocks
-  int32_t allocation_count GUARDED_BY(mu);
+  int32_t allocation_count ABSL_GUARDED_BY(mu);
   // flags passed to NewArena
   const uint32_t flags;
   // Result of sysconf(_SC_PAGESIZE)
   const size_t pagesize;
   // Lowest power of two >= max(16, sizeof(AllocList))
-  const size_t roundup;
+  const size_t round_up;
   // Smallest allocation block size
   const size_t min_size;
   // PRNG state
-  uint32_t random GUARDED_BY(mu);
+  uint32_t random ABSL_GUARDED_BY(mu);
 };
 
 namespace {
-using ArenaStorage = std::aligned_storage<sizeof(LowLevelAlloc::Arena),
-                                          alignof(LowLevelAlloc::Arena)>::type;
-
 // Static storage space for the lazily-constructed, default global arena
 // instances.  We require this space because the whole point of LowLevelAlloc
 // is to avoid relying on malloc/new.
-ArenaStorage default_arena_storage;
-ArenaStorage unhooked_arena_storage;
+alignas(LowLevelAlloc::Arena) unsigned char default_arena_storage[sizeof(
+    LowLevelAlloc::Arena)];
+alignas(LowLevelAlloc::Arena) unsigned char unhooked_arena_storage[sizeof(
+    LowLevelAlloc::Arena)];
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
-ArenaStorage unhooked_async_sig_safe_arena_storage;
+alignas(
+    LowLevelAlloc::Arena) unsigned char unhooked_async_sig_safe_arena_storage
+    [sizeof(LowLevelAlloc::Arena)];
 #endif
 
 // We must use LowLevelCallOnce here to construct the global arenas, rather than
@@ -276,10 +277,10 @@
 static const uintptr_t kMagicUnallocated = ~kMagicAllocated;
 
 namespace {
-class SCOPED_LOCKABLE ArenaLock {
+class ABSL_SCOPED_LOCKABLE ArenaLock {
  public:
   explicit ArenaLock(LowLevelAlloc::Arena *arena)
-      EXCLUSIVE_LOCK_FUNCTION(arena->mu)
+      ABSL_EXCLUSIVE_LOCK_FUNCTION(arena->mu)
       : arena_(arena) {
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
     if ((arena->flags & LowLevelAlloc::kAsyncSignalSafe) != 0) {
@@ -291,7 +292,7 @@
     arena_->mu.Lock();
   }
   ~ArenaLock() { ABSL_RAW_CHECK(left_, "haven't left Arena region"); }
-  void Leave() UNLOCK_FUNCTION() {
+  void Leave() ABSL_UNLOCK_FUNCTION() {
     arena_->mu.Unlock();
 #ifndef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
     if (mask_valid_) {
@@ -337,11 +338,11 @@
 
 size_t RoundedUpBlockSize() {
   // Round up block sizes to a power of two close to the header size.
-  size_t roundup = 16;
-  while (roundup < sizeof(AllocList::Header)) {
-    roundup += roundup;
+  size_t round_up = 16;
+  while (round_up < sizeof(AllocList::Header)) {
+    round_up += round_up;
   }
-  return roundup;
+  return round_up;
 }
 
 }  // namespace
@@ -351,8 +352,8 @@
       allocation_count(0),
       flags(flags_value),
       pagesize(GetPageSize()),
-      roundup(RoundedUpBlockSize()),
-      min_size(2 * roundup),
+      round_up(RoundedUpBlockSize()),
+      min_size(2 * round_up),
       random(0) {
   freelist.header.size = 0;
   freelist.header.magic =
@@ -448,7 +449,7 @@
 // that the freelist is in the correct order, that it
 // consists of regions marked "unallocated", and that no two regions
 // are adjacent in memory (they should have been coalesced).
-// L < arena->mu
+// L >= arena->mu
 static AllocList *Next(int i, AllocList *prev, LowLevelAlloc::Arena *arena) {
   ABSL_RAW_CHECK(i < prev->levels, "too few levels in Next()");
   AllocList *next = prev->next[i];
@@ -509,8 +510,6 @@
   if (v != nullptr) {
     AllocList *f = reinterpret_cast<AllocList *>(
                         reinterpret_cast<char *>(v) - sizeof (f->header));
-    ABSL_RAW_CHECK(f->header.magic == Magic(kMagicAllocated, &f->header),
-                   "bad magic number in Free()");
     LowLevelAlloc::Arena *arena = f->header.arena;
     ArenaLock section(arena);
     AddToFreelist(v, arena);
@@ -529,7 +528,7 @@
     ArenaLock section(arena);
     // round up with header
     size_t req_rnd = RoundUp(CheckedAdd(request, sizeof (s->header)),
-                             arena->roundup);
+                             arena->round_up);
     for (;;) {      // loop until we find a suitable region
       // find the minimum levels that a block of this size must have
       int i = LLA_SkiplistLevels(req_rnd, arena->min_size, nullptr) - 1;
@@ -615,7 +614,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/base/internal/low_level_alloc.h b/absl/base/internal/low_level_alloc.h
index 32b6ec1..db91951 100644
--- a/absl/base/internal/low_level_alloc.h
+++ b/absl/base/internal/low_level_alloc.h
@@ -55,7 +55,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 class LowLevelAlloc {
@@ -120,7 +120,7 @@
 };
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_LOW_LEVEL_ALLOC_H_
diff --git a/absl/base/internal/low_level_alloc_test.cc b/absl/base/internal/low_level_alloc_test.cc
index b6eb8b3..7abbbf9 100644
--- a/absl/base/internal/low_level_alloc_test.cc
+++ b/absl/base/internal/low_level_alloc_test.cc
@@ -22,7 +22,7 @@
 #include <utility>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -150,7 +150,7 @@
 
 }  // namespace
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int argc, char *argv[]) {
diff --git a/absl/base/internal/low_level_scheduling.h b/absl/base/internal/low_level_scheduling.h
index b762279..961cc98 100644
--- a/absl/base/internal/low_level_scheduling.h
+++ b/absl/base/internal/low_level_scheduling.h
@@ -28,7 +28,7 @@
 extern "C" void __google_enable_rescheduling(bool disable_result);
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 class SchedulingHelper;  // To allow use of SchedulingGuard.
@@ -101,7 +101,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_LOW_LEVEL_SCHEDULING_H_
diff --git a/absl/base/internal/periodic_sampler.cc b/absl/base/internal/periodic_sampler.cc
new file mode 100644
index 0000000..520dabb
--- /dev/null
+++ b/absl/base/internal/periodic_sampler.cc
@@ -0,0 +1,53 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/periodic_sampler.h"
+
+#include <atomic>
+
+#include "absl/base/internal/exponential_biased.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+int64_t PeriodicSamplerBase::GetExponentialBiased(int period) noexcept {
+  return rng_.GetStride(period);
+}
+
+bool PeriodicSamplerBase::SubtleConfirmSample() noexcept {
+  int current_period = period();
+
+  // Deal with period case 0 (always off) and 1 (always on)
+  if (ABSL_PREDICT_FALSE(current_period < 2)) {
+    stride_ = 0;
+    return current_period == 1;
+  }
+
+  // Check if this is the first call to Sample()
+  if (ABSL_PREDICT_FALSE(stride_ == 1)) {
+    stride_ = static_cast<uint64_t>(-GetExponentialBiased(current_period));
+    if (static_cast<int64_t>(stride_) < -1) {
+      ++stride_;
+      return false;
+    }
+  }
+
+  stride_ = static_cast<uint64_t>(-GetExponentialBiased(current_period));
+  return true;
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/periodic_sampler.h b/absl/base/internal/periodic_sampler.h
new file mode 100644
index 0000000..f8a8679
--- /dev/null
+++ b/absl/base/internal/periodic_sampler.h
@@ -0,0 +1,211 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
+#define ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
+
+#include <stdint.h>
+
+#include <atomic>
+
+#include "absl/base/internal/exponential_biased.h"
+#include "absl/base/optimization.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+
+// PeriodicSamplerBase provides the basic period sampler implementation.
+//
+// This is the base class for the templated PeriodicSampler class, which holds
+// a global std::atomic value identified by a user defined tag, such that
+// each specific PeriodSampler implementation holds its own global period.
+//
+// PeriodicSamplerBase is thread-compatible except where stated otherwise.
+class PeriodicSamplerBase {
+ public:
+  // PeriodicSamplerBase is trivial / copyable / movable / destructible.
+  PeriodicSamplerBase() = default;
+  PeriodicSamplerBase(PeriodicSamplerBase&&) = default;
+  PeriodicSamplerBase(const PeriodicSamplerBase&) = default;
+
+  // Returns true roughly once every `period` calls. This is established by a
+  // randomly picked `stride` that is counted down on each call to `Sample`.
+  // This stride is picked such that the probability of `Sample()` returning
+  // true is 1 in `period`.
+  inline bool Sample() noexcept;
+
+  // The below methods are intended for optimized use cases where the
+  // size of the inlined fast path code is highly important. Applications
+  // should use the `Sample()` method unless they have proof that their
+  // specific use case requires the optimizations offered by these methods.
+  //
+  // An example of such a use case is SwissTable sampling. All sampling checks
+  // are in inlined SwissTable methods, and the number of call sites is huge.
+  // In this case, the inlined code size added to each translation unit calling
+  // SwissTable methods is non-trivial.
+  //
+  // The `SubtleMaybeSample()` function spuriously returns true even if the
+  // function should not be sampled, applications MUST match each call to
+  // 'SubtleMaybeSample()' returning true with a `SubtleConfirmSample()` call,
+  // and use the result of the latter as the sampling decision.
+  // In other words: the code should logically be equivalent to:
+  //
+  //    if (SubtleMaybeSample() && SubtleConfirmSample()) {
+  //      // Sample this call
+  //    }
+  //
+  // In the 'inline-size' optimized case, the `SubtleConfirmSample()` call can
+  // be placed out of line, for example, the typical use case looks as follows:
+  //
+  //   // --- frobber.h -----------
+  //   void FrobberSampled();
+  //
+  //   inline void FrobberImpl() {
+  //     // ...
+  //   }
+  //
+  //   inline void Frobber() {
+  //     if (ABSL_PREDICT_FALSE(sampler.SubtleMaybeSample())) {
+  //       FrobberSampled();
+  //     } else {
+  //       FrobberImpl();
+  //     }
+  //   }
+  //
+  //   // --- frobber.cc -----------
+  //   void FrobberSampled() {
+  //     if (!sampler.SubtleConfirmSample())) {
+  //       // Spurious false positive
+  //       FrobberImpl();
+  //       return;
+  //     }
+  //
+  //     // Sampled execution
+  //     // ...
+  //   }
+  inline bool SubtleMaybeSample() noexcept;
+  bool SubtleConfirmSample() noexcept;
+
+ protected:
+  // We explicitly don't use a virtual destructor as this class is never
+  // virtually destroyed, and it keeps the class trivial, which avoids TLS
+  // prologue and epilogue code for our TLS instances.
+  ~PeriodicSamplerBase() = default;
+
+  // Returns the next stride for our sampler.
+  // This function is virtual for testing purposes only.
+  virtual int64_t GetExponentialBiased(int period) noexcept;
+
+ private:
+  // Returns the current period of this sampler. Thread-safe.
+  virtual int period() const noexcept = 0;
+
+  // Keep and decrement stride_ as an unsigned integer, but compare the value
+  // to zero casted as a signed int. clang and msvc do not create optimum code
+  // if we use signed for the combined decrement and sign comparison.
+  //
+  // Below 3 alternative options, all compiles generate the best code
+  // using the unsigned increment <---> signed int comparison option.
+  //
+  // Option 1:
+  //   int64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(++stride_ < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/R5MzzA
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/z7NZAt
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/t4gPsd
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/rE6s8W
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/ARMXqS
+  //
+  // Option 2:
+  //   int64_t stride_ = 0;
+  //   if (ABSL_PREDICT_TRUE(--stride_ >= 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/jSQxYK
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/VJdYaA
+  //   Clang x64 (BAD): https://gcc.godbolt.org/z/Xm4NjX
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/4snaFd
+  //   MSVC  x64 (BAD): https://gcc.godbolt.org/z/BgnEKE
+  //
+  // Option 3:
+  //   uint64_t stride_;
+  //   if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) { ... }
+  //
+  //   GCC   x64 (OK) : https://gcc.godbolt.org/z/bFbfPy
+  //   GCC   ppc (OK) : https://gcc.godbolt.org/z/S9KkUE
+  //   Clang x64 (OK) : https://gcc.godbolt.org/z/UYzRb4
+  //   ICC   x64 (OK) : https://gcc.godbolt.org/z/ptTNfD
+  //   MSVC  x64 (OK) : https://gcc.godbolt.org/z/76j4-5
+  uint64_t stride_ = 0;
+  ExponentialBiased rng_;
+};
+
+inline bool PeriodicSamplerBase::SubtleMaybeSample() noexcept {
+  // See comments on `stride_` for the unsigned increment / signed compare.
+  if (ABSL_PREDICT_TRUE(static_cast<int64_t>(++stride_) < 0)) {
+    return false;
+  }
+  return true;
+}
+
+inline bool PeriodicSamplerBase::Sample() noexcept {
+  return ABSL_PREDICT_FALSE(SubtleMaybeSample()) ? SubtleConfirmSample()
+                                                 : false;
+}
+
+// PeriodicSampler is a concreted periodic sampler implementation.
+// The user provided Tag identifies the implementation, and is required to
+// isolate the global state of this instance from other instances.
+//
+// Typical use case:
+//
+//   struct HashTablezTag {};
+//   thread_local PeriodicSampler sampler;
+//
+//   void HashTableSamplingLogic(...) {
+//     if (sampler.Sample()) {
+//       HashTableSlowSamplePath(...);
+//     }
+//   }
+//
+template <typename Tag, int default_period = 0>
+class PeriodicSampler final : public PeriodicSamplerBase {
+ public:
+  ~PeriodicSampler() = default;
+
+  int period() const noexcept final {
+    return period_.load(std::memory_order_relaxed);
+  }
+
+  // Sets the global period for this sampler. Thread-safe.
+  // Setting a period of 0 disables the sampler, i.e., every call to Sample()
+  // will return false. Setting a period of 1 puts the sampler in 'always on'
+  // mode, i.e., every call to Sample() returns true.
+  static void SetGlobalPeriod(int period) {
+    period_.store(period, std::memory_order_relaxed);
+  }
+
+ private:
+  static std::atomic<int> period_;
+};
+
+template <typename Tag, int default_period>
+std::atomic<int> PeriodicSampler<Tag, default_period>::period_(default_period);
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_BASE_INTERNAL_PERIODIC_SAMPLER_H_
diff --git a/absl/base/internal/periodic_sampler_benchmark.cc b/absl/base/internal/periodic_sampler_benchmark.cc
new file mode 100644
index 0000000..5ad469c
--- /dev/null
+++ b/absl/base/internal/periodic_sampler_benchmark.cc
@@ -0,0 +1,79 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "benchmark/benchmark.h"
+#include "absl/base/internal/periodic_sampler.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+template <typename Sampler>
+void BM_Sample(Sampler* sampler, benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(sampler);
+    benchmark::DoNotOptimize(sampler->Sample());
+  }
+}
+
+template <typename Sampler>
+void BM_SampleMinunumInlined(Sampler* sampler, benchmark::State& state) {
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(sampler);
+    if (ABSL_PREDICT_FALSE(sampler->SubtleMaybeSample())) {
+      benchmark::DoNotOptimize(sampler->SubtleConfirmSample());
+    }
+  }
+}
+
+void BM_PeriodicSampler_TinySample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 10> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_TinySample);
+
+void BM_PeriodicSampler_ShortSample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_ShortSample);
+
+void BM_PeriodicSampler_LongSample(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024 * 1024> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_LongSample);
+
+void BM_PeriodicSampler_LongSampleMinunumInlined(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 1024 * 1024> sampler;
+  BM_SampleMinunumInlined(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_LongSampleMinunumInlined);
+
+void BM_PeriodicSampler_Disabled(benchmark::State& state) {
+  struct Tag {};
+  PeriodicSampler<Tag, 0> sampler;
+  BM_Sample(&sampler, state);
+}
+BENCHMARK(BM_PeriodicSampler_Disabled);
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/periodic_sampler_test.cc b/absl/base/internal/periodic_sampler_test.cc
new file mode 100644
index 0000000..3b301e3
--- /dev/null
+++ b/absl/base/internal/periodic_sampler_test.cc
@@ -0,0 +1,177 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/periodic_sampler.h"
+
+#include <thread>  // NOLINT(build/c++11)
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/base/macros.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+
+using testing::Eq;
+using testing::Return;
+using testing::StrictMock;
+
+class MockPeriodicSampler : public PeriodicSamplerBase {
+ public:
+  virtual ~MockPeriodicSampler() = default;
+
+  MOCK_METHOD(int, period, (), (const, noexcept));
+  MOCK_METHOD(int64_t, GetExponentialBiased, (int), (noexcept));
+};
+
+TEST(PeriodicSamplerBaseTest, Sample) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .WillOnce(Return(2))
+      .WillOnce(Return(3))
+      .WillOnce(Return(4));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, ImmediatelySample) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .WillOnce(Return(1))
+      .WillOnce(Return(2))
+      .WillOnce(Return(3));
+
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Disabled) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(0));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, AlwaysOn) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).Times(3).WillRepeatedly(Return(1));
+
+  EXPECT_TRUE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Disable) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).WillOnce(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16)).WillOnce(Return(3));
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(0));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerBaseTest, Enable) {
+  StrictMock<MockPeriodicSampler> sampler;
+
+  EXPECT_CALL(sampler, period()).WillOnce(Return(0));
+  EXPECT_FALSE(sampler.Sample());
+
+  EXPECT_CALL(sampler, period()).Times(2).WillRepeatedly(Return(16));
+  EXPECT_CALL(sampler, GetExponentialBiased(16))
+      .Times(2)
+      .WillRepeatedly(Return(3));
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_TRUE(sampler.Sample());
+
+  EXPECT_FALSE(sampler.Sample());
+  EXPECT_FALSE(sampler.Sample());
+}
+
+TEST(PeriodicSamplerTest, ConstructConstInit) {
+  struct Tag {};
+  ABSL_CONST_INIT static PeriodicSampler<Tag> sampler;
+  (void)sampler;
+}
+
+TEST(PeriodicSamplerTest, DefaultPeriod0) {
+  struct Tag {};
+  PeriodicSampler<Tag> sampler;
+  EXPECT_THAT(sampler.period(), Eq(0));
+}
+
+TEST(PeriodicSamplerTest, DefaultPeriod) {
+  struct Tag {};
+  PeriodicSampler<Tag, 100> sampler;
+  EXPECT_THAT(sampler.period(), Eq(100));
+}
+
+TEST(PeriodicSamplerTest, SetGlobalPeriod) {
+  struct Tag1 {};
+  struct Tag2 {};
+  PeriodicSampler<Tag1, 25> sampler1;
+  PeriodicSampler<Tag2, 50> sampler2;
+
+  EXPECT_THAT(sampler1.period(), Eq(25));
+  EXPECT_THAT(sampler2.period(), Eq(50));
+
+  std::thread thread([] {
+    PeriodicSampler<Tag1, 25> sampler1;
+    PeriodicSampler<Tag2, 50> sampler2;
+    EXPECT_THAT(sampler1.period(), Eq(25));
+    EXPECT_THAT(sampler2.period(), Eq(50));
+    sampler1.SetGlobalPeriod(10);
+    sampler2.SetGlobalPeriod(20);
+  });
+  thread.join();
+
+  EXPECT_THAT(sampler1.period(), Eq(10));
+  EXPECT_THAT(sampler2.period(), Eq(20));
+}
+
+}  // namespace
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/base/internal/raw_logging.cc b/absl/base/internal/raw_logging.cc
index 1a36d5b..40cea55 100644
--- a/absl/base/internal/raw_logging.cc
+++ b/absl/base/internal/raw_logging.cc
@@ -37,9 +37,9 @@
 // this, consider moving both to config.h instead.
 #if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
     defined(__Fuchsia__) || defined(__native_client__) || \
-    defined(__EMSCRIPTEN__)
-#include <unistd.h>
+    defined(__EMSCRIPTEN__) || defined(__ASYLO__)
 
+#include <unistd.h>
 
 #define ABSL_HAVE_POSIX_WRITE 1
 #define ABSL_LOW_LEVEL_WRITE_SUPPORTED 1
@@ -71,10 +71,12 @@
 // Explicitly #error out when not ABSL_LOW_LEVEL_WRITE_SUPPORTED, except for a
 // whitelisted set of platforms for which we expect not to be able to raw log.
 
-ABSL_CONST_INIT static absl::base_internal::AtomicHook<
-    absl::raw_logging_internal::LogPrefixHook> log_prefix_hook;
-ABSL_CONST_INIT static absl::base_internal::AtomicHook<
-    absl::raw_logging_internal::AbortHook> abort_hook;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+    absl::raw_logging_internal::LogPrefixHook>
+    log_prefix_hook;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static absl::base_internal::AtomicHook<
+    absl::raw_logging_internal::AbortHook>
+    abort_hook;
 
 #ifdef ABSL_LOW_LEVEL_WRITE_SUPPORTED
 static const char kTruncated[] = " ... (message truncated)\n";
@@ -182,7 +184,7 @@
 }  // namespace
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace raw_logging_internal {
 void SafeWriteToStderr(const char *s, size_t len) {
 #if defined(ABSL_HAVE_SYSCALL_WRITE)
@@ -225,13 +227,14 @@
 #endif  // !ABSL_LOW_LEVEL_WRITE_SUPPORTED
 }
 
-ABSL_CONST_INIT absl::base_internal::AtomicHook<InternalLogFunction>
-    internal_log_function(DefaultInternalLog);
+ABSL_DLL ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+    absl::base_internal::AtomicHook<InternalLogFunction>
+        internal_log_function(DefaultInternalLog);
 
 void RegisterInternalLogFunction(InternalLogFunction func) {
   internal_log_function.Store(func);
 }
 
 }  // namespace raw_logging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/raw_logging.h b/absl/base/internal/raw_logging.h
index 8e5a34e..418d6c8 100644
--- a/absl/base/internal/raw_logging.h
+++ b/absl/base/internal/raw_logging.h
@@ -22,9 +22,11 @@
 #include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/base/internal/atomic_hook.h"
 #include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
+#include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
 // This is similar to LOG(severity) << format..., but
@@ -70,14 +72,10 @@
 //
 // The API is a subset of the above: each macro only takes two arguments.  Use
 // StrCat if you need to build a richer message.
-#define ABSL_INTERNAL_LOG(severity, message)                          \
-  do {                                                                \
-    constexpr const char* absl_raw_logging_internal_basename =        \
-        ::absl::raw_logging_internal::Basename(__FILE__,              \
-                                               sizeof(__FILE__) - 1); \
-    ::absl::raw_logging_internal::internal_log_function(              \
-        ABSL_RAW_LOGGING_INTERNAL_##severity,                         \
-        absl_raw_logging_internal_basename, __LINE__, message);       \
+#define ABSL_INTERNAL_LOG(severity, message)                                \
+  do {                                                                      \
+    ::absl::raw_logging_internal::internal_log_function(                    \
+        ABSL_RAW_LOGGING_INTERNAL_##severity, __FILE__, __LINE__, message); \
   } while (0)
 
 #define ABSL_INTERNAL_CHECK(condition, message)                    \
@@ -97,7 +95,7 @@
   ::absl::NormalizeLogSeverity(severity)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace raw_logging_internal {
 
 // Helper function to implement ABSL_RAW_LOG
@@ -158,7 +156,7 @@
 //
 // 'file' and 'line' are the file and line number where the ABSL_RAW_LOG macro
 // was located.
-// The null-terminated logged message lives in the buffer between 'buf_start'
+// The NUL-terminated logged message lives in the buffer between 'buf_start'
 // and 'buf_end'.  'prefix_end' points to the first non-prefix character of the
 // buffer (as written by the LogPrefixHook.)
 using AbortHook = void (*)(const char* file, int line, const char* buf_start,
@@ -172,12 +170,14 @@
                                      const char* file, int line,
                                      const std::string& message);
 
-extern base_internal::AtomicHook<InternalLogFunction> internal_log_function;
+ABSL_DLL ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES extern base_internal::AtomicHook<
+    InternalLogFunction>
+    internal_log_function;
 
 void RegisterInternalLogFunction(InternalLogFunction func);
 
 }  // namespace raw_logging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_RAW_LOGGING_H_
diff --git a/absl/base/internal/scheduling_mode.h b/absl/base/internal/scheduling_mode.h
index 4876792..8be5ab6 100644
--- a/absl/base/internal/scheduling_mode.h
+++ b/absl/base/internal/scheduling_mode.h
@@ -18,8 +18,10 @@
 #ifndef ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
 #define ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Used to describe how a thread may be scheduled.  Typically associated with
@@ -50,7 +52,7 @@
 };
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SCHEDULING_MODE_H_
diff --git a/absl/base/internal/scoped_set_env.cc b/absl/base/internal/scoped_set_env.cc
index eb7a011..8a934cb 100644
--- a/absl/base/internal/scoped_set_env.cc
+++ b/absl/base/internal/scoped_set_env.cc
@@ -23,7 +23,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 namespace {
@@ -77,5 +77,5 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/scoped_set_env.h b/absl/base/internal/scoped_set_env.h
index 709843b..19ec7b5 100644
--- a/absl/base/internal/scoped_set_env.h
+++ b/absl/base/internal/scoped_set_env.h
@@ -19,8 +19,10 @@
 
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 class ScopedSetEnv {
@@ -37,7 +39,7 @@
 };
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SCOPED_SET_ENV_H_
diff --git a/absl/base/internal/spinlock.cc b/absl/base/internal/spinlock.cc
index 5f443bf..830d472 100644
--- a/absl/base/internal/spinlock.cc
+++ b/absl/base/internal/spinlock.cc
@@ -54,11 +54,11 @@
 //          holder to acquire the lock.  There may be outstanding waiter(s).
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-ABSL_CONST_INIT static base_internal::AtomicHook<void (*)(const void *lock,
-                                                          int64_t wait_cycles)>
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES static base_internal::AtomicHook<void (*)(
+    const void *lock, int64_t wait_cycles)>
     submit_profile_data;
 
 void RegisterSpinLockProfiler(void (*fn)(const void *contendedlock,
@@ -229,5 +229,5 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/spinlock.h b/absl/base/internal/spinlock.h
index df94766..24e2e9a 100644
--- a/absl/base/internal/spinlock.h
+++ b/absl/base/internal/spinlock.h
@@ -46,10 +46,10 @@
 #include "absl/base/thread_annotations.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-class LOCKABLE SpinLock {
+class ABSL_LOCKABLE SpinLock {
  public:
   SpinLock() : lockword_(kSpinLockCooperative) {
     ABSL_TSAN_MUTEX_CREATE(this, __tsan_mutex_not_static);
@@ -80,7 +80,7 @@
   ~SpinLock() { ABSL_TSAN_MUTEX_DESTROY(this, __tsan_mutex_not_static); }
 
   // Acquire this SpinLock.
-  inline void Lock() EXCLUSIVE_LOCK_FUNCTION() {
+  inline void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     ABSL_TSAN_MUTEX_PRE_LOCK(this, 0);
     if (!TryLockImpl()) {
       SlowLock();
@@ -92,7 +92,7 @@
   // acquisition was successful.  If the lock was not acquired, false is
   // returned.  If this SpinLock is free at the time of the call, TryLock
   // will return true with high probability.
-  inline bool TryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+  inline bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
     ABSL_TSAN_MUTEX_PRE_LOCK(this, __tsan_mutex_try_lock);
     bool res = TryLockImpl();
     ABSL_TSAN_MUTEX_POST_LOCK(
@@ -102,7 +102,7 @@
   }
 
   // Release this SpinLock, which must be held by the calling thread.
-  inline void Unlock() UNLOCK_FUNCTION() {
+  inline void Unlock() ABSL_UNLOCK_FUNCTION() {
     ABSL_TSAN_MUTEX_PRE_UNLOCK(this, 0);
     uint32_t lock_value = lockword_.load(std::memory_order_relaxed);
     lock_value = lockword_.exchange(lock_value & kSpinLockCooperative,
@@ -180,13 +180,13 @@
 
 // Corresponding locker object that arranges to acquire a spinlock for
 // the duration of a C++ scope.
-class SCOPED_LOCKABLE SpinLockHolder {
+class ABSL_SCOPED_LOCKABLE SpinLockHolder {
  public:
-  inline explicit SpinLockHolder(SpinLock* l) EXCLUSIVE_LOCK_FUNCTION(l)
+  inline explicit SpinLockHolder(SpinLock* l) ABSL_EXCLUSIVE_LOCK_FUNCTION(l)
       : lock_(l) {
     l->Lock();
   }
-  inline ~SpinLockHolder() UNLOCK_FUNCTION() { lock_->Unlock(); }
+  inline ~SpinLockHolder() ABSL_UNLOCK_FUNCTION() { lock_->Unlock(); }
 
   SpinLockHolder(const SpinLockHolder&) = delete;
   SpinLockHolder& operator=(const SpinLockHolder&) = delete;
@@ -226,11 +226,10 @@
     }
   }
 
-  if (lockword_.compare_exchange_strong(
+  if (!lockword_.compare_exchange_strong(
           lock_value,
           kSpinLockHeld | lock_value | wait_cycles | sched_disabled_bit,
           std::memory_order_acquire, std::memory_order_relaxed)) {
-  } else {
     base_internal::SchedulingGuard::EnableRescheduling(sched_disabled_bit != 0);
   }
 
@@ -238,7 +237,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SPINLOCK_H_
diff --git a/absl/base/internal/spinlock_linux.inc b/absl/base/internal/spinlock_linux.inc
index 28e29d1..323edd6 100644
--- a/absl/base/internal/spinlock_linux.inc
+++ b/absl/base/internal/spinlock_linux.inc
@@ -19,12 +19,12 @@
 #include <unistd.h>
 
 #include <atomic>
-#include <cerrno>
 #include <climits>
 #include <cstdint>
 #include <ctime>
 
 #include "absl/base/attributes.h"
+#include "absl/base/internal/errno_saver.h"
 
 // The SpinLock lockword is `std::atomic<uint32_t>`. Here we assert that
 // `std::atomic<uint32_t>` is bitwise equivalent of the `int` expected
@@ -51,12 +51,11 @@
 ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
     std::atomic<uint32_t> *w, uint32_t value, int loop,
     absl::base_internal::SchedulingMode) {
-  int save_errno = errno;
+  absl::base_internal::ErrnoSaver errno_saver;
   struct timespec tm;
   tm.tv_sec = 0;
   tm.tv_nsec = absl::base_internal::SpinLockSuggestedDelayNS(loop);
   syscall(SYS_futex, w, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, value, &tm);
-  errno = save_errno;
 }
 
 ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(std::atomic<uint32_t> *w,
diff --git a/absl/base/internal/spinlock_posix.inc b/absl/base/internal/spinlock_posix.inc
index f025b5f..fcd21b1 100644
--- a/absl/base/internal/spinlock_posix.inc
+++ b/absl/base/internal/spinlock_posix.inc
@@ -15,10 +15,11 @@
 // This file is a Posix-specific part of spinlock_wait.cc
 
 #include <sched.h>
+
 #include <atomic>
 #include <ctime>
-#include <cerrno>
 
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/scheduling_mode.h"
 #include "absl/base/port.h"
 
@@ -27,7 +28,7 @@
 ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockDelay(
     std::atomic<uint32_t>* /* lock_word */, uint32_t /* value */, int loop,
     absl::base_internal::SchedulingMode /* mode */) {
-  int save_errno = errno;
+  absl::base_internal::ErrnoSaver errno_saver;
   if (loop == 0) {
   } else if (loop == 1) {
     sched_yield();
@@ -37,7 +38,6 @@
     tm.tv_nsec = absl::base_internal::SpinLockSuggestedDelayNS(loop);
     nanosleep(&tm, nullptr);
   }
-  errno = save_errno;
 }
 
 ABSL_ATTRIBUTE_WEAK void AbslInternalSpinLockWake(
diff --git a/absl/base/internal/spinlock_wait.cc b/absl/base/internal/spinlock_wait.cc
index 60a8519..fa824be 100644
--- a/absl/base/internal/spinlock_wait.cc
+++ b/absl/base/internal/spinlock_wait.cc
@@ -32,7 +32,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // See spinlock_wait.h for spec.
@@ -77,5 +77,5 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/spinlock_wait.h b/absl/base/internal/spinlock_wait.h
index 7e139de..169bc74 100644
--- a/absl/base/internal/spinlock_wait.h
+++ b/absl/base/internal/spinlock_wait.h
@@ -24,7 +24,7 @@
 #include "absl/base/internal/scheduling_mode.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // SpinLockWait() waits until it can perform one of several transitions from
@@ -63,7 +63,7 @@
 int SpinLockSuggestedDelayNS(int loop);
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
diff --git a/absl/base/internal/sysinfo.cc b/absl/base/internal/sysinfo.cc
index 0d5cf82..a0930e9 100644
--- a/absl/base/internal/sysinfo.cc
+++ b/absl/base/internal/sysinfo.cc
@@ -17,7 +17,6 @@
 #include "absl/base/attributes.h"
 
 #ifdef _WIN32
-#include <shlwapi.h>
 #include <windows.h>
 #else
 #include <fcntl.h>
@@ -56,13 +55,9 @@
 #include "absl/base/internal/unscaledcycleclock.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
-static once_flag init_system_info_once;
-static int num_cpus = 0;
-static double nominal_cpu_frequency = 1.0;  // 0.0 might be dangerous.
-
 static int GetNumCPUs() {
 #if defined(__myriad2__)
   return 1;
@@ -77,14 +72,23 @@
 #if defined(_WIN32)
 
 static double GetNominalCPUFrequency() {
-  DWORD data;
-  DWORD data_size = sizeof(data);
-  #pragma comment(lib, "shlwapi.lib")  // For SHGetValue().
-  if (SUCCEEDED(
-          SHGetValueA(HKEY_LOCAL_MACHINE,
-                      "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0",
-                      "~MHz", nullptr, &data, &data_size))) {
-    return data * 1e6;  // Value is MHz.
+#pragma comment(lib, "advapi32.lib")  // For Reg* functions.
+  HKEY key;
+  // Use the Reg* functions rather than the SH functions because shlwapi.dll
+  // pulls in gdi32.dll which makes process destruction much more costly.
+  if (RegOpenKeyExA(HKEY_LOCAL_MACHINE,
+                    "HARDWARE\\DESCRIPTION\\System\\CentralProcessor\\0", 0,
+                    KEY_READ, &key) == ERROR_SUCCESS) {
+    DWORD type = 0;
+    DWORD data = 0;
+    DWORD data_size = sizeof(data);
+    auto result = RegQueryValueExA(key, "~MHz", 0, &type,
+                                   reinterpret_cast<LPBYTE>(&data), &data_size);
+    RegCloseKey(key);
+    if (result == ERROR_SUCCESS && type == REG_DWORD &&
+        data_size == sizeof(data)) {
+      return data * 1e6;  // Value is MHz.
+    }
   }
   return 1.0;
 }
@@ -257,28 +261,34 @@
 
 #endif
 
-// InitializeSystemInfo() may be called before main() and before
-// malloc is properly initialized, therefore this must not allocate
-// memory.
-static void InitializeSystemInfo() {
-  num_cpus = GetNumCPUs();
-  nominal_cpu_frequency = GetNominalCPUFrequency();
-}
+ABSL_CONST_INIT static once_flag init_num_cpus_once;
+ABSL_CONST_INIT static int num_cpus = 0;
 
+// NumCPUs() may be called before main() and before malloc is properly
+// initialized, therefore this must not allocate memory.
 int NumCPUs() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(
+      &init_num_cpus_once, []() { num_cpus = GetNumCPUs(); });
   return num_cpus;
 }
 
+// A default frequency of 0.0 might be dangerous if it is used in division.
+ABSL_CONST_INIT static once_flag init_nominal_cpu_frequency_once;
+ABSL_CONST_INIT static double nominal_cpu_frequency = 1.0;
+
+// NominalCPUFrequency() may be called before main() and before malloc is
+// properly initialized, therefore this must not allocate memory.
 double NominalCPUFrequency() {
-  base_internal::LowLevelCallOnce(&init_system_info_once, InitializeSystemInfo);
+  base_internal::LowLevelCallOnce(
+      &init_nominal_cpu_frequency_once,
+      []() { nominal_cpu_frequency = GetNominalCPUFrequency(); });
   return nominal_cpu_frequency;
 }
 
 #if defined(_WIN32)
 
 pid_t GetTID() {
-  return GetCurrentThreadId();
+  return pid_t{GetCurrentThreadId()};
 }
 
 #elif defined(__linux__)
@@ -402,5 +412,5 @@
 #endif
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/sysinfo.h b/absl/base/internal/sysinfo.h
index 22739ae..7246d5d 100644
--- a/absl/base/internal/sysinfo.h
+++ b/absl/base/internal/sysinfo.h
@@ -26,14 +26,14 @@
 
 #ifndef _WIN32
 #include <sys/types.h>
-#else
-#include <intsafe.h>
 #endif
 
+#include <cstdint>
+
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Nominal core processor cycles per second of each processor.   This is _not_
@@ -52,14 +52,15 @@
 // On Linux, you may send a signal to the resulting ID with kill().  However,
 // it is recommended for portability that you use pthread_kill() instead.
 #ifdef _WIN32
-// On Windows, process id and thread id are of the same type according to
-// the return types of GetProcessId() and GetThreadId() are both DWORD.
-using pid_t = DWORD;
+// On Windows, process id and thread id are of the same type according to the
+// return types of GetProcessId() and GetThreadId() are both DWORD, an unsigned
+// 32-bit type.
+using pid_t = uint32_t;
 #endif
 pid_t GetTID();
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_SYSINFO_H_
diff --git a/absl/base/internal/sysinfo_test.cc b/absl/base/internal/sysinfo_test.cc
index c8323e5..fa8b88b 100644
--- a/absl/base/internal/sysinfo_test.cc
+++ b/absl/base/internal/sysinfo_test.cc
@@ -28,7 +28,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -59,8 +59,8 @@
 #endif
   // Test that TIDs are unique to each thread.
   // Uses a few loops to exercise implementations that reallocate IDs.
-  for (int i = 0; i < 32; ++i) {
-    constexpr int kNumThreads = 64;
+  for (int i = 0; i < 10; ++i) {
+    constexpr int kNumThreads = 10;
     Barrier all_threads_done(kNumThreads);
     std::vector<std::thread> threads;
 
@@ -96,5 +96,5 @@
 
 }  // namespace
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/thread_identity.cc b/absl/base/internal/thread_identity.cc
index dbec326..d63a04a 100644
--- a/absl/base/internal/thread_identity.cc
+++ b/absl/base/internal/thread_identity.cc
@@ -28,7 +28,7 @@
 #include "absl/base/internal/spinlock.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if ABSL_THREAD_IDENTITY_MODE != ABSL_THREAD_IDENTITY_MODE_USE_CPP11
@@ -56,7 +56,12 @@
 #ifdef __GNUC__
 __attribute__((visibility("protected")))
 #endif  // __GNUC__
-  ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr;
+#if ABSL_PER_THREAD_TLS
+// Prefer __thread to thread_local as benchmarks indicate it is a bit faster.
+ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr = nullptr;
+#elif defined(ABSL_HAVE_THREAD_LOCAL)
+thread_local ThreadIdentity* thread_identity_ptr = nullptr;
+#endif  // ABSL_PER_THREAD_TLS
 #endif  // TLS or CPP11
 
 void SetCurrentThreadIdentity(
@@ -70,8 +75,8 @@
   absl::call_once(init_thread_identity_key_once, AllocateThreadIdentityKey,
                   reclaimer);
 
-#ifdef __EMSCRIPTEN__
-  // Emscripten PThread implementation does not support signals.
+#if defined(__EMSCRIPTEN__) || defined(__MINGW32__)
+  // Emscripten and MinGW pthread implementations does not support signals.
   // See https://kripken.github.io/emscripten-site/docs/porting/pthreads.html
   // for more information.
   pthread_setspecific(thread_identity_pthread_key,
@@ -90,7 +95,7 @@
   pthread_setspecific(thread_identity_pthread_key,
                       reinterpret_cast<void*>(identity));
   pthread_sigmask(SIG_SETMASK, &curr_signals, nullptr);
-#endif  // !__EMSCRIPTEN__
+#endif  // !__EMSCRIPTEN__ && !__MINGW32__
 
 #elif ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS
   // NOTE: Not async-safe.  But can be open-coded.
@@ -108,6 +113,18 @@
 #endif
 }
 
+#if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
+    ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
+
+// Please see the comment on `CurrentThreadIdentityIfPresent` in
+// thread_identity.h. Because DLLs cannot expose thread_local variables in
+// headers, we opt for the correct-but-slower option of placing the definition
+// of this function only in a translation unit inside DLL.
+#if defined(ABSL_BUILD_DLL) || defined(ABSL_CONSUME_DLL)
+ThreadIdentity* CurrentThreadIdentityIfPresent() { return thread_identity_ptr; }
+#endif
+#endif
+
 void ClearCurrentThreadIdentity() {
 #if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
@@ -131,5 +148,5 @@
 #endif
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/thread_identity.h b/absl/base/internal/thread_identity.h
index bb5aa07..ceb109b 100644
--- a/absl/base/internal/thread_identity.h
+++ b/absl/base/internal/thread_identity.h
@@ -30,10 +30,11 @@
 #include <atomic>
 #include <cstdint>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/per_thread_tls.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 struct SynchLocksHeld;
 struct SynchWaitParams;
@@ -209,7 +210,7 @@
 #error ABSL_THREAD_IDENTITY_MODE cannot be direcly set
 #elif defined(ABSL_FORCE_THREAD_IDENTITY_MODE)
 #define ABSL_THREAD_IDENTITY_MODE ABSL_FORCE_THREAD_IDENTITY_MODE
-#elif defined(_WIN32)
+#elif defined(_WIN32) && !defined(__MINGW32__)
 #define ABSL_THREAD_IDENTITY_MODE ABSL_THREAD_IDENTITY_MODE_USE_CPP11
 #elif ABSL_PER_THREAD_TLS && defined(__GOOGLE_GRTE_VERSION__) && \
     (__GOOGLE_GRTE_VERSION__ >= 20140228L)
@@ -225,11 +226,26 @@
 #if ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_TLS || \
     ABSL_THREAD_IDENTITY_MODE == ABSL_THREAD_IDENTITY_MODE_USE_CPP11
 
-extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity* thread_identity_ptr;
+#if ABSL_PER_THREAD_TLS
+ABSL_CONST_INIT extern ABSL_PER_THREAD_TLS_KEYWORD ThreadIdentity*
+    thread_identity_ptr;
+#elif defined(ABSL_HAVE_THREAD_LOCAL)
+ABSL_CONST_INIT extern thread_local ThreadIdentity* thread_identity_ptr;
+#else
+#error Thread-local storage not detected on this platform
+#endif
 
+// thread_local variables cannot be in headers exposed by DLLs. However, it is
+// important for performance reasons in general that
+// `CurrentThreadIdentityIfPresent` be inlined. This is not possible across a
+// DLL boundary so, with DLLs, we opt to have the function not be inlined. Note
+// that `CurrentThreadIdentityIfPresent` is declared above so we can exclude
+// this entire inline definition when compiling as a DLL.
+#if !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL)
 inline ThreadIdentity* CurrentThreadIdentityIfPresent() {
   return thread_identity_ptr;
 }
+#endif
 
 #elif ABSL_THREAD_IDENTITY_MODE != \
     ABSL_THREAD_IDENTITY_MODE_USE_POSIX_SETSPECIFIC
@@ -237,7 +253,7 @@
 #endif
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_THREAD_IDENTITY_H_
diff --git a/absl/base/internal/thread_identity_test.cc b/absl/base/internal/thread_identity_test.cc
index 8de6d9e..3685779 100644
--- a/absl/base/internal/thread_identity_test.cc
+++ b/absl/base/internal/thread_identity_test.cc
@@ -25,7 +25,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -124,5 +124,5 @@
 
 }  // namespace
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/throw_delegate.cc b/absl/base/internal/throw_delegate.cc
index bee404d..c055f75 100644
--- a/absl/base/internal/throw_delegate.cc
+++ b/absl/base/internal/throw_delegate.cc
@@ -22,7 +22,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 namespace {
@@ -104,5 +104,5 @@
 void ThrowStdBadAlloc() { Throw(std::bad_alloc()); }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/internal/throw_delegate.h b/absl/base/internal/throw_delegate.h
index 60ed4f3..075f527 100644
--- a/absl/base/internal/throw_delegate.h
+++ b/absl/base/internal/throw_delegate.h
@@ -19,8 +19,10 @@
 
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Helper functions that allow throwing exceptions consistently from anywhere.
@@ -67,7 +69,7 @@
 // [[noreturn]] void ThrowStdBadArrayNewLength();
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_THROW_DELEGATE_H_
diff --git a/absl/base/internal/unaligned_access.h b/absl/base/internal/unaligned_access.h
index a709a44..6be56c8 100644
--- a/absl/base/internal/unaligned_access.h
+++ b/absl/base/internal/unaligned_access.h
@@ -18,9 +18,11 @@
 #define ABSL_BASE_INTERNAL_UNALIGNED_ACCESS_H_
 
 #include <string.h>
+
 #include <cstdint>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 
 // unaligned APIs
 
@@ -56,7 +58,7 @@
 }  // extern "C"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 inline uint16_t UnalignedLoad16(const void *p) {
@@ -84,7 +86,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #define ABSL_INTERNAL_UNALIGNED_LOAD16(_p) \
@@ -104,7 +106,7 @@
 #else
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 inline uint16_t UnalignedLoad16(const void *p) {
@@ -132,7 +134,7 @@
 inline void UnalignedStore64(void *p, uint64_t v) { memcpy(p, &v, sizeof v); }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #define ABSL_INTERNAL_UNALIGNED_LOAD16(_p) \
diff --git a/absl/base/internal/unscaledcycleclock.cc b/absl/base/internal/unscaledcycleclock.cc
index 29a927d..f1e7bbe 100644
--- a/absl/base/internal/unscaledcycleclock.cc
+++ b/absl/base/internal/unscaledcycleclock.cc
@@ -21,13 +21,18 @@
 #endif
 
 #if defined(__powerpc__) || defined(__ppc__)
+#ifdef __GLIBC__
 #include <sys/platform/ppc.h>
+#elif defined(__FreeBSD__)
+#include <sys/sysctl.h>
+#include <sys/types.h>
+#endif
 #endif
 
 #include "absl/base/internal/sysinfo.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 #if defined(__i386__)
@@ -57,11 +62,43 @@
 #elif defined(__powerpc__) || defined(__ppc__)
 
 int64_t UnscaledCycleClock::Now() {
+#ifdef __GLIBC__
   return __ppc_get_timebase();
+#else
+#ifdef __powerpc64__
+  int64_t tbr;
+  asm volatile("mfspr %0, 268" : "=r"(tbr));
+  return tbr;
+#else
+  int32_t tbu, tbl, tmp;
+  asm volatile(
+      "0:\n"
+      "mftbu %[hi32]\n"
+      "mftb %[lo32]\n"
+      "mftbu %[tmp]\n"
+      "cmpw %[tmp],%[hi32]\n"
+      "bne 0b\n"
+      : [ hi32 ] "=r"(tbu), [ lo32 ] "=r"(tbl), [ tmp ] "=r"(tmp));
+  return (static_cast<int64_t>(tbu) << 32) | tbl;
+#endif
+#endif
 }
 
 double UnscaledCycleClock::Frequency() {
+#ifdef __GLIBC__
   return __ppc_get_timebase_freq();
+#elif defined(__FreeBSD__)
+  static once_flag init_timebase_frequency_once;
+  static double timebase_frequency = 0.0;
+  base_internal::LowLevelCallOnce(&init_timebase_frequency_once, [&]() {
+    size_t length = sizeof(timebase_frequency);
+    sysctlbyname("kern.timecounter.tc.timebase.frequency", &timebase_frequency,
+                 &length, nullptr, 0);
+  });
+  return timebase_frequency;
+#else
+#error Must implement UnscaledCycleClock::Frequency()
+#endif
 }
 
 #elif defined(__aarch64__)
@@ -97,7 +134,7 @@
 #endif
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_USE_UNSCALED_CYCLECLOCK
diff --git a/absl/base/internal/unscaledcycleclock.h b/absl/base/internal/unscaledcycleclock.h
index e6fc910..cdce9bf 100644
--- a/absl/base/internal/unscaledcycleclock.h
+++ b/absl/base/internal/unscaledcycleclock.h
@@ -86,7 +86,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 class UnscaledCycleClockWrapperForGetCurrentTime;
 }  // namespace time_internal
@@ -116,7 +116,7 @@
 };
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_USE_UNSCALED_CYCLECLOCK
diff --git a/absl/base/invoke_test.cc b/absl/base/invoke_test.cc
index 685501a..6aa613c 100644
--- a/absl/base/invoke_test.cc
+++ b/absl/base/invoke_test.cc
@@ -25,7 +25,7 @@
 #include "absl/strings/str_cat.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 namespace {
 
@@ -219,5 +219,5 @@
 
 }  // namespace
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/log_severity.cc b/absl/base/log_severity.cc
index 8109da1..72312af 100644
--- a/absl/base/log_severity.cc
+++ b/absl/base/log_severity.cc
@@ -17,11 +17,11 @@
 #include <ostream>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 std::ostream& operator<<(std::ostream& os, absl::LogSeverity s) {
   if (s == absl::NormalizeLogSeverity(s)) return os << absl::LogSeverityName(s);
   return os << "absl::LogSeverity(" << static_cast<int>(s) << ")";
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/log_severity.h b/absl/base/log_severity.h
index 45f79cc..65a3b16 100644
--- a/absl/base/log_severity.h
+++ b/absl/base/log_severity.h
@@ -19,13 +19,53 @@
 #include <ostream>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
-// Four severity levels are defined.  Logging APIs should terminate the program
+// absl::LogSeverity
+//
+// Four severity levels are defined. Logging APIs should terminate the program
 // when a message is logged at severity `kFatal`; the other levels have no
 // special semantics.
+//
+// Values other than the four defined levels (e.g. produced by `static_cast`)
+// are valid, but their semantics when passed to a function, macro, or flag
+// depend on the function, macro, or flag. The usual behavior is to normalize
+// such values to a defined severity level, however in some cases values other
+// than the defined levels are useful for comparison.
+//
+// Exmaple:
+//
+//   // Effectively disables all logging:
+//   SetMinLogLevel(static_cast<absl::LogSeverity>(100));
+//
+// Abseil flags may be defined with type `LogSeverity`. Dependency layering
+// constraints require that the `AbslParseFlag()` overload be declared and
+// defined in the flags library itself rather than here. The `AbslUnparseFlag()`
+// overload is defined there as well for consistency.
+//
+// absl::LogSeverity Flag String Representation
+//
+// An `absl::LogSeverity` has a string representation used for parsing
+// command-line flags based on the enumerator name (e.g. `kFatal`) or
+// its unprefixed name (without the `k`) in any case-insensitive form. (E.g.
+// "FATAL", "fatal" or "Fatal" are all valid.) Unparsing such flags produces an
+// unprefixed string representation in all caps (e.g. "FATAL") or an integer.
+//
+// Additionally, the parser accepts arbitrary integers (as if the type were
+// `int`).
+//
+// Examples:
+//
+//   --my_log_level=kInfo
+//   --my_log_level=INFO
+//   --my_log_level=info
+//   --my_log_level=0
+//
+// Unparsing a flag produces the same result as `absl::LogSeverityName()` for
+// the standard levels and a base-ten integer otherwise.
 enum class LogSeverity : int {
   kInfo = 0,
   kWarning = 1,
@@ -33,6 +73,8 @@
   kFatal = 3,
 };
 
+// LogSeverities()
+//
 // Returns an iterable of all standard `absl::LogSeverity` values, ordered from
 // least to most severe.
 constexpr std::array<absl::LogSeverity, 4> LogSeverities() {
@@ -40,8 +82,10 @@
            absl::LogSeverity::kError, absl::LogSeverity::kFatal}};
 }
 
+// LogSeverityName()
+//
 // Returns the all-caps string representation (e.g. "INFO") of the specified
-// severity level if it is one of the normal levels and "UNKNOWN" otherwise.
+// severity level if it is one of the standard levels and "UNKNOWN" otherwise.
 constexpr const char* LogSeverityName(absl::LogSeverity s) {
   return s == absl::LogSeverity::kInfo
              ? "INFO"
@@ -52,6 +96,8 @@
                          : s == absl::LogSeverity::kFatal ? "FATAL" : "UNKNOWN";
 }
 
+// NormalizeLogSeverity()
+//
 // Values less than `kInfo` normalize to `kInfo`; values greater than `kFatal`
 // normalize to `kError` (**NOT** `kFatal`).
 constexpr absl::LogSeverity NormalizeLogSeverity(absl::LogSeverity s) {
@@ -60,14 +106,16 @@
              : s > absl::LogSeverity::kFatal ? absl::LogSeverity::kError : s;
 }
 constexpr absl::LogSeverity NormalizeLogSeverity(int s) {
-  return NormalizeLogSeverity(static_cast<absl::LogSeverity>(s));
+  return absl::NormalizeLogSeverity(static_cast<absl::LogSeverity>(s));
 }
 
+// operator<<
+//
 // The exact representation of a streamed `absl::LogSeverity` is deliberately
 // unspecified; do not rely on it.
 std::ostream& operator<<(std::ostream& os, absl::LogSeverity s);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_INTERNAL_LOG_SEVERITY_H_
diff --git a/absl/base/log_severity_test.cc b/absl/base/log_severity_test.cc
index 1de2d10..2302aa1 100644
--- a/absl/base/log_severity_test.cc
+++ b/absl/base/log_severity_test.cc
@@ -14,14 +14,26 @@
 
 #include "absl/base/log_severity.h"
 
+#include <cstdint>
+#include <ios>
+#include <limits>
+#include <ostream>
 #include <sstream>
 #include <string>
+#include <tuple>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/marshalling.h"
+#include "absl/strings/str_cat.h"
 
 namespace {
-using testing::Eq;
+using ::testing::Eq;
+using ::testing::IsFalse;
+using ::testing::IsTrue;
+using ::testing::TestWithParam;
+using ::testing::Values;
 
 std::string StreamHelper(absl::LogSeverity value) {
   std::ostringstream stream;
@@ -40,4 +52,153 @@
               Eq("absl::LogSeverity(4)"));
 }
 
+static_assert(
+    absl::flags_internal::IsAtomicFlagTypeTrait<absl::LogSeverity>::value,
+    "Flags of type absl::LogSeverity ought to be lock-free.");
+
+using ParseFlagFromOutOfRangeIntegerTest = TestWithParam<int64_t>;
+INSTANTIATE_TEST_SUITE_P(
+    Instantiation, ParseFlagFromOutOfRangeIntegerTest,
+    Values(static_cast<int64_t>(std::numeric_limits<int>::min()) - 1,
+           static_cast<int64_t>(std::numeric_limits<int>::max()) + 1));
+TEST_P(ParseFlagFromOutOfRangeIntegerTest, ReturnsError) {
+  const std::string to_parse = absl::StrCat(GetParam());
+  absl::LogSeverity value;
+  std::string error;
+  EXPECT_THAT(absl::ParseFlag(to_parse, &value, &error), IsFalse()) << value;
+}
+
+using ParseFlagFromAlmostOutOfRangeIntegerTest = TestWithParam<int>;
+INSTANTIATE_TEST_SUITE_P(Instantiation,
+                         ParseFlagFromAlmostOutOfRangeIntegerTest,
+                         Values(std::numeric_limits<int>::min(),
+                                std::numeric_limits<int>::max()));
+TEST_P(ParseFlagFromAlmostOutOfRangeIntegerTest, YieldsExpectedValue) {
+  const auto expected = static_cast<absl::LogSeverity>(GetParam());
+  const std::string to_parse = absl::StrCat(GetParam());
+  absl::LogSeverity value;
+  std::string error;
+  ASSERT_THAT(absl::ParseFlag(to_parse, &value, &error), IsTrue()) << error;
+  EXPECT_THAT(value, Eq(expected));
+}
+
+using ParseFlagFromIntegerMatchingEnumeratorTest =
+    TestWithParam<std::tuple<absl::string_view, absl::LogSeverity>>;
+INSTANTIATE_TEST_SUITE_P(
+    Instantiation, ParseFlagFromIntegerMatchingEnumeratorTest,
+    Values(std::make_tuple("0", absl::LogSeverity::kInfo),
+           std::make_tuple(" 0", absl::LogSeverity::kInfo),
+           std::make_tuple("-0", absl::LogSeverity::kInfo),
+           std::make_tuple("+0", absl::LogSeverity::kInfo),
+           std::make_tuple("00", absl::LogSeverity::kInfo),
+           std::make_tuple("0 ", absl::LogSeverity::kInfo),
+           std::make_tuple("0x0", absl::LogSeverity::kInfo),
+           std::make_tuple("1", absl::LogSeverity::kWarning),
+           std::make_tuple("+1", absl::LogSeverity::kWarning),
+           std::make_tuple("2", absl::LogSeverity::kError),
+           std::make_tuple("3", absl::LogSeverity::kFatal)));
+TEST_P(ParseFlagFromIntegerMatchingEnumeratorTest, YieldsExpectedValue) {
+  const absl::string_view to_parse = std::get<0>(GetParam());
+  const absl::LogSeverity expected = std::get<1>(GetParam());
+  absl::LogSeverity value;
+  std::string error;
+  ASSERT_THAT(absl::ParseFlag(to_parse, &value, &error), IsTrue()) << error;
+  EXPECT_THAT(value, Eq(expected));
+}
+
+using ParseFlagFromOtherIntegerTest =
+    TestWithParam<std::tuple<absl::string_view, int>>;
+INSTANTIATE_TEST_SUITE_P(Instantiation, ParseFlagFromOtherIntegerTest,
+                         Values(std::make_tuple("-1", -1),
+                                std::make_tuple("4", 4),
+                                std::make_tuple("010", 10),
+                                std::make_tuple("0x10", 16)));
+TEST_P(ParseFlagFromOtherIntegerTest, YieldsExpectedValue) {
+  const absl::string_view to_parse = std::get<0>(GetParam());
+  const auto expected = static_cast<absl::LogSeverity>(std::get<1>(GetParam()));
+  absl::LogSeverity value;
+  std::string error;
+  ASSERT_THAT(absl::ParseFlag(to_parse, &value, &error), IsTrue()) << error;
+  EXPECT_THAT(value, Eq(expected));
+}
+
+using ParseFlagFromEnumeratorTest =
+    TestWithParam<std::tuple<absl::string_view, absl::LogSeverity>>;
+INSTANTIATE_TEST_SUITE_P(
+    Instantiation, ParseFlagFromEnumeratorTest,
+    Values(std::make_tuple("INFO", absl::LogSeverity::kInfo),
+           std::make_tuple("info", absl::LogSeverity::kInfo),
+           std::make_tuple("kInfo", absl::LogSeverity::kInfo),
+           std::make_tuple("iNfO", absl::LogSeverity::kInfo),
+           std::make_tuple("kInFo", absl::LogSeverity::kInfo),
+           std::make_tuple("WARNING", absl::LogSeverity::kWarning),
+           std::make_tuple("warning", absl::LogSeverity::kWarning),
+           std::make_tuple("kWarning", absl::LogSeverity::kWarning),
+           std::make_tuple("WaRnInG", absl::LogSeverity::kWarning),
+           std::make_tuple("KwArNiNg", absl::LogSeverity::kWarning),
+           std::make_tuple("ERROR", absl::LogSeverity::kError),
+           std::make_tuple("error", absl::LogSeverity::kError),
+           std::make_tuple("kError", absl::LogSeverity::kError),
+           std::make_tuple("eRrOr", absl::LogSeverity::kError),
+           std::make_tuple("kErRoR", absl::LogSeverity::kError),
+           std::make_tuple("FATAL", absl::LogSeverity::kFatal),
+           std::make_tuple("fatal", absl::LogSeverity::kFatal),
+           std::make_tuple("kFatal", absl::LogSeverity::kFatal),
+           std::make_tuple("FaTaL", absl::LogSeverity::kFatal),
+           std::make_tuple("KfAtAl", absl::LogSeverity::kFatal)));
+TEST_P(ParseFlagFromEnumeratorTest, YieldsExpectedValue) {
+  const absl::string_view to_parse = std::get<0>(GetParam());
+  const absl::LogSeverity expected = std::get<1>(GetParam());
+  absl::LogSeverity value;
+  std::string error;
+  ASSERT_THAT(absl::ParseFlag(to_parse, &value, &error), IsTrue()) << error;
+  EXPECT_THAT(value, Eq(expected));
+}
+
+using ParseFlagFromGarbageTest = TestWithParam<absl::string_view>;
+INSTANTIATE_TEST_SUITE_P(Instantiation, ParseFlagFromGarbageTest,
+                         Values("", "\0", " ", "garbage", "kkinfo", "I"));
+TEST_P(ParseFlagFromGarbageTest, ReturnsError) {
+  const absl::string_view to_parse = GetParam();
+  absl::LogSeverity value;
+  std::string error;
+  EXPECT_THAT(absl::ParseFlag(to_parse, &value, &error), IsFalse()) << value;
+}
+
+using UnparseFlagToEnumeratorTest =
+    TestWithParam<std::tuple<absl::LogSeverity, absl::string_view>>;
+INSTANTIATE_TEST_SUITE_P(
+    Instantiation, UnparseFlagToEnumeratorTest,
+    Values(std::make_tuple(absl::LogSeverity::kInfo, "INFO"),
+           std::make_tuple(absl::LogSeverity::kWarning, "WARNING"),
+           std::make_tuple(absl::LogSeverity::kError, "ERROR"),
+           std::make_tuple(absl::LogSeverity::kFatal, "FATAL")));
+TEST_P(UnparseFlagToEnumeratorTest, ReturnsExpectedValueAndRoundTrips) {
+  const absl::LogSeverity to_unparse = std::get<0>(GetParam());
+  const absl::string_view expected = std::get<1>(GetParam());
+  const std::string stringified_value = absl::UnparseFlag(to_unparse);
+  EXPECT_THAT(stringified_value, Eq(expected));
+  absl::LogSeverity reparsed_value;
+  std::string error;
+  EXPECT_THAT(absl::ParseFlag(stringified_value, &reparsed_value, &error),
+              IsTrue());
+  EXPECT_THAT(reparsed_value, Eq(to_unparse));
+}
+
+using UnparseFlagToOtherIntegerTest = TestWithParam<int>;
+INSTANTIATE_TEST_SUITE_P(Instantiation, UnparseFlagToOtherIntegerTest,
+                         Values(std::numeric_limits<int>::min(), -1, 4,
+                                std::numeric_limits<int>::max()));
+TEST_P(UnparseFlagToOtherIntegerTest, ReturnsExpectedValueAndRoundTrips) {
+  const absl::LogSeverity to_unparse =
+      static_cast<absl::LogSeverity>(GetParam());
+  const std::string expected = absl::StrCat(GetParam());
+  const std::string stringified_value = absl::UnparseFlag(to_unparse);
+  EXPECT_THAT(stringified_value, Eq(expected));
+  absl::LogSeverity reparsed_value;
+  std::string error;
+  EXPECT_THAT(absl::ParseFlag(stringified_value, &reparsed_value, &error),
+              IsTrue());
+  EXPECT_THAT(reparsed_value, Eq(to_unparse));
+}
 }  // namespace
diff --git a/absl/base/macros.h b/absl/base/macros.h
index 3121088..547f93b 100644
--- a/absl/base/macros.h
+++ b/absl/base/macros.h
@@ -31,6 +31,7 @@
 #include <cassert>
 #include <cstddef>
 
+#include "absl/base/attributes.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
@@ -43,14 +44,14 @@
   (sizeof(::absl::macros_internal::ArraySizeHelper(array)))
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace macros_internal {
 // Note: this internal template function declaration is used by ABSL_ARRAYSIZE.
 // The function doesn't need a definition, as we only use its type.
 template <typename T, size_t N>
 auto ArraySizeHelper(const T (&array)[N]) -> char (&)[N];
 }  // namespace macros_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // kLinkerInitialized
@@ -74,13 +75,13 @@
 //       // Invocation
 //       static MyClass my_global(absl::base_internal::kLinkerInitialized);
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 enum LinkerInitialized {
   kLinkerInitialized = 0,
 };
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // ABSL_FALLTHROUGH_INTENDED
@@ -111,7 +112,7 @@
 // when  performing switch labels fall-through diagnostic
 // (`-Wimplicit-fallthrough`). See clang documentation on language extensions
 // for details:
-// http://clang.llvm.org/docs/AttributeReference.html#fallthrough-clang-fallthrough
+// https://clang.llvm.org/docs/AttributeReference.html#fallthrough-clang-fallthrough
 //
 // When used with unsupported compilers, the ABSL_FALLTHROUGH_INTENDED macro
 // has no effect on diagnostics. In any case this macro has no effect on runtime
@@ -141,10 +142,15 @@
 // declarations. The macro argument is used as a custom diagnostic message (e.g.
 // suggestion of a better alternative).
 //
-// Example:
+// Examples:
 //
 //   class ABSL_DEPRECATED("Use Bar instead") Foo {...};
-//   ABSL_DEPRECATED("Use Baz instead") void Bar() {...}
+//
+//   ABSL_DEPRECATED("Use Baz() instead") void Bar() {...}
+//
+//   template <typename T>
+//   ABSL_DEPRECATED("Use DoThat() instead")
+//   void DoThis();
 //
 // Every usage of a deprecated entity will trigger a warning when compiled with
 // clang's `-Wdeprecated-declarations` option. This option is turned off by
@@ -162,7 +168,7 @@
 // Used on a function overload to trap bad calls: any call that matches the
 // overload will cause a compile-time error. This macro uses a clang-specific
 // "enable_if" attribute, as described at
-// http://clang.llvm.org/docs/AttributeReference.html#enable-if
+// https://clang.llvm.org/docs/AttributeReference.html#enable-if
 //
 // Overloads which use this macro should be bracketed by
 // `#ifdef ABSL_BAD_CALL_IF`.
@@ -175,12 +181,9 @@
 //     ABSL_BAD_CALL_IF(c <= -1 || c > 255,
 //                       "'c' must have the value of an unsigned char or EOF");
 //   #endif // ABSL_BAD_CALL_IF
-
-#if defined(__clang__)
-# if __has_attribute(enable_if)
-#  define ABSL_BAD_CALL_IF(expr, msg) \
-    __attribute__((enable_if(expr, "Bad call trap"), unavailable(msg)))
-# endif
+#if ABSL_HAVE_ATTRIBUTE(enable_if)
+#define ABSL_BAD_CALL_IF(expr, msg) \
+  __attribute__((enable_if(expr, "Bad call trap"), unavailable(msg)))
 #endif
 
 // ABSL_ASSERT()
diff --git a/absl/base/optimization.h b/absl/base/optimization.h
index 0dcbef3..646523b 100644
--- a/absl/base/optimization.h
+++ b/absl/base/optimization.h
@@ -172,7 +172,7 @@
 #if ABSL_HAVE_BUILTIN(__builtin_expect) || \
     (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_PREDICT_FALSE(x) (__builtin_expect(x, 0))
-#define ABSL_PREDICT_TRUE(x) (__builtin_expect(!!(x), 1))
+#define ABSL_PREDICT_TRUE(x) (__builtin_expect(false || (x), true))
 #else
 #define ABSL_PREDICT_FALSE(x) (x)
 #define ABSL_PREDICT_TRUE(x) (x)
diff --git a/absl/base/options.h b/absl/base/options.h
new file mode 100644
index 0000000..50f26e2
--- /dev/null
+++ b/absl/base/options.h
@@ -0,0 +1,211 @@
+#ifndef ABSL_BASE_OPTIONS_H_
+#define ABSL_BASE_OPTIONS_H_
+
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: options.h
+// -----------------------------------------------------------------------------
+//
+// This file contains Abseil configuration options for setting specific
+// implementations instead of letting Abseil determine which implementation to
+// use at compile-time. Setting these options may be useful for package or build
+// managers who wish to guarantee ABI stability within binary builds (which are
+// otherwise difficult to enforce).
+//
+// *** IMPORTANT NOTICE FOR PACKAGE MANAGERS:  It is important that
+// maintainers of package managers who wish to package Abseil read and
+// understand this file! ***
+//
+// Abseil contains a number of possible configuration endpoints, based on
+// parameters such as the detected platform, language version, or command-line
+// flags used to invoke the underlying binary. As is the case with all
+// libraries, binaries which contain Abseil code must ensure that separate
+// packages use the same compiled copy of Abseil to avoid a diamond dependency
+// problem, which can occur if two packages built with different Abseil
+// configuration settings are linked together. Diamond dependency problems in
+// C++ may manifest as violations to the One Definition Rule (ODR) (resulting in
+// linker errors), or undefined behavior (resulting in crashes).
+//
+// Diamond dependency problems can be avoided if all packages utilize the same
+// exact version of Abseil. Building from source code with the same compilation
+// parameters is the easiest way to avoid such dependency problems. However, for
+// package managers who cannot control such compilation parameters, we are
+// providing the file to allow you to inject ABI (Application Binary Interface)
+// stability across builds. Settings options in this file will neither change
+// API nor ABI, providing a stable copy of Abseil between packages.
+//
+// Care must be taken to keep options within these configurations isolated
+// from any other dynamic settings, such as command-line flags which could alter
+// these options. This file is provided specifically to help build and package
+// managers provide a stable copy of Abseil within their libraries and binaries;
+// other developers should not have need to alter the contents of this file.
+//
+// -----------------------------------------------------------------------------
+// Usage
+// -----------------------------------------------------------------------------
+//
+// For any particular package release, set the appropriate definitions within
+// this file to whatever value makes the most sense for your package(s). Note
+// that, by default, most of these options, at the moment, affect the
+// implementation of types; future options may affect other implementation
+// details.
+//
+// NOTE: the defaults within this file all assume that Abseil can select the
+// proper Abseil implementation at compile-time, which will not be sufficient
+// to guarantee ABI stability to package managers.
+
+// Include a standard library header to allow configuration based on the
+// standard library in use.
+#ifdef __cplusplus
+#include <ciso646>
+#endif
+
+// -----------------------------------------------------------------------------
+// Type Compatibility Options
+// -----------------------------------------------------------------------------
+//
+// ABSL_OPTION_USE_STD_ANY
+//
+// This option controls whether absl::any is implemented as an alias to
+// std::any, or as an independent implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++11
+// support, and is expected to work on every toolchain we support.
+//
+// A value of 1 means to use an alias to std::any.  This requires that all code
+// using Abseil is built in C++17 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if a working std::any is available.  This option is
+// useful when you are building your entire program, including all of its
+// dependencies, from source.  It should not be used otherwise -- for example,
+// if you are distributing Abseil in a binary package manager -- since in
+// mode 2, absl::any will name a different type, with a different mangled name
+// and binary layout, depending on the compiler flags passed by the end user.
+// For more info, see https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// absl::any is a typedef of std::any, use the feature macro ABSL_USES_STD_ANY.
+
+#define ABSL_OPTION_USE_STD_ANY 2
+
+
+// ABSL_OPTION_USE_STD_OPTIONAL
+//
+// This option controls whether absl::optional is implemented as an alias to
+// std::optional, or as an independent implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++11
+// support, and is expected to work on every toolchain we support.
+//
+// A value of 1 means to use an alias to std::optional.  This requires that all
+// code using Abseil is built in C++17 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if a working std::optional is available.  This option
+// is useful when you are building your program from source.  It should not be
+// used otherwise -- for example, if you are distributing Abseil in a binary
+// package manager -- since in mode 2, absl::optional will name a different
+// type, with a different mangled name and binary layout, depending on the
+// compiler flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+
+// User code should not inspect this macro.  To check in the preprocessor if
+// absl::optional is a typedef of std::optional, use the feature macro
+// ABSL_USES_STD_OPTIONAL.
+
+#define ABSL_OPTION_USE_STD_OPTIONAL 2
+
+
+// ABSL_OPTION_USE_STD_STRING_VIEW
+//
+// This option controls whether absl::string_view is implemented as an alias to
+// std::string_view, or as an independent implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++11
+// support, and is expected to work on every toolchain we support.
+//
+// A value of 1 means to use an alias to std::string_view.  This requires that
+// all code using Abseil is built in C++17 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if a working std::string_view is available.  This
+// option is useful when you are building your program from source.  It should
+// not be used otherwise -- for example, if you are distributing Abseil in a
+// binary package manager -- since in mode 2, absl::string_view will name a
+// different type, with a different mangled name and binary layout, depending on
+// the compiler flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// absl::string_view is a typedef of std::string_view, use the feature macro
+// ABSL_USES_STD_STRING_VIEW.
+
+#define ABSL_OPTION_USE_STD_STRING_VIEW 2
+
+// ABSL_OPTION_USE_STD_VARIANT
+//
+// This option controls whether absl::variant is implemented as an alias to
+// std::variant, or as an independent implementation.
+//
+// A value of 0 means to use Abseil's implementation.  This requires only C++11
+// support, and is expected to work on every toolchain we support.
+//
+// A value of 1 means to use an alias to std::variant.  This requires that all
+// code using Abseil is built in C++17 mode or later.
+//
+// A value of 2 means to detect the C++ version being used to compile Abseil,
+// and use an alias only if a working std::variant is available.  This option
+// is useful when you are building your program from source.  It should not be
+// used otherwise -- for example, if you are distributing Abseil in a binary
+// package manager -- since in mode 2, absl::variant will name a different
+// type, with a different mangled name and binary layout, depending on the
+// compiler flags passed by the end user.  For more info, see
+// https://abseil.io/about/design/dropin-types.
+//
+// User code should not inspect this macro.  To check in the preprocessor if
+// absl::variant is a typedef of std::variant, use the feature macro
+// ABSL_USES_STD_VARIANT.
+
+#define ABSL_OPTION_USE_STD_VARIANT 2
+
+
+// ABSL_OPTION_USE_INLINE_NAMESPACE
+// ABSL_OPTION_INLINE_NAMESPACE_NAME
+//
+// These options controls whether all entities in the absl namespace are
+// contained within an inner inline namespace.  This does not affect the
+// user-visible API of Abseil, but it changes the mangled names of all symbols.
+//
+// This can be useful as a version tag if you are distributing Abseil in
+// precompiled form.  This will prevent a binary library build of Abseil with
+// one inline namespace being used with headers configured with a different
+// inline namespace name.  Binary packagers are reminded that Abseil does not
+// guarantee any ABI stability in Abseil, so any update of Abseil or
+// configuration change in such a binary package should be combined with a
+// new, unique value for the inline namespace name.
+//
+// A value of 0 means not to use inline namespaces.
+//
+// A value of 1 means to use an inline namespace with the given name inside
+// namespace absl.  If this is set, ABSL_OPTION_INLINE_NAMESPACE_NAME must also
+// be changed to a new, unique identifier name.  In particular "head" is not
+// allowed.
+
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME lts_2020_02_25
+
+#endif  // ABSL_BASE_OPTIONS_H_
diff --git a/absl/base/policy_checks.h b/absl/base/policy_checks.h
index 699fb1a..4dfa49e 100644
--- a/absl/base/policy_checks.h
+++ b/absl/base/policy_checks.h
@@ -82,16 +82,6 @@
 // Standard Library Check
 // -----------------------------------------------------------------------------
 
-// We have chosen glibc 2.12 as the minimum as it was tagged for release
-// in May, 2010 and includes some functionality used in Google software
-// (for instance pthread_setname_np):
-// https://sourceware.org/ml/libc-alpha/2010-05/msg00000.html
-#if defined(__GLIBC__) && defined(__GLIBC_PREREQ)
-#if !__GLIBC_PREREQ(2, 12)
-#error "Minimum required version of glibc is 2.12."
-#endif
-#endif
-
 #if defined(_STLPORT_VERSION)
 #error "STLPort is not supported."
 #endif
diff --git a/absl/base/spinlock_test_common.cc b/absl/base/spinlock_test_common.cc
index 06860e7..08f61ba 100644
--- a/absl/base/spinlock_test_common.cc
+++ b/absl/base/spinlock_test_common.cc
@@ -36,7 +36,7 @@
 constexpr int32_t kIters = 1000;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // This is defined outside of anonymous namespace so that it can be
@@ -267,5 +267,5 @@
 
 }  // namespace
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/base/thread_annotations.h b/absl/base/thread_annotations.h
index f3e9658..5f51c0c 100644
--- a/absl/base/thread_annotations.h
+++ b/absl/base/thread_annotations.h
@@ -34,6 +34,7 @@
 #ifndef ABSL_BASE_THREAD_ANNOTATIONS_H_
 #define ABSL_BASE_THREAD_ANNOTATIONS_H_
 
+#include "absl/base/config.h"
 // TODO(mbonadei): Remove after the backward compatibility period.
 #include "absl/base/internal/thread_annotations.h"  // IWYU pragma: export
 
@@ -256,7 +257,7 @@
 #define ABSL_TS_UNCHECKED_READ(x) absl::base_internal::ts_unchecked_read(x)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace base_internal {
 
 // Takes a reference to a guarded data member, and returns an unguarded
@@ -273,7 +274,7 @@
 }
 
 }  // namespace base_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_BASE_THREAD_ANNOTATIONS_H_
diff --git a/absl/base/throw_delegate_test.cc b/absl/base/throw_delegate_test.cc
index a74dd3c..5ba4ce5 100644
--- a/absl/base/throw_delegate_test.cc
+++ b/absl/base/throw_delegate_test.cc
@@ -18,6 +18,7 @@
 #include <new>
 #include <stdexcept>
 
+#include "absl/base/config.h"
 #include "gtest/gtest.h"
 
 namespace {
@@ -38,31 +39,43 @@
 
 template <typename E>
 void ExpectThrowChar(void (*f)(const char*)) {
+#ifdef ABSL_HAVE_EXCEPTIONS
   try {
     f(what_arg);
     FAIL() << "Didn't throw";
   } catch (const E& e) {
     EXPECT_STREQ(e.what(), what_arg);
   }
+#else
+  EXPECT_DEATH_IF_SUPPORTED(f(what_arg), what_arg);
+#endif
 }
 
 template <typename E>
 void ExpectThrowString(void (*f)(const std::string&)) {
+#ifdef ABSL_HAVE_EXCEPTIONS
   try {
     f(what_arg);
     FAIL() << "Didn't throw";
   } catch (const E& e) {
     EXPECT_STREQ(e.what(), what_arg);
   }
+#else
+  EXPECT_DEATH_IF_SUPPORTED(f(what_arg), what_arg);
+#endif
 }
 
 template <typename E>
 void ExpectThrowNoWhat(void (*f)()) {
+#ifdef ABSL_HAVE_EXCEPTIONS
   try {
     f();
     FAIL() << "Didn't throw";
   } catch (const E& e) {
   }
+#else
+  EXPECT_DEATH_IF_SUPPORTED(f(), "");
+#endif
 }
 
 TEST(ThrowHelper, Test) {
diff --git a/absl/container/BUILD.bazel b/absl/container/BUILD.bazel
index 9e2a5b1..f221714 100644
--- a/absl/container/BUILD.bazel
+++ b/absl/container/BUILD.bazel
@@ -14,12 +14,11 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
@@ -71,20 +70,6 @@
 cc_test(
     name = "fixed_array_test",
     srcs = ["fixed_array_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":fixed_array",
-        "//absl/base:exception_testing",
-        "//absl/hash:hash_testing",
-        "//absl/memory",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "fixed_array_test_noexceptions",
-    srcs = ["fixed_array_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
@@ -99,10 +84,11 @@
 cc_test(
     name = "fixed_array_exception_safety_test",
     srcs = ["fixed_array_exception_safety_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":fixed_array",
+        "//absl/base:config",
         "//absl/base:exception_safety_testing",
         "@com_google_googletest//:gtest_main",
     ],
@@ -155,39 +141,21 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
     name = "inlined_vector_test",
     srcs = ["inlined_vector_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":counting_allocator",
-        ":inlined_vector",
-        ":test_instance_tracker",
-        "//absl/base",
-        "//absl/base:core_headers",
-        "//absl/base:exception_testing",
-        "//absl/hash:hash_testing",
-        "//absl/memory",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "inlined_vector_test_noexceptions",
-    srcs = ["inlined_vector_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":counting_allocator",
         ":inlined_vector",
         ":test_instance_tracker",
-        "//absl/base",
         "//absl/base:core_headers",
         "//absl/base:exception_testing",
+        "//absl/base:raw_logging_internal",
         "//absl/hash:hash_testing",
         "//absl/memory",
         "//absl/strings",
@@ -203,8 +171,8 @@
     tags = ["benchmark"],
     deps = [
         ":inlined_vector",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "@com_github_google_benchmark//:benchmark_main",
     ],
@@ -213,9 +181,10 @@
 cc_test(
     name = "inlined_vector_exception_safety_test",
     srcs = ["inlined_vector_exception_safety_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
+    copts = ABSL_TEST_COPTS,
     deps = [
         ":inlined_vector",
+        "//absl/base:config",
         "//absl/base:exception_safety_testing",
         "@com_google_googletest//:gtest_main",
     ],
@@ -447,6 +416,7 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":hash_policy_testing",
+        "//absl/memory",
         "//absl/meta:type_traits",
         "//absl/strings",
     ],
@@ -509,6 +479,9 @@
     hdrs = ["internal/hashtable_debug_hooks.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_library(
@@ -524,6 +497,7 @@
         ":have_sse",
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:exponential_biased",
         "//absl/debugging:stacktrace",
         "//absl/memory",
         "//absl/synchronization",
@@ -551,6 +525,7 @@
     hdrs = ["internal/node_hash_policy.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
@@ -635,6 +610,7 @@
         ":raw_hash_set",
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
@@ -678,8 +654,8 @@
     visibility = ["//visibility:private"],
     deps = [
         ":layout",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/types:span",
         "@com_google_googletest//:gtest_main",
     ],
@@ -691,6 +667,9 @@
     hdrs = ["internal/tracked.h"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_library(
@@ -825,3 +804,99 @@
         "@com_google_googletest//:gtest_main",
     ],
 )
+
+cc_library(
+    name = "btree",
+    srcs = [
+        "internal/btree.h",
+        "internal/btree_container.h",
+    ],
+    hdrs = [
+        "btree_map.h",
+        "btree_set.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:public"],
+    deps = [
+        ":common",
+        ":compressed_tuple",
+        ":container_memory",
+        ":layout",
+        "//absl/base:core_headers",
+        "//absl/base:throw_delegate",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/types:compare",
+        "//absl/utility",
+    ],
+)
+
+cc_library(
+    name = "btree_test_common",
+    testonly = 1,
+    hdrs = ["btree_test.h"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":btree",
+        ":flat_hash_set",
+        "//absl/strings",
+        "//absl/time",
+    ],
+)
+
+cc_test(
+    name = "btree_test",
+    size = "large",
+    srcs = [
+        "btree_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    shard_count = 10,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":btree",
+        ":btree_test_common",
+        ":counting_allocator",
+        ":test_instance_tracker",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
+        "//absl/flags:flag",
+        "//absl/hash:hash_testing",
+        "//absl/memory",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/types:compare",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_binary(
+    name = "btree_benchmark",
+    testonly = 1,
+    srcs = [
+        "btree_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":btree",
+        ":btree_test_common",
+        ":flat_hash_map",
+        ":flat_hash_set",
+        ":hashtable_debug",
+        "//absl/base:raw_logging_internal",
+        "//absl/flags:flag",
+        "//absl/hash",
+        "//absl/memory",
+        "//absl/strings:str_format",
+        "//absl/time",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
diff --git a/absl/container/CMakeLists.txt b/absl/container/CMakeLists.txt
index 7988b12..e702ba8 100644
--- a/absl/container/CMakeLists.txt
+++ b/absl/container/CMakeLists.txt
@@ -25,6 +25,73 @@
 
 absl_cc_library(
   NAME
+    btree
+  HDRS
+    "btree_map.h"
+    "btree_set.h"
+    "internal/btree.h"
+    "internal/btree_container.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::container_common
+    absl::compare
+    absl::compressed_tuple
+    absl::container_memory
+    absl::core_headers
+    absl::layout
+    absl::memory
+    absl::strings
+    absl::throw_delegate
+    absl::type_traits
+    absl::utility
+)
+
+absl_cc_library(
+  NAME
+    btree_test_common
+  hdrs
+    "btree_test.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::btree
+    absl::flat_hash_set
+    absl::strings
+    absl::time
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    btree_test
+  SRCS
+    "btree_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::btree
+    absl::btree_test_common
+    absl::compare
+    absl::core_headers
+    absl::counting_allocator
+    absl::flags
+    absl::hash_testing
+    absl::raw_logging_internal
+    absl::strings
+    absl::test_instance_tracker
+    absl::type_traits
+    gmock_main
+)
+
+absl_cc_library(
+  NAME
     compressed_tuple
   HDRS
     "internal/compressed_tuple.h"
@@ -76,24 +143,6 @@
     "fixed_array_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
-  DEPS
-    absl::fixed_array
-    absl::exception_testing
-    absl::hash_testing
-    absl::memory
-    gmock_main
-)
-
-absl_cc_test(
-  NAME
-    fixed_array_test_noexceptions
-  SRCS
-    "fixed_array_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
   DEPS
     absl::fixed_array
     absl::exception_testing
@@ -109,11 +158,9 @@
     "fixed_array_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::fixed_array
+    absl::config
     absl::exception_safety_testing
     gmock_main
 )
@@ -157,6 +204,8 @@
     "internal/counting_allocator.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
 )
 
 absl_cc_test(
@@ -166,37 +215,15 @@
     "inlined_vector_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::counting_allocator
     absl::inlined_vector
     absl::test_instance_tracker
-    absl::base
     absl::core_headers
     absl::exception_testing
     absl::hash_testing
     absl::memory
-    absl::strings
-    gmock_main
-)
-
-absl_cc_test(
-  NAME
-    inlined_vector_test_noexceptions
-  SRCS
-    "inlined_vector_test.cc"
-  COPTS
-    ${ABSL_TEST_COPTS}
-  DEPS
-    absl::inlined_vector
-    absl::test_instance_tracker
-    absl::base
-    absl::core_headers
-    absl::exception_testing
-    absl::hash_testing
-    absl::memory
+    absl::raw_logging_internal
     absl::strings
     gmock_main
 )
@@ -208,11 +235,9 @@
     "inlined_vector_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::inlined_vector
+    absl::config
     absl::exception_safety_testing
     gmock_main
 )
@@ -448,6 +473,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::hash_policy_testing
+    absl::memory
     absl::meta
     absl::strings
   TESTONLY
@@ -514,6 +540,7 @@
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
+    absl::exponential_biased
     absl::have_sse
     absl::synchronization
 )
@@ -549,6 +576,8 @@
     "internal/hashtable_debug_hooks.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   PUBLIC
 )
 
@@ -568,6 +597,8 @@
     "internal/node_hash_policy.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
   PUBLIC
 )
 
@@ -602,7 +633,7 @@
   NAME
     container_common
   HDRS
-    "internal/commom.h"
+    "internal/common.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
@@ -653,6 +684,7 @@
     absl::raw_hash_set
     absl::base
     absl::core_headers
+    absl::raw_logging_internal
     absl::strings
     gmock_main
 )
@@ -696,8 +728,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::layout
-    absl::base
     absl::core_headers
+    absl::raw_logging_internal
     absl::span
     gmock_main
 )
@@ -709,6 +741,8 @@
     "internal/tracked.h"
   COPTS
     ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
diff --git a/absl/container/btree_benchmark.cc b/absl/container/btree_benchmark.cc
new file mode 100644
index 0000000..4af92f9
--- /dev/null
+++ b/absl/container/btree_benchmark.cc
@@ -0,0 +1,707 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <stdint.h>
+
+#include <algorithm>
+#include <functional>
+#include <map>
+#include <numeric>
+#include <random>
+#include <set>
+#include <string>
+#include <type_traits>
+#include <unordered_map>
+#include <unordered_set>
+#include <vector>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/btree_test.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/container/internal/hashtable_debug.h"
+#include "absl/flags/flag.h"
+#include "absl/hash/hash.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/str_format.h"
+#include "absl/time/time.h"
+#include "benchmark/benchmark.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+constexpr size_t kBenchmarkValues = 1 << 20;
+
+// How many times we add and remove sub-batches in one batch of *AddRem
+// benchmarks.
+constexpr size_t kAddRemBatchSize = 1 << 2;
+
+// Generates n values in the range [0, 4 * n].
+template <typename V>
+std::vector<V> GenerateValues(int n) {
+  constexpr int kSeed = 23;
+  return GenerateValuesWithSeed<V>(n, 4 * n, kSeed);
+}
+
+// Benchmark insertion of values into a container.
+template <typename T>
+void BM_InsertImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    state.PauseTiming();
+    const auto i = static_cast<int>(state.iterations());
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.erase(key_of_value(values[x]));
+    }
+
+    state.ResumeTiming();
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.insert(values[x]);
+    }
+  }
+}
+
+template <typename T>
+void BM_Insert(benchmark::State& state) {
+  BM_InsertImpl<T>(state, false);
+}
+
+template <typename T>
+void BM_InsertSorted(benchmark::State& state) {
+  BM_InsertImpl<T>(state, true);
+}
+
+// container::insert sometimes returns a pair<iterator, bool> and sometimes
+// returns an iterator (for multi- containers).
+template <typename Iter>
+Iter GetIterFromInsert(const std::pair<Iter, bool>& pair) {
+  return pair.first;
+}
+template <typename Iter>
+Iter GetIterFromInsert(const Iter iter) {
+  return iter;
+}
+
+// Benchmark insertion of values into a container at the end.
+template <typename T>
+void BM_InsertEnd(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  T container;
+  const int kSize = 10000;
+  for (int i = 0; i < kSize; ++i) {
+    container.insert(Generator<V>(kSize)(i));
+  }
+  V v = Generator<V>(kSize)(kSize - 1);
+  typename T::key_type k = key_of_value(v);
+
+  auto it = container.find(k);
+  while (state.KeepRunning()) {
+    // Repeatedly removing then adding v.
+    container.erase(it);
+    it = GetIterFromInsert(container.insert(v));
+  }
+}
+
+template <typename T>
+void BM_LookupImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  T container(values.begin(), values.end());
+
+  while (state.KeepRunning()) {
+    int idx = state.iterations() % kBenchmarkValues;
+    benchmark::DoNotOptimize(container.find(key_of_value(values[idx])));
+  }
+}
+
+// Benchmark lookup of values in a container.
+template <typename T>
+void BM_Lookup(benchmark::State& state) {
+  BM_LookupImpl<T>(state, false);
+}
+
+// Benchmark lookup of values in a full container, meaning that values
+// are inserted in-order to take advantage of biased insertion, which
+// yields a full tree.
+template <typename T>
+void BM_FullLookup(benchmark::State& state) {
+  BM_LookupImpl<T>(state, true);
+}
+
+// Benchmark deletion of values from a container.
+template <typename T>
+void BM_Delete(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    const int i = state.iterations();
+
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.erase(key_of_value(values[x]));
+    }
+
+    state.PauseTiming();
+    for (int j = i; j < i + batch_size; j++) {
+      int x = j % kBenchmarkValues;
+      container.insert(values[x]);
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Benchmark deletion of multiple values from a container.
+template <typename T>
+void BM_DeleteRange(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  // Remove and re-insert 10% of the keys per batch.
+  const int batch_size = (kBenchmarkValues + 9) / 10;
+  while (state.KeepRunningBatch(batch_size)) {
+    const int i = state.iterations();
+
+    const int start_index = i % kBenchmarkValues;
+
+    state.PauseTiming();
+    {
+      std::vector<V> removed;
+      removed.reserve(batch_size);
+      auto itr = container.find(key_of_value(values[start_index]));
+      auto start = itr;
+      for (int j = 0; j < batch_size; j++) {
+        if (itr == container.end()) {
+          state.ResumeTiming();
+          container.erase(start, itr);
+          state.PauseTiming();
+          itr = container.begin();
+          start = itr;
+        }
+        removed.push_back(*itr++);
+      }
+
+      state.ResumeTiming();
+      container.erase(start, itr);
+      state.PauseTiming();
+
+      container.insert(removed.begin(), removed.end());
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Benchmark steady-state insert (into first half of range) and remove (from
+// second half of range), treating the container approximately like a queue with
+// log-time access for all elements. This benchmark does not test the case where
+// insertion and removal happen in the same region of the tree.  This benchmark
+// counts two value constructors.
+template <typename T>
+void BM_QueueAddRem(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  ABSL_RAW_CHECK(kBenchmarkValues % 2 == 0, "for performance");
+
+  T container;
+
+  const size_t half = kBenchmarkValues / 2;
+  std::vector<int> remove_keys(half);
+  std::vector<int> add_keys(half);
+
+  // We want to do the exact same work repeatedly, and the benchmark can end
+  // after a different number of iterations depending on the speed of the
+  // individual run so we use a large batch size here and ensure that we do
+  // deterministic work every batch.
+  while (state.KeepRunningBatch(half * kAddRemBatchSize)) {
+    state.PauseTiming();
+
+    container.clear();
+
+    for (size_t i = 0; i < half; ++i) {
+      remove_keys[i] = i;
+      add_keys[i] = i;
+    }
+    constexpr int kSeed = 5;
+    std::mt19937_64 rand(kSeed);
+    std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+    std::shuffle(add_keys.begin(), add_keys.end(), rand);
+
+    // Note needs lazy generation of values.
+    Generator<V> g(kBenchmarkValues * kAddRemBatchSize);
+
+    for (size_t i = 0; i < half; ++i) {
+      container.insert(g(add_keys[i]));
+      container.insert(g(half + remove_keys[i]));
+    }
+
+    // There are three parts each of size "half":
+    // 1 is being deleted from  [offset - half, offset)
+    // 2 is standing            [offset, offset + half)
+    // 3 is being inserted into [offset + half, offset + 2 * half)
+    size_t offset = 0;
+
+    for (size_t i = 0; i < kAddRemBatchSize; ++i) {
+      std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+      std::shuffle(add_keys.begin(), add_keys.end(), rand);
+      offset += half;
+
+      state.ResumeTiming();
+      for (size_t idx = 0; idx < half; ++idx) {
+        container.erase(key_of_value(g(offset - half + remove_keys[idx])));
+        container.insert(g(offset + half + add_keys[idx]));
+      }
+      state.PauseTiming();
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Mixed insertion and deletion in the same range using pre-constructed values.
+template <typename T>
+void BM_MixedAddRem(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  ABSL_RAW_CHECK(kBenchmarkValues % 2 == 0, "for performance");
+
+  T container;
+
+  // Create two random shuffles
+  std::vector<int> remove_keys(kBenchmarkValues);
+  std::vector<int> add_keys(kBenchmarkValues);
+
+  // We want to do the exact same work repeatedly, and the benchmark can end
+  // after a different number of iterations depending on the speed of the
+  // individual run so we use a large batch size here and ensure that we do
+  // deterministic work every batch.
+  while (state.KeepRunningBatch(kBenchmarkValues * kAddRemBatchSize)) {
+    state.PauseTiming();
+
+    container.clear();
+
+    constexpr int kSeed = 7;
+    std::mt19937_64 rand(kSeed);
+
+    std::vector<V> values = GenerateValues<V>(kBenchmarkValues * 2);
+
+    // Insert the first half of the values (already in random order)
+    container.insert(values.begin(), values.begin() + kBenchmarkValues);
+
+    // Insert the first half of the values (already in random order)
+    for (size_t i = 0; i < kBenchmarkValues; ++i) {
+      // remove_keys and add_keys will be swapped before each round,
+      // therefore fill add_keys here w/ the keys being inserted, so
+      // they'll be the first to be removed.
+      remove_keys[i] = i + kBenchmarkValues;
+      add_keys[i] = i;
+    }
+
+    for (size_t i = 0; i < kAddRemBatchSize; ++i) {
+      remove_keys.swap(add_keys);
+      std::shuffle(remove_keys.begin(), remove_keys.end(), rand);
+      std::shuffle(add_keys.begin(), add_keys.end(), rand);
+
+      state.ResumeTiming();
+      for (size_t idx = 0; idx < kBenchmarkValues; ++idx) {
+        container.erase(key_of_value(values[remove_keys[idx]]));
+        container.insert(values[add_keys[idx]]);
+      }
+      state.PauseTiming();
+    }
+    state.ResumeTiming();
+  }
+}
+
+// Insertion at end, removal from the beginning.  This benchmark
+// counts two value constructors.
+// TODO(ezb): we could add a GenerateNext version of generator that could reduce
+// noise for string-like types.
+template <typename T>
+void BM_Fifo(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  T container;
+  // Need lazy generation of values as state.max_iterations is large.
+  Generator<V> g(kBenchmarkValues + state.max_iterations);
+
+  for (int i = 0; i < kBenchmarkValues; i++) {
+    container.insert(g(i));
+  }
+
+  while (state.KeepRunning()) {
+    container.erase(container.begin());
+    container.insert(container.end(), g(state.iterations() + kBenchmarkValues));
+  }
+}
+
+// Iteration (forward) through the tree
+template <typename T>
+void BM_FwdIter(benchmark::State& state) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  using R = typename T::value_type const*;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  T container(values.begin(), values.end());
+
+  auto iter = container.end();
+
+  R r = nullptr;
+
+  while (state.KeepRunning()) {
+    if (iter == container.end()) iter = container.begin();
+    r = &(*iter);
+    ++iter;
+  }
+
+  benchmark::DoNotOptimize(r);
+}
+
+// Benchmark random range-construction of a container.
+template <typename T>
+void BM_RangeConstructionImpl(benchmark::State& state, bool sorted) {
+  using V = typename remove_pair_const<typename T::value_type>::type;
+
+  std::vector<V> values = GenerateValues<V>(kBenchmarkValues);
+  if (sorted) {
+    std::sort(values.begin(), values.end());
+  }
+  {
+    T container(values.begin(), values.end());
+  }
+
+  while (state.KeepRunning()) {
+    T container(values.begin(), values.end());
+    benchmark::DoNotOptimize(container);
+  }
+}
+
+template <typename T>
+void BM_InsertRangeRandom(benchmark::State& state) {
+  BM_RangeConstructionImpl<T>(state, false);
+}
+
+template <typename T>
+void BM_InsertRangeSorted(benchmark::State& state) {
+  BM_RangeConstructionImpl<T>(state, true);
+}
+
+#define STL_ORDERED_TYPES(value)                     \
+  using stl_set_##value = std::set<value>;           \
+  using stl_map_##value = std::map<value, intptr_t>; \
+  using stl_multiset_##value = std::multiset<value>; \
+  using stl_multimap_##value = std::multimap<value, intptr_t>
+
+using StdString = std::string;
+STL_ORDERED_TYPES(int32_t);
+STL_ORDERED_TYPES(int64_t);
+STL_ORDERED_TYPES(StdString);
+STL_ORDERED_TYPES(Time);
+
+#define STL_UNORDERED_TYPES(value)                                       \
+  using stl_unordered_set_##value = std::unordered_set<value>;           \
+  using stl_unordered_map_##value = std::unordered_map<value, intptr_t>; \
+  using flat_hash_set_##value = flat_hash_set<value>;                    \
+  using flat_hash_map_##value = flat_hash_map<value, intptr_t>;          \
+  using stl_unordered_multiset_##value = std::unordered_multiset<value>; \
+  using stl_unordered_multimap_##value =                                 \
+      std::unordered_multimap<value, intptr_t>
+
+#define STL_UNORDERED_TYPES_CUSTOM_HASH(value, hash)                           \
+  using stl_unordered_set_##value = std::unordered_set<value, hash>;           \
+  using stl_unordered_map_##value = std::unordered_map<value, intptr_t, hash>; \
+  using flat_hash_set_##value = flat_hash_set<value, hash>;                    \
+  using flat_hash_map_##value = flat_hash_map<value, intptr_t, hash>;          \
+  using stl_unordered_multiset_##value = std::unordered_multiset<value, hash>; \
+  using stl_unordered_multimap_##value =                                       \
+      std::unordered_multimap<value, intptr_t, hash>
+
+STL_UNORDERED_TYPES(int32_t);
+STL_UNORDERED_TYPES(int64_t);
+STL_UNORDERED_TYPES(StdString);
+STL_UNORDERED_TYPES_CUSTOM_HASH(Time, absl::Hash<absl::Time>);
+
+#define BTREE_TYPES(value)                                            \
+  using btree_256_set_##value =                                       \
+      btree_set<value, std::less<value>, std::allocator<value>>;      \
+  using btree_256_map_##value =                                       \
+      btree_map<value, intptr_t, std::less<value>,                    \
+                std::allocator<std::pair<const value, intptr_t>>>;    \
+  using btree_256_multiset_##value =                                  \
+      btree_multiset<value, std::less<value>, std::allocator<value>>; \
+  using btree_256_multimap_##value =                                  \
+      btree_multimap<value, intptr_t, std::less<value>,               \
+                     std::allocator<std::pair<const value, intptr_t>>>
+
+BTREE_TYPES(int32_t);
+BTREE_TYPES(int64_t);
+BTREE_TYPES(StdString);
+BTREE_TYPES(Time);
+
+#define MY_BENCHMARK4(type, func)                                              \
+  void BM_##type##_##func(benchmark::State& state) { BM_##func<type>(state); } \
+  BENCHMARK(BM_##type##_##func)
+
+#define MY_BENCHMARK3(type)               \
+  MY_BENCHMARK4(type, Insert);            \
+  MY_BENCHMARK4(type, InsertSorted);      \
+  MY_BENCHMARK4(type, InsertEnd);         \
+  MY_BENCHMARK4(type, Lookup);            \
+  MY_BENCHMARK4(type, FullLookup);        \
+  MY_BENCHMARK4(type, Delete);            \
+  MY_BENCHMARK4(type, DeleteRange);       \
+  MY_BENCHMARK4(type, QueueAddRem);       \
+  MY_BENCHMARK4(type, MixedAddRem);       \
+  MY_BENCHMARK4(type, Fifo);              \
+  MY_BENCHMARK4(type, FwdIter);           \
+  MY_BENCHMARK4(type, InsertRangeRandom); \
+  MY_BENCHMARK4(type, InsertRangeSorted)
+
+#define MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(type) \
+  MY_BENCHMARK3(stl_##type);                    \
+  MY_BENCHMARK3(stl_unordered_##type);          \
+  MY_BENCHMARK3(btree_256_##type)
+
+#define MY_BENCHMARK2(type)                \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(type); \
+  MY_BENCHMARK3(flat_hash_##type)
+
+// Define MULTI_TESTING to see benchmarks for multi-containers also.
+//
+// You can use --copt=-DMULTI_TESTING.
+#ifdef MULTI_TESTING
+#define MY_BENCHMARK(type)                            \
+  MY_BENCHMARK2(set_##type);                          \
+  MY_BENCHMARK2(map_##type);                          \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(multiset_##type); \
+  MY_BENCHMARK2_SUPPORTS_MULTI_ONLY(multimap_##type)
+#else
+#define MY_BENCHMARK(type)   \
+  MY_BENCHMARK2(set_##type); \
+  MY_BENCHMARK2(map_##type)
+#endif
+
+MY_BENCHMARK(int32_t);
+MY_BENCHMARK(int64_t);
+MY_BENCHMARK(StdString);
+MY_BENCHMARK(Time);
+
+// Define a type whose size and cost of moving are independently customizable.
+// When sizeof(value_type) increases, we expect btree to no longer have as much
+// cache-locality advantage over STL. When cost of moving increases, we expect
+// btree to actually do more work than STL because it has to move values around
+// and STL doesn't have to.
+template <int Size, int Copies>
+struct BigType {
+  BigType() : BigType(0) {}
+  explicit BigType(int x) { std::iota(values.begin(), values.end(), x); }
+
+  void Copy(const BigType& x) {
+    for (int i = 0; i < Size && i < Copies; ++i) values[i] = x.values[i];
+    // If Copies > Size, do extra copies.
+    for (int i = Size, idx = 0; i < Copies; ++i) {
+      int64_t tmp = x.values[idx];
+      benchmark::DoNotOptimize(tmp);
+      idx = idx + 1 == Size ? 0 : idx + 1;
+    }
+  }
+
+  BigType(const BigType& x) { Copy(x); }
+  BigType& operator=(const BigType& x) {
+    Copy(x);
+    return *this;
+  }
+
+  // Compare only the first Copies elements if Copies is less than Size.
+  bool operator<(const BigType& other) const {
+    return std::lexicographical_compare(
+        values.begin(), values.begin() + std::min(Size, Copies),
+        other.values.begin(), other.values.begin() + std::min(Size, Copies));
+  }
+  bool operator==(const BigType& other) const {
+    return std::equal(values.begin(), values.begin() + std::min(Size, Copies),
+                      other.values.begin());
+  }
+
+  // Support absl::Hash.
+  template <typename State>
+  friend State AbslHashValue(State h, const BigType& b) {
+    for (int i = 0; i < Size && i < Copies; ++i)
+      h = State::combine(std::move(h), b.values[i]);
+    return h;
+  }
+
+  std::array<int64_t, Size> values;
+};
+
+#define BIG_TYPE_BENCHMARKS(SIZE, COPIES)                                     \
+  using stl_set_size##SIZE##copies##COPIES = std::set<BigType<SIZE, COPIES>>; \
+  using stl_map_size##SIZE##copies##COPIES =                                  \
+      std::map<BigType<SIZE, COPIES>, intptr_t>;                              \
+  using stl_multiset_size##SIZE##copies##COPIES =                             \
+      std::multiset<BigType<SIZE, COPIES>>;                                   \
+  using stl_multimap_size##SIZE##copies##COPIES =                             \
+      std::multimap<BigType<SIZE, COPIES>, intptr_t>;                         \
+  using stl_unordered_set_size##SIZE##copies##COPIES =                        \
+      std::unordered_set<BigType<SIZE, COPIES>,                               \
+                         absl::Hash<BigType<SIZE, COPIES>>>;                  \
+  using stl_unordered_map_size##SIZE##copies##COPIES =                        \
+      std::unordered_map<BigType<SIZE, COPIES>, intptr_t,                     \
+                         absl::Hash<BigType<SIZE, COPIES>>>;                  \
+  using flat_hash_set_size##SIZE##copies##COPIES =                            \
+      flat_hash_set<BigType<SIZE, COPIES>>;                                   \
+  using flat_hash_map_size##SIZE##copies##COPIES =                            \
+      flat_hash_map<BigType<SIZE, COPIES>, intptr_t>;                         \
+  using stl_unordered_multiset_size##SIZE##copies##COPIES =                   \
+      std::unordered_multiset<BigType<SIZE, COPIES>,                          \
+                              absl::Hash<BigType<SIZE, COPIES>>>;             \
+  using stl_unordered_multimap_size##SIZE##copies##COPIES =                   \
+      std::unordered_multimap<BigType<SIZE, COPIES>, intptr_t,                \
+                              absl::Hash<BigType<SIZE, COPIES>>>;             \
+  using btree_256_set_size##SIZE##copies##COPIES =                            \
+      btree_set<BigType<SIZE, COPIES>>;                                       \
+  using btree_256_map_size##SIZE##copies##COPIES =                            \
+      btree_map<BigType<SIZE, COPIES>, intptr_t>;                             \
+  using btree_256_multiset_size##SIZE##copies##COPIES =                       \
+      btree_multiset<BigType<SIZE, COPIES>>;                                  \
+  using btree_256_multimap_size##SIZE##copies##COPIES =                       \
+      btree_multimap<BigType<SIZE, COPIES>, intptr_t>;                        \
+  MY_BENCHMARK(size##SIZE##copies##COPIES)
+
+// Define BIG_TYPE_TESTING to see benchmarks for more big types.
+//
+// You can use --copt=-DBIG_TYPE_TESTING.
+#ifndef NODESIZE_TESTING
+#ifdef BIG_TYPE_TESTING
+BIG_TYPE_BENCHMARKS(1, 4);
+BIG_TYPE_BENCHMARKS(4, 1);
+BIG_TYPE_BENCHMARKS(4, 4);
+BIG_TYPE_BENCHMARKS(1, 8);
+BIG_TYPE_BENCHMARKS(8, 1);
+BIG_TYPE_BENCHMARKS(8, 8);
+BIG_TYPE_BENCHMARKS(1, 16);
+BIG_TYPE_BENCHMARKS(16, 1);
+BIG_TYPE_BENCHMARKS(16, 16);
+BIG_TYPE_BENCHMARKS(1, 32);
+BIG_TYPE_BENCHMARKS(32, 1);
+BIG_TYPE_BENCHMARKS(32, 32);
+#else
+BIG_TYPE_BENCHMARKS(32, 32);
+#endif
+#endif
+
+// Benchmark using unique_ptrs to large value types. In order to be able to use
+// the same benchmark code as the other types, use a type that holds a
+// unique_ptr and has a copy constructor.
+template <int Size>
+struct BigTypePtr {
+  BigTypePtr() : BigTypePtr(0) {}
+  explicit BigTypePtr(int x) {
+    ptr = absl::make_unique<BigType<Size, Size>>(x);
+  }
+  BigTypePtr(const BigTypePtr& x) {
+    ptr = absl::make_unique<BigType<Size, Size>>(*x.ptr);
+  }
+  BigTypePtr(BigTypePtr&& x) noexcept = default;
+  BigTypePtr& operator=(const BigTypePtr& x) {
+    ptr = absl::make_unique<BigType<Size, Size>>(*x.ptr);
+  }
+  BigTypePtr& operator=(BigTypePtr&& x) noexcept = default;
+
+  bool operator<(const BigTypePtr& other) const { return *ptr < *other.ptr; }
+  bool operator==(const BigTypePtr& other) const { return *ptr == *other.ptr; }
+
+  std::unique_ptr<BigType<Size, Size>> ptr;
+};
+
+template <int Size>
+double ContainerInfo(const btree_set<BigTypePtr<Size>>& b) {
+  const double bytes_used =
+      b.bytes_used() + b.size() * sizeof(BigType<Size, Size>);
+  const double bytes_per_value = bytes_used / b.size();
+  BtreeContainerInfoLog(b, bytes_used, bytes_per_value);
+  return bytes_per_value;
+}
+template <int Size>
+double ContainerInfo(const btree_map<int, BigTypePtr<Size>>& b) {
+  const double bytes_used =
+      b.bytes_used() + b.size() * sizeof(BigType<Size, Size>);
+  const double bytes_per_value = bytes_used / b.size();
+  BtreeContainerInfoLog(b, bytes_used, bytes_per_value);
+  return bytes_per_value;
+}
+
+#define BIG_TYPE_PTR_BENCHMARKS(SIZE)                                          \
+  using stl_set_size##SIZE##copies##SIZE##ptr = std::set<BigType<SIZE, SIZE>>; \
+  using stl_map_size##SIZE##copies##SIZE##ptr =                                \
+      std::map<int, BigType<SIZE, SIZE>>;                                      \
+  using stl_unordered_set_size##SIZE##copies##SIZE##ptr =                      \
+      std::unordered_set<BigType<SIZE, SIZE>,                                  \
+                         absl::Hash<BigType<SIZE, SIZE>>>;                     \
+  using stl_unordered_map_size##SIZE##copies##SIZE##ptr =                      \
+      std::unordered_map<int, BigType<SIZE, SIZE>>;                            \
+  using flat_hash_set_size##SIZE##copies##SIZE##ptr =                          \
+      flat_hash_set<BigType<SIZE, SIZE>>;                                      \
+  using flat_hash_map_size##SIZE##copies##SIZE##ptr =                          \
+      flat_hash_map<int, BigTypePtr<SIZE>>;                                    \
+  using btree_256_set_size##SIZE##copies##SIZE##ptr =                          \
+      btree_set<BigTypePtr<SIZE>>;                                             \
+  using btree_256_map_size##SIZE##copies##SIZE##ptr =                          \
+      btree_map<int, BigTypePtr<SIZE>>;                                        \
+  MY_BENCHMARK3(stl_set_size##SIZE##copies##SIZE##ptr);                        \
+  MY_BENCHMARK3(stl_unordered_set_size##SIZE##copies##SIZE##ptr);              \
+  MY_BENCHMARK3(flat_hash_set_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3(btree_256_set_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3(stl_map_size##SIZE##copies##SIZE##ptr);                        \
+  MY_BENCHMARK3(stl_unordered_map_size##SIZE##copies##SIZE##ptr);              \
+  MY_BENCHMARK3(flat_hash_map_size##SIZE##copies##SIZE##ptr);                  \
+  MY_BENCHMARK3(btree_256_map_size##SIZE##copies##SIZE##ptr)
+
+BIG_TYPE_PTR_BENCHMARKS(32);
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/btree_map.h b/absl/container/btree_map.h
new file mode 100644
index 0000000..d23f4ee
--- /dev/null
+++ b/absl/container/btree_map.h
@@ -0,0 +1,759 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: btree_map.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines B-tree maps: sorted associative containers mapping
+// keys to values.
+//
+//     * `absl::btree_map<>`
+//     * `absl::btree_multimap<>`
+//
+// These B-tree types are similar to the corresponding types in the STL
+// (`std::map` and `std::multimap`) and generally conform to the STL interfaces
+// of those types. However, because they are implemented using B-trees, they
+// are more efficient in most situations.
+//
+// Unlike `std::map` and `std::multimap`, which are commonly implemented using
+// red-black tree nodes, B-tree maps use more generic B-tree nodes able to hold
+// multiple values per node. Holding multiple values per node often makes
+// B-tree maps perform better than their `std::map` counterparts, because
+// multiple entries can be checked within the same cache hit.
+//
+// However, these types should not be considered drop-in replacements for
+// `std::map` and `std::multimap` as there are some API differences, which are
+// noted in this header file.
+//
+// Importantly, insertions and deletions may invalidate outstanding iterators,
+// pointers, and references to elements. Such invalidations are typically only
+// an issue if insertion and deletion operations are interleaved with the use of
+// more than one iterator, pointer, or reference simultaneously. For this
+// reason, `insert()` and `erase()` return a valid iterator at the current
+// position.
+
+#ifndef ABSL_CONTAINER_BTREE_MAP_H_
+#define ABSL_CONTAINER_BTREE_MAP_H_
+
+#include "absl/container/internal/btree.h"  // IWYU pragma: export
+#include "absl/container/internal/btree_container.h"  // IWYU pragma: export
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::btree_map<>
+//
+// An `absl::btree_map<K, V>` is an ordered associative container of
+// unique keys and associated values designed to be a more efficient replacement
+// for `std::map` (in most cases).
+//
+// Keys are sorted using an (optional) comparison function, which defaults to
+// `std::less<K>`.
+//
+// An `absl::btree_map<K, V>` uses a default allocator of
+// `std::allocator<std::pair<const K, V>>` to allocate (and deallocate)
+// nodes, and construct and destruct values within those nodes. You may
+// instead specify a custom allocator `A` (which in turn requires specifying a
+// custom comparator `C`) as in `absl::btree_map<K, V, C, A>`.
+//
+template <typename Key, typename Value, typename Compare = std::less<Key>,
+          typename Alloc = std::allocator<std::pair<const Key, Value>>>
+class btree_map
+    : public container_internal::btree_map_container<
+          container_internal::btree<container_internal::map_params<
+              Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
+              /*Multi=*/false>>> {
+  using Base = typename btree_map::btree_map_container;
+
+ public:
+  // Constructors and Assignment Operators
+  //
+  // A `btree_map` supports the same overload set as `std::map`
+  // for construction and assignment:
+  //
+  // * Default constructor
+  //
+  //   absl::btree_map<int, std::string> map1;
+  //
+  // * Initializer List constructor
+  //
+  //   absl::btree_map<int, std::string> map2 =
+  //       {{1, "huey"}, {2, "dewey"}, {3, "louie"},};
+  //
+  // * Copy constructor
+  //
+  //   absl::btree_map<int, std::string> map3(map2);
+  //
+  // * Copy assignment operator
+  //
+  //  absl::btree_map<int, std::string> map4;
+  //  map4 = map3;
+  //
+  // * Move constructor
+  //
+  //   // Move is guaranteed efficient
+  //   absl::btree_map<int, std::string> map5(std::move(map4));
+  //
+  // * Move assignment operator
+  //
+  //   // May be efficient if allocators are compatible
+  //   absl::btree_map<int, std::string> map6;
+  //   map6 = std::move(map5);
+  //
+  // * Range constructor
+  //
+  //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
+  //   absl::btree_map<int, std::string> map7(v.begin(), v.end());
+  btree_map() {}
+  using Base::Base;
+
+  // btree_map::begin()
+  //
+  // Returns an iterator to the beginning of the `btree_map`.
+  using Base::begin;
+
+  // btree_map::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `btree_map`.
+  using Base::cbegin;
+
+  // btree_map::end()
+  //
+  // Returns an iterator to the end of the `btree_map`.
+  using Base::end;
+
+  // btree_map::cend()
+  //
+  // Returns a const iterator to the end of the `btree_map`.
+  using Base::cend;
+
+  // btree_map::empty()
+  //
+  // Returns whether or not the `btree_map` is empty.
+  using Base::empty;
+
+  // btree_map::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `btree_map` under current memory constraints. This value can be thought
+  // of as the largest value of `std::distance(begin(), end())` for a
+  // `btree_map<Key, T>`.
+  using Base::max_size;
+
+  // btree_map::size()
+  //
+  // Returns the number of elements currently within the `btree_map`.
+  using Base::size;
+
+  // btree_map::clear()
+  //
+  // Removes all elements from the `btree_map`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  using Base::clear;
+
+  // btree_map::erase()
+  //
+  // Erases elements within the `btree_map`. If an erase occurs, any references,
+  // pointers, or iterators are invalidated.
+  // Overloads are listed below.
+  //
+  // iterator erase(iterator position):
+  // iterator erase(const_iterator position):
+  //
+  //   Erases the element at `position` of the `btree_map`, returning
+  //   the iterator pointing to the element after the one that was erased
+  //   (or end() if none exists).
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning
+  //   the iterator pointing to the element after the interval that was erased
+  //   (or end() if none exists).
+  //
+  // template <typename K> size_type erase(const K& key):
+  //
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased.
+  using Base::erase;
+
+  // btree_map::insert()
+  //
+  // Inserts an element of the specified value into the `btree_map`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If an insertion
+  // occurs, any references, pointers, or iterators are invalidated.
+  // Overloads are listed below.
+  //
+  // std::pair<iterator,bool> insert(const value_type& value):
+  //
+  //   Inserts a value into the `btree_map`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a bool denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(value_type&& value):
+  //
+  //   Inserts a moveable value into the `btree_map`. Returns a pair
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a bool denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const value_type& value):
+  // iterator insert(const_iterator hint, value_type&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  // void insert(std::initializer_list<init_type> ilist):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  using Base::insert;
+
+  // btree_map::insert_or_assign()
+  //
+  // Inserts an element of the specified value into the `btree_map` provided
+  // that a value with the given key does not already exist, or replaces the
+  // corresponding mapped type with the forwarded `obj` argument if a key for
+  // that value already exists, returning an iterator pointing to the newly
+  // inserted element. Overloads are listed below.
+  //
+  // pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj):
+  // pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `btree_map`. If the returned bool is true, insertion took place, and if
+  //   it's false, assignment took place.
+  //
+  // iterator insert_or_assign(const_iterator hint,
+  //                           const key_type& k, M&& obj):
+  // iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj):
+  //
+  //   Inserts/Assigns (or moves) the element of the specified key into the
+  //   `btree_map` using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search.
+  using Base::insert_or_assign;
+
+  // btree_map::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_map`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace;
+
+  // btree_map::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_map`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately. Prefer `try_emplace()` unless your key is not
+  // copyable or moveable.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace_hint;
+
+  // btree_map::try_emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_map`, provided that no element with the given key
+  // already exists. Unlike `emplace()`, if an element with the given key
+  // already exists, we guarantee that no element is constructed.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated.
+  //
+  // Overloads are listed below.
+  //
+  //   std::pair<iterator, bool> try_emplace(const key_type& k, Args&&... args):
+  //   std::pair<iterator, bool> try_emplace(key_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `btree_map`.
+  //
+  //   iterator try_emplace(const_iterator hint,
+  //                        const key_type& k, Args&&... args):
+  //   iterator try_emplace(const_iterator hint, key_type&& k, Args&&... args):
+  //
+  // Inserts (via copy or move) the element of the specified key into the
+  // `btree_map` using the position of `hint` as a non-binding suggestion
+  // for where to begin the insertion search.
+  using Base::try_emplace;
+
+  // btree_map::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // template <typename K> node_type extract(const K& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `btree_map`
+  //   does not contain an element with a matching key, this function returns an
+  //   empty node handle.
+  //
+  // NOTE: In this context, `node_type` refers to the C++17 concept of a
+  // move-only type that owns and provides access to the elements in associative
+  // containers (https://en.cppreference.com/w/cpp/container/node_handle).
+  // It does NOT refer to the data layout of the underlying btree.
+  using Base::extract;
+
+  // btree_map::merge()
+  //
+  // Extracts elements from a given `source` btree_map into this
+  // `btree_map`. If the destination `btree_map` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // btree_map::swap(btree_map& other)
+  //
+  // Exchanges the contents of this `btree_map` with those of the `other`
+  // btree_map, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `btree_map` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  using Base::swap;
+
+  // btree_map::at()
+  //
+  // Returns a reference to the mapped value of the element with key equivalent
+  // to the passed key.
+  using Base::at;
+
+  // btree_map::contains()
+  //
+  // template <typename K> bool contains(const K& key) const:
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `btree_map`, returning `true` if so or `false` otherwise.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::contains;
+
+  // btree_map::count()
+  //
+  // template <typename K> size_type count(const K& key) const:
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `btree_map`. Note that this function will return either `1` or `0`
+  // since duplicate elements are not allowed within a `btree_map`.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::count;
+
+  // btree_map::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `btree_map`.
+  using Base::equal_range;
+
+  // btree_map::find()
+  //
+  // template <typename K> iterator find(const K& key):
+  // template <typename K> const_iterator find(const K& key) const:
+  //
+  // Finds an element with the passed `key` within the `btree_map`.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::find;
+
+  // btree_map::operator[]()
+  //
+  // Returns a reference to the value mapped to the passed key within the
+  // `btree_map`, performing an `insert()` if the key does not already
+  // exist.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated. Otherwise iterators are not affected and references are not
+  // invalidated. Overloads are listed below.
+  //
+  // T& operator[](key_type&& key):
+  // T& operator[](const key_type& key):
+  //
+  //   Inserts a value_type object constructed in-place if the element with the
+  //   given key does not exist.
+  using Base::operator[];
+
+  // btree_map::get_allocator()
+  //
+  // Returns the allocator function associated with this `btree_map`.
+  using Base::get_allocator;
+
+  // btree_map::key_comp();
+  //
+  // Returns the key comparator associated with this `btree_map`.
+  using Base::key_comp;
+
+  // btree_map::value_comp();
+  //
+  // Returns the value comparator associated with this `btree_map`.
+  using Base::value_comp;
+};
+
+// absl::swap(absl::btree_map<>, absl::btree_map<>)
+//
+// Swaps the contents of two `absl::btree_map` containers.
+template <typename K, typename V, typename C, typename A>
+void swap(btree_map<K, V, C, A> &x, btree_map<K, V, C, A> &y) {
+  return x.swap(y);
+}
+
+// absl::erase_if(absl::btree_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+template <typename K, typename V, typename C, typename A, typename Pred>
+void erase_if(btree_map<K, V, C, A> &map, Pred pred) {
+  for (auto it = map.begin(); it != map.end();) {
+    if (pred(*it)) {
+      it = map.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}
+
+// absl::btree_multimap
+//
+// An `absl::btree_multimap<K, V>` is an ordered associative container of
+// keys and associated values designed to be a more efficient replacement for
+// `std::multimap` (in most cases). Unlike `absl::btree_map`, a B-tree multimap
+// allows multiple elements with equivalent keys.
+//
+// Keys are sorted using an (optional) comparison function, which defaults to
+// `std::less<K>`.
+//
+// An `absl::btree_multimap<K, V>` uses a default allocator of
+// `std::allocator<std::pair<const K, V>>` to allocate (and deallocate)
+// nodes, and construct and destruct values within those nodes. You may
+// instead specify a custom allocator `A` (which in turn requires specifying a
+// custom comparator `C`) as in `absl::btree_multimap<K, V, C, A>`.
+//
+template <typename Key, typename Value, typename Compare = std::less<Key>,
+          typename Alloc = std::allocator<std::pair<const Key, Value>>>
+class btree_multimap
+    : public container_internal::btree_multimap_container<
+          container_internal::btree<container_internal::map_params<
+              Key, Value, Compare, Alloc, /*TargetNodeSize=*/256,
+              /*Multi=*/true>>> {
+  using Base = typename btree_multimap::btree_multimap_container;
+
+ public:
+  // Constructors and Assignment Operators
+  //
+  // A `btree_multimap` supports the same overload set as `std::multimap`
+  // for construction and assignment:
+  //
+  // * Default constructor
+  //
+  //   absl::btree_multimap<int, std::string> map1;
+  //
+  // * Initializer List constructor
+  //
+  //   absl::btree_multimap<int, std::string> map2 =
+  //       {{1, "huey"}, {2, "dewey"}, {3, "louie"},};
+  //
+  // * Copy constructor
+  //
+  //   absl::btree_multimap<int, std::string> map3(map2);
+  //
+  // * Copy assignment operator
+  //
+  //  absl::btree_multimap<int, std::string> map4;
+  //  map4 = map3;
+  //
+  // * Move constructor
+  //
+  //   // Move is guaranteed efficient
+  //   absl::btree_multimap<int, std::string> map5(std::move(map4));
+  //
+  // * Move assignment operator
+  //
+  //   // May be efficient if allocators are compatible
+  //   absl::btree_multimap<int, std::string> map6;
+  //   map6 = std::move(map5);
+  //
+  // * Range constructor
+  //
+  //   std::vector<std::pair<int, std::string>> v = {{1, "a"}, {2, "b"}};
+  //   absl::btree_multimap<int, std::string> map7(v.begin(), v.end());
+  btree_multimap() {}
+  using Base::Base;
+
+  // btree_multimap::begin()
+  //
+  // Returns an iterator to the beginning of the `btree_multimap`.
+  using Base::begin;
+
+  // btree_multimap::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `btree_multimap`.
+  using Base::cbegin;
+
+  // btree_multimap::end()
+  //
+  // Returns an iterator to the end of the `btree_multimap`.
+  using Base::end;
+
+  // btree_multimap::cend()
+  //
+  // Returns a const iterator to the end of the `btree_multimap`.
+  using Base::cend;
+
+  // btree_multimap::empty()
+  //
+  // Returns whether or not the `btree_multimap` is empty.
+  using Base::empty;
+
+  // btree_multimap::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `btree_multimap` under current memory constraints. This value can be
+  // thought of as the largest value of `std::distance(begin(), end())` for a
+  // `btree_multimap<Key, T>`.
+  using Base::max_size;
+
+  // btree_multimap::size()
+  //
+  // Returns the number of elements currently within the `btree_multimap`.
+  using Base::size;
+
+  // btree_multimap::clear()
+  //
+  // Removes all elements from the `btree_multimap`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  using Base::clear;
+
+  // btree_multimap::erase()
+  //
+  // Erases elements within the `btree_multimap`. If an erase occurs, any
+  // references, pointers, or iterators are invalidated.
+  // Overloads are listed below.
+  //
+  // iterator erase(iterator position):
+  // iterator erase(const_iterator position):
+  //
+  //   Erases the element at `position` of the `btree_multimap`, returning
+  //   the iterator pointing to the element after the one that was erased
+  //   (or end() if none exists).
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning
+  //   the iterator pointing to the element after the interval that was erased
+  //   (or end() if none exists).
+  //
+  // template <typename K> size_type erase(const K& key):
+  //
+  //   Erases the elements matching the key, if any exist, returning the
+  //   number of elements erased.
+  using Base::erase;
+
+  // btree_multimap::insert()
+  //
+  // Inserts an element of the specified value into the `btree_multimap`,
+  // returning an iterator pointing to the newly inserted element.
+  // Any references, pointers, or iterators are invalidated.  Overloads are
+  // listed below.
+  //
+  // iterator insert(const value_type& value):
+  //
+  //   Inserts a value into the `btree_multimap`, returning an iterator to the
+  //   inserted element.
+  //
+  // iterator insert(value_type&& value):
+  //
+  //   Inserts a moveable value into the `btree_multimap`, returning an iterator
+  //   to the inserted element.
+  //
+  // iterator insert(const_iterator hint, const value_type& value):
+  // iterator insert(const_iterator hint, value_type&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element.
+  //
+  // void insert(InputIterator first, InputIterator last):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  // void insert(std::initializer_list<init_type> ilist):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  using Base::insert;
+
+  // btree_multimap::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_multimap`. Any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace;
+
+  // btree_multimap::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_multimap`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search.
+  //
+  // Any references, pointers, or iterators are invalidated.
+  using Base::emplace_hint;
+
+  // btree_multimap::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // template <typename K> node_type extract(const K& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `btree_multimap`
+  //   does not contain an element with a matching key, this function returns an
+  //   empty node handle.
+  //
+  // NOTE: In this context, `node_type` refers to the C++17 concept of a
+  // move-only type that owns and provides access to the elements in associative
+  // containers (https://en.cppreference.com/w/cpp/container/node_handle).
+  // It does NOT refer to the data layout of the underlying btree.
+  using Base::extract;
+
+  // btree_multimap::merge()
+  //
+  // Extracts elements from a given `source` btree_multimap into this
+  // `btree_multimap`. If the destination `btree_multimap` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // btree_multimap::swap(btree_multimap& other)
+  //
+  // Exchanges the contents of this `btree_multimap` with those of the `other`
+  // btree_multimap, avoiding invocation of any move, copy, or swap operations
+  // on individual elements.
+  //
+  // All iterators and references on the `btree_multimap` remain valid,
+  // excepting for the past-the-end iterator, which is invalidated.
+  using Base::swap;
+
+  // btree_multimap::contains()
+  //
+  // template <typename K> bool contains(const K& key) const:
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `btree_multimap`, returning `true` if so or `false` otherwise.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::contains;
+
+  // btree_multimap::count()
+  //
+  // template <typename K> size_type count(const K& key) const:
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `btree_multimap`.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::count;
+
+  // btree_multimap::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `btree_multimap`.
+  using Base::equal_range;
+
+  // btree_multimap::find()
+  //
+  // template <typename K> iterator find(const K& key):
+  // template <typename K> const_iterator find(const K& key) const:
+  //
+  // Finds an element with the passed `key` within the `btree_multimap`.
+  //
+  // Supports heterogeneous lookup, provided that the map is provided a
+  // compatible heterogeneous comparator.
+  using Base::find;
+
+  // btree_multimap::get_allocator()
+  //
+  // Returns the allocator function associated with this `btree_multimap`.
+  using Base::get_allocator;
+
+  // btree_multimap::key_comp();
+  //
+  // Returns the key comparator associated with this `btree_multimap`.
+  using Base::key_comp;
+
+  // btree_multimap::value_comp();
+  //
+  // Returns the value comparator associated with this `btree_multimap`.
+  using Base::value_comp;
+};
+
+// absl::swap(absl::btree_multimap<>, absl::btree_multimap<>)
+//
+// Swaps the contents of two `absl::btree_multimap` containers.
+template <typename K, typename V, typename C, typename A>
+void swap(btree_multimap<K, V, C, A> &x, btree_multimap<K, V, C, A> &y) {
+  return x.swap(y);
+}
+
+// absl::erase_if(absl::btree_multimap<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+template <typename K, typename V, typename C, typename A, typename Pred>
+void erase_if(btree_multimap<K, V, C, A> &map, Pred pred) {
+  for (auto it = map.begin(); it != map.end();) {
+    if (pred(*it)) {
+      it = map.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_BTREE_MAP_H_
diff --git a/absl/container/btree_set.h b/absl/container/btree_set.h
new file mode 100644
index 0000000..127fb94
--- /dev/null
+++ b/absl/container/btree_set.h
@@ -0,0 +1,683 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: btree_set.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines B-tree sets: sorted associative containers of
+// values.
+//
+//     * `absl::btree_set<>`
+//     * `absl::btree_multiset<>`
+//
+// These B-tree types are similar to the corresponding types in the STL
+// (`std::set` and `std::multiset`) and generally conform to the STL interfaces
+// of those types. However, because they are implemented using B-trees, they
+// are more efficient in most situations.
+//
+// Unlike `std::set` and `std::multiset`, which are commonly implemented using
+// red-black tree nodes, B-tree sets use more generic B-tree nodes able to hold
+// multiple values per node. Holding multiple values per node often makes
+// B-tree sets perform better than their `std::set` counterparts, because
+// multiple entries can be checked within the same cache hit.
+//
+// However, these types should not be considered drop-in replacements for
+// `std::set` and `std::multiset` as there are some API differences, which are
+// noted in this header file.
+//
+// Importantly, insertions and deletions may invalidate outstanding iterators,
+// pointers, and references to elements. Such invalidations are typically only
+// an issue if insertion and deletion operations are interleaved with the use of
+// more than one iterator, pointer, or reference simultaneously. For this
+// reason, `insert()` and `erase()` return a valid iterator at the current
+// position.
+
+#ifndef ABSL_CONTAINER_BTREE_SET_H_
+#define ABSL_CONTAINER_BTREE_SET_H_
+
+#include "absl/container/internal/btree.h"  // IWYU pragma: export
+#include "absl/container/internal/btree_container.h"  // IWYU pragma: export
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// absl::btree_set<>
+//
+// An `absl::btree_set<K>` is an ordered associative container of unique key
+// values designed to be a more efficient replacement for `std::set` (in most
+// cases).
+//
+// Keys are sorted using an (optional) comparison function, which defaults to
+// `std::less<K>`.
+//
+// An `absl::btree_set<K>` uses a default allocator of `std::allocator<K>` to
+// allocate (and deallocate) nodes, and construct and destruct values within
+// those nodes. You may instead specify a custom allocator `A` (which in turn
+// requires specifying a custom comparator `C`) as in
+// `absl::btree_set<K, C, A>`.
+//
+template <typename Key, typename Compare = std::less<Key>,
+          typename Alloc = std::allocator<Key>>
+class btree_set
+    : public container_internal::btree_set_container<
+          container_internal::btree<container_internal::set_params<
+              Key, Compare, Alloc, /*TargetNodeSize=*/256,
+              /*Multi=*/false>>> {
+  using Base = typename btree_set::btree_set_container;
+
+ public:
+  // Constructors and Assignment Operators
+  //
+  // A `btree_set` supports the same overload set as `std::set`
+  // for construction and assignment:
+  //
+  // * Default constructor
+  //
+  //   absl::btree_set<std::string> set1;
+  //
+  // * Initializer List constructor
+  //
+  //   absl::btree_set<std::string> set2 =
+  //       {{"huey"}, {"dewey"}, {"louie"},};
+  //
+  // * Copy constructor
+  //
+  //   absl::btree_set<std::string> set3(set2);
+  //
+  // * Copy assignment operator
+  //
+  //  absl::btree_set<std::string> set4;
+  //  set4 = set3;
+  //
+  // * Move constructor
+  //
+  //   // Move is guaranteed efficient
+  //   absl::btree_set<std::string> set5(std::move(set4));
+  //
+  // * Move assignment operator
+  //
+  //   // May be efficient if allocators are compatible
+  //   absl::btree_set<std::string> set6;
+  //   set6 = std::move(set5);
+  //
+  // * Range constructor
+  //
+  //   std::vector<std::string> v = {"a", "b"};
+  //   absl::btree_set<std::string> set7(v.begin(), v.end());
+  btree_set() {}
+  using Base::Base;
+
+  // btree_set::begin()
+  //
+  // Returns an iterator to the beginning of the `btree_set`.
+  using Base::begin;
+
+  // btree_set::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `btree_set`.
+  using Base::cbegin;
+
+  // btree_set::end()
+  //
+  // Returns an iterator to the end of the `btree_set`.
+  using Base::end;
+
+  // btree_set::cend()
+  //
+  // Returns a const iterator to the end of the `btree_set`.
+  using Base::cend;
+
+  // btree_set::empty()
+  //
+  // Returns whether or not the `btree_set` is empty.
+  using Base::empty;
+
+  // btree_set::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `btree_set` under current memory constraints. This value can be thought
+  // of as the largest value of `std::distance(begin(), end())` for a
+  // `btree_set<Key>`.
+  using Base::max_size;
+
+  // btree_set::size()
+  //
+  // Returns the number of elements currently within the `btree_set`.
+  using Base::size;
+
+  // btree_set::clear()
+  //
+  // Removes all elements from the `btree_set`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  using Base::clear;
+
+  // btree_set::erase()
+  //
+  // Erases elements within the `btree_set`. Overloads are listed below.
+  //
+  // iterator erase(iterator position):
+  // iterator erase(const_iterator position):
+  //
+  //   Erases the element at `position` of the `btree_set`, returning
+  //   the iterator pointing to the element after the one that was erased
+  //   (or end() if none exists).
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning
+  //   the iterator pointing to the element after the interval that was erased
+  //   (or end() if none exists).
+  //
+  // template <typename K> size_type erase(const K& key):
+  //
+  //   Erases the element with the matching key, if it exists, returning the
+  //   number of elements erased.
+  using Base::erase;
+
+  // btree_set::insert()
+  //
+  // Inserts an element of the specified value into the `btree_set`,
+  // returning an iterator pointing to the newly inserted element, provided that
+  // an element with the given key does not already exist. If an insertion
+  // occurs, any references, pointers, or iterators are invalidated.
+  // Overloads are listed below.
+  //
+  // std::pair<iterator,bool> insert(const value_type& value):
+  //
+  //   Inserts a value into the `btree_set`. Returns a pair consisting of an
+  //   iterator to the inserted element (or to the element that prevented the
+  //   insertion) and a bool denoting whether the insertion took place.
+  //
+  // std::pair<iterator,bool> insert(value_type&& value):
+  //
+  //   Inserts a moveable value into the `btree_set`. Returns a pair
+  //   consisting of an iterator to the inserted element (or to the element that
+  //   prevented the insertion) and a bool denoting whether the insertion took
+  //   place.
+  //
+  // iterator insert(const_iterator hint, const value_type& value):
+  // iterator insert(const_iterator hint, value_type&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element, or to the existing element that prevented the
+  //   insertion.
+  //
+  // void insert(InputIterator first, InputIterator last):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  // void insert(std::initializer_list<init_type> ilist):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  using Base::insert;
+
+  // btree_set::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_set`, provided that no element with the given key
+  // already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace;
+
+  // btree_set::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_set`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search, and only inserts
+  // provided that no element with the given key already exists.
+  //
+  // The element may be constructed even if there already is an element with the
+  // key in the container, in which case the newly constructed element will be
+  // destroyed immediately.
+  //
+  // If an insertion occurs, any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace_hint;
+
+  // btree_set::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // template <typename K> node_type extract(const K& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `btree_set`
+  //   does not contain an element with a matching key, this function returns an
+  //   empty node handle.
+  //
+  // NOTE: In this context, `node_type` refers to the C++17 concept of a
+  // move-only type that owns and provides access to the elements in associative
+  // containers (https://en.cppreference.com/w/cpp/container/node_handle).
+  // It does NOT refer to the data layout of the underlying btree.
+  using Base::extract;
+
+  // btree_set::merge()
+  //
+  // Extracts elements from a given `source` btree_set into this
+  // `btree_set`. If the destination `btree_set` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // btree_set::swap(btree_set& other)
+  //
+  // Exchanges the contents of this `btree_set` with those of the `other`
+  // btree_set, avoiding invocation of any move, copy, or swap operations on
+  // individual elements.
+  //
+  // All iterators and references on the `btree_set` remain valid, excepting
+  // for the past-the-end iterator, which is invalidated.
+  using Base::swap;
+
+  // btree_set::contains()
+  //
+  // template <typename K> bool contains(const K& key) const:
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `btree_set`, returning `true` if so or `false` otherwise.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::contains;
+
+  // btree_set::count()
+  //
+  // template <typename K> size_type count(const K& key) const:
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `btree_set`. Note that this function will return either `1` or `0`
+  // since duplicate elements are not allowed within a `btree_set`.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::count;
+
+  // btree_set::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `btree_set`.
+  using Base::equal_range;
+
+  // btree_set::find()
+  //
+  // template <typename K> iterator find(const K& key):
+  // template <typename K> const_iterator find(const K& key) const:
+  //
+  // Finds an element with the passed `key` within the `btree_set`.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::find;
+
+  // btree_set::get_allocator()
+  //
+  // Returns the allocator function associated with this `btree_set`.
+  using Base::get_allocator;
+
+  // btree_set::key_comp();
+  //
+  // Returns the key comparator associated with this `btree_set`.
+  using Base::key_comp;
+
+  // btree_set::value_comp();
+  //
+  // Returns the value comparator associated with this `btree_set`. The keys to
+  // sort the elements are the values themselves, therefore `value_comp` and its
+  // sibling member function `key_comp` are equivalent.
+  using Base::value_comp;
+};
+
+// absl::swap(absl::btree_set<>, absl::btree_set<>)
+//
+// Swaps the contents of two `absl::btree_set` containers.
+template <typename K, typename C, typename A>
+void swap(btree_set<K, C, A> &x, btree_set<K, C, A> &y) {
+  return x.swap(y);
+}
+
+// absl::erase_if(absl::btree_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+template <typename K, typename C, typename A, typename Pred>
+void erase_if(btree_set<K, C, A> &set, Pred pred) {
+  for (auto it = set.begin(); it != set.end();) {
+    if (pred(*it)) {
+      it = set.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}
+
+// absl::btree_multiset<>
+//
+// An `absl::btree_multiset<K>` is an ordered associative container of
+// keys and associated values designed to be a more efficient replacement
+// for `std::multiset` (in most cases). Unlike `absl::btree_set`, a B-tree
+// multiset allows equivalent elements.
+//
+// Keys are sorted using an (optional) comparison function, which defaults to
+// `std::less<K>`.
+//
+// An `absl::btree_multiset<K>` uses a default allocator of `std::allocator<K>`
+// to allocate (and deallocate) nodes, and construct and destruct values within
+// those nodes. You may instead specify a custom allocator `A` (which in turn
+// requires specifying a custom comparator `C`) as in
+// `absl::btree_multiset<K, C, A>`.
+//
+template <typename Key, typename Compare = std::less<Key>,
+          typename Alloc = std::allocator<Key>>
+class btree_multiset
+    : public container_internal::btree_multiset_container<
+          container_internal::btree<container_internal::set_params<
+              Key, Compare, Alloc, /*TargetNodeSize=*/256,
+              /*Multi=*/true>>> {
+  using Base = typename btree_multiset::btree_multiset_container;
+
+ public:
+  // Constructors and Assignment Operators
+  //
+  // A `btree_multiset` supports the same overload set as `std::set`
+  // for construction and assignment:
+  //
+  // * Default constructor
+  //
+  //   absl::btree_multiset<std::string> set1;
+  //
+  // * Initializer List constructor
+  //
+  //   absl::btree_multiset<std::string> set2 =
+  //       {{"huey"}, {"dewey"}, {"louie"},};
+  //
+  // * Copy constructor
+  //
+  //   absl::btree_multiset<std::string> set3(set2);
+  //
+  // * Copy assignment operator
+  //
+  //  absl::btree_multiset<std::string> set4;
+  //  set4 = set3;
+  //
+  // * Move constructor
+  //
+  //   // Move is guaranteed efficient
+  //   absl::btree_multiset<std::string> set5(std::move(set4));
+  //
+  // * Move assignment operator
+  //
+  //   // May be efficient if allocators are compatible
+  //   absl::btree_multiset<std::string> set6;
+  //   set6 = std::move(set5);
+  //
+  // * Range constructor
+  //
+  //   std::vector<std::string> v = {"a", "b"};
+  //   absl::btree_multiset<std::string> set7(v.begin(), v.end());
+  btree_multiset() {}
+  using Base::Base;
+
+  // btree_multiset::begin()
+  //
+  // Returns an iterator to the beginning of the `btree_multiset`.
+  using Base::begin;
+
+  // btree_multiset::cbegin()
+  //
+  // Returns a const iterator to the beginning of the `btree_multiset`.
+  using Base::cbegin;
+
+  // btree_multiset::end()
+  //
+  // Returns an iterator to the end of the `btree_multiset`.
+  using Base::end;
+
+  // btree_multiset::cend()
+  //
+  // Returns a const iterator to the end of the `btree_multiset`.
+  using Base::cend;
+
+  // btree_multiset::empty()
+  //
+  // Returns whether or not the `btree_multiset` is empty.
+  using Base::empty;
+
+  // btree_multiset::max_size()
+  //
+  // Returns the largest theoretical possible number of elements within a
+  // `btree_multiset` under current memory constraints. This value can be
+  // thought of as the largest value of `std::distance(begin(), end())` for a
+  // `btree_multiset<Key>`.
+  using Base::max_size;
+
+  // btree_multiset::size()
+  //
+  // Returns the number of elements currently within the `btree_multiset`.
+  using Base::size;
+
+  // btree_multiset::clear()
+  //
+  // Removes all elements from the `btree_multiset`. Invalidates any references,
+  // pointers, or iterators referring to contained elements.
+  using Base::clear;
+
+  // btree_multiset::erase()
+  //
+  // Erases elements within the `btree_multiset`. Overloads are listed below.
+  //
+  // iterator erase(iterator position):
+  // iterator erase(const_iterator position):
+  //
+  //   Erases the element at `position` of the `btree_multiset`, returning
+  //   the iterator pointing to the element after the one that was erased
+  //   (or end() if none exists).
+  //
+  // iterator erase(const_iterator first, const_iterator last):
+  //
+  //   Erases the elements in the open interval [`first`, `last`), returning
+  //   the iterator pointing to the element after the interval that was erased
+  //   (or end() if none exists).
+  //
+  // template <typename K> size_type erase(const K& key):
+  //
+  //   Erases the elements matching the key, if any exist, returning the
+  //   number of elements erased.
+  using Base::erase;
+
+  // btree_multiset::insert()
+  //
+  // Inserts an element of the specified value into the `btree_multiset`,
+  // returning an iterator pointing to the newly inserted element.
+  // Any references, pointers, or iterators are invalidated.  Overloads are
+  // listed below.
+  //
+  // iterator insert(const value_type& value):
+  //
+  //   Inserts a value into the `btree_multiset`, returning an iterator to the
+  //   inserted element.
+  //
+  // iterator insert(value_type&& value):
+  //
+  //   Inserts a moveable value into the `btree_multiset`, returning an iterator
+  //   to the inserted element.
+  //
+  // iterator insert(const_iterator hint, const value_type& value):
+  // iterator insert(const_iterator hint, value_type&& value):
+  //
+  //   Inserts a value, using the position of `hint` as a non-binding suggestion
+  //   for where to begin the insertion search. Returns an iterator to the
+  //   inserted element.
+  //
+  // void insert(InputIterator first, InputIterator last):
+  //
+  //   Inserts a range of values [`first`, `last`).
+  //
+  // void insert(std::initializer_list<init_type> ilist):
+  //
+  //   Inserts the elements within the initializer list `ilist`.
+  using Base::insert;
+
+  // btree_multiset::emplace()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_multiset`. Any references, pointers, or iterators are
+  // invalidated.
+  using Base::emplace;
+
+  // btree_multiset::emplace_hint()
+  //
+  // Inserts an element of the specified value by constructing it in-place
+  // within the `btree_multiset`, using the position of `hint` as a non-binding
+  // suggestion for where to begin the insertion search.
+  //
+  // Any references, pointers, or iterators are invalidated.
+  using Base::emplace_hint;
+
+  // btree_multiset::extract()
+  //
+  // Extracts the indicated element, erasing it in the process, and returns it
+  // as a C++17-compatible node handle. Overloads are listed below.
+  //
+  // node_type extract(const_iterator position):
+  //
+  //   Extracts the element at the indicated position and returns a node handle
+  //   owning that extracted data.
+  //
+  // template <typename K> node_type extract(const K& x):
+  //
+  //   Extracts the element with the key matching the passed key value and
+  //   returns a node handle owning that extracted data. If the `btree_multiset`
+  //   does not contain an element with a matching key, this function returns an
+  //   empty node handle.
+  //
+  // NOTE: In this context, `node_type` refers to the C++17 concept of a
+  // move-only type that owns and provides access to the elements in associative
+  // containers (https://en.cppreference.com/w/cpp/container/node_handle).
+  // It does NOT refer to the data layout of the underlying btree.
+  using Base::extract;
+
+  // btree_multiset::merge()
+  //
+  // Extracts elements from a given `source` btree_multiset into this
+  // `btree_multiset`. If the destination `btree_multiset` already contains an
+  // element with an equivalent key, that element is not extracted.
+  using Base::merge;
+
+  // btree_multiset::swap(btree_multiset& other)
+  //
+  // Exchanges the contents of this `btree_multiset` with those of the `other`
+  // btree_multiset, avoiding invocation of any move, copy, or swap operations
+  // on individual elements.
+  //
+  // All iterators and references on the `btree_multiset` remain valid,
+  // excepting for the past-the-end iterator, which is invalidated.
+  using Base::swap;
+
+  // btree_multiset::contains()
+  //
+  // template <typename K> bool contains(const K& key) const:
+  //
+  // Determines whether an element comparing equal to the given `key` exists
+  // within the `btree_multiset`, returning `true` if so or `false` otherwise.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::contains;
+
+  // btree_multiset::count()
+  //
+  // template <typename K> size_type count(const K& key) const:
+  //
+  // Returns the number of elements comparing equal to the given `key` within
+  // the `btree_multiset`.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::count;
+
+  // btree_multiset::equal_range()
+  //
+  // Returns a closed range [first, last], defined by a `std::pair` of two
+  // iterators, containing all elements with the passed key in the
+  // `btree_multiset`.
+  using Base::equal_range;
+
+  // btree_multiset::find()
+  //
+  // template <typename K> iterator find(const K& key):
+  // template <typename K> const_iterator find(const K& key) const:
+  //
+  // Finds an element with the passed `key` within the `btree_multiset`.
+  //
+  // Supports heterogeneous lookup, provided that the set is provided a
+  // compatible heterogeneous comparator.
+  using Base::find;
+
+  // btree_multiset::get_allocator()
+  //
+  // Returns the allocator function associated with this `btree_multiset`.
+  using Base::get_allocator;
+
+  // btree_multiset::key_comp();
+  //
+  // Returns the key comparator associated with this `btree_multiset`.
+  using Base::key_comp;
+
+  // btree_multiset::value_comp();
+  //
+  // Returns the value comparator associated with this `btree_multiset`. The
+  // keys to sort the elements are the values themselves, therefore `value_comp`
+  // and its sibling member function `key_comp` are equivalent.
+  using Base::value_comp;
+};
+
+// absl::swap(absl::btree_multiset<>, absl::btree_multiset<>)
+//
+// Swaps the contents of two `absl::btree_multiset` containers.
+template <typename K, typename C, typename A>
+void swap(btree_multiset<K, C, A> &x, btree_multiset<K, C, A> &y) {
+  return x.swap(y);
+}
+
+// absl::erase_if(absl::btree_multiset<>, Pred)
+//
+// Erases all elements that satisfy the predicate pred from the container.
+template <typename K, typename C, typename A, typename Pred>
+void erase_if(btree_multiset<K, C, A> &set, Pred pred) {
+  for (auto it = set.begin(); it != set.end();) {
+    if (pred(*it)) {
+      it = set.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_BTREE_SET_H_
diff --git a/absl/container/btree_test.cc b/absl/container/btree_test.cc
new file mode 100644
index 0000000..9edf38f
--- /dev/null
+++ b/absl/container/btree_test.cc
@@ -0,0 +1,2404 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/container/btree_test.h"
+
+#include <cstdint>
+#include <map>
+#include <memory>
+#include <stdexcept>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/macros.h"
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/internal/counting_allocator.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/flags/flag.h"
+#include "absl/hash/hash_testing.h"
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/compare.h"
+
+ABSL_FLAG(int, test_values, 10000, "The number of values to use for tests");
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+namespace {
+
+using ::absl::test_internal::CopyableMovableInstance;
+using ::absl::test_internal::InstanceTracker;
+using ::absl::test_internal::MovableOnlyInstance;
+using ::testing::ElementsAre;
+using ::testing::ElementsAreArray;
+using ::testing::IsEmpty;
+using ::testing::Pair;
+
+template <typename T, typename U>
+void CheckPairEquals(const T &x, const U &y) {
+  ABSL_INTERNAL_CHECK(x == y, "Values are unequal.");
+}
+
+template <typename T, typename U, typename V, typename W>
+void CheckPairEquals(const std::pair<T, U> &x, const std::pair<V, W> &y) {
+  CheckPairEquals(x.first, y.first);
+  CheckPairEquals(x.second, y.second);
+}
+}  // namespace
+
+// The base class for a sorted associative container checker. TreeType is the
+// container type to check and CheckerType is the container type to check
+// against. TreeType is expected to be btree_{set,map,multiset,multimap} and
+// CheckerType is expected to be {set,map,multiset,multimap}.
+template <typename TreeType, typename CheckerType>
+class base_checker {
+ public:
+  using key_type = typename TreeType::key_type;
+  using value_type = typename TreeType::value_type;
+  using key_compare = typename TreeType::key_compare;
+  using pointer = typename TreeType::pointer;
+  using const_pointer = typename TreeType::const_pointer;
+  using reference = typename TreeType::reference;
+  using const_reference = typename TreeType::const_reference;
+  using size_type = typename TreeType::size_type;
+  using difference_type = typename TreeType::difference_type;
+  using iterator = typename TreeType::iterator;
+  using const_iterator = typename TreeType::const_iterator;
+  using reverse_iterator = typename TreeType::reverse_iterator;
+  using const_reverse_iterator = typename TreeType::const_reverse_iterator;
+
+ public:
+  base_checker() : const_tree_(tree_) {}
+  base_checker(const base_checker &x)
+      : tree_(x.tree_), const_tree_(tree_), checker_(x.checker_) {}
+  template <typename InputIterator>
+  base_checker(InputIterator b, InputIterator e)
+      : tree_(b, e), const_tree_(tree_), checker_(b, e) {}
+
+  iterator begin() { return tree_.begin(); }
+  const_iterator begin() const { return tree_.begin(); }
+  iterator end() { return tree_.end(); }
+  const_iterator end() const { return tree_.end(); }
+  reverse_iterator rbegin() { return tree_.rbegin(); }
+  const_reverse_iterator rbegin() const { return tree_.rbegin(); }
+  reverse_iterator rend() { return tree_.rend(); }
+  const_reverse_iterator rend() const { return tree_.rend(); }
+
+  template <typename IterType, typename CheckerIterType>
+  IterType iter_check(IterType tree_iter, CheckerIterType checker_iter) const {
+    if (tree_iter == tree_.end()) {
+      ABSL_INTERNAL_CHECK(checker_iter == checker_.end(),
+                          "Checker iterator not at end.");
+    } else {
+      CheckPairEquals(*tree_iter, *checker_iter);
+    }
+    return tree_iter;
+  }
+  template <typename IterType, typename CheckerIterType>
+  IterType riter_check(IterType tree_iter, CheckerIterType checker_iter) const {
+    if (tree_iter == tree_.rend()) {
+      ABSL_INTERNAL_CHECK(checker_iter == checker_.rend(),
+                          "Checker iterator not at rend.");
+    } else {
+      CheckPairEquals(*tree_iter, *checker_iter);
+    }
+    return tree_iter;
+  }
+  void value_check(const value_type &x) {
+    typename KeyOfValue<typename TreeType::key_type,
+                        typename TreeType::value_type>::type key_of_value;
+    const key_type &key = key_of_value(x);
+    CheckPairEquals(*find(key), x);
+    lower_bound(key);
+    upper_bound(key);
+    equal_range(key);
+    contains(key);
+    count(key);
+  }
+  void erase_check(const key_type &key) {
+    EXPECT_FALSE(tree_.contains(key));
+    EXPECT_EQ(tree_.find(key), const_tree_.end());
+    EXPECT_FALSE(const_tree_.contains(key));
+    EXPECT_EQ(const_tree_.find(key), tree_.end());
+    EXPECT_EQ(tree_.equal_range(key).first,
+              const_tree_.equal_range(key).second);
+  }
+
+  iterator lower_bound(const key_type &key) {
+    return iter_check(tree_.lower_bound(key), checker_.lower_bound(key));
+  }
+  const_iterator lower_bound(const key_type &key) const {
+    return iter_check(tree_.lower_bound(key), checker_.lower_bound(key));
+  }
+  iterator upper_bound(const key_type &key) {
+    return iter_check(tree_.upper_bound(key), checker_.upper_bound(key));
+  }
+  const_iterator upper_bound(const key_type &key) const {
+    return iter_check(tree_.upper_bound(key), checker_.upper_bound(key));
+  }
+  std::pair<iterator, iterator> equal_range(const key_type &key) {
+    std::pair<typename CheckerType::iterator, typename CheckerType::iterator>
+        checker_res = checker_.equal_range(key);
+    std::pair<iterator, iterator> tree_res = tree_.equal_range(key);
+    iter_check(tree_res.first, checker_res.first);
+    iter_check(tree_res.second, checker_res.second);
+    return tree_res;
+  }
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_type &key) const {
+    std::pair<typename CheckerType::const_iterator,
+              typename CheckerType::const_iterator>
+        checker_res = checker_.equal_range(key);
+    std::pair<const_iterator, const_iterator> tree_res = tree_.equal_range(key);
+    iter_check(tree_res.first, checker_res.first);
+    iter_check(tree_res.second, checker_res.second);
+    return tree_res;
+  }
+  iterator find(const key_type &key) {
+    return iter_check(tree_.find(key), checker_.find(key));
+  }
+  const_iterator find(const key_type &key) const {
+    return iter_check(tree_.find(key), checker_.find(key));
+  }
+  bool contains(const key_type &key) const { return find(key) != end(); }
+  size_type count(const key_type &key) const {
+    size_type res = checker_.count(key);
+    EXPECT_EQ(res, tree_.count(key));
+    return res;
+  }
+
+  base_checker &operator=(const base_checker &x) {
+    tree_ = x.tree_;
+    checker_ = x.checker_;
+    return *this;
+  }
+
+  int erase(const key_type &key) {
+    int size = tree_.size();
+    int res = checker_.erase(key);
+    EXPECT_EQ(res, tree_.count(key));
+    EXPECT_EQ(res, tree_.erase(key));
+    EXPECT_EQ(tree_.count(key), 0);
+    EXPECT_EQ(tree_.size(), size - res);
+    erase_check(key);
+    return res;
+  }
+  iterator erase(iterator iter) {
+    key_type key = iter.key();
+    int size = tree_.size();
+    int count = tree_.count(key);
+    auto checker_iter = checker_.lower_bound(key);
+    for (iterator tmp(tree_.lower_bound(key)); tmp != iter; ++tmp) {
+      ++checker_iter;
+    }
+    auto checker_next = checker_iter;
+    ++checker_next;
+    checker_.erase(checker_iter);
+    iter = tree_.erase(iter);
+    EXPECT_EQ(tree_.size(), checker_.size());
+    EXPECT_EQ(tree_.size(), size - 1);
+    EXPECT_EQ(tree_.count(key), count - 1);
+    if (count == 1) {
+      erase_check(key);
+    }
+    return iter_check(iter, checker_next);
+  }
+
+  void erase(iterator begin, iterator end) {
+    int size = tree_.size();
+    int count = std::distance(begin, end);
+    auto checker_begin = checker_.lower_bound(begin.key());
+    for (iterator tmp(tree_.lower_bound(begin.key())); tmp != begin; ++tmp) {
+      ++checker_begin;
+    }
+    auto checker_end =
+        end == tree_.end() ? checker_.end() : checker_.lower_bound(end.key());
+    if (end != tree_.end()) {
+      for (iterator tmp(tree_.lower_bound(end.key())); tmp != end; ++tmp) {
+        ++checker_end;
+      }
+    }
+    const auto checker_ret = checker_.erase(checker_begin, checker_end);
+    const auto tree_ret = tree_.erase(begin, end);
+    EXPECT_EQ(std::distance(checker_.begin(), checker_ret),
+              std::distance(tree_.begin(), tree_ret));
+    EXPECT_EQ(tree_.size(), checker_.size());
+    EXPECT_EQ(tree_.size(), size - count);
+  }
+
+  void clear() {
+    tree_.clear();
+    checker_.clear();
+  }
+  void swap(base_checker &x) {
+    tree_.swap(x.tree_);
+    checker_.swap(x.checker_);
+  }
+
+  void verify() const {
+    tree_.verify();
+    EXPECT_EQ(tree_.size(), checker_.size());
+
+    // Move through the forward iterators using increment.
+    auto checker_iter = checker_.begin();
+    const_iterator tree_iter(tree_.begin());
+    for (; tree_iter != tree_.end(); ++tree_iter, ++checker_iter) {
+      CheckPairEquals(*tree_iter, *checker_iter);
+    }
+
+    // Move through the forward iterators using decrement.
+    for (int n = tree_.size() - 1; n >= 0; --n) {
+      iter_check(tree_iter, checker_iter);
+      --tree_iter;
+      --checker_iter;
+    }
+    EXPECT_EQ(tree_iter, tree_.begin());
+    EXPECT_EQ(checker_iter, checker_.begin());
+
+    // Move through the reverse iterators using increment.
+    auto checker_riter = checker_.rbegin();
+    const_reverse_iterator tree_riter(tree_.rbegin());
+    for (; tree_riter != tree_.rend(); ++tree_riter, ++checker_riter) {
+      CheckPairEquals(*tree_riter, *checker_riter);
+    }
+
+    // Move through the reverse iterators using decrement.
+    for (int n = tree_.size() - 1; n >= 0; --n) {
+      riter_check(tree_riter, checker_riter);
+      --tree_riter;
+      --checker_riter;
+    }
+    EXPECT_EQ(tree_riter, tree_.rbegin());
+    EXPECT_EQ(checker_riter, checker_.rbegin());
+  }
+
+  const TreeType &tree() const { return tree_; }
+
+  size_type size() const {
+    EXPECT_EQ(tree_.size(), checker_.size());
+    return tree_.size();
+  }
+  size_type max_size() const { return tree_.max_size(); }
+  bool empty() const {
+    EXPECT_EQ(tree_.empty(), checker_.empty());
+    return tree_.empty();
+  }
+
+ protected:
+  TreeType tree_;
+  const TreeType &const_tree_;
+  CheckerType checker_;
+};
+
+namespace {
+// A checker for unique sorted associative containers. TreeType is expected to
+// be btree_{set,map} and CheckerType is expected to be {set,map}.
+template <typename TreeType, typename CheckerType>
+class unique_checker : public base_checker<TreeType, CheckerType> {
+  using super_type = base_checker<TreeType, CheckerType>;
+
+ public:
+  using iterator = typename super_type::iterator;
+  using value_type = typename super_type::value_type;
+
+ public:
+  unique_checker() : super_type() {}
+  unique_checker(const unique_checker &x) : super_type(x) {}
+  template <class InputIterator>
+  unique_checker(InputIterator b, InputIterator e) : super_type(b, e) {}
+  unique_checker &operator=(const unique_checker &) = default;
+
+  // Insertion routines.
+  std::pair<iterator, bool> insert(const value_type &x) {
+    int size = this->tree_.size();
+    std::pair<typename CheckerType::iterator, bool> checker_res =
+        this->checker_.insert(x);
+    std::pair<iterator, bool> tree_res = this->tree_.insert(x);
+    CheckPairEquals(*tree_res.first, *checker_res.first);
+    EXPECT_EQ(tree_res.second, checker_res.second);
+    EXPECT_EQ(this->tree_.size(), this->checker_.size());
+    EXPECT_EQ(this->tree_.size(), size + tree_res.second);
+    return tree_res;
+  }
+  iterator insert(iterator position, const value_type &x) {
+    int size = this->tree_.size();
+    std::pair<typename CheckerType::iterator, bool> checker_res =
+        this->checker_.insert(x);
+    iterator tree_res = this->tree_.insert(position, x);
+    CheckPairEquals(*tree_res, *checker_res.first);
+    EXPECT_EQ(this->tree_.size(), this->checker_.size());
+    EXPECT_EQ(this->tree_.size(), size + checker_res.second);
+    return tree_res;
+  }
+  template <typename InputIterator>
+  void insert(InputIterator b, InputIterator e) {
+    for (; b != e; ++b) {
+      insert(*b);
+    }
+  }
+};
+
+// A checker for multiple sorted associative containers. TreeType is expected
+// to be btree_{multiset,multimap} and CheckerType is expected to be
+// {multiset,multimap}.
+template <typename TreeType, typename CheckerType>
+class multi_checker : public base_checker<TreeType, CheckerType> {
+  using super_type = base_checker<TreeType, CheckerType>;
+
+ public:
+  using iterator = typename super_type::iterator;
+  using value_type = typename super_type::value_type;
+
+ public:
+  multi_checker() : super_type() {}
+  multi_checker(const multi_checker &x) : super_type(x) {}
+  template <class InputIterator>
+  multi_checker(InputIterator b, InputIterator e) : super_type(b, e) {}
+  multi_checker &operator=(const multi_checker &) = default;
+
+  // Insertion routines.
+  iterator insert(const value_type &x) {
+    int size = this->tree_.size();
+    auto checker_res = this->checker_.insert(x);
+    iterator tree_res = this->tree_.insert(x);
+    CheckPairEquals(*tree_res, *checker_res);
+    EXPECT_EQ(this->tree_.size(), this->checker_.size());
+    EXPECT_EQ(this->tree_.size(), size + 1);
+    return tree_res;
+  }
+  iterator insert(iterator position, const value_type &x) {
+    int size = this->tree_.size();
+    auto checker_res = this->checker_.insert(x);
+    iterator tree_res = this->tree_.insert(position, x);
+    CheckPairEquals(*tree_res, *checker_res);
+    EXPECT_EQ(this->tree_.size(), this->checker_.size());
+    EXPECT_EQ(this->tree_.size(), size + 1);
+    return tree_res;
+  }
+  template <typename InputIterator>
+  void insert(InputIterator b, InputIterator e) {
+    for (; b != e; ++b) {
+      insert(*b);
+    }
+  }
+};
+
+template <typename T, typename V>
+void DoTest(const char *name, T *b, const std::vector<V> &values) {
+  typename KeyOfValue<typename T::key_type, V>::type key_of_value;
+
+  T &mutable_b = *b;
+  const T &const_b = *b;
+
+  // Test insert.
+  for (int i = 0; i < values.size(); ++i) {
+    mutable_b.insert(values[i]);
+    mutable_b.value_check(values[i]);
+  }
+  ASSERT_EQ(mutable_b.size(), values.size());
+
+  const_b.verify();
+
+  // Test copy constructor.
+  T b_copy(const_b);
+  EXPECT_EQ(b_copy.size(), const_b.size());
+  for (int i = 0; i < values.size(); ++i) {
+    CheckPairEquals(*b_copy.find(key_of_value(values[i])), values[i]);
+  }
+
+  // Test range constructor.
+  T b_range(const_b.begin(), const_b.end());
+  EXPECT_EQ(b_range.size(), const_b.size());
+  for (int i = 0; i < values.size(); ++i) {
+    CheckPairEquals(*b_range.find(key_of_value(values[i])), values[i]);
+  }
+
+  // Test range insertion for values that already exist.
+  b_range.insert(b_copy.begin(), b_copy.end());
+  b_range.verify();
+
+  // Test range insertion for new values.
+  b_range.clear();
+  b_range.insert(b_copy.begin(), b_copy.end());
+  EXPECT_EQ(b_range.size(), b_copy.size());
+  for (int i = 0; i < values.size(); ++i) {
+    CheckPairEquals(*b_range.find(key_of_value(values[i])), values[i]);
+  }
+
+  // Test assignment to self. Nothing should change.
+  b_range.operator=(b_range);
+  EXPECT_EQ(b_range.size(), b_copy.size());
+
+  // Test assignment of new values.
+  b_range.clear();
+  b_range = b_copy;
+  EXPECT_EQ(b_range.size(), b_copy.size());
+
+  // Test swap.
+  b_range.clear();
+  b_range.swap(b_copy);
+  EXPECT_EQ(b_copy.size(), 0);
+  EXPECT_EQ(b_range.size(), const_b.size());
+  for (int i = 0; i < values.size(); ++i) {
+    CheckPairEquals(*b_range.find(key_of_value(values[i])), values[i]);
+  }
+  b_range.swap(b_copy);
+
+  // Test non-member function swap.
+  swap(b_range, b_copy);
+  EXPECT_EQ(b_copy.size(), 0);
+  EXPECT_EQ(b_range.size(), const_b.size());
+  for (int i = 0; i < values.size(); ++i) {
+    CheckPairEquals(*b_range.find(key_of_value(values[i])), values[i]);
+  }
+  swap(b_range, b_copy);
+
+  // Test erase via values.
+  for (int i = 0; i < values.size(); ++i) {
+    mutable_b.erase(key_of_value(values[i]));
+    // Erasing a non-existent key should have no effect.
+    ASSERT_EQ(mutable_b.erase(key_of_value(values[i])), 0);
+  }
+
+  const_b.verify();
+  EXPECT_EQ(const_b.size(), 0);
+
+  // Test erase via iterators.
+  mutable_b = b_copy;
+  for (int i = 0; i < values.size(); ++i) {
+    mutable_b.erase(mutable_b.find(key_of_value(values[i])));
+  }
+
+  const_b.verify();
+  EXPECT_EQ(const_b.size(), 0);
+
+  // Test insert with hint.
+  for (int i = 0; i < values.size(); i++) {
+    mutable_b.insert(mutable_b.upper_bound(key_of_value(values[i])), values[i]);
+  }
+
+  const_b.verify();
+
+  // Test range erase.
+  mutable_b.erase(mutable_b.begin(), mutable_b.end());
+  EXPECT_EQ(mutable_b.size(), 0);
+  const_b.verify();
+
+  // First half.
+  mutable_b = b_copy;
+  typename T::iterator mutable_iter_end = mutable_b.begin();
+  for (int i = 0; i < values.size() / 2; ++i) ++mutable_iter_end;
+  mutable_b.erase(mutable_b.begin(), mutable_iter_end);
+  EXPECT_EQ(mutable_b.size(), values.size() - values.size() / 2);
+  const_b.verify();
+
+  // Second half.
+  mutable_b = b_copy;
+  typename T::iterator mutable_iter_begin = mutable_b.begin();
+  for (int i = 0; i < values.size() / 2; ++i) ++mutable_iter_begin;
+  mutable_b.erase(mutable_iter_begin, mutable_b.end());
+  EXPECT_EQ(mutable_b.size(), values.size() / 2);
+  const_b.verify();
+
+  // Second quarter.
+  mutable_b = b_copy;
+  mutable_iter_begin = mutable_b.begin();
+  for (int i = 0; i < values.size() / 4; ++i) ++mutable_iter_begin;
+  mutable_iter_end = mutable_iter_begin;
+  for (int i = 0; i < values.size() / 4; ++i) ++mutable_iter_end;
+  mutable_b.erase(mutable_iter_begin, mutable_iter_end);
+  EXPECT_EQ(mutable_b.size(), values.size() - values.size() / 4);
+  const_b.verify();
+
+  mutable_b.clear();
+}
+
+template <typename T>
+void ConstTest() {
+  using value_type = typename T::value_type;
+  typename KeyOfValue<typename T::key_type, value_type>::type key_of_value;
+
+  T mutable_b;
+  const T &const_b = mutable_b;
+
+  // Insert a single value into the container and test looking it up.
+  value_type value = Generator<value_type>(2)(2);
+  mutable_b.insert(value);
+  EXPECT_TRUE(mutable_b.contains(key_of_value(value)));
+  EXPECT_NE(mutable_b.find(key_of_value(value)), const_b.end());
+  EXPECT_TRUE(const_b.contains(key_of_value(value)));
+  EXPECT_NE(const_b.find(key_of_value(value)), mutable_b.end());
+  EXPECT_EQ(*const_b.lower_bound(key_of_value(value)), value);
+  EXPECT_EQ(const_b.upper_bound(key_of_value(value)), const_b.end());
+  EXPECT_EQ(*const_b.equal_range(key_of_value(value)).first, value);
+
+  // We can only create a non-const iterator from a non-const container.
+  typename T::iterator mutable_iter(mutable_b.begin());
+  EXPECT_EQ(mutable_iter, const_b.begin());
+  EXPECT_NE(mutable_iter, const_b.end());
+  EXPECT_EQ(const_b.begin(), mutable_iter);
+  EXPECT_NE(const_b.end(), mutable_iter);
+  typename T::reverse_iterator mutable_riter(mutable_b.rbegin());
+  EXPECT_EQ(mutable_riter, const_b.rbegin());
+  EXPECT_NE(mutable_riter, const_b.rend());
+  EXPECT_EQ(const_b.rbegin(), mutable_riter);
+  EXPECT_NE(const_b.rend(), mutable_riter);
+
+  // We can create a const iterator from a non-const iterator.
+  typename T::const_iterator const_iter(mutable_iter);
+  EXPECT_EQ(const_iter, mutable_b.begin());
+  EXPECT_NE(const_iter, mutable_b.end());
+  EXPECT_EQ(mutable_b.begin(), const_iter);
+  EXPECT_NE(mutable_b.end(), const_iter);
+  typename T::const_reverse_iterator const_riter(mutable_riter);
+  EXPECT_EQ(const_riter, mutable_b.rbegin());
+  EXPECT_NE(const_riter, mutable_b.rend());
+  EXPECT_EQ(mutable_b.rbegin(), const_riter);
+  EXPECT_NE(mutable_b.rend(), const_riter);
+
+  // Make sure various methods can be invoked on a const container.
+  const_b.verify();
+  ASSERT_TRUE(!const_b.empty());
+  EXPECT_EQ(const_b.size(), 1);
+  EXPECT_GT(const_b.max_size(), 0);
+  EXPECT_TRUE(const_b.contains(key_of_value(value)));
+  EXPECT_EQ(const_b.count(key_of_value(value)), 1);
+}
+
+template <typename T, typename C>
+void BtreeTest() {
+  ConstTest<T>();
+
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  const std::vector<V> random_values = GenerateValuesWithSeed<V>(
+      absl::GetFlag(FLAGS_test_values), 4 * absl::GetFlag(FLAGS_test_values),
+      testing::GTEST_FLAG(random_seed));
+
+  unique_checker<T, C> container;
+
+  // Test key insertion/deletion in sorted order.
+  std::vector<V> sorted_values(random_values);
+  std::sort(sorted_values.begin(), sorted_values.end());
+  DoTest("sorted:    ", &container, sorted_values);
+
+  // Test key insertion/deletion in reverse sorted order.
+  std::reverse(sorted_values.begin(), sorted_values.end());
+  DoTest("rsorted:   ", &container, sorted_values);
+
+  // Test key insertion/deletion in random order.
+  DoTest("random:    ", &container, random_values);
+}
+
+template <typename T, typename C>
+void BtreeMultiTest() {
+  ConstTest<T>();
+
+  using V = typename remove_pair_const<typename T::value_type>::type;
+  const std::vector<V> random_values = GenerateValuesWithSeed<V>(
+      absl::GetFlag(FLAGS_test_values), 4 * absl::GetFlag(FLAGS_test_values),
+      testing::GTEST_FLAG(random_seed));
+
+  multi_checker<T, C> container;
+
+  // Test keys in sorted order.
+  std::vector<V> sorted_values(random_values);
+  std::sort(sorted_values.begin(), sorted_values.end());
+  DoTest("sorted:    ", &container, sorted_values);
+
+  // Test keys in reverse sorted order.
+  std::reverse(sorted_values.begin(), sorted_values.end());
+  DoTest("rsorted:   ", &container, sorted_values);
+
+  // Test keys in random order.
+  DoTest("random:    ", &container, random_values);
+
+  // Test keys in random order w/ duplicates.
+  std::vector<V> duplicate_values(random_values);
+  duplicate_values.insert(duplicate_values.end(), random_values.begin(),
+                          random_values.end());
+  DoTest("duplicates:", &container, duplicate_values);
+
+  // Test all identical keys.
+  std::vector<V> identical_values(100);
+  std::fill(identical_values.begin(), identical_values.end(),
+            Generator<V>(2)(2));
+  DoTest("identical: ", &container, identical_values);
+}
+
+template <typename T>
+struct PropagatingCountingAlloc : public CountingAllocator<T> {
+  using propagate_on_container_copy_assignment = std::true_type;
+  using propagate_on_container_move_assignment = std::true_type;
+  using propagate_on_container_swap = std::true_type;
+
+  using Base = CountingAllocator<T>;
+  using Base::Base;
+
+  template <typename U>
+  explicit PropagatingCountingAlloc(const PropagatingCountingAlloc<U> &other)
+      : Base(other.bytes_used_) {}
+
+  template <typename U>
+  struct rebind {
+    using other = PropagatingCountingAlloc<U>;
+  };
+};
+
+template <typename T>
+void BtreeAllocatorTest() {
+  using value_type = typename T::value_type;
+
+  int64_t bytes1 = 0, bytes2 = 0;
+  PropagatingCountingAlloc<T> allocator1(&bytes1);
+  PropagatingCountingAlloc<T> allocator2(&bytes2);
+  Generator<value_type> generator(1000);
+
+  // Test that we allocate properly aligned memory. If we don't, then Layout
+  // will assert fail.
+  auto unused1 = allocator1.allocate(1);
+  auto unused2 = allocator2.allocate(1);
+
+  // Test copy assignment
+  {
+    T b1(typename T::key_compare(), allocator1);
+    T b2(typename T::key_compare(), allocator2);
+
+    int64_t original_bytes1 = bytes1;
+    b1.insert(generator(0));
+    EXPECT_GT(bytes1, original_bytes1);
+
+    // This should propagate the allocator.
+    b1 = b2;
+    EXPECT_EQ(b1.size(), 0);
+    EXPECT_EQ(b2.size(), 0);
+    EXPECT_EQ(bytes1, original_bytes1);
+
+    for (int i = 1; i < 1000; i++) {
+      b1.insert(generator(i));
+    }
+
+    // We should have allocated out of allocator2.
+    EXPECT_GT(bytes2, bytes1);
+  }
+
+  // Test move assignment
+  {
+    T b1(typename T::key_compare(), allocator1);
+    T b2(typename T::key_compare(), allocator2);
+
+    int64_t original_bytes1 = bytes1;
+    b1.insert(generator(0));
+    EXPECT_GT(bytes1, original_bytes1);
+
+    // This should propagate the allocator.
+    b1 = std::move(b2);
+    EXPECT_EQ(b1.size(), 0);
+    EXPECT_EQ(bytes1, original_bytes1);
+
+    for (int i = 1; i < 1000; i++) {
+      b1.insert(generator(i));
+    }
+
+    // We should have allocated out of allocator2.
+    EXPECT_GT(bytes2, bytes1);
+  }
+
+  // Test swap
+  {
+    T b1(typename T::key_compare(), allocator1);
+    T b2(typename T::key_compare(), allocator2);
+
+    int64_t original_bytes1 = bytes1;
+    b1.insert(generator(0));
+    EXPECT_GT(bytes1, original_bytes1);
+
+    // This should swap the allocators.
+    swap(b1, b2);
+    EXPECT_EQ(b1.size(), 0);
+    EXPECT_EQ(b2.size(), 1);
+    EXPECT_GT(bytes1, original_bytes1);
+
+    for (int i = 1; i < 1000; i++) {
+      b1.insert(generator(i));
+    }
+
+    // We should have allocated out of allocator2.
+    EXPECT_GT(bytes2, bytes1);
+  }
+
+  allocator1.deallocate(unused1, 1);
+  allocator2.deallocate(unused2, 1);
+}
+
+template <typename T>
+void BtreeMapTest() {
+  using value_type = typename T::value_type;
+  using mapped_type = typename T::mapped_type;
+
+  mapped_type m = Generator<mapped_type>(0)(0);
+  (void)m;
+
+  T b;
+
+  // Verify we can insert using operator[].
+  for (int i = 0; i < 1000; i++) {
+    value_type v = Generator<value_type>(1000)(i);
+    b[v.first] = v.second;
+  }
+  EXPECT_EQ(b.size(), 1000);
+
+  // Test whether we can use the "->" operator on iterators and
+  // reverse_iterators. This stresses the btree_map_params::pair_pointer
+  // mechanism.
+  EXPECT_EQ(b.begin()->first, Generator<value_type>(1000)(0).first);
+  EXPECT_EQ(b.begin()->second, Generator<value_type>(1000)(0).second);
+  EXPECT_EQ(b.rbegin()->first, Generator<value_type>(1000)(999).first);
+  EXPECT_EQ(b.rbegin()->second, Generator<value_type>(1000)(999).second);
+}
+
+template <typename T>
+void BtreeMultiMapTest() {
+  using mapped_type = typename T::mapped_type;
+  mapped_type m = Generator<mapped_type>(0)(0);
+  (void)m;
+}
+
+template <typename K, int N = 256>
+void SetTest() {
+  EXPECT_EQ(
+      sizeof(absl::btree_set<K>),
+      2 * sizeof(void *) + sizeof(typename absl::btree_set<K>::size_type));
+  using BtreeSet = absl::btree_set<K>;
+  using CountingBtreeSet =
+      absl::btree_set<K, std::less<K>, PropagatingCountingAlloc<K>>;
+  BtreeTest<BtreeSet, std::set<K>>();
+  BtreeAllocatorTest<CountingBtreeSet>();
+}
+
+template <typename K, int N = 256>
+void MapTest() {
+  EXPECT_EQ(
+      sizeof(absl::btree_map<K, K>),
+      2 * sizeof(void *) + sizeof(typename absl::btree_map<K, K>::size_type));
+  using BtreeMap = absl::btree_map<K, K>;
+  using CountingBtreeMap =
+      absl::btree_map<K, K, std::less<K>,
+                      PropagatingCountingAlloc<std::pair<const K, K>>>;
+  BtreeTest<BtreeMap, std::map<K, K>>();
+  BtreeAllocatorTest<CountingBtreeMap>();
+  BtreeMapTest<BtreeMap>();
+}
+
+TEST(Btree, set_int32) { SetTest<int32_t>(); }
+TEST(Btree, set_int64) { SetTest<int64_t>(); }
+TEST(Btree, set_string) { SetTest<std::string>(); }
+TEST(Btree, set_pair) { SetTest<std::pair<int, int>>(); }
+TEST(Btree, map_int32) { MapTest<int32_t>(); }
+TEST(Btree, map_int64) { MapTest<int64_t>(); }
+TEST(Btree, map_string) { MapTest<std::string>(); }
+TEST(Btree, map_pair) { MapTest<std::pair<int, int>>(); }
+
+template <typename K, int N = 256>
+void MultiSetTest() {
+  EXPECT_EQ(
+      sizeof(absl::btree_multiset<K>),
+      2 * sizeof(void *) + sizeof(typename absl::btree_multiset<K>::size_type));
+  using BtreeMSet = absl::btree_multiset<K>;
+  using CountingBtreeMSet =
+      absl::btree_multiset<K, std::less<K>, PropagatingCountingAlloc<K>>;
+  BtreeMultiTest<BtreeMSet, std::multiset<K>>();
+  BtreeAllocatorTest<CountingBtreeMSet>();
+}
+
+template <typename K, int N = 256>
+void MultiMapTest() {
+  EXPECT_EQ(sizeof(absl::btree_multimap<K, K>),
+            2 * sizeof(void *) +
+                sizeof(typename absl::btree_multimap<K, K>::size_type));
+  using BtreeMMap = absl::btree_multimap<K, K>;
+  using CountingBtreeMMap =
+      absl::btree_multimap<K, K, std::less<K>,
+                           PropagatingCountingAlloc<std::pair<const K, K>>>;
+  BtreeMultiTest<BtreeMMap, std::multimap<K, K>>();
+  BtreeMultiMapTest<BtreeMMap>();
+  BtreeAllocatorTest<CountingBtreeMMap>();
+}
+
+TEST(Btree, multiset_int32) { MultiSetTest<int32_t>(); }
+TEST(Btree, multiset_int64) { MultiSetTest<int64_t>(); }
+TEST(Btree, multiset_string) { MultiSetTest<std::string>(); }
+TEST(Btree, multiset_pair) { MultiSetTest<std::pair<int, int>>(); }
+TEST(Btree, multimap_int32) { MultiMapTest<int32_t>(); }
+TEST(Btree, multimap_int64) { MultiMapTest<int64_t>(); }
+TEST(Btree, multimap_string) { MultiMapTest<std::string>(); }
+TEST(Btree, multimap_pair) { MultiMapTest<std::pair<int, int>>(); }
+
+struct CompareIntToString {
+  bool operator()(const std::string &a, const std::string &b) const {
+    return a < b;
+  }
+  bool operator()(const std::string &a, int b) const {
+    return a < absl::StrCat(b);
+  }
+  bool operator()(int a, const std::string &b) const {
+    return absl::StrCat(a) < b;
+  }
+  using is_transparent = void;
+};
+
+struct NonTransparentCompare {
+  template <typename T, typename U>
+  bool operator()(const T &t, const U &u) const {
+    // Treating all comparators as transparent can cause inefficiencies (see
+    // N3657 C++ proposal). Test that for comparators without 'is_transparent'
+    // alias (like this one), we do not attempt heterogeneous lookup.
+    EXPECT_TRUE((std::is_same<T, U>()));
+    return t < u;
+  }
+};
+
+template <typename T>
+bool CanEraseWithEmptyBrace(T t, decltype(t.erase({})) *) {
+  return true;
+}
+
+template <typename T>
+bool CanEraseWithEmptyBrace(T, ...) {
+  return false;
+}
+
+template <typename T>
+void TestHeterogeneous(T table) {
+  auto lb = table.lower_bound("3");
+  EXPECT_EQ(lb, table.lower_bound(3));
+  EXPECT_NE(lb, table.lower_bound(4));
+  EXPECT_EQ(lb, table.lower_bound({"3"}));
+  EXPECT_NE(lb, table.lower_bound({}));
+
+  auto ub = table.upper_bound("3");
+  EXPECT_EQ(ub, table.upper_bound(3));
+  EXPECT_NE(ub, table.upper_bound(5));
+  EXPECT_EQ(ub, table.upper_bound({"3"}));
+  EXPECT_NE(ub, table.upper_bound({}));
+
+  auto er = table.equal_range("3");
+  EXPECT_EQ(er, table.equal_range(3));
+  EXPECT_NE(er, table.equal_range(4));
+  EXPECT_EQ(er, table.equal_range({"3"}));
+  EXPECT_NE(er, table.equal_range({}));
+
+  auto it = table.find("3");
+  EXPECT_EQ(it, table.find(3));
+  EXPECT_NE(it, table.find(4));
+  EXPECT_EQ(it, table.find({"3"}));
+  EXPECT_NE(it, table.find({}));
+
+  EXPECT_TRUE(table.contains(3));
+  EXPECT_FALSE(table.contains(4));
+  EXPECT_TRUE(table.count({"3"}));
+  EXPECT_FALSE(table.contains({}));
+
+  EXPECT_EQ(1, table.count(3));
+  EXPECT_EQ(0, table.count(4));
+  EXPECT_EQ(1, table.count({"3"}));
+  EXPECT_EQ(0, table.count({}));
+
+  auto copy = table;
+  copy.erase(3);
+  EXPECT_EQ(table.size() - 1, copy.size());
+  copy.erase(4);
+  EXPECT_EQ(table.size() - 1, copy.size());
+  copy.erase({"5"});
+  EXPECT_EQ(table.size() - 2, copy.size());
+  EXPECT_FALSE(CanEraseWithEmptyBrace(table, nullptr));
+
+  // Also run it with const T&.
+  if (std::is_class<T>()) TestHeterogeneous<const T &>(table);
+}
+
+TEST(Btree, HeterogeneousLookup) {
+  TestHeterogeneous(btree_set<std::string, CompareIntToString>{"1", "3", "5"});
+  TestHeterogeneous(btree_map<std::string, int, CompareIntToString>{
+      {"1", 1}, {"3", 3}, {"5", 5}});
+  TestHeterogeneous(
+      btree_multiset<std::string, CompareIntToString>{"1", "3", "5"});
+  TestHeterogeneous(btree_multimap<std::string, int, CompareIntToString>{
+      {"1", 1}, {"3", 3}, {"5", 5}});
+
+  // Only maps have .at()
+  btree_map<std::string, int, CompareIntToString> map{
+      {"", -1}, {"1", 1}, {"3", 3}, {"5", 5}};
+  EXPECT_EQ(1, map.at(1));
+  EXPECT_EQ(3, map.at({"3"}));
+  EXPECT_EQ(-1, map.at({}));
+  const auto &cmap = map;
+  EXPECT_EQ(1, cmap.at(1));
+  EXPECT_EQ(3, cmap.at({"3"}));
+  EXPECT_EQ(-1, cmap.at({}));
+}
+
+TEST(Btree, NoHeterogeneousLookupWithoutAlias) {
+  using StringSet = absl::btree_set<std::string, NonTransparentCompare>;
+  StringSet s;
+  ASSERT_TRUE(s.insert("hello").second);
+  ASSERT_TRUE(s.insert("world").second);
+  EXPECT_TRUE(s.end() == s.find("blah"));
+  EXPECT_TRUE(s.begin() == s.lower_bound("hello"));
+  EXPECT_EQ(1, s.count("world"));
+  EXPECT_TRUE(s.contains("hello"));
+  EXPECT_TRUE(s.contains("world"));
+  EXPECT_FALSE(s.contains("blah"));
+
+  using StringMultiSet =
+      absl::btree_multiset<std::string, NonTransparentCompare>;
+  StringMultiSet ms;
+  ms.insert("hello");
+  ms.insert("world");
+  ms.insert("world");
+  EXPECT_TRUE(ms.end() == ms.find("blah"));
+  EXPECT_TRUE(ms.begin() == ms.lower_bound("hello"));
+  EXPECT_EQ(2, ms.count("world"));
+  EXPECT_TRUE(ms.contains("hello"));
+  EXPECT_TRUE(ms.contains("world"));
+  EXPECT_FALSE(ms.contains("blah"));
+}
+
+TEST(Btree, DefaultTransparent) {
+  {
+    // `int` does not have a default transparent comparator.
+    // The input value is converted to key_type.
+    btree_set<int> s = {1};
+    double d = 1.1;
+    EXPECT_EQ(s.begin(), s.find(d));
+    EXPECT_TRUE(s.contains(d));
+  }
+
+  {
+    // `std::string` has heterogeneous support.
+    btree_set<std::string> s = {"A"};
+    EXPECT_EQ(s.begin(), s.find(absl::string_view("A")));
+    EXPECT_TRUE(s.contains(absl::string_view("A")));
+  }
+}
+
+class StringLike {
+ public:
+  StringLike() = default;
+
+  StringLike(const char *s) : s_(s) {  // NOLINT
+    ++constructor_calls_;
+  }
+
+  bool operator<(const StringLike &a) const { return s_ < a.s_; }
+
+  static void clear_constructor_call_count() { constructor_calls_ = 0; }
+
+  static int constructor_calls() { return constructor_calls_; }
+
+ private:
+  static int constructor_calls_;
+  std::string s_;
+};
+
+int StringLike::constructor_calls_ = 0;
+
+TEST(Btree, HeterogeneousLookupDoesntDegradePerformance) {
+  using StringSet = absl::btree_set<StringLike>;
+  StringSet s;
+  for (int i = 0; i < 100; ++i) {
+    ASSERT_TRUE(s.insert(absl::StrCat(i).c_str()).second);
+  }
+  StringLike::clear_constructor_call_count();
+  s.find("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.contains("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.count("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.lower_bound("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.upper_bound("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.equal_range("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+
+  StringLike::clear_constructor_call_count();
+  s.erase("50");
+  ASSERT_EQ(1, StringLike::constructor_calls());
+}
+
+// Verify that swapping btrees swaps the key comparison functors and that we can
+// use non-default constructible comparators.
+struct SubstringLess {
+  SubstringLess() = delete;
+  explicit SubstringLess(int length) : n(length) {}
+  bool operator()(const std::string &a, const std::string &b) const {
+    return absl::string_view(a).substr(0, n) <
+           absl::string_view(b).substr(0, n);
+  }
+  int n;
+};
+
+TEST(Btree, SwapKeyCompare) {
+  using SubstringSet = absl::btree_set<std::string, SubstringLess>;
+  SubstringSet s1(SubstringLess(1), SubstringSet::allocator_type());
+  SubstringSet s2(SubstringLess(2), SubstringSet::allocator_type());
+
+  ASSERT_TRUE(s1.insert("a").second);
+  ASSERT_FALSE(s1.insert("aa").second);
+
+  ASSERT_TRUE(s2.insert("a").second);
+  ASSERT_TRUE(s2.insert("aa").second);
+  ASSERT_FALSE(s2.insert("aaa").second);
+
+  swap(s1, s2);
+
+  ASSERT_TRUE(s1.insert("b").second);
+  ASSERT_TRUE(s1.insert("bb").second);
+  ASSERT_FALSE(s1.insert("bbb").second);
+
+  ASSERT_TRUE(s2.insert("b").second);
+  ASSERT_FALSE(s2.insert("bb").second);
+}
+
+TEST(Btree, UpperBoundRegression) {
+  // Regress a bug where upper_bound would default-construct a new key_compare
+  // instead of copying the existing one.
+  using SubstringSet = absl::btree_set<std::string, SubstringLess>;
+  SubstringSet my_set(SubstringLess(3));
+  my_set.insert("aab");
+  my_set.insert("abb");
+  // We call upper_bound("aaa").  If this correctly uses the length 3
+  // comparator, aaa < aab < abb, so we should get aab as the result.
+  // If it instead uses the default-constructed length 2 comparator,
+  // aa == aa < ab, so we'll get abb as our result.
+  SubstringSet::iterator it = my_set.upper_bound("aaa");
+  ASSERT_TRUE(it != my_set.end());
+  EXPECT_EQ("aab", *it);
+}
+
+TEST(Btree, Comparison) {
+  const int kSetSize = 1201;
+  absl::btree_set<int64_t> my_set;
+  for (int i = 0; i < kSetSize; ++i) {
+    my_set.insert(i);
+  }
+  absl::btree_set<int64_t> my_set_copy(my_set);
+  EXPECT_TRUE(my_set_copy == my_set);
+  EXPECT_TRUE(my_set == my_set_copy);
+  EXPECT_FALSE(my_set_copy != my_set);
+  EXPECT_FALSE(my_set != my_set_copy);
+
+  my_set.insert(kSetSize);
+  EXPECT_FALSE(my_set_copy == my_set);
+  EXPECT_FALSE(my_set == my_set_copy);
+  EXPECT_TRUE(my_set_copy != my_set);
+  EXPECT_TRUE(my_set != my_set_copy);
+
+  my_set.erase(kSetSize - 1);
+  EXPECT_FALSE(my_set_copy == my_set);
+  EXPECT_FALSE(my_set == my_set_copy);
+  EXPECT_TRUE(my_set_copy != my_set);
+  EXPECT_TRUE(my_set != my_set_copy);
+
+  absl::btree_map<std::string, int64_t> my_map;
+  for (int i = 0; i < kSetSize; ++i) {
+    my_map[std::string(i, 'a')] = i;
+  }
+  absl::btree_map<std::string, int64_t> my_map_copy(my_map);
+  EXPECT_TRUE(my_map_copy == my_map);
+  EXPECT_TRUE(my_map == my_map_copy);
+  EXPECT_FALSE(my_map_copy != my_map);
+  EXPECT_FALSE(my_map != my_map_copy);
+
+  ++my_map_copy[std::string(7, 'a')];
+  EXPECT_FALSE(my_map_copy == my_map);
+  EXPECT_FALSE(my_map == my_map_copy);
+  EXPECT_TRUE(my_map_copy != my_map);
+  EXPECT_TRUE(my_map != my_map_copy);
+
+  my_map_copy = my_map;
+  my_map["hello"] = kSetSize;
+  EXPECT_FALSE(my_map_copy == my_map);
+  EXPECT_FALSE(my_map == my_map_copy);
+  EXPECT_TRUE(my_map_copy != my_map);
+  EXPECT_TRUE(my_map != my_map_copy);
+
+  my_map.erase(std::string(kSetSize - 1, 'a'));
+  EXPECT_FALSE(my_map_copy == my_map);
+  EXPECT_FALSE(my_map == my_map_copy);
+  EXPECT_TRUE(my_map_copy != my_map);
+  EXPECT_TRUE(my_map != my_map_copy);
+}
+
+TEST(Btree, RangeCtorSanity) {
+  std::vector<int> ivec;
+  ivec.push_back(1);
+  std::map<int, int> imap;
+  imap.insert(std::make_pair(1, 2));
+  absl::btree_multiset<int> tmset(ivec.begin(), ivec.end());
+  absl::btree_multimap<int, int> tmmap(imap.begin(), imap.end());
+  absl::btree_set<int> tset(ivec.begin(), ivec.end());
+  absl::btree_map<int, int> tmap(imap.begin(), imap.end());
+  EXPECT_EQ(1, tmset.size());
+  EXPECT_EQ(1, tmmap.size());
+  EXPECT_EQ(1, tset.size());
+  EXPECT_EQ(1, tmap.size());
+}
+
+TEST(Btree, BtreeMapCanHoldMoveOnlyTypes) {
+  absl::btree_map<std::string, std::unique_ptr<std::string>> m;
+
+  std::unique_ptr<std::string> &v = m["A"];
+  EXPECT_TRUE(v == nullptr);
+  v.reset(new std::string("X"));
+
+  auto iter = m.find("A");
+  EXPECT_EQ("X", *iter->second);
+}
+
+TEST(Btree, InitializerListConstructor) {
+  absl::btree_set<std::string> set({"a", "b"});
+  EXPECT_EQ(set.count("a"), 1);
+  EXPECT_EQ(set.count("b"), 1);
+
+  absl::btree_multiset<int> mset({1, 1, 4});
+  EXPECT_EQ(mset.count(1), 2);
+  EXPECT_EQ(mset.count(4), 1);
+
+  absl::btree_map<int, int> map({{1, 5}, {2, 10}});
+  EXPECT_EQ(map[1], 5);
+  EXPECT_EQ(map[2], 10);
+
+  absl::btree_multimap<int, int> mmap({{1, 5}, {1, 10}});
+  auto range = mmap.equal_range(1);
+  auto it = range.first;
+  ASSERT_NE(it, range.second);
+  EXPECT_EQ(it->second, 5);
+  ASSERT_NE(++it, range.second);
+  EXPECT_EQ(it->second, 10);
+  EXPECT_EQ(++it, range.second);
+}
+
+TEST(Btree, InitializerListInsert) {
+  absl::btree_set<std::string> set;
+  set.insert({"a", "b"});
+  EXPECT_EQ(set.count("a"), 1);
+  EXPECT_EQ(set.count("b"), 1);
+
+  absl::btree_multiset<int> mset;
+  mset.insert({1, 1, 4});
+  EXPECT_EQ(mset.count(1), 2);
+  EXPECT_EQ(mset.count(4), 1);
+
+  absl::btree_map<int, int> map;
+  map.insert({{1, 5}, {2, 10}});
+  // Test that inserting one element using an initializer list also works.
+  map.insert({3, 15});
+  EXPECT_EQ(map[1], 5);
+  EXPECT_EQ(map[2], 10);
+  EXPECT_EQ(map[3], 15);
+
+  absl::btree_multimap<int, int> mmap;
+  mmap.insert({{1, 5}, {1, 10}});
+  auto range = mmap.equal_range(1);
+  auto it = range.first;
+  ASSERT_NE(it, range.second);
+  EXPECT_EQ(it->second, 5);
+  ASSERT_NE(++it, range.second);
+  EXPECT_EQ(it->second, 10);
+  EXPECT_EQ(++it, range.second);
+}
+
+template <typename Compare, typename K>
+void AssertKeyCompareToAdapted() {
+  using Adapted = typename key_compare_to_adapter<Compare>::type;
+  static_assert(!std::is_same<Adapted, Compare>::value,
+                "key_compare_to_adapter should have adapted this comparator.");
+  static_assert(
+      std::is_same<absl::weak_ordering,
+                   absl::result_of_t<Adapted(const K &, const K &)>>::value,
+      "Adapted comparator should be a key-compare-to comparator.");
+}
+template <typename Compare, typename K>
+void AssertKeyCompareToNotAdapted() {
+  using Unadapted = typename key_compare_to_adapter<Compare>::type;
+  static_assert(
+      std::is_same<Unadapted, Compare>::value,
+      "key_compare_to_adapter shouldn't have adapted this comparator.");
+  static_assert(
+      std::is_same<bool,
+                   absl::result_of_t<Unadapted(const K &, const K &)>>::value,
+      "Un-adapted comparator should return bool.");
+}
+
+TEST(Btree, KeyCompareToAdapter) {
+  AssertKeyCompareToAdapted<std::less<std::string>, std::string>();
+  AssertKeyCompareToAdapted<std::greater<std::string>, std::string>();
+  AssertKeyCompareToAdapted<std::less<absl::string_view>, absl::string_view>();
+  AssertKeyCompareToAdapted<std::greater<absl::string_view>,
+                            absl::string_view>();
+  AssertKeyCompareToNotAdapted<std::less<int>, int>();
+  AssertKeyCompareToNotAdapted<std::greater<int>, int>();
+}
+
+TEST(Btree, RValueInsert) {
+  InstanceTracker tracker;
+
+  absl::btree_set<MovableOnlyInstance> set;
+  set.insert(MovableOnlyInstance(1));
+  set.insert(MovableOnlyInstance(3));
+  MovableOnlyInstance two(2);
+  set.insert(set.find(MovableOnlyInstance(3)), std::move(two));
+  auto it = set.find(MovableOnlyInstance(2));
+  ASSERT_NE(it, set.end());
+  ASSERT_NE(++it, set.end());
+  EXPECT_EQ(it->value(), 3);
+
+  absl::btree_multiset<MovableOnlyInstance> mset;
+  MovableOnlyInstance zero(0);
+  MovableOnlyInstance zero2(0);
+  mset.insert(std::move(zero));
+  mset.insert(mset.find(MovableOnlyInstance(0)), std::move(zero2));
+  EXPECT_EQ(mset.count(MovableOnlyInstance(0)), 2);
+
+  absl::btree_map<int, MovableOnlyInstance> map;
+  std::pair<const int, MovableOnlyInstance> p1 = {1, MovableOnlyInstance(5)};
+  std::pair<const int, MovableOnlyInstance> p2 = {2, MovableOnlyInstance(10)};
+  std::pair<const int, MovableOnlyInstance> p3 = {3, MovableOnlyInstance(15)};
+  map.insert(std::move(p1));
+  map.insert(std::move(p3));
+  map.insert(map.find(3), std::move(p2));
+  ASSERT_NE(map.find(2), map.end());
+  EXPECT_EQ(map.find(2)->second.value(), 10);
+
+  absl::btree_multimap<int, MovableOnlyInstance> mmap;
+  std::pair<const int, MovableOnlyInstance> p4 = {1, MovableOnlyInstance(5)};
+  std::pair<const int, MovableOnlyInstance> p5 = {1, MovableOnlyInstance(10)};
+  mmap.insert(std::move(p4));
+  mmap.insert(mmap.find(1), std::move(p5));
+  auto range = mmap.equal_range(1);
+  auto it1 = range.first;
+  ASSERT_NE(it1, range.second);
+  EXPECT_EQ(it1->second.value(), 10);
+  ASSERT_NE(++it1, range.second);
+  EXPECT_EQ(it1->second.value(), 5);
+  EXPECT_EQ(++it1, range.second);
+
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.swaps(), 0);
+}
+
+}  // namespace
+
+class BtreeNodePeer {
+ public:
+  // Yields the size of a leaf node with a specific number of values.
+  template <typename ValueType>
+  constexpr static size_t GetTargetNodeSize(size_t target_values_per_node) {
+    return btree_node<
+        set_params<ValueType, std::less<ValueType>, std::allocator<ValueType>,
+                   /*TargetNodeSize=*/256,  // This parameter isn't used here.
+                   /*Multi=*/false>>::SizeWithNValues(target_values_per_node);
+  }
+
+  // Yields the number of values in a (non-root) leaf node for this set.
+  template <typename Set>
+  constexpr static size_t GetNumValuesPerNode() {
+    return btree_node<typename Set::params_type>::kNodeValues;
+  }
+};
+
+namespace {
+
+// A btree set with a specific number of values per node.
+template <typename Key, int TargetValuesPerNode, typename Cmp = std::less<Key>>
+class SizedBtreeSet
+    : public btree_set_container<btree<
+          set_params<Key, Cmp, std::allocator<Key>,
+                     BtreeNodePeer::GetTargetNodeSize<Key>(TargetValuesPerNode),
+                     /*Multi=*/false>>> {
+  using Base = typename SizedBtreeSet::btree_set_container;
+
+ public:
+  SizedBtreeSet() {}
+  using Base::Base;
+};
+
+template <typename Set>
+void ExpectOperationCounts(const int expected_moves,
+                           const int expected_comparisons,
+                           const std::vector<int> &values,
+                           InstanceTracker *tracker, Set *set) {
+  for (const int v : values) set->insert(MovableOnlyInstance(v));
+  set->clear();
+  EXPECT_EQ(tracker->moves(), expected_moves);
+  EXPECT_EQ(tracker->comparisons(), expected_comparisons);
+  EXPECT_EQ(tracker->copies(), 0);
+  EXPECT_EQ(tracker->swaps(), 0);
+  tracker->ResetCopiesMovesSwaps();
+}
+
+// Note: when the values in this test change, it is expected to have an impact
+// on performance.
+TEST(Btree, MovesComparisonsCopiesSwapsTracking) {
+  InstanceTracker tracker;
+  // Note: this is minimum number of values per node.
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/3> set3;
+  // Note: this is the default number of values per node for a set of int32s
+  // (with 64-bit pointers).
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/61> set61;
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/100> set100;
+
+  // Don't depend on flags for random values because then the expectations will
+  // fail if the flags change.
+  std::vector<int> values =
+      GenerateValuesWithSeed<int>(10000, 1 << 22, /*seed=*/23);
+
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set3)>(), 3);
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>(), 61);
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set100)>(), 100);
+  if (sizeof(void *) == 8) {
+    EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<absl::btree_set<int32_t>>(),
+              BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>());
+  }
+
+  // Test key insertion/deletion in random order.
+  ExpectOperationCounts(45281, 132551, values, &tracker, &set3);
+  ExpectOperationCounts(386718, 129807, values, &tracker, &set61);
+  ExpectOperationCounts(586761, 130310, values, &tracker, &set100);
+
+  // Test key insertion/deletion in sorted order.
+  std::sort(values.begin(), values.end());
+  ExpectOperationCounts(26638, 92134, values, &tracker, &set3);
+  ExpectOperationCounts(20208, 87757, values, &tracker, &set61);
+  ExpectOperationCounts(20124, 96583, values, &tracker, &set100);
+
+  // Test key insertion/deletion in reverse sorted order.
+  std::reverse(values.begin(), values.end());
+  ExpectOperationCounts(49951, 119325, values, &tracker, &set3);
+  ExpectOperationCounts(338813, 118266, values, &tracker, &set61);
+  ExpectOperationCounts(534529, 125279, values, &tracker, &set100);
+}
+
+struct MovableOnlyInstanceThreeWayCompare {
+  absl::weak_ordering operator()(const MovableOnlyInstance &a,
+                                 const MovableOnlyInstance &b) const {
+    return a.compare(b);
+  }
+};
+
+// Note: when the values in this test change, it is expected to have an impact
+// on performance.
+TEST(Btree, MovesComparisonsCopiesSwapsTrackingThreeWayCompare) {
+  InstanceTracker tracker;
+  // Note: this is minimum number of values per node.
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/3,
+                MovableOnlyInstanceThreeWayCompare>
+      set3;
+  // Note: this is the default number of values per node for a set of int32s
+  // (with 64-bit pointers).
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/61,
+                MovableOnlyInstanceThreeWayCompare>
+      set61;
+  SizedBtreeSet<MovableOnlyInstance, /*TargetValuesPerNode=*/100,
+                MovableOnlyInstanceThreeWayCompare>
+      set100;
+
+  // Don't depend on flags for random values because then the expectations will
+  // fail if the flags change.
+  std::vector<int> values =
+      GenerateValuesWithSeed<int>(10000, 1 << 22, /*seed=*/23);
+
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set3)>(), 3);
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>(), 61);
+  EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<decltype(set100)>(), 100);
+  if (sizeof(void *) == 8) {
+    EXPECT_EQ(BtreeNodePeer::GetNumValuesPerNode<absl::btree_set<int32_t>>(),
+              BtreeNodePeer::GetNumValuesPerNode<decltype(set61)>());
+  }
+
+  // Test key insertion/deletion in random order.
+  ExpectOperationCounts(45281, 122560, values, &tracker, &set3);
+  ExpectOperationCounts(386718, 119816, values, &tracker, &set61);
+  ExpectOperationCounts(586761, 120319, values, &tracker, &set100);
+
+  // Test key insertion/deletion in sorted order.
+  std::sort(values.begin(), values.end());
+  ExpectOperationCounts(26638, 92134, values, &tracker, &set3);
+  ExpectOperationCounts(20208, 87757, values, &tracker, &set61);
+  ExpectOperationCounts(20124, 96583, values, &tracker, &set100);
+
+  // Test key insertion/deletion in reverse sorted order.
+  std::reverse(values.begin(), values.end());
+  ExpectOperationCounts(49951, 109326, values, &tracker, &set3);
+  ExpectOperationCounts(338813, 108267, values, &tracker, &set61);
+  ExpectOperationCounts(534529, 115280, values, &tracker, &set100);
+}
+
+struct NoDefaultCtor {
+  int num;
+  explicit NoDefaultCtor(int i) : num(i) {}
+
+  friend bool operator<(const NoDefaultCtor &a, const NoDefaultCtor &b) {
+    return a.num < b.num;
+  }
+};
+
+TEST(Btree, BtreeMapCanHoldNoDefaultCtorTypes) {
+  absl::btree_map<NoDefaultCtor, NoDefaultCtor> m;
+
+  for (int i = 1; i <= 99; ++i) {
+    SCOPED_TRACE(i);
+    EXPECT_TRUE(m.emplace(NoDefaultCtor(i), NoDefaultCtor(100 - i)).second);
+  }
+  EXPECT_FALSE(m.emplace(NoDefaultCtor(78), NoDefaultCtor(0)).second);
+
+  auto iter99 = m.find(NoDefaultCtor(99));
+  ASSERT_NE(iter99, m.end());
+  EXPECT_EQ(iter99->second.num, 1);
+
+  auto iter1 = m.find(NoDefaultCtor(1));
+  ASSERT_NE(iter1, m.end());
+  EXPECT_EQ(iter1->second.num, 99);
+
+  auto iter50 = m.find(NoDefaultCtor(50));
+  ASSERT_NE(iter50, m.end());
+  EXPECT_EQ(iter50->second.num, 50);
+
+  auto iter25 = m.find(NoDefaultCtor(25));
+  ASSERT_NE(iter25, m.end());
+  EXPECT_EQ(iter25->second.num, 75);
+}
+
+TEST(Btree, BtreeMultimapCanHoldNoDefaultCtorTypes) {
+  absl::btree_multimap<NoDefaultCtor, NoDefaultCtor> m;
+
+  for (int i = 1; i <= 99; ++i) {
+    SCOPED_TRACE(i);
+    m.emplace(NoDefaultCtor(i), NoDefaultCtor(100 - i));
+  }
+
+  auto iter99 = m.find(NoDefaultCtor(99));
+  ASSERT_NE(iter99, m.end());
+  EXPECT_EQ(iter99->second.num, 1);
+
+  auto iter1 = m.find(NoDefaultCtor(1));
+  ASSERT_NE(iter1, m.end());
+  EXPECT_EQ(iter1->second.num, 99);
+
+  auto iter50 = m.find(NoDefaultCtor(50));
+  ASSERT_NE(iter50, m.end());
+  EXPECT_EQ(iter50->second.num, 50);
+
+  auto iter25 = m.find(NoDefaultCtor(25));
+  ASSERT_NE(iter25, m.end());
+  EXPECT_EQ(iter25->second.num, 75);
+}
+
+TEST(Btree, MapAt) {
+  absl::btree_map<int, int> map = {{1, 2}, {2, 4}};
+  EXPECT_EQ(map.at(1), 2);
+  EXPECT_EQ(map.at(2), 4);
+  map.at(2) = 8;
+  const absl::btree_map<int, int> &const_map = map;
+  EXPECT_EQ(const_map.at(1), 2);
+  EXPECT_EQ(const_map.at(2), 8);
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(map.at(3), std::out_of_range);
+#else
+  EXPECT_DEATH(map.at(3), "absl::btree_map::at");
+#endif
+}
+
+TEST(Btree, BtreeMultisetEmplace) {
+  const int value_to_insert = 123456;
+  absl::btree_multiset<int> s;
+  auto iter = s.emplace(value_to_insert);
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(*iter, value_to_insert);
+  auto iter2 = s.emplace(value_to_insert);
+  EXPECT_NE(iter2, iter);
+  ASSERT_NE(iter2, s.end());
+  EXPECT_EQ(*iter2, value_to_insert);
+  auto result = s.equal_range(value_to_insert);
+  EXPECT_EQ(std::distance(result.first, result.second), 2);
+}
+
+TEST(Btree, BtreeMultisetEmplaceHint) {
+  const int value_to_insert = 123456;
+  absl::btree_multiset<int> s;
+  auto iter = s.emplace(value_to_insert);
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(*iter, value_to_insert);
+  auto emplace_iter = s.emplace_hint(iter, value_to_insert);
+  EXPECT_NE(emplace_iter, iter);
+  ASSERT_NE(emplace_iter, s.end());
+  EXPECT_EQ(*emplace_iter, value_to_insert);
+}
+
+TEST(Btree, BtreeMultimapEmplace) {
+  const int key_to_insert = 123456;
+  const char value0[] = "a";
+  absl::btree_multimap<int, std::string> s;
+  auto iter = s.emplace(key_to_insert, value0);
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(iter->first, key_to_insert);
+  EXPECT_EQ(iter->second, value0);
+  const char value1[] = "b";
+  auto iter2 = s.emplace(key_to_insert, value1);
+  EXPECT_NE(iter2, iter);
+  ASSERT_NE(iter2, s.end());
+  EXPECT_EQ(iter2->first, key_to_insert);
+  EXPECT_EQ(iter2->second, value1);
+  auto result = s.equal_range(key_to_insert);
+  EXPECT_EQ(std::distance(result.first, result.second), 2);
+}
+
+TEST(Btree, BtreeMultimapEmplaceHint) {
+  const int key_to_insert = 123456;
+  const char value0[] = "a";
+  absl::btree_multimap<int, std::string> s;
+  auto iter = s.emplace(key_to_insert, value0);
+  ASSERT_NE(iter, s.end());
+  EXPECT_EQ(iter->first, key_to_insert);
+  EXPECT_EQ(iter->second, value0);
+  const char value1[] = "b";
+  auto emplace_iter = s.emplace_hint(iter, key_to_insert, value1);
+  EXPECT_NE(emplace_iter, iter);
+  ASSERT_NE(emplace_iter, s.end());
+  EXPECT_EQ(emplace_iter->first, key_to_insert);
+  EXPECT_EQ(emplace_iter->second, value1);
+}
+
+TEST(Btree, ConstIteratorAccessors) {
+  absl::btree_set<int> set;
+  for (int i = 0; i < 100; ++i) {
+    set.insert(i);
+  }
+
+  auto it = set.cbegin();
+  auto r_it = set.crbegin();
+  for (int i = 0; i < 100; ++i, ++it, ++r_it) {
+    ASSERT_EQ(*it, i);
+    ASSERT_EQ(*r_it, 99 - i);
+  }
+  EXPECT_EQ(it, set.cend());
+  EXPECT_EQ(r_it, set.crend());
+}
+
+TEST(Btree, StrSplitCompatible) {
+  const absl::btree_set<std::string> split_set = absl::StrSplit("a,b,c", ',');
+  const absl::btree_set<std::string> expected_set = {"a", "b", "c"};
+
+  EXPECT_EQ(split_set, expected_set);
+}
+
+// We can't use EXPECT_EQ/etc. to compare absl::weak_ordering because they
+// convert literal 0 to int and absl::weak_ordering can only be compared with
+// literal 0. Defining this function allows for avoiding ClangTidy warnings.
+bool Identity(const bool b) { return b; }
+
+TEST(Btree, ValueComp) {
+  absl::btree_set<int> s;
+  EXPECT_TRUE(s.value_comp()(1, 2));
+  EXPECT_FALSE(s.value_comp()(2, 2));
+  EXPECT_FALSE(s.value_comp()(2, 1));
+
+  absl::btree_map<int, int> m1;
+  EXPECT_TRUE(m1.value_comp()(std::make_pair(1, 0), std::make_pair(2, 0)));
+  EXPECT_FALSE(m1.value_comp()(std::make_pair(2, 0), std::make_pair(2, 0)));
+  EXPECT_FALSE(m1.value_comp()(std::make_pair(2, 0), std::make_pair(1, 0)));
+
+  absl::btree_map<std::string, int> m2;
+  EXPECT_TRUE(Identity(
+      m2.value_comp()(std::make_pair("a", 0), std::make_pair("b", 0)) < 0));
+  EXPECT_TRUE(Identity(
+      m2.value_comp()(std::make_pair("b", 0), std::make_pair("b", 0)) == 0));
+  EXPECT_TRUE(Identity(
+      m2.value_comp()(std::make_pair("b", 0), std::make_pair("a", 0)) > 0));
+}
+
+TEST(Btree, DefaultConstruction) {
+  absl::btree_set<int> s;
+  absl::btree_map<int, int> m;
+  absl::btree_multiset<int> ms;
+  absl::btree_multimap<int, int> mm;
+
+  EXPECT_TRUE(s.empty());
+  EXPECT_TRUE(m.empty());
+  EXPECT_TRUE(ms.empty());
+  EXPECT_TRUE(mm.empty());
+}
+
+TEST(Btree, SwissTableHashable) {
+  static constexpr int kValues = 10000;
+  std::vector<int> values(kValues);
+  std::iota(values.begin(), values.end(), 0);
+  std::vector<std::pair<int, int>> map_values;
+  for (int v : values) map_values.emplace_back(v, -v);
+
+  using set = absl::btree_set<int>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      set{},
+      set{1},
+      set{2},
+      set{1, 2},
+      set{2, 1},
+      set(values.begin(), values.end()),
+      set(values.rbegin(), values.rend()),
+  }));
+
+  using mset = absl::btree_multiset<int>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      mset{},
+      mset{1},
+      mset{1, 1},
+      mset{2},
+      mset{2, 2},
+      mset{1, 2},
+      mset{1, 1, 2},
+      mset{1, 2, 2},
+      mset{1, 1, 2, 2},
+      mset(values.begin(), values.end()),
+      mset(values.rbegin(), values.rend()),
+  }));
+
+  using map = absl::btree_map<int, int>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      map{},
+      map{{1, 0}},
+      map{{1, 1}},
+      map{{2, 0}},
+      map{{2, 2}},
+      map{{1, 0}, {2, 1}},
+      map(map_values.begin(), map_values.end()),
+      map(map_values.rbegin(), map_values.rend()),
+  }));
+
+  using mmap = absl::btree_multimap<int, int>;
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      mmap{},
+      mmap{{1, 0}},
+      mmap{{1, 1}},
+      mmap{{1, 0}, {1, 1}},
+      mmap{{1, 1}, {1, 0}},
+      mmap{{2, 0}},
+      mmap{{2, 2}},
+      mmap{{1, 0}, {2, 1}},
+      mmap(map_values.begin(), map_values.end()),
+      mmap(map_values.rbegin(), map_values.rend()),
+  }));
+}
+
+TEST(Btree, ComparableSet) {
+  absl::btree_set<int> s1 = {1, 2};
+  absl::btree_set<int> s2 = {2, 3};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_LE(s1, s1);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+  EXPECT_GE(s1, s1);
+}
+
+TEST(Btree, ComparableSetsDifferentLength) {
+  absl::btree_set<int> s1 = {1, 2};
+  absl::btree_set<int> s2 = {1, 2, 3};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+}
+
+TEST(Btree, ComparableMultiset) {
+  absl::btree_multiset<int> s1 = {1, 2};
+  absl::btree_multiset<int> s2 = {2, 3};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_LE(s1, s1);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+  EXPECT_GE(s1, s1);
+}
+
+TEST(Btree, ComparableMap) {
+  absl::btree_map<int, int> s1 = {{1, 2}};
+  absl::btree_map<int, int> s2 = {{2, 3}};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_LE(s1, s1);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+  EXPECT_GE(s1, s1);
+}
+
+TEST(Btree, ComparableMultimap) {
+  absl::btree_multimap<int, int> s1 = {{1, 2}};
+  absl::btree_multimap<int, int> s2 = {{2, 3}};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_LE(s1, s1);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+  EXPECT_GE(s1, s1);
+}
+
+TEST(Btree, ComparableSetWithCustomComparator) {
+  // As specified by
+  // http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf section
+  // [container.requirements.general].12, ordering associative containers always
+  // uses default '<' operator
+  // - even if otherwise the container uses custom functor.
+  absl::btree_set<int, std::greater<int>> s1 = {1, 2};
+  absl::btree_set<int, std::greater<int>> s2 = {2, 3};
+  EXPECT_LT(s1, s2);
+  EXPECT_LE(s1, s2);
+  EXPECT_LE(s1, s1);
+  EXPECT_GT(s2, s1);
+  EXPECT_GE(s2, s1);
+  EXPECT_GE(s1, s1);
+}
+
+TEST(Btree, EraseReturnsIterator) {
+  absl::btree_set<int> set = {1, 2, 3, 4, 5};
+  auto result_it = set.erase(set.begin(), set.find(3));
+  EXPECT_EQ(result_it, set.find(3));
+  result_it = set.erase(set.find(5));
+  EXPECT_EQ(result_it, set.end());
+}
+
+TEST(Btree, ExtractAndInsertNodeHandleSet) {
+  absl::btree_set<int> src1 = {1, 2, 3, 4, 5};
+  auto nh = src1.extract(src1.find(3));
+  EXPECT_THAT(src1, ElementsAre(1, 2, 4, 5));
+  absl::btree_set<int> other;
+  absl::btree_set<int>::insert_return_type res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(3));
+  EXPECT_EQ(res.position, other.find(3));
+  EXPECT_TRUE(res.inserted);
+  EXPECT_TRUE(res.node.empty());
+
+  absl::btree_set<int> src2 = {3, 4};
+  nh = src2.extract(src2.find(3));
+  EXPECT_THAT(src2, ElementsAre(4));
+  res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(3));
+  EXPECT_EQ(res.position, other.find(3));
+  EXPECT_FALSE(res.inserted);
+  ASSERT_FALSE(res.node.empty());
+  EXPECT_EQ(res.node.value(), 3);
+}
+
+template <typename Set>
+void TestExtractWithTrackingForSet() {
+  InstanceTracker tracker;
+  {
+    Set s;
+    // Add enough elements to make sure we test internal nodes too.
+    const size_t kSize = 1000;
+    while (s.size() < kSize) {
+      s.insert(MovableOnlyInstance(s.size()));
+    }
+    for (int i = 0; i < kSize; ++i) {
+      // Extract with key
+      auto nh = s.extract(MovableOnlyInstance(i));
+      EXPECT_EQ(s.size(), kSize - 1);
+      EXPECT_EQ(nh.value().value(), i);
+      // Insert with node
+      s.insert(std::move(nh));
+      EXPECT_EQ(s.size(), kSize);
+
+      // Extract with iterator
+      auto it = s.find(MovableOnlyInstance(i));
+      nh = s.extract(it);
+      EXPECT_EQ(s.size(), kSize - 1);
+      EXPECT_EQ(nh.value().value(), i);
+      // Insert with node and hint
+      s.insert(s.begin(), std::move(nh));
+      EXPECT_EQ(s.size(), kSize);
+    }
+  }
+  EXPECT_EQ(0, tracker.instances());
+}
+
+template <typename Map>
+void TestExtractWithTrackingForMap() {
+  InstanceTracker tracker;
+  {
+    Map m;
+    // Add enough elements to make sure we test internal nodes too.
+    const size_t kSize = 1000;
+    while (m.size() < kSize) {
+      m.insert(
+          {CopyableMovableInstance(m.size()), MovableOnlyInstance(m.size())});
+    }
+    for (int i = 0; i < kSize; ++i) {
+      // Extract with key
+      auto nh = m.extract(CopyableMovableInstance(i));
+      EXPECT_EQ(m.size(), kSize - 1);
+      EXPECT_EQ(nh.key().value(), i);
+      EXPECT_EQ(nh.mapped().value(), i);
+      // Insert with node
+      m.insert(std::move(nh));
+      EXPECT_EQ(m.size(), kSize);
+
+      // Extract with iterator
+      auto it = m.find(CopyableMovableInstance(i));
+      nh = m.extract(it);
+      EXPECT_EQ(m.size(), kSize - 1);
+      EXPECT_EQ(nh.key().value(), i);
+      EXPECT_EQ(nh.mapped().value(), i);
+      // Insert with node and hint
+      m.insert(m.begin(), std::move(nh));
+      EXPECT_EQ(m.size(), kSize);
+    }
+  }
+  EXPECT_EQ(0, tracker.instances());
+}
+
+TEST(Btree, ExtractTracking) {
+  TestExtractWithTrackingForSet<absl::btree_set<MovableOnlyInstance>>();
+  TestExtractWithTrackingForSet<absl::btree_multiset<MovableOnlyInstance>>();
+  TestExtractWithTrackingForMap<
+      absl::btree_map<CopyableMovableInstance, MovableOnlyInstance>>();
+  TestExtractWithTrackingForMap<
+      absl::btree_multimap<CopyableMovableInstance, MovableOnlyInstance>>();
+}
+
+TEST(Btree, ExtractAndInsertNodeHandleMultiSet) {
+  absl::btree_multiset<int> src1 = {1, 2, 3, 3, 4, 5};
+  auto nh = src1.extract(src1.find(3));
+  EXPECT_THAT(src1, ElementsAre(1, 2, 3, 4, 5));
+  absl::btree_multiset<int> other;
+  auto res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(3));
+  EXPECT_EQ(res, other.find(3));
+
+  absl::btree_multiset<int> src2 = {3, 4};
+  nh = src2.extract(src2.find(3));
+  EXPECT_THAT(src2, ElementsAre(4));
+  res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(3, 3));
+  EXPECT_EQ(res, ++other.find(3));
+}
+
+TEST(Btree, ExtractAndInsertNodeHandleMap) {
+  absl::btree_map<int, int> src1 = {{1, 2}, {3, 4}, {5, 6}};
+  auto nh = src1.extract(src1.find(3));
+  EXPECT_THAT(src1, ElementsAre(Pair(1, 2), Pair(5, 6)));
+  absl::btree_map<int, int> other;
+  absl::btree_map<int, int>::insert_return_type res =
+      other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(Pair(3, 4)));
+  EXPECT_EQ(res.position, other.find(3));
+  EXPECT_TRUE(res.inserted);
+  EXPECT_TRUE(res.node.empty());
+
+  absl::btree_map<int, int> src2 = {{3, 6}};
+  nh = src2.extract(src2.find(3));
+  EXPECT_TRUE(src2.empty());
+  res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(Pair(3, 4)));
+  EXPECT_EQ(res.position, other.find(3));
+  EXPECT_FALSE(res.inserted);
+  ASSERT_FALSE(res.node.empty());
+  EXPECT_EQ(res.node.key(), 3);
+  EXPECT_EQ(res.node.mapped(), 6);
+}
+
+TEST(Btree, ExtractAndInsertNodeHandleMultiMap) {
+  absl::btree_multimap<int, int> src1 = {{1, 2}, {3, 4}, {5, 6}};
+  auto nh = src1.extract(src1.find(3));
+  EXPECT_THAT(src1, ElementsAre(Pair(1, 2), Pair(5, 6)));
+  absl::btree_multimap<int, int> other;
+  auto res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(Pair(3, 4)));
+  EXPECT_EQ(res, other.find(3));
+
+  absl::btree_multimap<int, int> src2 = {{3, 6}};
+  nh = src2.extract(src2.find(3));
+  EXPECT_TRUE(src2.empty());
+  res = other.insert(std::move(nh));
+  EXPECT_THAT(other, ElementsAre(Pair(3, 4), Pair(3, 6)));
+  EXPECT_EQ(res, ++other.begin());
+}
+
+// For multisets, insert with hint also affects correctness because we need to
+// insert immediately before the hint if possible.
+struct InsertMultiHintData {
+  int key;
+  int not_key;
+  bool operator==(const InsertMultiHintData other) const {
+    return key == other.key && not_key == other.not_key;
+  }
+};
+
+struct InsertMultiHintDataKeyCompare {
+  using is_transparent = void;
+  bool operator()(const InsertMultiHintData a,
+                  const InsertMultiHintData b) const {
+    return a.key < b.key;
+  }
+  bool operator()(const int a, const InsertMultiHintData b) const {
+    return a < b.key;
+  }
+  bool operator()(const InsertMultiHintData a, const int b) const {
+    return a.key < b;
+  }
+};
+
+TEST(Btree, InsertHintNodeHandle) {
+  // For unique sets, insert with hint is just a performance optimization.
+  // Test that insert works correctly when the hint is right or wrong.
+  {
+    absl::btree_set<int> src = {1, 2, 3, 4, 5};
+    auto nh = src.extract(src.find(3));
+    EXPECT_THAT(src, ElementsAre(1, 2, 4, 5));
+    absl::btree_set<int> other = {0, 100};
+    // Test a correct hint.
+    auto it = other.insert(other.lower_bound(3), std::move(nh));
+    EXPECT_THAT(other, ElementsAre(0, 3, 100));
+    EXPECT_EQ(it, other.find(3));
+
+    nh = src.extract(src.find(5));
+    // Test an incorrect hint.
+    it = other.insert(other.end(), std::move(nh));
+    EXPECT_THAT(other, ElementsAre(0, 3, 5, 100));
+    EXPECT_EQ(it, other.find(5));
+  }
+
+  absl::btree_multiset<InsertMultiHintData, InsertMultiHintDataKeyCompare> src =
+      {{1, 2}, {3, 4}, {3, 5}};
+  auto nh = src.extract(src.lower_bound(3));
+  EXPECT_EQ(nh.value(), (InsertMultiHintData{3, 4}));
+  absl::btree_multiset<InsertMultiHintData, InsertMultiHintDataKeyCompare>
+      other = {{3, 1}, {3, 2}, {3, 3}};
+  auto it = other.insert(--other.end(), std::move(nh));
+  EXPECT_THAT(
+      other, ElementsAre(InsertMultiHintData{3, 1}, InsertMultiHintData{3, 2},
+                         InsertMultiHintData{3, 4}, InsertMultiHintData{3, 3}));
+  EXPECT_EQ(it, --(--other.end()));
+
+  nh = src.extract(src.find(3));
+  EXPECT_EQ(nh.value(), (InsertMultiHintData{3, 5}));
+  it = other.insert(other.begin(), std::move(nh));
+  EXPECT_THAT(other,
+              ElementsAre(InsertMultiHintData{3, 5}, InsertMultiHintData{3, 1},
+                          InsertMultiHintData{3, 2}, InsertMultiHintData{3, 4},
+                          InsertMultiHintData{3, 3}));
+  EXPECT_EQ(it, other.begin());
+}
+
+struct IntCompareToCmp {
+  absl::weak_ordering operator()(int a, int b) const {
+    if (a < b) return absl::weak_ordering::less;
+    if (a > b) return absl::weak_ordering::greater;
+    return absl::weak_ordering::equivalent;
+  }
+};
+
+TEST(Btree, MergeIntoUniqueContainers) {
+  absl::btree_set<int, IntCompareToCmp> src1 = {1, 2, 3};
+  absl::btree_multiset<int> src2 = {3, 4, 4, 5};
+  absl::btree_set<int> dst;
+
+  dst.merge(src1);
+  EXPECT_TRUE(src1.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3));
+  dst.merge(src2);
+  EXPECT_THAT(src2, ElementsAre(3, 4));
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(Btree, MergeIntoUniqueContainersWithCompareTo) {
+  absl::btree_set<int, IntCompareToCmp> src1 = {1, 2, 3};
+  absl::btree_multiset<int> src2 = {3, 4, 4, 5};
+  absl::btree_set<int, IntCompareToCmp> dst;
+
+  dst.merge(src1);
+  EXPECT_TRUE(src1.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3));
+  dst.merge(src2);
+  EXPECT_THAT(src2, ElementsAre(3, 4));
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3, 4, 5));
+}
+
+TEST(Btree, MergeIntoMultiContainers) {
+  absl::btree_set<int, IntCompareToCmp> src1 = {1, 2, 3};
+  absl::btree_multiset<int> src2 = {3, 4, 4, 5};
+  absl::btree_multiset<int> dst;
+
+  dst.merge(src1);
+  EXPECT_TRUE(src1.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3));
+  dst.merge(src2);
+  EXPECT_TRUE(src2.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3, 3, 4, 4, 5));
+}
+
+TEST(Btree, MergeIntoMultiContainersWithCompareTo) {
+  absl::btree_set<int, IntCompareToCmp> src1 = {1, 2, 3};
+  absl::btree_multiset<int> src2 = {3, 4, 4, 5};
+  absl::btree_multiset<int, IntCompareToCmp> dst;
+
+  dst.merge(src1);
+  EXPECT_TRUE(src1.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3));
+  dst.merge(src2);
+  EXPECT_TRUE(src2.empty());
+  EXPECT_THAT(dst, ElementsAre(1, 2, 3, 3, 4, 4, 5));
+}
+
+TEST(Btree, MergeIntoMultiMapsWithDifferentComparators) {
+  absl::btree_map<int, int, IntCompareToCmp> src1 = {{1, 1}, {2, 2}, {3, 3}};
+  absl::btree_multimap<int, int, std::greater<int>> src2 = {
+      {5, 5}, {4, 1}, {4, 4}, {3, 2}};
+  absl::btree_multimap<int, int> dst;
+
+  dst.merge(src1);
+  EXPECT_TRUE(src1.empty());
+  EXPECT_THAT(dst, ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3)));
+  dst.merge(src2);
+  EXPECT_TRUE(src2.empty());
+  EXPECT_THAT(dst, ElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3), Pair(3, 2),
+                               Pair(4, 1), Pair(4, 4), Pair(5, 5)));
+}
+
+struct KeyCompareToWeakOrdering {
+  template <typename T>
+  absl::weak_ordering operator()(const T &a, const T &b) const {
+    return a < b ? absl::weak_ordering::less
+                 : a == b ? absl::weak_ordering::equivalent
+                          : absl::weak_ordering::greater;
+  }
+};
+
+struct KeyCompareToStrongOrdering {
+  template <typename T>
+  absl::strong_ordering operator()(const T &a, const T &b) const {
+    return a < b ? absl::strong_ordering::less
+                 : a == b ? absl::strong_ordering::equal
+                          : absl::strong_ordering::greater;
+  }
+};
+
+TEST(Btree, UserProvidedKeyCompareToComparators) {
+  absl::btree_set<int, KeyCompareToWeakOrdering> weak_set = {1, 2, 3};
+  EXPECT_TRUE(weak_set.contains(2));
+  EXPECT_FALSE(weak_set.contains(4));
+
+  absl::btree_set<int, KeyCompareToStrongOrdering> strong_set = {1, 2, 3};
+  EXPECT_TRUE(strong_set.contains(2));
+  EXPECT_FALSE(strong_set.contains(4));
+}
+
+TEST(Btree, TryEmplaceBasicTest) {
+  absl::btree_map<int, std::string> m;
+
+  // Should construct a std::string from the literal.
+  m.try_emplace(1, "one");
+  EXPECT_EQ(1, m.size());
+
+  // Try other std::string constructors and const lvalue key.
+  const int key(42);
+  m.try_emplace(key, 3, 'a');
+  m.try_emplace(2, std::string("two"));
+
+  EXPECT_TRUE(std::is_sorted(m.begin(), m.end()));
+  EXPECT_THAT(m, ElementsAreArray(std::vector<std::pair<int, std::string>>{
+                     {1, "one"}, {2, "two"}, {42, "aaa"}}));
+}
+
+TEST(Btree, TryEmplaceWithHintWorks) {
+  // Use a counting comparator here to verify that hint is used.
+  int calls = 0;
+  auto cmp = [&calls](int x, int y) {
+    ++calls;
+    return x < y;
+  };
+  using Cmp = decltype(cmp);
+
+  absl::btree_map<int, int, Cmp> m(cmp);
+  for (int i = 0; i < 128; ++i) {
+    m.emplace(i, i);
+  }
+
+  // Sanity check for the comparator
+  calls = 0;
+  m.emplace(127, 127);
+  EXPECT_GE(calls, 4);
+
+  // Try with begin hint:
+  calls = 0;
+  auto it = m.try_emplace(m.begin(), -1, -1);
+  EXPECT_EQ(129, m.size());
+  EXPECT_EQ(it, m.begin());
+  EXPECT_LE(calls, 2);
+
+  // Try with end hint:
+  calls = 0;
+  std::pair<int, int> pair1024 = {1024, 1024};
+  it = m.try_emplace(m.end(), pair1024.first, pair1024.second);
+  EXPECT_EQ(130, m.size());
+  EXPECT_EQ(it, --m.end());
+  EXPECT_LE(calls, 2);
+
+  // Try value already present, bad hint; ensure no duplicate added:
+  calls = 0;
+  it = m.try_emplace(m.end(), 16, 17);
+  EXPECT_EQ(130, m.size());
+  EXPECT_GE(calls, 4);
+  EXPECT_EQ(it, m.find(16));
+
+  // Try value already present, hint points directly to it:
+  calls = 0;
+  it = m.try_emplace(it, 16, 17);
+  EXPECT_EQ(130, m.size());
+  EXPECT_LE(calls, 2);
+  EXPECT_EQ(it, m.find(16));
+
+  m.erase(2);
+  EXPECT_EQ(129, m.size());
+  auto hint = m.find(3);
+  // Try emplace in the middle of two other elements.
+  calls = 0;
+  m.try_emplace(hint, 2, 2);
+  EXPECT_EQ(130, m.size());
+  EXPECT_LE(calls, 2);
+
+  EXPECT_TRUE(std::is_sorted(m.begin(), m.end()));
+}
+
+TEST(Btree, TryEmplaceWithBadHint) {
+  absl::btree_map<int, int> m = {{1, 1}, {9, 9}};
+
+  // Bad hint (too small), should still emplace:
+  auto it = m.try_emplace(m.begin(), 2, 2);
+  EXPECT_EQ(it, ++m.begin());
+  EXPECT_THAT(m, ElementsAreArray(
+                     std::vector<std::pair<int, int>>{{1, 1}, {2, 2}, {9, 9}}));
+
+  // Bad hint, too large this time:
+  it = m.try_emplace(++(++m.begin()), 0, 0);
+  EXPECT_EQ(it, m.begin());
+  EXPECT_THAT(m, ElementsAreArray(std::vector<std::pair<int, int>>{
+                     {0, 0}, {1, 1}, {2, 2}, {9, 9}}));
+}
+
+TEST(Btree, TryEmplaceMaintainsSortedOrder) {
+  absl::btree_map<int, std::string> m;
+  std::pair<int, std::string> pair5 = {5, "five"};
+
+  // Test both lvalue & rvalue emplace.
+  m.try_emplace(10, "ten");
+  m.try_emplace(pair5.first, pair5.second);
+  EXPECT_EQ(2, m.size());
+  EXPECT_TRUE(std::is_sorted(m.begin(), m.end()));
+
+  int int100{100};
+  m.try_emplace(int100, "hundred");
+  m.try_emplace(1, "one");
+  EXPECT_EQ(4, m.size());
+  EXPECT_TRUE(std::is_sorted(m.begin(), m.end()));
+}
+
+TEST(Btree, TryEmplaceWithHintAndNoValueArgsWorks) {
+  absl::btree_map<int, int> m;
+  m.try_emplace(m.end(), 1);
+  EXPECT_EQ(0, m[1]);
+}
+
+TEST(Btree, TryEmplaceWithHintAndMultipleValueArgsWorks) {
+  absl::btree_map<int, std::string> m;
+  m.try_emplace(m.end(), 1, 10, 'a');
+  EXPECT_EQ(std::string(10, 'a'), m[1]);
+}
+
+TEST(Btree, MoveAssignmentAllocatorPropagation) {
+  InstanceTracker tracker;
+
+  int64_t bytes1 = 0, bytes2 = 0;
+  PropagatingCountingAlloc<MovableOnlyInstance> allocator1(&bytes1);
+  PropagatingCountingAlloc<MovableOnlyInstance> allocator2(&bytes2);
+  std::less<MovableOnlyInstance> cmp;
+
+  // Test propagating allocator_type.
+  {
+    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
+                    PropagatingCountingAlloc<MovableOnlyInstance>>
+        set1(cmp, allocator1), set2(cmp, allocator2);
+
+    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+
+    tracker.ResetCopiesMovesSwaps();
+    set2 = std::move(set1);
+    EXPECT_EQ(tracker.moves(), 0);
+  }
+  // Test non-propagating allocator_type with equal allocators.
+  {
+    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
+                    CountingAllocator<MovableOnlyInstance>>
+        set1(cmp, allocator1), set2(cmp, allocator1);
+
+    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+
+    tracker.ResetCopiesMovesSwaps();
+    set2 = std::move(set1);
+    EXPECT_EQ(tracker.moves(), 0);
+  }
+  // Test non-propagating allocator_type with different allocators.
+  {
+    absl::btree_set<MovableOnlyInstance, std::less<MovableOnlyInstance>,
+                    CountingAllocator<MovableOnlyInstance>>
+        set1(cmp, allocator1), set2(cmp, allocator2);
+
+    for (int i = 0; i < 100; ++i) set1.insert(MovableOnlyInstance(i));
+
+    tracker.ResetCopiesMovesSwaps();
+    set2 = std::move(set1);
+    EXPECT_GE(tracker.moves(), 100);
+  }
+}
+
+TEST(Btree, EmptyTree) {
+  absl::btree_set<int> s;
+  EXPECT_TRUE(s.empty());
+  EXPECT_EQ(s.size(), 0);
+  EXPECT_GT(s.max_size(), 0);
+}
+
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(Btree, EraseIf) {
+  // Test that erase_if works with all the container types and supports lambdas.
+  {
+    absl::btree_set<int> s = {1, 3, 5, 6, 100};
+    erase_if(s, [](int k) { return k > 3; });
+    EXPECT_THAT(s, ElementsAre(1, 3));
+  }
+  {
+    absl::btree_multiset<int> s = {1, 3, 3, 5, 6, 6, 100};
+    erase_if(s, [](int k) { return k <= 3; });
+    EXPECT_THAT(s, ElementsAre(5, 6, 6, 100));
+  }
+  {
+    absl::btree_map<int, int> m = {{1, 1}, {3, 3}, {6, 6}, {100, 100}};
+    erase_if(m, [](std::pair<const int, int> kv) { return kv.first > 3; });
+    EXPECT_THAT(m, ElementsAre(Pair(1, 1), Pair(3, 3)));
+  }
+  {
+    absl::btree_multimap<int, int> m = {{1, 1}, {3, 3}, {3, 6},
+                                        {6, 6}, {6, 7}, {100, 6}};
+    erase_if(m, [](std::pair<const int, int> kv) { return kv.second == 6; });
+    EXPECT_THAT(m, ElementsAre(Pair(1, 1), Pair(3, 3), Pair(6, 7)));
+  }
+  // Test that erasing all elements from a large set works and test support for
+  // function pointers.
+  {
+    absl::btree_set<int> s;
+    for (int i = 0; i < 1000; ++i) s.insert(2 * i);
+    erase_if(s, IsEven);
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Test that erase_if supports other format of function pointers.
+  {
+    absl::btree_set<int> s = {1, 3, 5, 6, 100};
+    erase_if(s, &IsEven);
+    EXPECT_THAT(s, ElementsAre(1, 3, 5));
+  }
+}
+
+TEST(Btree, InsertOrAssign) {
+  absl::btree_map<int, int> m = {{1, 1}, {3, 3}};
+  using value_type = typename decltype(m)::value_type;
+
+  auto ret = m.insert_or_assign(4, 4);
+  EXPECT_EQ(*ret.first, value_type(4, 4));
+  EXPECT_TRUE(ret.second);
+  ret = m.insert_or_assign(3, 100);
+  EXPECT_EQ(*ret.first, value_type(3, 100));
+  EXPECT_FALSE(ret.second);
+
+  auto hint_ret = m.insert_or_assign(ret.first, 3, 200);
+  EXPECT_EQ(*hint_ret, value_type(3, 200));
+  hint_ret = m.insert_or_assign(m.find(1), 0, 1);
+  EXPECT_EQ(*hint_ret, value_type(0, 1));
+  // Test with bad hint.
+  hint_ret = m.insert_or_assign(m.end(), -1, 1);
+  EXPECT_EQ(*hint_ret, value_type(-1, 1));
+
+  EXPECT_THAT(m, ElementsAre(Pair(-1, 1), Pair(0, 1), Pair(1, 1), Pair(3, 200),
+                             Pair(4, 4)));
+}
+
+TEST(Btree, InsertOrAssignMovableOnly) {
+  absl::btree_map<int, MovableOnlyInstance> m;
+  using value_type = typename decltype(m)::value_type;
+
+  auto ret = m.insert_or_assign(4, MovableOnlyInstance(4));
+  EXPECT_EQ(*ret.first, value_type(4, MovableOnlyInstance(4)));
+  EXPECT_TRUE(ret.second);
+  ret = m.insert_or_assign(4, MovableOnlyInstance(100));
+  EXPECT_EQ(*ret.first, value_type(4, MovableOnlyInstance(100)));
+  EXPECT_FALSE(ret.second);
+
+  auto hint_ret = m.insert_or_assign(ret.first, 3, MovableOnlyInstance(200));
+  EXPECT_EQ(*hint_ret, value_type(3, MovableOnlyInstance(200)));
+
+  EXPECT_EQ(m.size(), 2);
+}
+
+TEST(Btree, BitfieldArgument) {
+  union {
+    int n : 1;
+  };
+  n = 0;
+  absl::btree_map<int, int> m;
+  m.erase(n);
+  m.count(n);
+  m.find(n);
+  m.contains(n);
+  m.equal_range(n);
+  m.insert_or_assign(n, n);
+  m.insert_or_assign(m.end(), n, n);
+  m.try_emplace(n);
+  m.try_emplace(m.end(), n);
+  m.at(n);
+  m[n];
+}
+
+}  // namespace
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/container/btree_test.h b/absl/container/btree_test.h
new file mode 100644
index 0000000..218ba41
--- /dev/null
+++ b/absl/container/btree_test.h
@@ -0,0 +1,155 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_BTREE_TEST_H_
+#define ABSL_CONTAINER_BTREE_TEST_H_
+
+#include <algorithm>
+#include <cassert>
+#include <random>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/container/btree_map.h"
+#include "absl/container/btree_set.h"
+#include "absl/container/flat_hash_set.h"
+#include "absl/time/time.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// Like remove_const but propagates the removal through std::pair.
+template <typename T>
+struct remove_pair_const {
+  using type = typename std::remove_const<T>::type;
+};
+template <typename T, typename U>
+struct remove_pair_const<std::pair<T, U> > {
+  using type = std::pair<typename remove_pair_const<T>::type,
+                         typename remove_pair_const<U>::type>;
+};
+
+// Utility class to provide an accessor for a key given a value. The default
+// behavior is to treat the value as a pair and return the first element.
+template <typename K, typename V>
+struct KeyOfValue {
+  struct type {
+    const K& operator()(const V& p) const { return p.first; }
+  };
+};
+
+// Partial specialization of KeyOfValue class for when the key and value are
+// the same type such as in set<> and btree_set<>.
+template <typename K>
+struct KeyOfValue<K, K> {
+  struct type {
+    const K& operator()(const K& k) const { return k; }
+  };
+};
+
+inline char* GenerateDigits(char buf[16], unsigned val, unsigned maxval) {
+  assert(val <= maxval);
+  constexpr unsigned kBase = 64;  // avoid integer division.
+  unsigned p = 15;
+  buf[p--] = 0;
+  while (maxval > 0) {
+    buf[p--] = ' ' + (val % kBase);
+    val /= kBase;
+    maxval /= kBase;
+  }
+  return buf + p + 1;
+}
+
+template <typename K>
+struct Generator {
+  int maxval;
+  explicit Generator(int m) : maxval(m) {}
+  K operator()(int i) const {
+    assert(i <= maxval);
+    return K(i);
+  }
+};
+
+template <>
+struct Generator<absl::Time> {
+  int maxval;
+  explicit Generator(int m) : maxval(m) {}
+  absl::Time operator()(int i) const { return absl::FromUnixMillis(i); }
+};
+
+template <>
+struct Generator<std::string> {
+  int maxval;
+  explicit Generator(int m) : maxval(m) {}
+  std::string operator()(int i) const {
+    char buf[16];
+    return GenerateDigits(buf, i, maxval);
+  }
+};
+
+template <typename T, typename U>
+struct Generator<std::pair<T, U> > {
+  Generator<typename remove_pair_const<T>::type> tgen;
+  Generator<typename remove_pair_const<U>::type> ugen;
+
+  explicit Generator(int m) : tgen(m), ugen(m) {}
+  std::pair<T, U> operator()(int i) const {
+    return std::make_pair(tgen(i), ugen(i));
+  }
+};
+
+// Generate n values for our tests and benchmarks. Value range is [0, maxval].
+inline std::vector<int> GenerateNumbersWithSeed(int n, int maxval, int seed) {
+  // NOTE: Some tests rely on generated numbers not changing between test runs.
+  // We use std::minstd_rand0 because it is well-defined, but don't use
+  // std::uniform_int_distribution because platforms use different algorithms.
+  std::minstd_rand0 rng(seed);
+
+  std::vector<int> values;
+  absl::flat_hash_set<int> unique_values;
+  if (values.size() < n) {
+    for (int i = values.size(); i < n; i++) {
+      int value;
+      do {
+        value = static_cast<int>(rng()) % (maxval + 1);
+      } while (!unique_values.insert(value).second);
+
+      values.push_back(value);
+    }
+  }
+  return values;
+}
+
+// Generates n values in the range [0, maxval].
+template <typename V>
+std::vector<V> GenerateValuesWithSeed(int n, int maxval, int seed) {
+  const std::vector<int> nums = GenerateNumbersWithSeed(n, maxval, seed);
+  Generator<V> gen(maxval);
+  std::vector<V> vec;
+
+  vec.reserve(n);
+  for (int i = 0; i < n; i++) {
+    vec.push_back(gen(nums[i]));
+  }
+
+  return vec;
+}
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_BTREE_TEST_H_
diff --git a/absl/container/fixed_array.h b/absl/container/fixed_array.h
index 1e0da5e..a9ce99b 100644
--- a/absl/container/fixed_array.h
+++ b/absl/container/fixed_array.h
@@ -31,7 +31,6 @@
 #define ABSL_CONTAINER_FIXED_ARRAY_H_
 
 #include <algorithm>
-#include <array>
 #include <cassert>
 #include <cstddef>
 #include <initializer_list>
@@ -51,7 +50,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
 
@@ -387,8 +386,7 @@
   //     error: call to int __builtin___sprintf_chk(etc...)
   //     will always overflow destination buffer [-Werror]
   //
-  template <typename OuterT = value_type,
-            typename InnerT = absl::remove_extent_t<OuterT>,
+  template <typename OuterT, typename InnerT = absl::remove_extent_t<OuterT>,
             size_t InnerN = std::extent<OuterT>::value>
   struct StorageElementWrapper {
     InnerT array[InnerN];
@@ -397,8 +395,6 @@
   using StorageElement =
       absl::conditional_t<std::is_array<value_type>::value,
                           StorageElementWrapper<value_type>, value_type>;
-  using StorageElementBuffer =
-      absl::aligned_storage_t<sizeof(StorageElement), alignof(StorageElement)>;
 
   static pointer AsValueType(pointer ptr) { return ptr; }
   static pointer AsValueType(StorageElementWrapper<value_type>* ptr) {
@@ -408,25 +404,25 @@
   static_assert(sizeof(StorageElement) == sizeof(value_type), "");
   static_assert(alignof(StorageElement) == alignof(value_type), "");
 
-  struct NonEmptyInlinedStorage {
-    StorageElement* data() {
-      return reinterpret_cast<StorageElement*>(inlined_storage_.data());
-    }
+  class NonEmptyInlinedStorage {
+   public:
+    StorageElement* data() { return reinterpret_cast<StorageElement*>(buff_); }
+    void AnnotateConstruct(size_type n);
+    void AnnotateDestruct(size_type n);
 
 #ifdef ADDRESS_SANITIZER
     void* RedzoneBegin() { return &redzone_begin_; }
     void* RedzoneEnd() { return &redzone_end_ + 1; }
 #endif  // ADDRESS_SANITIZER
 
-    void AnnotateConstruct(size_type);
-    void AnnotateDestruct(size_type);
-
+   private:
     ADDRESS_SANITIZER_REDZONE(redzone_begin_);
-    std::array<StorageElementBuffer, inline_elements> inlined_storage_;
+    alignas(StorageElement) char buff_[sizeof(StorageElement[inline_elements])];
     ADDRESS_SANITIZER_REDZONE(redzone_end_);
   };
 
-  struct EmptyInlinedStorage {
+  class EmptyInlinedStorage {
+   public:
     StorageElement* data() { return nullptr; }
     void AnnotateConstruct(size_type) {}
     void AnnotateDestruct(size_type) {}
@@ -460,9 +456,7 @@
     size_type size() const { return size_alloc_.template get<0>(); }
     StorageElement* begin() const { return data_; }
     StorageElement* end() const { return begin() + size(); }
-    allocator_type& alloc() {
-      return size_alloc_.template get<1>();
-    }
+    allocator_type& alloc() { return size_alloc_.template get<1>(); }
 
    private:
     static bool UsingInlinedStorage(size_type n) {
@@ -515,7 +509,7 @@
 #endif                   // ADDRESS_SANITIZER
   static_cast<void>(n);  // Mark used when not in asan mode
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FIXED_ARRAY_H_
diff --git a/absl/container/fixed_array_exception_safety_test.cc b/absl/container/fixed_array_exception_safety_test.cc
index 4a67bb4..a5bb009 100644
--- a/absl/container/fixed_array_exception_safety_test.cc
+++ b/absl/container/fixed_array_exception_safety_test.cc
@@ -12,14 +12,18 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "absl/base/config.h"
+#include "absl/container/fixed_array.h"
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+
 #include <initializer_list>
 
 #include "gtest/gtest.h"
 #include "absl/base/internal/exception_safety_testing.h"
-#include "absl/container/fixed_array.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -33,10 +37,19 @@
 using ::testing::TestThrowingCtor;
 
 using Thrower = testing::ThrowingValue<testing::TypeSpec::kEverythingThrows>;
-using FixedArr = absl::FixedArray<Thrower, kInlined>;
-
+using ThrowAlloc =
+    testing::ThrowingAllocator<Thrower, testing::AllocSpec::kEverythingThrows>;
 using MoveThrower = testing::ThrowingValue<testing::TypeSpec::kNoThrowMove>;
+using MoveThrowAlloc =
+    testing::ThrowingAllocator<MoveThrower,
+                               testing::AllocSpec::kEverythingThrows>;
+
+using FixedArr = absl::FixedArray<Thrower, kInlined>;
+using FixedArrWithAlloc = absl::FixedArray<Thrower, kInlined, ThrowAlloc>;
+
 using MoveFixedArr = absl::FixedArray<MoveThrower, kInlined>;
+using MoveFixedArrWithAlloc =
+    absl::FixedArray<MoveThrower, kInlined, MoveThrowAlloc>;
 
 TEST(FixedArrayExceptionSafety, CopyConstructor) {
   auto small = FixedArr(kSmallSize);
@@ -46,6 +59,14 @@
   TestThrowingCtor<FixedArr>(large);
 }
 
+TEST(FixedArrayExceptionSafety, CopyConstructorWithAlloc) {
+  auto small = FixedArrWithAlloc(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(small);
+
+  auto large = FixedArrWithAlloc(kLargeSize);
+  TestThrowingCtor<FixedArrWithAlloc>(large);
+}
+
 TEST(FixedArrayExceptionSafety, MoveConstructor) {
   TestThrowingCtor<FixedArr>(FixedArr(kSmallSize));
   TestThrowingCtor<FixedArr>(FixedArr(kLargeSize));
@@ -55,16 +76,35 @@
   TestThrowingCtor<MoveFixedArr>(MoveFixedArr(kLargeSize));
 }
 
+TEST(FixedArrayExceptionSafety, MoveConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(FixedArrWithAlloc(kSmallSize));
+  TestThrowingCtor<FixedArrWithAlloc>(FixedArrWithAlloc(kLargeSize));
+
+  // TypeSpec::kNoThrowMove
+  TestThrowingCtor<MoveFixedArrWithAlloc>(MoveFixedArrWithAlloc(kSmallSize));
+  TestThrowingCtor<MoveFixedArrWithAlloc>(MoveFixedArrWithAlloc(kLargeSize));
+}
+
 TEST(FixedArrayExceptionSafety, SizeConstructor) {
   TestThrowingCtor<FixedArr>(kSmallSize);
   TestThrowingCtor<FixedArr>(kLargeSize);
 }
 
+TEST(FixedArrayExceptionSafety, SizeConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(kLargeSize);
+}
+
 TEST(FixedArrayExceptionSafety, SizeValueConstructor) {
   TestThrowingCtor<FixedArr>(kSmallSize, Thrower());
   TestThrowingCtor<FixedArr>(kLargeSize, Thrower());
 }
 
+TEST(FixedArrayExceptionSafety, SizeValueConstructorWithAlloc) {
+  TestThrowingCtor<FixedArrWithAlloc>(kSmallSize, Thrower());
+  TestThrowingCtor<FixedArrWithAlloc>(kLargeSize, Thrower());
+}
+
 TEST(FixedArrayExceptionSafety, IteratorConstructor) {
   auto small = FixedArr(kSmallSize);
   TestThrowingCtor<FixedArr>(small.begin(), small.end());
@@ -73,6 +113,14 @@
   TestThrowingCtor<FixedArr>(large.begin(), large.end());
 }
 
+TEST(FixedArrayExceptionSafety, IteratorConstructorWithAlloc) {
+  auto small = FixedArrWithAlloc(kSmallSize);
+  TestThrowingCtor<FixedArrWithAlloc>(small.begin(), small.end());
+
+  auto large = FixedArrWithAlloc(kLargeSize);
+  TestThrowingCtor<FixedArrWithAlloc>(large.begin(), large.end());
+}
+
 TEST(FixedArrayExceptionSafety, InitListConstructor) {
   constexpr int small_inlined = 3;
   using SmallFixedArr = absl::FixedArray<Thrower, small_inlined>;
@@ -86,7 +134,22 @@
       Thrower{}, Thrower{}, Thrower{}, Thrower{}, Thrower{}});
 }
 
-testing::AssertionResult ReadMemory(FixedArr* fixed_arr) {
+TEST(FixedArrayExceptionSafety, InitListConstructorWithAlloc) {
+  constexpr int small_inlined = 3;
+  using SmallFixedArrWithAlloc =
+      absl::FixedArray<Thrower, small_inlined, ThrowAlloc>;
+
+  TestThrowingCtor<SmallFixedArrWithAlloc>(std::initializer_list<Thrower>{});
+  // Test inlined allocation
+  TestThrowingCtor<SmallFixedArrWithAlloc>(
+      std::initializer_list<Thrower>{Thrower{}, Thrower{}});
+  // Test out of line allocation
+  TestThrowingCtor<SmallFixedArrWithAlloc>(std::initializer_list<Thrower>{
+      Thrower{}, Thrower{}, Thrower{}, Thrower{}, Thrower{}});
+}
+
+template <typename FixedArrT>
+testing::AssertionResult ReadMemory(FixedArrT* fixed_arr) {
   // Marked volatile to prevent optimization. Used for running asan tests.
   volatile int sum = 0;
   for (const auto& thrower : *fixed_arr) {
@@ -97,7 +160,7 @@
 
 TEST(FixedArrayExceptionSafety, Fill) {
   auto test_fill = testing::MakeExceptionSafetyTester()
-                       .WithContracts(ReadMemory)
+                       .WithContracts(ReadMemory<FixedArr>)
                        .WithOperation([&](FixedArr* fixed_arr_ptr) {
                          auto thrower =
                              Thrower(kUpdatedValue, testing::nothrow_ctor);
@@ -112,7 +175,28 @@
           .Test());
 }
 
+TEST(FixedArrayExceptionSafety, FillWithAlloc) {
+  auto test_fill = testing::MakeExceptionSafetyTester()
+                       .WithContracts(ReadMemory<FixedArrWithAlloc>)
+                       .WithOperation([&](FixedArrWithAlloc* fixed_arr_ptr) {
+                         auto thrower =
+                             Thrower(kUpdatedValue, testing::nothrow_ctor);
+                         fixed_arr_ptr->fill(thrower);
+                       });
+
+  EXPECT_TRUE(test_fill
+                  .WithInitialValue(
+                      FixedArrWithAlloc(kSmallSize, Thrower(kInitialValue)))
+                  .Test());
+  EXPECT_TRUE(test_fill
+                  .WithInitialValue(
+                      FixedArrWithAlloc(kLargeSize, Thrower(kInitialValue)))
+                  .Test());
+}
+
 }  // namespace
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
+
+#endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/container/fixed_array_test.cc b/absl/container/fixed_array_test.cc
index 2b1cf47..c960fe5 100644
--- a/absl/container/fixed_array_test.cc
+++ b/absl/container/fixed_array_test.cc
@@ -604,19 +604,16 @@
   empty.fill(fill_val);
 }
 
-// TODO(johnsoncj): Investigate InlinedStorage default initialization in GCC 4.x
 #ifndef __GNUC__
 TEST(FixedArrayTest, DefaultCtorDoesNotValueInit) {
   using T = char;
   constexpr auto capacity = 10;
   using FixedArrType = absl::FixedArray<T, capacity>;
-  using FixedArrBuffType =
-      absl::aligned_storage_t<sizeof(FixedArrType), alignof(FixedArrType)>;
   constexpr auto scrubbed_bits = 0x95;
   constexpr auto length = capacity / 2;
 
-  FixedArrBuffType buff;
-  std::memset(std::addressof(buff), scrubbed_bits, sizeof(FixedArrBuffType));
+  alignas(FixedArrType) unsigned char buff[sizeof(FixedArrType)];
+  std::memset(std::addressof(buff), scrubbed_bits, sizeof(FixedArrType));
 
   FixedArrType* arr =
       ::new (static_cast<void*>(std::addressof(buff))) FixedArrType(length);
diff --git a/absl/container/flat_hash_map.h b/absl/container/flat_hash_map.h
index a711398..fcb70d8 100644
--- a/absl/container/flat_hash_map.h
+++ b/absl/container/flat_hash_map.h
@@ -42,7 +42,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <class K, class V>
 struct FlatHashMapPolicy;
@@ -361,6 +361,10 @@
   // Inserts (via copy or move) the element of the specified key into the
   // `flat_hash_map` using the position of `hint` as a non-binding suggestion
   // for where to begin the insertion search.
+  //
+  // All `try_emplace()` overloads make the same guarantees regarding rvalue
+  // arguments as `std::unordered_map::try_emplace()`, namely that these
+  // functions will not move from rvalue arguments if insertions do not happen.
   using Base::try_emplace;
 
   // flat_hash_map::extract()
@@ -398,7 +402,7 @@
   // for the past-the-end iterator, which is invalidated.
   //
   // `swap()` requires that the flat hash map's hashing and key equivalence
-  // functions be Swappable, and are exchaged using unqualified calls to
+  // functions be Swappable, and are exchanged using unqualified calls to
   // non-member `swap()`. If the map's allocator has
   // `std::allocator_traits<allocator_type>::propagate_on_container_swap::value`
   // set to `true`, the allocators are also exchanged using an unqualified call
@@ -528,6 +532,15 @@
   using Base::key_eq;
 };
 
+// erase_if(flat_hash_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Predicate>
+void erase_if(flat_hash_map<K, V, H, E, A>& c, Predicate pred) {
+  container_internal::EraseIf(pred, &c);
+}
+
 namespace container_internal {
 
 template <class K, class V>
@@ -581,7 +594,7 @@
 
 }  // namespace container_algorithm_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FLAT_HASH_MAP_H_
diff --git a/absl/container/flat_hash_map_test.cc b/absl/container/flat_hash_map_test.cc
index 3f11a52..728b693 100644
--- a/absl/container/flat_hash_map_test.cc
+++ b/absl/container/flat_hash_map_test.cc
@@ -14,6 +14,8 @@
 
 #include "absl/container/flat_hash_map.h"
 
+#include <memory>
+
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/unordered_map_constructor_test.h"
 #include "absl/container/internal/unordered_map_lookup_test.h"
@@ -22,12 +24,13 @@
 #include "absl/types/any.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 using ::absl::container_internal::hash_internal::Enum;
 using ::absl::container_internal::hash_internal::EnumClass;
 using ::testing::_;
+using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
@@ -47,6 +50,11 @@
 INSTANTIATE_TYPED_TEST_SUITE_P(FlatHashMap, MembersTest, MapTypes);
 INSTANTIATE_TYPED_TEST_SUITE_P(FlatHashMap, ModifiersTest, MapTypes);
 
+using UniquePtrMapTypes = ::testing::Types<Map<int, std::unique_ptr<int>>>;
+
+INSTANTIATE_TYPED_TEST_SUITE_P(FlatHashMap, UniquePtrModifiersTest,
+                               UniquePtrMapTypes);
+
 TEST(FlatHashMap, StandardLayout) {
   struct Int {
     explicit Int(size_t value) : value(value) {}
@@ -208,42 +216,44 @@
   EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 17), Pair(2, 9)));
 }
 
-#if (defined(ABSL_HAVE_STD_ANY) || !defined(_LIBCPP_VERSION)) && \
-    !defined(__EMSCRIPTEN__)
-TEST(FlatHashMap, Any) {
-  absl::flat_hash_map<int, absl::any> m;
-  m.emplace(1, 7);
-  auto it = m.find(1);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(7, absl::any_cast<int>(it->second));
+bool FirstIsEven(std::pair<const int, int> p) { return p.first % 2 == 0; }
 
-  m.emplace(std::piecewise_construct, std::make_tuple(2), std::make_tuple(8));
-  it = m.find(2);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(8, absl::any_cast<int>(it->second));
-
-  m.emplace(std::piecewise_construct, std::make_tuple(3),
-            std::make_tuple(absl::any(9)));
-  it = m.find(3);
-  ASSERT_NE(it, m.end());
-  EXPECT_EQ(9, absl::any_cast<int>(it->second));
-
-  struct H {
-    size_t operator()(const absl::any&) const { return 0; }
-  };
-  struct E {
-    bool operator()(const absl::any&, const absl::any&) const { return true; }
-  };
-  absl::flat_hash_map<absl::any, int, H, E> m2;
-  m2.emplace(1, 7);
-  auto it2 = m2.find(1);
-  ASSERT_NE(it2, m2.end());
-  EXPECT_EQ(7, it2->second);
+TEST(FlatHashMap, EraseIf) {
+  // Erase all elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, [](std::pair<const int, int>) { return true; });
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, [](std::pair<const int, int>) { return false; });
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3),
+                                        Pair(4, 4), Pair(5, 5)));
+  }
+  // Erase specific elements.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s,
+             [](std::pair<const int, int> kvp) { return kvp.first % 2 == 1; });
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(2, 2), Pair(4, 4)));
+  }
+  // Predicate is function reference.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, FirstIsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+  // Predicate is function pointer.
+  {
+    flat_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, &FirstIsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
 }
-#endif  // (defined(ABSL_HAVE_STD_ANY) || !defined(_LIBCPP_VERSION)) &&
-        // !defined(__EMSCRIPTEN__)
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/flat_hash_set.h b/absl/container/flat_hash_set.h
index 8adbbcd..94be6e3 100644
--- a/absl/container/flat_hash_set.h
+++ b/absl/container/flat_hash_set.h
@@ -40,7 +40,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <typename T>
 struct FlatHashSetPolicy;
@@ -439,6 +439,14 @@
   using Base::key_eq;
 };
 
+// erase_if(flat_hash_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename T, typename H, typename E, typename A, typename Predicate>
+void erase_if(flat_hash_set<T, H, E, A>& c, Predicate pred) {
+  container_internal::EraseIf(pred, &c);
+}
+
 namespace container_internal {
 
 template <class T>
@@ -489,7 +497,7 @@
 
 }  // namespace container_algorithm_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_FLAT_HASH_SET_H_
diff --git a/absl/container/flat_hash_set_test.cc b/absl/container/flat_hash_set_test.cc
index 56140bb..40d7f85 100644
--- a/absl/container/flat_hash_set_test.cc
+++ b/absl/container/flat_hash_set_test.cc
@@ -25,12 +25,13 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
 using ::absl::container_internal::hash_internal::Enum;
 using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::IsEmpty;
 using ::testing::Pointee;
 using ::testing::UnorderedElementsAre;
 using ::testing::UnorderedElementsAreArray;
@@ -124,7 +125,42 @@
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
 }
 
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(FlatHashSet, EraseIf) {
+  // Erase all elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int) { return true; });
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int) { return false; });
+    EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+  }
+  // Erase specific elements.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int k) { return k % 2 == 1; });
+    EXPECT_THAT(s, UnorderedElementsAre(2, 4));
+  }
+  // Predicate is function reference.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, IsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+  // Predicate is function pointer.
+  {
+    flat_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, &IsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+}
+
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/inlined_vector.h b/absl/container/inlined_vector.h
index 27186b1..2388d47 100644
--- a/absl/container/inlined_vector.h
+++ b/absl/container/inlined_vector.h
@@ -54,7 +54,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 // -----------------------------------------------------------------------------
 // InlinedVector
 // -----------------------------------------------------------------------------
@@ -70,9 +70,10 @@
   static_assert(N > 0, "`absl::InlinedVector` requires an inlined capacity.");
 
   using Storage = inlined_vector_internal::Storage<T, N, A>;
-  using rvalue_reference = typename Storage::rvalue_reference;
-  using MoveIterator = typename Storage::MoveIterator;
+
   using AllocatorTraits = typename Storage::AllocatorTraits;
+  using RValueReference = typename Storage::RValueReference;
+  using MoveIterator = typename Storage::MoveIterator;
   using IsMemcpyOk = typename Storage::IsMemcpyOk;
 
   template <typename Iterator>
@@ -93,10 +94,10 @@
   using value_type = typename Storage::value_type;
   using pointer = typename Storage::pointer;
   using const_pointer = typename Storage::const_pointer;
-  using reference = typename Storage::reference;
-  using const_reference = typename Storage::const_reference;
   using size_type = typename Storage::size_type;
   using difference_type = typename Storage::difference_type;
+  using reference = typename Storage::reference;
+  using const_reference = typename Storage::const_reference;
   using iterator = typename Storage::iterator;
   using const_iterator = typename Storage::const_iterator;
   using reverse_iterator = typename Storage::reverse_iterator;
@@ -534,7 +535,6 @@
     }
 
     erase(data() + i, data() + size());
-
     std::copy(first, last, std::back_inserter(*this));
   }
 
@@ -565,7 +565,7 @@
 
   // Overload of `InlinedVector::insert(...)` that inserts `v` at `pos` using
   // move semantics, returning an `iterator` to the newly inserted element.
-  iterator insert(const_iterator pos, rvalue_reference v) {
+  iterator insert(const_iterator pos, RValueReference v) {
     return emplace(pos, std::move(v));
   }
 
@@ -662,7 +662,7 @@
 
   // Overload of `InlinedVector::push_back(...)` for inserting `v` at `end()`
   // using move semantics.
-  void push_back(rvalue_reference v) {
+  void push_back(RValueReference v) {
     static_cast<void>(emplace_back(std::move(v)));
   }
 
@@ -714,6 +714,7 @@
     inlined_vector_internal::DestroyElements(storage_.GetAllocPtr(), data(),
                                              size());
     storage_.DeallocateIfAllocated();
+
     storage_.SetInlinedSize(0);
   }
 
@@ -841,7 +842,7 @@
   return H::combine(H::combine_contiguous(std::move(h), a.data(), size), size);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INLINED_VECTOR_H_
diff --git a/absl/container/inlined_vector_benchmark.cc b/absl/container/inlined_vector_benchmark.cc
index b99bbd6..3f2b4ed 100644
--- a/absl/container/inlined_vector_benchmark.cc
+++ b/absl/container/inlined_vector_benchmark.cc
@@ -25,42 +25,45 @@
 namespace {
 
 void BM_InlinedVectorFill(benchmark::State& state) {
+  const int len = state.range(0);
   absl::InlinedVector<int, 8> v;
-  int val = 10;
+  v.reserve(len);
   for (auto _ : state) {
+    v.resize(0);  // Use resize(0) as InlinedVector releases storage on clear().
+    for (int i = 0; i < len; ++i) {
+      v.push_back(i);
+    }
     benchmark::DoNotOptimize(v);
-    v.push_back(val);
   }
 }
-BENCHMARK(BM_InlinedVectorFill)->Range(0, 1024);
+BENCHMARK(BM_InlinedVectorFill)->Range(1, 256);
 
 void BM_InlinedVectorFillRange(benchmark::State& state) {
   const int len = state.range(0);
-  std::unique_ptr<int[]> ia(new int[len]);
-  for (int i = 0; i < len; i++) {
-    ia[i] = i;
-  }
-  auto* from = ia.get();
-  auto* to = from + len;
+  const std::vector<int> src(len, len);
+  absl::InlinedVector<int, 8> v;
+  v.reserve(len);
   for (auto _ : state) {
-    benchmark::DoNotOptimize(from);
-    benchmark::DoNotOptimize(to);
-    absl::InlinedVector<int, 8> v(from, to);
+    benchmark::DoNotOptimize(src);
+    v.assign(src.begin(), src.end());
     benchmark::DoNotOptimize(v);
   }
 }
-BENCHMARK(BM_InlinedVectorFillRange)->Range(0, 1024);
+BENCHMARK(BM_InlinedVectorFillRange)->Range(1, 256);
 
 void BM_StdVectorFill(benchmark::State& state) {
+  const int len = state.range(0);
   std::vector<int> v;
-  int val = 10;
+  v.reserve(len);
   for (auto _ : state) {
+    v.clear();
+    for (int i = 0; i < len; ++i) {
+      v.push_back(i);
+    }
     benchmark::DoNotOptimize(v);
-    benchmark::DoNotOptimize(val);
-    v.push_back(val);
   }
 }
-BENCHMARK(BM_StdVectorFill)->Range(0, 1024);
+BENCHMARK(BM_StdVectorFill)->Range(1, 256);
 
 // The purpose of the next two benchmarks is to verify that
 // absl::InlinedVector is efficient when moving is more efficent than
diff --git a/absl/container/inlined_vector_exception_safety_test.cc b/absl/container/inlined_vector_exception_safety_test.cc
index ff0da75..0e6a05b 100644
--- a/absl/container/inlined_vector_exception_safety_test.cc
+++ b/absl/container/inlined_vector_exception_safety_test.cc
@@ -12,6 +12,12 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include "absl/container/inlined_vector.h"
+
+#include "absl/base/config.h"
+
+#if defined(ABSL_HAVE_EXCEPTIONS)
+
 #include <array>
 #include <initializer_list>
 #include <iterator>
@@ -20,7 +26,6 @@
 
 #include "gtest/gtest.h"
 #include "absl/base/internal/exception_safety_testing.h"
-#include "absl/container/inlined_vector.h"
 
 namespace {
 
@@ -57,8 +62,8 @@
                                                                \
        : std::initializer_list<T>{T(0, testing::nothrow_ctor), \
                                   T(1, testing::nothrow_ctor)})
-static_assert((kLargeSize == 8 || kSmallSize == 2),
-              "Must update ABSL_INTERNAL_MAKE_INIT_LIST(...).");
+static_assert(kLargeSize == 8, "Must update ABSL_INTERNAL_MAKE_INIT_LIST(...)");
+static_assert(kSmallSize == 2, "Must update ABSL_INTERNAL_MAKE_INIT_LIST(...)");
 
 template <typename TheVecT, size_t... TheSizes>
 class TestParams {
@@ -359,9 +364,11 @@
   using VecT = typename TypeParam::VecT;
   constexpr static auto size = TypeParam::GetSizeAt(0);
 
+  // For testing calls to `emplace_back(...)` that reallocate.
   VecT full_vec{size};
   full_vec.resize(full_vec.capacity());
 
+  // For testing calls to `emplace_back(...)` that don't reallocate.
   VecT nonfull_vec{size};
   nonfull_vec.reserve(size + 1);
 
@@ -369,12 +376,11 @@
       InlinedVectorInvariants<VecT>);
 
   EXPECT_TRUE(tester.WithInitialValue(nonfull_vec).Test([](VecT* vec) {
-    vec->emplace_back();  //
+    vec->emplace_back();
   }));
 
-  EXPECT_TRUE(tester.WithInitialValue(full_vec).Test([](VecT* vec) {
-    vec->emplace_back();  //
-  }));
+  EXPECT_TRUE(tester.WithInitialValue(full_vec).Test(
+      [](VecT* vec) { vec->emplace_back(); }));
 }
 
 TYPED_TEST(OneSizeTest, PopBack) {
@@ -413,6 +419,19 @@
 
   EXPECT_TRUE(tester.Test([](VecT* vec) {
     auto it = vec->begin();
+    vec->erase(it, it);
+  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin() + (vec->size() / 2);
+    vec->erase(it, it);
+  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin() + (vec->size() - 1);
+    vec->erase(it, it);
+  }));
+
+  EXPECT_TRUE(tester.Test([](VecT* vec) {
+    auto it = vec->begin();
     vec->erase(it, it + 1);
   }));
   EXPECT_TRUE(tester.Test([](VecT* vec) {
@@ -447,9 +466,7 @@
                     .WithInitialValue(VecT{from_size})
                     .WithContracts(InlinedVectorInvariants<VecT>);
 
-  EXPECT_TRUE(tester.Test([](VecT* vec) {
-    vec->reserve(to_capacity);  //
-  }));
+  EXPECT_TRUE(tester.Test([](VecT* vec) { vec->reserve(to_capacity); }));
 }
 
 TYPED_TEST(OneSizeTest, ShrinkToFit) {
@@ -487,3 +504,5 @@
 }
 
 }  // namespace
+
+#endif  // defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/container/inlined_vector_test.cc b/absl/container/inlined_vector_test.cc
index bada4fe..2c9b0d0 100644
--- a/absl/container/inlined_vector_test.cc
+++ b/absl/container/inlined_vector_test.cc
@@ -1689,7 +1689,11 @@
   inlined_case.emplace_back();
 
   int64_t absl_responsible_for_count = total_allocated_byte_count;
+
+  // MSVC's allocator preemptively allocates in debug mode
+#if !defined(_MSC_VER)
   EXPECT_EQ(absl_responsible_for_count, 0);
+#endif  // !defined(_MSC_VER)
 
   inlined_case[0].emplace_back();
   EXPECT_GT(total_allocated_byte_count, absl_responsible_for_count);
@@ -1750,6 +1754,30 @@
   }
 }
 
+TEST(InlinedVectorTest, MinimumAllocatorCompilesUsingTraits) {
+  using T = int;
+  using A = std::allocator<T>;
+  using ATraits = absl::allocator_traits<A>;
+
+  struct MinimumAllocator {
+    using value_type = T;
+
+    value_type* allocate(size_t n) {
+      A a;
+      return ATraits::allocate(a, n);
+    }
+
+    void deallocate(value_type* p, size_t n) {
+      A a;
+      ATraits::deallocate(a, p, n);
+    }
+  };
+
+  absl::InlinedVector<T, 1, MinimumAllocator> vec;
+  vec.emplace_back();
+  vec.resize(0);
+}
+
 TEST(InlinedVectorTest, AbslHashValueWorks) {
   using V = absl::InlinedVector<int, 4>;
   std::vector<V> cases;
diff --git a/absl/container/internal/btree.h b/absl/container/internal/btree.h
new file mode 100644
index 0000000..fd5c0e7
--- /dev/null
+++ b/absl/container/internal/btree.h
@@ -0,0 +1,2614 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// A btree implementation of the STL set and map interfaces. A btree is smaller
+// and generally also faster than STL set/map (refer to the benchmarks below).
+// The red-black tree implementation of STL set/map has an overhead of 3
+// pointers (left, right and parent) plus the node color information for each
+// stored value. So a set<int32_t> consumes 40 bytes for each value stored in
+// 64-bit mode. This btree implementation stores multiple values on fixed
+// size nodes (usually 256 bytes) and doesn't store child pointers for leaf
+// nodes. The result is that a btree_set<int32_t> may use much less memory per
+// stored value. For the random insertion benchmark in btree_bench.cc, a
+// btree_set<int32_t> with node-size of 256 uses 5.1 bytes per stored value.
+//
+// The packing of multiple values on to each node of a btree has another effect
+// besides better space utilization: better cache locality due to fewer cache
+// lines being accessed. Better cache locality translates into faster
+// operations.
+//
+// CAVEATS
+//
+// Insertions and deletions on a btree can cause splitting, merging or
+// rebalancing of btree nodes. And even without these operations, insertions
+// and deletions on a btree will move values around within a node. In both
+// cases, the result is that insertions and deletions can invalidate iterators
+// pointing to values other than the one being inserted/deleted. Therefore, this
+// container does not provide pointer stability. This is notably different from
+// STL set/map which takes care to not invalidate iterators on insert/erase
+// except, of course, for iterators pointing to the value being erased.  A
+// partial workaround when erasing is available: erase() returns an iterator
+// pointing to the item just after the one that was erased (or end() if none
+// exists).
+
+#ifndef ABSL_CONTAINER_INTERNAL_BTREE_H_
+#define ABSL_CONTAINER_INTERNAL_BTREE_H_
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <functional>
+#include <iterator>
+#include <limits>
+#include <new>
+#include <string>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/macros.h"
+#include "absl/container/internal/common.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/container/internal/container_memory.h"
+#include "absl/container/internal/layout.h"
+#include "absl/memory/memory.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/compare.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// A helper class that indicates if the Compare parameter is a key-compare-to
+// comparator.
+template <typename Compare, typename T>
+using btree_is_key_compare_to =
+    std::is_convertible<absl::result_of_t<Compare(const T &, const T &)>,
+                        absl::weak_ordering>;
+
+struct StringBtreeDefaultLess {
+  using is_transparent = void;
+
+  StringBtreeDefaultLess() = default;
+
+  // Compatibility constructor.
+  StringBtreeDefaultLess(std::less<std::string>) {}  // NOLINT
+  StringBtreeDefaultLess(std::less<string_view>) {}  // NOLINT
+
+  absl::weak_ordering operator()(absl::string_view lhs,
+                                 absl::string_view rhs) const {
+    return compare_internal::compare_result_as_ordering(lhs.compare(rhs));
+  }
+};
+
+struct StringBtreeDefaultGreater {
+  using is_transparent = void;
+
+  StringBtreeDefaultGreater() = default;
+
+  StringBtreeDefaultGreater(std::greater<std::string>) {}  // NOLINT
+  StringBtreeDefaultGreater(std::greater<string_view>) {}  // NOLINT
+
+  absl::weak_ordering operator()(absl::string_view lhs,
+                                 absl::string_view rhs) const {
+    return compare_internal::compare_result_as_ordering(rhs.compare(lhs));
+  }
+};
+
+// A helper class to convert a boolean comparison into a three-way "compare-to"
+// comparison that returns a negative value to indicate less-than, zero to
+// indicate equality and a positive value to indicate greater-than. This helper
+// class is specialized for less<std::string>, greater<std::string>,
+// less<string_view>, and greater<string_view>.
+//
+// key_compare_to_adapter is provided so that btree users
+// automatically get the more efficient compare-to code when using common
+// google string types with common comparison functors.
+// These string-like specializations also turn on heterogeneous lookup by
+// default.
+template <typename Compare>
+struct key_compare_to_adapter {
+  using type = Compare;
+};
+
+template <>
+struct key_compare_to_adapter<std::less<std::string>> {
+  using type = StringBtreeDefaultLess;
+};
+
+template <>
+struct key_compare_to_adapter<std::greater<std::string>> {
+  using type = StringBtreeDefaultGreater;
+};
+
+template <>
+struct key_compare_to_adapter<std::less<absl::string_view>> {
+  using type = StringBtreeDefaultLess;
+};
+
+template <>
+struct key_compare_to_adapter<std::greater<absl::string_view>> {
+  using type = StringBtreeDefaultGreater;
+};
+
+template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
+          bool Multi, typename SlotPolicy>
+struct common_params {
+  // If Compare is a common comparator for a std::string-like type, then we adapt it
+  // to use heterogeneous lookup and to be a key-compare-to comparator.
+  using key_compare = typename key_compare_to_adapter<Compare>::type;
+  // A type which indicates if we have a key-compare-to functor or a plain old
+  // key-compare functor.
+  using is_key_compare_to = btree_is_key_compare_to<key_compare, Key>;
+
+  using allocator_type = Alloc;
+  using key_type = Key;
+  using size_type = std::make_signed<size_t>::type;
+  using difference_type = ptrdiff_t;
+
+  // True if this is a multiset or multimap.
+  using is_multi_container = std::integral_constant<bool, Multi>;
+
+  using slot_policy = SlotPolicy;
+  using slot_type = typename slot_policy::slot_type;
+  using value_type = typename slot_policy::value_type;
+  using init_type = typename slot_policy::mutable_value_type;
+  using pointer = value_type *;
+  using const_pointer = const value_type *;
+  using reference = value_type &;
+  using const_reference = const value_type &;
+
+  enum {
+    kTargetNodeSize = TargetNodeSize,
+
+    // Upper bound for the available space for values. This is largest for leaf
+    // nodes, which have overhead of at least a pointer + 4 bytes (for storing
+    // 3 field_types and an enum).
+    kNodeValueSpace =
+        TargetNodeSize - /*minimum overhead=*/(sizeof(void *) + 4),
+  };
+
+  // This is an integral type large enough to hold as many
+  // ValueSize-values as will fit a node of TargetNodeSize bytes.
+  using node_count_type =
+      absl::conditional_t<(kNodeValueSpace / sizeof(value_type) >
+                           (std::numeric_limits<uint8_t>::max)()),
+                          uint16_t, uint8_t>;  // NOLINT
+
+  // The following methods are necessary for passing this struct as PolicyTraits
+  // for node_handle and/or are used within btree.
+  static value_type &element(slot_type *slot) {
+    return slot_policy::element(slot);
+  }
+  static const value_type &element(const slot_type *slot) {
+    return slot_policy::element(slot);
+  }
+  template <class... Args>
+  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
+    slot_policy::construct(alloc, slot, std::forward<Args>(args)...);
+  }
+  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
+    slot_policy::construct(alloc, slot, other);
+  }
+  static void destroy(Alloc *alloc, slot_type *slot) {
+    slot_policy::destroy(alloc, slot);
+  }
+  static void transfer(Alloc *alloc, slot_type *new_slot, slot_type *old_slot) {
+    construct(alloc, new_slot, old_slot);
+    destroy(alloc, old_slot);
+  }
+  static void swap(Alloc *alloc, slot_type *a, slot_type *b) {
+    slot_policy::swap(alloc, a, b);
+  }
+  static void move(Alloc *alloc, slot_type *src, slot_type *dest) {
+    slot_policy::move(alloc, src, dest);
+  }
+  static void move(Alloc *alloc, slot_type *first, slot_type *last,
+                   slot_type *result) {
+    slot_policy::move(alloc, first, last, result);
+  }
+};
+
+// A parameters structure for holding the type parameters for a btree_map.
+// Compare and Alloc should be nothrow copy-constructible.
+template <typename Key, typename Data, typename Compare, typename Alloc,
+          int TargetNodeSize, bool Multi>
+struct map_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
+                                  map_slot_policy<Key, Data>> {
+  using super_type = typename map_params::common_params;
+  using mapped_type = Data;
+  // This type allows us to move keys when it is safe to do so. It is safe
+  // for maps in which value_type and mutable_value_type are layout compatible.
+  using slot_policy = typename super_type::slot_policy;
+  using slot_type = typename super_type::slot_type;
+  using value_type = typename super_type::value_type;
+  using init_type = typename super_type::init_type;
+
+  using key_compare = typename super_type::key_compare;
+  // Inherit from key_compare for empty base class optimization.
+  struct value_compare : private key_compare {
+    value_compare() = default;
+    explicit value_compare(const key_compare &cmp) : key_compare(cmp) {}
+
+    template <typename T, typename U>
+    auto operator()(const T &left, const U &right) const
+        -> decltype(std::declval<key_compare>()(left.first, right.first)) {
+      return key_compare::operator()(left.first, right.first);
+    }
+  };
+  using is_map_container = std::true_type;
+
+  static const Key &key(const value_type &x) { return x.first; }
+  static const Key &key(const init_type &x) { return x.first; }
+  static const Key &key(const slot_type *x) { return slot_policy::key(x); }
+  static mapped_type &value(value_type *value) { return value->second; }
+};
+
+// This type implements the necessary functions from the
+// absl::container_internal::slot_type interface.
+template <typename Key>
+struct set_slot_policy {
+  using slot_type = Key;
+  using value_type = Key;
+  using mutable_value_type = Key;
+
+  static value_type &element(slot_type *slot) { return *slot; }
+  static const value_type &element(const slot_type *slot) { return *slot; }
+
+  template <typename Alloc, class... Args>
+  static void construct(Alloc *alloc, slot_type *slot, Args &&... args) {
+    absl::allocator_traits<Alloc>::construct(*alloc, slot,
+                                             std::forward<Args>(args)...);
+  }
+
+  template <typename Alloc>
+  static void construct(Alloc *alloc, slot_type *slot, slot_type *other) {
+    absl::allocator_traits<Alloc>::construct(*alloc, slot, std::move(*other));
+  }
+
+  template <typename Alloc>
+  static void destroy(Alloc *alloc, slot_type *slot) {
+    absl::allocator_traits<Alloc>::destroy(*alloc, slot);
+  }
+
+  template <typename Alloc>
+  static void swap(Alloc * /*alloc*/, slot_type *a, slot_type *b) {
+    using std::swap;
+    swap(*a, *b);
+  }
+
+  template <typename Alloc>
+  static void move(Alloc * /*alloc*/, slot_type *src, slot_type *dest) {
+    *dest = std::move(*src);
+  }
+
+  template <typename Alloc>
+  static void move(Alloc *alloc, slot_type *first, slot_type *last,
+                   slot_type *result) {
+    for (slot_type *src = first, *dest = result; src != last; ++src, ++dest)
+      move(alloc, src, dest);
+  }
+};
+
+// A parameters structure for holding the type parameters for a btree_set.
+// Compare and Alloc should be nothrow copy-constructible.
+template <typename Key, typename Compare, typename Alloc, int TargetNodeSize,
+          bool Multi>
+struct set_params : common_params<Key, Compare, Alloc, TargetNodeSize, Multi,
+                                  set_slot_policy<Key>> {
+  using value_type = Key;
+  using slot_type = typename set_params::common_params::slot_type;
+  using value_compare = typename set_params::common_params::key_compare;
+  using is_map_container = std::false_type;
+
+  static const Key &key(const value_type &x) { return x; }
+  static const Key &key(const slot_type *x) { return *x; }
+};
+
+// An adapter class that converts a lower-bound compare into an upper-bound
+// compare. Note: there is no need to make a version of this adapter specialized
+// for key-compare-to functors because the upper-bound (the first value greater
+// than the input) is never an exact match.
+template <typename Compare>
+struct upper_bound_adapter {
+  explicit upper_bound_adapter(const Compare &c) : comp(c) {}
+  template <typename K, typename LK>
+  bool operator()(const K &a, const LK &b) const {
+    // Returns true when a is not greater than b.
+    return !compare_internal::compare_result_as_less_than(comp(b, a));
+  }
+
+ private:
+  Compare comp;
+};
+
+enum class MatchKind : uint8_t { kEq, kNe };
+
+template <typename V, bool IsCompareTo>
+struct SearchResult {
+  V value;
+  MatchKind match;
+
+  static constexpr bool HasMatch() { return true; }
+  bool IsEq() const { return match == MatchKind::kEq; }
+};
+
+// When we don't use CompareTo, `match` is not present.
+// This ensures that callers can't use it accidentally when it provides no
+// useful information.
+template <typename V>
+struct SearchResult<V, false> {
+  V value;
+
+  static constexpr bool HasMatch() { return false; }
+  static constexpr bool IsEq() { return false; }
+};
+
+// A node in the btree holding. The same node type is used for both internal
+// and leaf nodes in the btree, though the nodes are allocated in such a way
+// that the children array is only valid in internal nodes.
+template <typename Params>
+class btree_node {
+  using is_key_compare_to = typename Params::is_key_compare_to;
+  using is_multi_container = typename Params::is_multi_container;
+  using field_type = typename Params::node_count_type;
+  using allocator_type = typename Params::allocator_type;
+  using slot_type = typename Params::slot_type;
+
+ public:
+  using params_type = Params;
+  using key_type = typename Params::key_type;
+  using value_type = typename Params::value_type;
+  using pointer = typename Params::pointer;
+  using const_pointer = typename Params::const_pointer;
+  using reference = typename Params::reference;
+  using const_reference = typename Params::const_reference;
+  using key_compare = typename Params::key_compare;
+  using size_type = typename Params::size_type;
+  using difference_type = typename Params::difference_type;
+
+  // Btree decides whether to use linear node search as follows:
+  //   - If the key is arithmetic and the comparator is std::less or
+  //     std::greater, choose linear.
+  //   - Otherwise, choose binary.
+  // TODO(ezb): Might make sense to add condition(s) based on node-size.
+  using use_linear_search = std::integral_constant<
+      bool,
+                std::is_arithmetic<key_type>::value &&
+                    (std::is_same<std::less<key_type>, key_compare>::value ||
+                     std::is_same<std::greater<key_type>, key_compare>::value)>;
+
+  // This class is organized by gtl::Layout as if it had the following
+  // structure:
+  //   // A pointer to the node's parent.
+  //   btree_node *parent;
+  //
+  //   // The position of the node in the node's parent.
+  //   field_type position;
+  //   // The index of the first populated value in `values`.
+  //   // TODO(ezb): right now, `start` is always 0. Update insertion/merge
+  //   // logic to allow for floating storage within nodes.
+  //   field_type start;
+  //   // The index after the last populated value in `values`. Currently, this
+  //   // is the same as the count of values.
+  //   field_type finish;
+  //   // The maximum number of values the node can hold. This is an integer in
+  //   // [1, kNodeValues] for root leaf nodes, kNodeValues for non-root leaf
+  //   // nodes, and kInternalNodeMaxCount (as a sentinel value) for internal
+  //   // nodes (even though there are still kNodeValues values in the node).
+  //   // TODO(ezb): make max_count use only 4 bits and record log2(capacity)
+  //   // to free extra bits for is_root, etc.
+  //   field_type max_count;
+  //
+  //   // The array of values. The capacity is `max_count` for leaf nodes and
+  //   // kNodeValues for internal nodes. Only the values in
+  //   // [start, finish) have been initialized and are valid.
+  //   slot_type values[max_count];
+  //
+  //   // The array of child pointers. The keys in children[i] are all less
+  //   // than key(i). The keys in children[i + 1] are all greater than key(i).
+  //   // There are 0 children for leaf nodes and kNodeValues + 1 children for
+  //   // internal nodes.
+  //   btree_node *children[kNodeValues + 1];
+  //
+  // This class is only constructed by EmptyNodeType. Normally, pointers to the
+  // layout above are allocated, cast to btree_node*, and de-allocated within
+  // the btree implementation.
+  ~btree_node() = default;
+  btree_node(btree_node const &) = delete;
+  btree_node &operator=(btree_node const &) = delete;
+
+  // Public for EmptyNodeType.
+  constexpr static size_type Alignment() {
+    static_assert(LeafLayout(1).Alignment() == InternalLayout().Alignment(),
+                  "Alignment of all nodes must be equal.");
+    return InternalLayout().Alignment();
+  }
+
+ protected:
+  btree_node() = default;
+
+ private:
+  using layout_type = absl::container_internal::Layout<btree_node *, field_type,
+                                                       slot_type, btree_node *>;
+  constexpr static size_type SizeWithNValues(size_type n) {
+    return layout_type(/*parent*/ 1,
+                       /*position, start, finish, max_count*/ 4,
+                       /*values*/ n,
+                       /*children*/ 0)
+        .AllocSize();
+  }
+  // A lower bound for the overhead of fields other than values in a leaf node.
+  constexpr static size_type MinimumOverhead() {
+    return SizeWithNValues(1) - sizeof(value_type);
+  }
+
+  // Compute how many values we can fit onto a leaf node taking into account
+  // padding.
+  constexpr static size_type NodeTargetValues(const int begin, const int end) {
+    return begin == end ? begin
+                        : SizeWithNValues((begin + end) / 2 + 1) >
+                                  params_type::kTargetNodeSize
+                              ? NodeTargetValues(begin, (begin + end) / 2)
+                              : NodeTargetValues((begin + end) / 2 + 1, end);
+  }
+
+  enum {
+    kTargetNodeSize = params_type::kTargetNodeSize,
+    kNodeTargetValues = NodeTargetValues(0, params_type::kTargetNodeSize),
+
+    // We need a minimum of 3 values per internal node in order to perform
+    // splitting (1 value for the two nodes involved in the split and 1 value
+    // propagated to the parent as the delimiter for the split).
+    kNodeValues = kNodeTargetValues >= 3 ? kNodeTargetValues : 3,
+
+    // The node is internal (i.e. is not a leaf node) if and only if `max_count`
+    // has this value.
+    kInternalNodeMaxCount = 0,
+  };
+
+  // Leaves can have less than kNodeValues values.
+  constexpr static layout_type LeafLayout(const int max_values = kNodeValues) {
+    return layout_type(/*parent*/ 1,
+                       /*position, start, finish, max_count*/ 4,
+                       /*values*/ max_values,
+                       /*children*/ 0);
+  }
+  constexpr static layout_type InternalLayout() {
+    return layout_type(/*parent*/ 1,
+                       /*position, start, finish, max_count*/ 4,
+                       /*values*/ kNodeValues,
+                       /*children*/ kNodeValues + 1);
+  }
+  constexpr static size_type LeafSize(const int max_values = kNodeValues) {
+    return LeafLayout(max_values).AllocSize();
+  }
+  constexpr static size_type InternalSize() {
+    return InternalLayout().AllocSize();
+  }
+
+  // N is the index of the type in the Layout definition.
+  // ElementType<N> is the Nth type in the Layout definition.
+  template <size_type N>
+  inline typename layout_type::template ElementType<N> *GetField() {
+    // We assert that we don't read from values that aren't there.
+    assert(N < 3 || !leaf());
+    return InternalLayout().template Pointer<N>(reinterpret_cast<char *>(this));
+  }
+  template <size_type N>
+  inline const typename layout_type::template ElementType<N> *GetField() const {
+    assert(N < 3 || !leaf());
+    return InternalLayout().template Pointer<N>(
+        reinterpret_cast<const char *>(this));
+  }
+  void set_parent(btree_node *p) { *GetField<0>() = p; }
+  field_type &mutable_finish() { return GetField<1>()[2]; }
+  slot_type *slot(int i) { return &GetField<2>()[i]; }
+  slot_type *start_slot() { return slot(start()); }
+  slot_type *finish_slot() { return slot(finish()); }
+  const slot_type *slot(int i) const { return &GetField<2>()[i]; }
+  void set_position(field_type v) { GetField<1>()[0] = v; }
+  void set_start(field_type v) { GetField<1>()[1] = v; }
+  void set_finish(field_type v) { GetField<1>()[2] = v; }
+  // This method is only called by the node init methods.
+  void set_max_count(field_type v) { GetField<1>()[3] = v; }
+
+ public:
+  // Whether this is a leaf node or not. This value doesn't change after the
+  // node is created.
+  bool leaf() const { return GetField<1>()[3] != kInternalNodeMaxCount; }
+
+  // Getter for the position of this node in its parent.
+  field_type position() const { return GetField<1>()[0]; }
+
+  // Getter for the offset of the first value in the `values` array.
+  field_type start() const {
+    // TODO(ezb): when floating storage is implemented, return GetField<1>()[1];
+    assert(GetField<1>()[1] == 0);
+    return 0;
+  }
+
+  // Getter for the offset after the last value in the `values` array.
+  field_type finish() const { return GetField<1>()[2]; }
+
+  // Getters for the number of values stored in this node.
+  field_type count() const {
+    assert(finish() >= start());
+    return finish() - start();
+  }
+  field_type max_count() const {
+    // Internal nodes have max_count==kInternalNodeMaxCount.
+    // Leaf nodes have max_count in [1, kNodeValues].
+    const field_type max_count = GetField<1>()[3];
+    return max_count == field_type{kInternalNodeMaxCount}
+               ? field_type{kNodeValues}
+               : max_count;
+  }
+
+  // Getter for the parent of this node.
+  btree_node *parent() const { return *GetField<0>(); }
+  // Getter for whether the node is the root of the tree. The parent of the
+  // root of the tree is the leftmost node in the tree which is guaranteed to
+  // be a leaf.
+  bool is_root() const { return parent()->leaf(); }
+  void make_root() {
+    assert(parent()->is_root());
+    set_parent(parent()->parent());
+  }
+
+  // Getters for the key/value at position i in the node.
+  const key_type &key(int i) const { return params_type::key(slot(i)); }
+  reference value(int i) { return params_type::element(slot(i)); }
+  const_reference value(int i) const { return params_type::element(slot(i)); }
+
+  // Getters/setter for the child at position i in the node.
+  btree_node *child(int i) const { return GetField<3>()[i]; }
+  btree_node *start_child() const { return child(start()); }
+  btree_node *&mutable_child(int i) { return GetField<3>()[i]; }
+  void clear_child(int i) {
+    absl::container_internal::SanitizerPoisonObject(&mutable_child(i));
+  }
+  void set_child(int i, btree_node *c) {
+    absl::container_internal::SanitizerUnpoisonObject(&mutable_child(i));
+    mutable_child(i) = c;
+    c->set_position(i);
+  }
+  void init_child(int i, btree_node *c) {
+    set_child(i, c);
+    c->set_parent(this);
+  }
+
+  // Returns the position of the first value whose key is not less than k.
+  template <typename K>
+  SearchResult<int, is_key_compare_to::value> lower_bound(
+      const K &k, const key_compare &comp) const {
+    return use_linear_search::value ? linear_search(k, comp)
+                                    : binary_search(k, comp);
+  }
+  // Returns the position of the first value whose key is greater than k.
+  template <typename K>
+  int upper_bound(const K &k, const key_compare &comp) const {
+    auto upper_compare = upper_bound_adapter<key_compare>(comp);
+    return use_linear_search::value ? linear_search(k, upper_compare).value
+                                    : binary_search(k, upper_compare).value;
+  }
+
+  template <typename K, typename Compare>
+  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
+  linear_search(const K &k, const Compare &comp) const {
+    return linear_search_impl(k, start(), finish(), comp,
+                              btree_is_key_compare_to<Compare, key_type>());
+  }
+
+  template <typename K, typename Compare>
+  SearchResult<int, btree_is_key_compare_to<Compare, key_type>::value>
+  binary_search(const K &k, const Compare &comp) const {
+    return binary_search_impl(k, start(), finish(), comp,
+                              btree_is_key_compare_to<Compare, key_type>());
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // linear search performed using plain compare.
+  template <typename K, typename Compare>
+  SearchResult<int, false> linear_search_impl(
+      const K &k, int s, const int e, const Compare &comp,
+      std::false_type /* IsCompareTo */) const {
+    while (s < e) {
+      if (!comp(key(s), k)) {
+        break;
+      }
+      ++s;
+    }
+    return {s};
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // linear search performed using compare-to.
+  template <typename K, typename Compare>
+  SearchResult<int, true> linear_search_impl(
+      const K &k, int s, const int e, const Compare &comp,
+      std::true_type /* IsCompareTo */) const {
+    while (s < e) {
+      const absl::weak_ordering c = comp(key(s), k);
+      if (c == 0) {
+        return {s, MatchKind::kEq};
+      } else if (c > 0) {
+        break;
+      }
+      ++s;
+    }
+    return {s, MatchKind::kNe};
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // binary search performed using plain compare.
+  template <typename K, typename Compare>
+  SearchResult<int, false> binary_search_impl(
+      const K &k, int s, int e, const Compare &comp,
+      std::false_type /* IsCompareTo */) const {
+    while (s != e) {
+      const int mid = (s + e) >> 1;
+      if (comp(key(mid), k)) {
+        s = mid + 1;
+      } else {
+        e = mid;
+      }
+    }
+    return {s};
+  }
+
+  // Returns the position of the first value whose key is not less than k using
+  // binary search performed using compare-to.
+  template <typename K, typename CompareTo>
+  SearchResult<int, true> binary_search_impl(
+      const K &k, int s, int e, const CompareTo &comp,
+      std::true_type /* IsCompareTo */) const {
+    if (is_multi_container::value) {
+      MatchKind exact_match = MatchKind::kNe;
+      while (s != e) {
+        const int mid = (s + e) >> 1;
+        const absl::weak_ordering c = comp(key(mid), k);
+        if (c < 0) {
+          s = mid + 1;
+        } else {
+          e = mid;
+          if (c == 0) {
+            // Need to return the first value whose key is not less than k,
+            // which requires continuing the binary search if this is a
+            // multi-container.
+            exact_match = MatchKind::kEq;
+          }
+        }
+      }
+      return {s, exact_match};
+    } else {  // Not a multi-container.
+      while (s != e) {
+        const int mid = (s + e) >> 1;
+        const absl::weak_ordering c = comp(key(mid), k);
+        if (c < 0) {
+          s = mid + 1;
+        } else if (c > 0) {
+          e = mid;
+        } else {
+          return {mid, MatchKind::kEq};
+        }
+      }
+      return {s, MatchKind::kNe};
+    }
+  }
+
+  // Emplaces a value at position i, shifting all existing values and
+  // children at positions >= i to the right by 1.
+  template <typename... Args>
+  void emplace_value(size_type i, allocator_type *alloc, Args &&... args);
+
+  // Removes the value at position i, shifting all existing values and children
+  // at positions > i to the left by 1.
+  void remove_value(int i, allocator_type *alloc);
+
+  // Removes the values at positions [i, i + to_erase), shifting all values
+  // after that range to the left by to_erase. Does not change children at all.
+  void remove_values_ignore_children(int i, int to_erase,
+                                     allocator_type *alloc);
+
+  // Rebalances a node with its right sibling.
+  void rebalance_right_to_left(int to_move, btree_node *right,
+                               allocator_type *alloc);
+  void rebalance_left_to_right(int to_move, btree_node *right,
+                               allocator_type *alloc);
+
+  // Splits a node, moving a portion of the node's values to its right sibling.
+  void split(int insert_position, btree_node *dest, allocator_type *alloc);
+
+  // Merges a node with its right sibling, moving all of the values and the
+  // delimiting key in the parent node onto itself.
+  void merge(btree_node *sibling, allocator_type *alloc);
+
+  // Swap the contents of "this" and "src".
+  void swap(btree_node *src, allocator_type *alloc);
+
+  // Node allocation/deletion routines.
+  static btree_node *init_leaf(btree_node *n, btree_node *parent,
+                               int max_count) {
+    n->set_parent(parent);
+    n->set_position(0);
+    n->set_start(0);
+    n->set_finish(0);
+    n->set_max_count(max_count);
+    absl::container_internal::SanitizerPoisonMemoryRegion(
+        n->start_slot(), max_count * sizeof(slot_type));
+    return n;
+  }
+  static btree_node *init_internal(btree_node *n, btree_node *parent) {
+    init_leaf(n, parent, kNodeValues);
+    // Set `max_count` to a sentinel value to indicate that this node is
+    // internal.
+    n->set_max_count(kInternalNodeMaxCount);
+    absl::container_internal::SanitizerPoisonMemoryRegion(
+        &n->mutable_child(n->start()),
+        (kNodeValues + 1) * sizeof(btree_node *));
+    return n;
+  }
+  void destroy(allocator_type *alloc) {
+    for (int i = start(); i < finish(); ++i) {
+      value_destroy(i, alloc);
+    }
+  }
+
+ public:
+  // Exposed only for tests.
+  static bool testonly_uses_linear_node_search() {
+    return use_linear_search::value;
+  }
+
+ private:
+  template <typename... Args>
+  void value_init(const size_type i, allocator_type *alloc, Args &&... args) {
+    absl::container_internal::SanitizerUnpoisonObject(slot(i));
+    params_type::construct(alloc, slot(i), std::forward<Args>(args)...);
+  }
+  void value_destroy(const size_type i, allocator_type *alloc) {
+    params_type::destroy(alloc, slot(i));
+    absl::container_internal::SanitizerPoisonObject(slot(i));
+  }
+
+  // Move n values starting at value i in this node into the values starting at
+  // value j in node x.
+  void uninitialized_move_n(const size_type n, const size_type i,
+                            const size_type j, btree_node *x,
+                            allocator_type *alloc) {
+    absl::container_internal::SanitizerUnpoisonMemoryRegion(
+        x->slot(j), n * sizeof(slot_type));
+    for (slot_type *src = slot(i), *end = src + n, *dest = x->slot(j);
+         src != end; ++src, ++dest) {
+      params_type::construct(alloc, dest, src);
+    }
+  }
+
+  // Destroys a range of n values, starting at index i.
+  void value_destroy_n(const size_type i, const size_type n,
+                       allocator_type *alloc) {
+    for (int j = 0; j < n; ++j) {
+      value_destroy(i + j, alloc);
+    }
+  }
+
+  template <typename P>
+  friend class btree;
+  template <typename N, typename R, typename P>
+  friend struct btree_iterator;
+  friend class BtreeNodePeer;
+};
+
+template <typename Node, typename Reference, typename Pointer>
+struct btree_iterator {
+ private:
+  using key_type = typename Node::key_type;
+  using size_type = typename Node::size_type;
+  using params_type = typename Node::params_type;
+
+  using node_type = Node;
+  using normal_node = typename std::remove_const<Node>::type;
+  using const_node = const Node;
+  using normal_pointer = typename params_type::pointer;
+  using normal_reference = typename params_type::reference;
+  using const_pointer = typename params_type::const_pointer;
+  using const_reference = typename params_type::const_reference;
+  using slot_type = typename params_type::slot_type;
+
+  using iterator =
+      btree_iterator<normal_node, normal_reference, normal_pointer>;
+  using const_iterator =
+      btree_iterator<const_node, const_reference, const_pointer>;
+
+ public:
+  // These aliases are public for std::iterator_traits.
+  using difference_type = typename Node::difference_type;
+  using value_type = typename params_type::value_type;
+  using pointer = Pointer;
+  using reference = Reference;
+  using iterator_category = std::bidirectional_iterator_tag;
+
+  btree_iterator() : node(nullptr), position(-1) {}
+  explicit btree_iterator(Node *n) : node(n), position(n->start()) {}
+  btree_iterator(Node *n, int p) : node(n), position(p) {}
+
+  // NOTE: this SFINAE allows for implicit conversions from iterator to
+  // const_iterator, but it specifically avoids defining copy constructors so
+  // that btree_iterator can be trivially copyable. This is for performance and
+  // binary size reasons.
+  template <typename N, typename R, typename P,
+            absl::enable_if_t<
+                std::is_same<btree_iterator<N, R, P>, iterator>::value &&
+                    std::is_same<btree_iterator, const_iterator>::value,
+                int> = 0>
+  btree_iterator(const btree_iterator<N, R, P> &x)  // NOLINT
+      : node(x.node), position(x.position) {}
+
+ private:
+  // This SFINAE allows explicit conversions from const_iterator to
+  // iterator, but also avoids defining a copy constructor.
+  // NOTE: the const_cast is safe because this constructor is only called by
+  // non-const methods and the container owns the nodes.
+  template <typename N, typename R, typename P,
+            absl::enable_if_t<
+                std::is_same<btree_iterator<N, R, P>, const_iterator>::value &&
+                    std::is_same<btree_iterator, iterator>::value,
+                int> = 0>
+  explicit btree_iterator(const btree_iterator<N, R, P> &x)
+      : node(const_cast<node_type *>(x.node)), position(x.position) {}
+
+  // Increment/decrement the iterator.
+  void increment() {
+    if (node->leaf() && ++position < node->finish()) {
+      return;
+    }
+    increment_slow();
+  }
+  void increment_slow();
+
+  void decrement() {
+    if (node->leaf() && --position >= node->start()) {
+      return;
+    }
+    decrement_slow();
+  }
+  void decrement_slow();
+
+ public:
+  bool operator==(const const_iterator &x) const {
+    return node == x.node && position == x.position;
+  }
+  bool operator!=(const const_iterator &x) const {
+    return node != x.node || position != x.position;
+  }
+
+  // Accessors for the key/value the iterator is pointing at.
+  reference operator*() const { return node->value(position); }
+  pointer operator->() const { return &node->value(position); }
+
+  btree_iterator &operator++() {
+    increment();
+    return *this;
+  }
+  btree_iterator &operator--() {
+    decrement();
+    return *this;
+  }
+  btree_iterator operator++(int) {
+    btree_iterator tmp = *this;
+    ++*this;
+    return tmp;
+  }
+  btree_iterator operator--(int) {
+    btree_iterator tmp = *this;
+    --*this;
+    return tmp;
+  }
+
+ private:
+  template <typename Params>
+  friend class btree;
+  template <typename Tree>
+  friend class btree_container;
+  template <typename Tree>
+  friend class btree_set_container;
+  template <typename Tree>
+  friend class btree_map_container;
+  template <typename Tree>
+  friend class btree_multiset_container;
+  template <typename N, typename R, typename P>
+  friend struct btree_iterator;
+  template <typename TreeType, typename CheckerType>
+  friend class base_checker;
+
+  const key_type &key() const { return node->key(position); }
+  slot_type *slot() { return node->slot(position); }
+
+  // The node in the tree the iterator is pointing at.
+  Node *node;
+  // The position within the node of the tree the iterator is pointing at.
+  // TODO(ezb): make this a field_type
+  int position;
+};
+
+template <typename Params>
+class btree {
+  using node_type = btree_node<Params>;
+  using is_key_compare_to = typename Params::is_key_compare_to;
+
+  // We use a static empty node for the root/leftmost/rightmost of empty btrees
+  // in order to avoid branching in begin()/end().
+  struct alignas(node_type::Alignment()) EmptyNodeType : node_type {
+    using field_type = typename node_type::field_type;
+    node_type *parent;
+    field_type position = 0;
+    field_type start = 0;
+    field_type finish = 0;
+    // max_count must be != kInternalNodeMaxCount (so that this node is regarded
+    // as a leaf node). max_count() is never called when the tree is empty.
+    field_type max_count = node_type::kInternalNodeMaxCount + 1;
+
+#ifdef _MSC_VER
+    // MSVC has constexpr code generations bugs here.
+    EmptyNodeType() : parent(this) {}
+#else
+    constexpr EmptyNodeType(node_type *p) : parent(p) {}
+#endif
+  };
+
+  static node_type *EmptyNode() {
+#ifdef _MSC_VER
+    static EmptyNodeType *empty_node = new EmptyNodeType;
+    // This assert fails on some other construction methods.
+    assert(empty_node->parent == empty_node);
+    return empty_node;
+#else
+    static constexpr EmptyNodeType empty_node(
+        const_cast<EmptyNodeType *>(&empty_node));
+    return const_cast<EmptyNodeType *>(&empty_node);
+#endif
+  }
+
+  enum {
+    kNodeValues = node_type::kNodeValues,
+    kMinNodeValues = kNodeValues / 2,
+  };
+
+  struct node_stats {
+    using size_type = typename Params::size_type;
+
+    node_stats(size_type l, size_type i) : leaf_nodes(l), internal_nodes(i) {}
+
+    node_stats &operator+=(const node_stats &x) {
+      leaf_nodes += x.leaf_nodes;
+      internal_nodes += x.internal_nodes;
+      return *this;
+    }
+
+    size_type leaf_nodes;
+    size_type internal_nodes;
+  };
+
+ public:
+  using key_type = typename Params::key_type;
+  using value_type = typename Params::value_type;
+  using size_type = typename Params::size_type;
+  using difference_type = typename Params::difference_type;
+  using key_compare = typename Params::key_compare;
+  using value_compare = typename Params::value_compare;
+  using allocator_type = typename Params::allocator_type;
+  using reference = typename Params::reference;
+  using const_reference = typename Params::const_reference;
+  using pointer = typename Params::pointer;
+  using const_pointer = typename Params::const_pointer;
+  using iterator = btree_iterator<node_type, reference, pointer>;
+  using const_iterator = typename iterator::const_iterator;
+  using reverse_iterator = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using node_handle_type = node_handle<Params, Params, allocator_type>;
+
+  // Internal types made public for use by btree_container types.
+  using params_type = Params;
+  using slot_type = typename Params::slot_type;
+
+ private:
+  // For use in copy_or_move_values_in_order.
+  const value_type &maybe_move_from_iterator(const_iterator x) { return *x; }
+  value_type &&maybe_move_from_iterator(iterator x) { return std::move(*x); }
+
+  // Copies or moves (depending on the template parameter) the values in
+  // x into this btree in their order in x. This btree must be empty before this
+  // method is called. This method is used in copy construction, copy
+  // assignment, and move assignment.
+  template <typename Btree>
+  void copy_or_move_values_in_order(Btree *x);
+
+  // Validates that various assumptions/requirements are true at compile time.
+  constexpr static bool static_assert_validation();
+
+ public:
+  btree(const key_compare &comp, const allocator_type &alloc);
+
+  btree(const btree &x);
+  btree(btree &&x) noexcept
+      : root_(std::move(x.root_)),
+        rightmost_(absl::exchange(x.rightmost_, EmptyNode())),
+        size_(absl::exchange(x.size_, 0)) {
+    x.mutable_root() = EmptyNode();
+  }
+
+  ~btree() {
+    // Put static_asserts in destructor to avoid triggering them before the type
+    // is complete.
+    static_assert(static_assert_validation(), "This call must be elided.");
+    clear();
+  }
+
+  // Assign the contents of x to *this.
+  btree &operator=(const btree &x);
+  btree &operator=(btree &&x) noexcept;
+
+  iterator begin() { return iterator(leftmost()); }
+  const_iterator begin() const { return const_iterator(leftmost()); }
+  iterator end() { return iterator(rightmost_, rightmost_->finish()); }
+  const_iterator end() const {
+    return const_iterator(rightmost_, rightmost_->finish());
+  }
+  reverse_iterator rbegin() { return reverse_iterator(end()); }
+  const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+  reverse_iterator rend() { return reverse_iterator(begin()); }
+  const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+
+  // Finds the first element whose key is not less than key.
+  template <typename K>
+  iterator lower_bound(const K &key) {
+    return internal_end(internal_lower_bound(key));
+  }
+  template <typename K>
+  const_iterator lower_bound(const K &key) const {
+    return internal_end(internal_lower_bound(key));
+  }
+
+  // Finds the first element whose key is greater than key.
+  template <typename K>
+  iterator upper_bound(const K &key) {
+    return internal_end(internal_upper_bound(key));
+  }
+  template <typename K>
+  const_iterator upper_bound(const K &key) const {
+    return internal_end(internal_upper_bound(key));
+  }
+
+  // Finds the range of values which compare equal to key. The first member of
+  // the returned pair is equal to lower_bound(key). The second member pair of
+  // the pair is equal to upper_bound(key).
+  template <typename K>
+  std::pair<iterator, iterator> equal_range(const K &key) {
+    return {lower_bound(key), upper_bound(key)};
+  }
+  template <typename K>
+  std::pair<const_iterator, const_iterator> equal_range(const K &key) const {
+    return {lower_bound(key), upper_bound(key)};
+  }
+
+  // Inserts a value into the btree only if it does not already exist. The
+  // boolean return value indicates whether insertion succeeded or failed.
+  // Requirement: if `key` already exists in the btree, does not consume `args`.
+  // Requirement: `key` is never referenced after consuming `args`.
+  template <typename... Args>
+  std::pair<iterator, bool> insert_unique(const key_type &key, Args &&... args);
+
+  // Inserts with hint. Checks to see if the value should be placed immediately
+  // before `position` in the tree. If so, then the insertion will take
+  // amortized constant time. If not, the insertion will take amortized
+  // logarithmic time as if a call to insert_unique() were made.
+  // Requirement: if `key` already exists in the btree, does not consume `args`.
+  // Requirement: `key` is never referenced after consuming `args`.
+  template <typename... Args>
+  std::pair<iterator, bool> insert_hint_unique(iterator position,
+                                               const key_type &key,
+                                               Args &&... args);
+
+  // Insert a range of values into the btree.
+  template <typename InputIterator>
+  void insert_iterator_unique(InputIterator b, InputIterator e);
+
+  // Inserts a value into the btree.
+  template <typename ValueType>
+  iterator insert_multi(const key_type &key, ValueType &&v);
+
+  // Inserts a value into the btree.
+  template <typename ValueType>
+  iterator insert_multi(ValueType &&v) {
+    return insert_multi(params_type::key(v), std::forward<ValueType>(v));
+  }
+
+  // Insert with hint. Check to see if the value should be placed immediately
+  // before position in the tree. If it does, then the insertion will take
+  // amortized constant time. If not, the insertion will take amortized
+  // logarithmic time as if a call to insert_multi(v) were made.
+  template <typename ValueType>
+  iterator insert_hint_multi(iterator position, ValueType &&v);
+
+  // Insert a range of values into the btree.
+  template <typename InputIterator>
+  void insert_iterator_multi(InputIterator b, InputIterator e);
+
+  // Erase the specified iterator from the btree. The iterator must be valid
+  // (i.e. not equal to end()).  Return an iterator pointing to the node after
+  // the one that was erased (or end() if none exists).
+  // Requirement: does not read the value at `*iter`.
+  iterator erase(iterator iter);
+
+  // Erases range. Returns the number of keys erased and an iterator pointing
+  // to the element after the last erased element.
+  std::pair<size_type, iterator> erase_range(iterator begin, iterator end);
+
+  // Erases the specified key from the btree. Returns 1 if an element was
+  // erased and 0 otherwise.
+  template <typename K>
+  size_type erase_unique(const K &key);
+
+  // Erases all of the entries matching the specified key from the
+  // btree. Returns the number of elements erased.
+  template <typename K>
+  size_type erase_multi(const K &key);
+
+  // Finds the iterator corresponding to a key or returns end() if the key is
+  // not present.
+  template <typename K>
+  iterator find(const K &key) {
+    return internal_end(internal_find(key));
+  }
+  template <typename K>
+  const_iterator find(const K &key) const {
+    return internal_end(internal_find(key));
+  }
+
+  // Returns a count of the number of times the key appears in the btree.
+  template <typename K>
+  size_type count_unique(const K &key) const {
+    const iterator begin = internal_find(key);
+    if (begin.node == nullptr) {
+      // The key doesn't exist in the tree.
+      return 0;
+    }
+    return 1;
+  }
+  // Returns a count of the number of times the key appears in the btree.
+  template <typename K>
+  size_type count_multi(const K &key) const {
+    const auto range = equal_range(key);
+    return std::distance(range.first, range.second);
+  }
+
+  // Clear the btree, deleting all of the values it contains.
+  void clear();
+
+  // Swap the contents of *this and x.
+  void swap(btree &x);
+
+  const key_compare &key_comp() const noexcept {
+    return root_.template get<0>();
+  }
+  template <typename K, typename LK>
+  bool compare_keys(const K &x, const LK &y) const {
+    return compare_internal::compare_result_as_less_than(key_comp()(x, y));
+  }
+
+  value_compare value_comp() const { return value_compare(key_comp()); }
+
+  // Verifies the structure of the btree.
+  void verify() const;
+
+  // Size routines.
+  size_type size() const { return size_; }
+  size_type max_size() const { return (std::numeric_limits<size_type>::max)(); }
+  bool empty() const { return size_ == 0; }
+
+  // The height of the btree. An empty tree will have height 0.
+  size_type height() const {
+    size_type h = 0;
+    if (!empty()) {
+      // Count the length of the chain from the leftmost node up to the
+      // root. We actually count from the root back around to the level below
+      // the root, but the calculation is the same because of the circularity
+      // of that traversal.
+      const node_type *n = root();
+      do {
+        ++h;
+        n = n->parent();
+      } while (n != root());
+    }
+    return h;
+  }
+
+  // The number of internal, leaf and total nodes used by the btree.
+  size_type leaf_nodes() const { return internal_stats(root()).leaf_nodes; }
+  size_type internal_nodes() const {
+    return internal_stats(root()).internal_nodes;
+  }
+  size_type nodes() const {
+    node_stats stats = internal_stats(root());
+    return stats.leaf_nodes + stats.internal_nodes;
+  }
+
+  // The total number of bytes used by the btree.
+  size_type bytes_used() const {
+    node_stats stats = internal_stats(root());
+    if (stats.leaf_nodes == 1 && stats.internal_nodes == 0) {
+      return sizeof(*this) + node_type::LeafSize(root()->max_count());
+    } else {
+      return sizeof(*this) + stats.leaf_nodes * node_type::LeafSize() +
+             stats.internal_nodes * node_type::InternalSize();
+    }
+  }
+
+  // The average number of bytes used per value stored in the btree.
+  static double average_bytes_per_value() {
+    // Returns the number of bytes per value on a leaf node that is 75%
+    // full. Experimentally, this matches up nicely with the computed number of
+    // bytes per value in trees that had their values inserted in random order.
+    return node_type::LeafSize() / (kNodeValues * 0.75);
+  }
+
+  // The fullness of the btree. Computed as the number of elements in the btree
+  // divided by the maximum number of elements a tree with the current number
+  // of nodes could hold. A value of 1 indicates perfect space
+  // utilization. Smaller values indicate space wastage.
+  // Returns 0 for empty trees.
+  double fullness() const {
+    if (empty()) return 0.0;
+    return static_cast<double>(size()) / (nodes() * kNodeValues);
+  }
+  // The overhead of the btree structure in bytes per node. Computed as the
+  // total number of bytes used by the btree minus the number of bytes used for
+  // storing elements divided by the number of elements.
+  // Returns 0 for empty trees.
+  double overhead() const {
+    if (empty()) return 0.0;
+    return (bytes_used() - size() * sizeof(value_type)) /
+           static_cast<double>(size());
+  }
+
+  // The allocator used by the btree.
+  allocator_type get_allocator() const { return allocator(); }
+
+ private:
+  // Internal accessor routines.
+  node_type *root() { return root_.template get<2>(); }
+  const node_type *root() const { return root_.template get<2>(); }
+  node_type *&mutable_root() noexcept { return root_.template get<2>(); }
+  key_compare *mutable_key_comp() noexcept { return &root_.template get<0>(); }
+
+  // The leftmost node is stored as the parent of the root node.
+  node_type *leftmost() { return root()->parent(); }
+  const node_type *leftmost() const { return root()->parent(); }
+
+  // Allocator routines.
+  allocator_type *mutable_allocator() noexcept {
+    return &root_.template get<1>();
+  }
+  const allocator_type &allocator() const noexcept {
+    return root_.template get<1>();
+  }
+
+  // Allocates a correctly aligned node of at least size bytes using the
+  // allocator.
+  node_type *allocate(const size_type size) {
+    return reinterpret_cast<node_type *>(
+        absl::container_internal::Allocate<node_type::Alignment()>(
+            mutable_allocator(), size));
+  }
+
+  // Node creation/deletion routines.
+  node_type *new_internal_node(node_type *parent) {
+    node_type *p = allocate(node_type::InternalSize());
+    return node_type::init_internal(p, parent);
+  }
+  node_type *new_leaf_node(node_type *parent) {
+    node_type *p = allocate(node_type::LeafSize());
+    return node_type::init_leaf(p, parent, kNodeValues);
+  }
+  node_type *new_leaf_root_node(const int max_count) {
+    node_type *p = allocate(node_type::LeafSize(max_count));
+    return node_type::init_leaf(p, p, max_count);
+  }
+
+  // Deletion helper routines.
+  void erase_same_node(iterator begin, iterator end);
+  iterator erase_from_leaf_node(iterator begin, size_type to_erase);
+  iterator rebalance_after_delete(iterator iter);
+
+  // Deallocates a node of a certain size in bytes using the allocator.
+  void deallocate(const size_type size, node_type *node) {
+    absl::container_internal::Deallocate<node_type::Alignment()>(
+        mutable_allocator(), node, size);
+  }
+
+  void delete_internal_node(node_type *node) {
+    node->destroy(mutable_allocator());
+    deallocate(node_type::InternalSize(), node);
+  }
+  void delete_leaf_node(node_type *node) {
+    node->destroy(mutable_allocator());
+    deallocate(node_type::LeafSize(node->max_count()), node);
+  }
+
+  // Rebalances or splits the node iter points to.
+  void rebalance_or_split(iterator *iter);
+
+  // Merges the values of left, right and the delimiting key on their parent
+  // onto left, removing the delimiting key and deleting right.
+  void merge_nodes(node_type *left, node_type *right);
+
+  // Tries to merge node with its left or right sibling, and failing that,
+  // rebalance with its left or right sibling. Returns true if a merge
+  // occurred, at which point it is no longer valid to access node. Returns
+  // false if no merging took place.
+  bool try_merge_or_rebalance(iterator *iter);
+
+  // Tries to shrink the height of the tree by 1.
+  void try_shrink();
+
+  iterator internal_end(iterator iter) {
+    return iter.node != nullptr ? iter : end();
+  }
+  const_iterator internal_end(const_iterator iter) const {
+    return iter.node != nullptr ? iter : end();
+  }
+
+  // Emplaces a value into the btree immediately before iter. Requires that
+  // key(v) <= iter.key() and (--iter).key() <= key(v).
+  template <typename... Args>
+  iterator internal_emplace(iterator iter, Args &&... args);
+
+  // Returns an iterator pointing to the first value >= the value "iter" is
+  // pointing at. Note that "iter" might be pointing to an invalid location such
+  // as iter.position == iter.node->finish(). This routine simply moves iter up
+  // in the tree to a valid location.
+  // Requires: iter.node is non-null.
+  template <typename IterType>
+  static IterType internal_last(IterType iter);
+
+  // Returns an iterator pointing to the leaf position at which key would
+  // reside in the tree. We provide 2 versions of internal_locate. The first
+  // version uses a less-than comparator and is incapable of distinguishing when
+  // there is an exact match. The second version is for the key-compare-to
+  // specialization and distinguishes exact matches. The key-compare-to
+  // specialization allows the caller to avoid a subsequent comparison to
+  // determine if an exact match was made, which is important for keys with
+  // expensive comparison, such as strings.
+  template <typename K>
+  SearchResult<iterator, is_key_compare_to::value> internal_locate(
+      const K &key) const;
+
+  template <typename K>
+  SearchResult<iterator, false> internal_locate_impl(
+      const K &key, std::false_type /* IsCompareTo */) const;
+
+  template <typename K>
+  SearchResult<iterator, true> internal_locate_impl(
+      const K &key, std::true_type /* IsCompareTo */) const;
+
+  // Internal routine which implements lower_bound().
+  template <typename K>
+  iterator internal_lower_bound(const K &key) const;
+
+  // Internal routine which implements upper_bound().
+  template <typename K>
+  iterator internal_upper_bound(const K &key) const;
+
+  // Internal routine which implements find().
+  template <typename K>
+  iterator internal_find(const K &key) const;
+
+  // Deletes a node and all of its children.
+  void internal_clear(node_type *node);
+
+  // Verifies the tree structure of node.
+  int internal_verify(const node_type *node, const key_type *lo,
+                      const key_type *hi) const;
+
+  node_stats internal_stats(const node_type *node) const {
+    // The root can be a static empty node.
+    if (node == nullptr || (node == root() && empty())) {
+      return node_stats(0, 0);
+    }
+    if (node->leaf()) {
+      return node_stats(1, 0);
+    }
+    node_stats res(0, 1);
+    for (int i = node->start(); i <= node->finish(); ++i) {
+      res += internal_stats(node->child(i));
+    }
+    return res;
+  }
+
+ public:
+  // Exposed only for tests.
+  static bool testonly_uses_linear_node_search() {
+    return node_type::testonly_uses_linear_node_search();
+  }
+
+ private:
+  // We use compressed tuple in order to save space because key_compare and
+  // allocator_type are usually empty.
+  absl::container_internal::CompressedTuple<key_compare, allocator_type,
+                                            node_type *>
+      root_;
+
+  // A pointer to the rightmost node. Note that the leftmost node is stored as
+  // the root's parent.
+  node_type *rightmost_;
+
+  // Number of values.
+  size_type size_;
+};
+
+////
+// btree_node methods
+template <typename P>
+template <typename... Args>
+inline void btree_node<P>::emplace_value(const size_type i,
+                                         allocator_type *alloc,
+                                         Args &&... args) {
+  assert(i >= start());
+  assert(i <= finish());
+  // Shift old values to create space for new value and then construct it in
+  // place.
+  if (i < finish()) {
+    value_init(finish(), alloc, slot(finish() - 1));
+    for (size_type j = finish() - 1; j > i; --j)
+      params_type::move(alloc, slot(j - 1), slot(j));
+    value_destroy(i, alloc);
+  }
+  value_init(i, alloc, std::forward<Args>(args)...);
+  set_finish(finish() + 1);
+
+  if (!leaf() && finish() > i + 1) {
+    for (int j = finish(); j > i + 1; --j) {
+      set_child(j, child(j - 1));
+    }
+    clear_child(i + 1);
+  }
+}
+
+template <typename P>
+inline void btree_node<P>::remove_value(const int i, allocator_type *alloc) {
+  if (!leaf() && finish() > i + 1) {
+    assert(child(i + 1)->count() == 0);
+    for (size_type j = i + 1; j < finish(); ++j) {
+      set_child(j, child(j + 1));
+    }
+    clear_child(finish());
+  }
+
+  remove_values_ignore_children(i, /*to_erase=*/1, alloc);
+}
+
+template <typename P>
+inline void btree_node<P>::remove_values_ignore_children(
+    const int i, const int to_erase, allocator_type *alloc) {
+  params_type::move(alloc, slot(i + to_erase), finish_slot(), slot(i));
+  value_destroy_n(finish() - to_erase, to_erase, alloc);
+  set_finish(finish() - to_erase);
+}
+
+template <typename P>
+void btree_node<P>::rebalance_right_to_left(const int to_move,
+                                            btree_node *right,
+                                            allocator_type *alloc) {
+  assert(parent() == right->parent());
+  assert(position() + 1 == right->position());
+  assert(right->count() >= count());
+  assert(to_move >= 1);
+  assert(to_move <= right->count());
+
+  // 1) Move the delimiting value in the parent to the left node.
+  value_init(finish(), alloc, parent()->slot(position()));
+
+  // 2) Move the (to_move - 1) values from the right node to the left node.
+  right->uninitialized_move_n(to_move - 1, right->start(), finish() + 1, this,
+                              alloc);
+
+  // 3) Move the new delimiting value to the parent from the right node.
+  params_type::move(alloc, right->slot(to_move - 1),
+                    parent()->slot(position()));
+
+  // 4) Shift the values in the right node to their correct position.
+  params_type::move(alloc, right->slot(to_move), right->finish_slot(),
+                    right->start_slot());
+
+  // 5) Destroy the now-empty to_move entries in the right node.
+  right->value_destroy_n(right->finish() - to_move, to_move, alloc);
+
+  if (!leaf()) {
+    // Move the child pointers from the right to the left node.
+    for (int i = 0; i < to_move; ++i) {
+      init_child(finish() + i + 1, right->child(i));
+    }
+    for (int i = right->start(); i <= right->finish() - to_move; ++i) {
+      assert(i + to_move <= right->max_count());
+      right->init_child(i, right->child(i + to_move));
+      right->clear_child(i + to_move);
+    }
+  }
+
+  // Fixup `finish` on the left and right nodes.
+  set_finish(finish() + to_move);
+  right->set_finish(right->finish() - to_move);
+}
+
+template <typename P>
+void btree_node<P>::rebalance_left_to_right(const int to_move,
+                                            btree_node *right,
+                                            allocator_type *alloc) {
+  assert(parent() == right->parent());
+  assert(position() + 1 == right->position());
+  assert(count() >= right->count());
+  assert(to_move >= 1);
+  assert(to_move <= count());
+
+  // Values in the right node are shifted to the right to make room for the
+  // new to_move values. Then, the delimiting value in the parent and the
+  // other (to_move - 1) values in the left node are moved into the right node.
+  // Lastly, a new delimiting value is moved from the left node into the
+  // parent, and the remaining empty left node entries are destroyed.
+
+  if (right->count() >= to_move) {
+    // The original location of the right->count() values are sufficient to hold
+    // the new to_move entries from the parent and left node.
+
+    // 1) Shift existing values in the right node to their correct positions.
+    right->uninitialized_move_n(to_move, right->finish() - to_move,
+                                right->finish(), right, alloc);
+    for (slot_type *src = right->slot(right->finish() - to_move - 1),
+                   *dest = right->slot(right->finish() - 1),
+                   *end = right->start_slot();
+         src >= end; --src, --dest) {
+      params_type::move(alloc, src, dest);
+    }
+
+    // 2) Move the delimiting value in the parent to the right node.
+    params_type::move(alloc, parent()->slot(position()),
+                      right->slot(to_move - 1));
+
+    // 3) Move the (to_move - 1) values from the left node to the right node.
+    params_type::move(alloc, slot(finish() - (to_move - 1)), finish_slot(),
+                      right->start_slot());
+  } else {
+    // The right node does not have enough initialized space to hold the new
+    // to_move entries, so part of them will move to uninitialized space.
+
+    // 1) Shift existing values in the right node to their correct positions.
+    right->uninitialized_move_n(right->count(), right->start(),
+                                right->start() + to_move, right, alloc);
+
+    // 2) Move the delimiting value in the parent to the right node.
+    right->value_init(to_move - 1, alloc, parent()->slot(position()));
+
+    // 3) Move the (to_move - 1) values from the left node to the right node.
+    const size_type uninitialized_remaining = to_move - right->count() - 1;
+    uninitialized_move_n(uninitialized_remaining,
+                         finish() - uninitialized_remaining, right->finish(),
+                         right, alloc);
+    params_type::move(alloc, slot(finish() - (to_move - 1)),
+                      slot(finish() - uninitialized_remaining),
+                      right->start_slot());
+  }
+
+  // 4) Move the new delimiting value to the parent from the left node.
+  params_type::move(alloc, slot(finish() - to_move),
+                    parent()->slot(position()));
+
+  // 5) Destroy the now-empty to_move entries in the left node.
+  value_destroy_n(finish() - to_move, to_move, alloc);
+
+  if (!leaf()) {
+    // Move the child pointers from the left to the right node.
+    for (int i = right->finish(); i >= right->start(); --i) {
+      right->init_child(i + to_move, right->child(i));
+      right->clear_child(i);
+    }
+    for (int i = 1; i <= to_move; ++i) {
+      right->init_child(i - 1, child(finish() - to_move + i));
+      clear_child(finish() - to_move + i);
+    }
+  }
+
+  // Fixup the counts on the left and right nodes.
+  set_finish(finish() - to_move);
+  right->set_finish(right->finish() + to_move);
+}
+
+template <typename P>
+void btree_node<P>::split(const int insert_position, btree_node *dest,
+                          allocator_type *alloc) {
+  assert(dest->count() == 0);
+  assert(max_count() == kNodeValues);
+
+  // We bias the split based on the position being inserted. If we're
+  // inserting at the beginning of the left node then bias the split to put
+  // more values on the right node. If we're inserting at the end of the
+  // right node then bias the split to put more values on the left node.
+  if (insert_position == start()) {
+    dest->set_finish(dest->start() + finish() - 1);
+  } else if (insert_position == kNodeValues) {
+    dest->set_finish(dest->start());
+  } else {
+    dest->set_finish(dest->start() + count() / 2);
+  }
+  set_finish(finish() - dest->count());
+  assert(count() >= 1);
+
+  // Move values from the left sibling to the right sibling.
+  uninitialized_move_n(dest->count(), finish(), dest->start(), dest, alloc);
+
+  // Destroy the now-empty entries in the left node.
+  value_destroy_n(finish(), dest->count(), alloc);
+
+  // The split key is the largest value in the left sibling.
+  --mutable_finish();
+  parent()->emplace_value(position(), alloc, finish_slot());
+  value_destroy(finish(), alloc);
+  parent()->init_child(position() + 1, dest);
+
+  if (!leaf()) {
+    for (int i = dest->start(), j = finish() + 1; i <= dest->finish();
+         ++i, ++j) {
+      assert(child(j) != nullptr);
+      dest->init_child(i, child(j));
+      clear_child(j);
+    }
+  }
+}
+
+template <typename P>
+void btree_node<P>::merge(btree_node *src, allocator_type *alloc) {
+  assert(parent() == src->parent());
+  assert(position() + 1 == src->position());
+
+  // Move the delimiting value to the left node.
+  value_init(finish(), alloc, parent()->slot(position()));
+
+  // Move the values from the right to the left node.
+  src->uninitialized_move_n(src->count(), src->start(), finish() + 1, this,
+                            alloc);
+
+  // Destroy the now-empty entries in the right node.
+  src->value_destroy_n(src->start(), src->count(), alloc);
+
+  if (!leaf()) {
+    // Move the child pointers from the right to the left node.
+    for (int i = src->start(), j = finish() + 1; i <= src->finish(); ++i, ++j) {
+      init_child(j, src->child(i));
+      src->clear_child(i);
+    }
+  }
+
+  // Fixup `finish` on the src and dest nodes.
+  set_finish(start() + 1 + count() + src->count());
+  src->set_finish(src->start());
+
+  // Remove the value on the parent node.
+  parent()->remove_value(position(), alloc);
+}
+
+template <typename P>
+void btree_node<P>::swap(btree_node *x, allocator_type *alloc) {
+  using std::swap;
+  assert(leaf() == x->leaf());
+
+  // Determine which is the smaller/larger node.
+  btree_node *smaller = this, *larger = x;
+  if (smaller->count() > larger->count()) {
+    swap(smaller, larger);
+  }
+
+  // Swap the values.
+  for (slot_type *a = smaller->start_slot(), *b = larger->start_slot(),
+                 *end = smaller->finish_slot();
+       a != end; ++a, ++b) {
+    params_type::swap(alloc, a, b);
+  }
+
+  // Move values that can't be swapped.
+  const size_type to_move = larger->count() - smaller->count();
+  larger->uninitialized_move_n(to_move, smaller->finish(), smaller->finish(),
+                               smaller, alloc);
+  larger->value_destroy_n(smaller->finish(), to_move, alloc);
+
+  if (!leaf()) {
+    // Swap the child pointers.
+    std::swap_ranges(&smaller->mutable_child(smaller->start()),
+                     &smaller->mutable_child(smaller->finish() + 1),
+                     &larger->mutable_child(larger->start()));
+    // Update swapped children's parent pointers.
+    int i = smaller->start();
+    int j = larger->start();
+    for (; i <= smaller->finish(); ++i, ++j) {
+      smaller->child(i)->set_parent(smaller);
+      larger->child(j)->set_parent(larger);
+    }
+    // Move the child pointers that couldn't be swapped.
+    for (; j <= larger->finish(); ++i, ++j) {
+      smaller->init_child(i, larger->child(j));
+      larger->clear_child(j);
+    }
+  }
+
+  // Swap the `finish`s.
+  // TODO(ezb): with floating storage, will also need to swap starts.
+  swap(mutable_finish(), x->mutable_finish());
+}
+
+////
+// btree_iterator methods
+template <typename N, typename R, typename P>
+void btree_iterator<N, R, P>::increment_slow() {
+  if (node->leaf()) {
+    assert(position >= node->finish());
+    btree_iterator save(*this);
+    while (position == node->finish() && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position();
+      node = node->parent();
+    }
+    if (position == node->finish()) {
+      *this = save;
+    }
+  } else {
+    assert(position < node->finish());
+    node = node->child(position + 1);
+    while (!node->leaf()) {
+      node = node->start_child();
+    }
+    position = node->start();
+  }
+}
+
+template <typename N, typename R, typename P>
+void btree_iterator<N, R, P>::decrement_slow() {
+  if (node->leaf()) {
+    assert(position <= -1);
+    btree_iterator save(*this);
+    while (position < node->start() && !node->is_root()) {
+      assert(node->parent()->child(node->position()) == node);
+      position = node->position() - 1;
+      node = node->parent();
+    }
+    if (position < node->start()) {
+      *this = save;
+    }
+  } else {
+    assert(position >= node->start());
+    node = node->child(position);
+    while (!node->leaf()) {
+      node = node->child(node->finish());
+    }
+    position = node->finish() - 1;
+  }
+}
+
+////
+// btree methods
+template <typename P>
+template <typename Btree>
+void btree<P>::copy_or_move_values_in_order(Btree *x) {
+  static_assert(std::is_same<btree, Btree>::value ||
+                    std::is_same<const btree, Btree>::value,
+                "Btree type must be same or const.");
+  assert(empty());
+
+  // We can avoid key comparisons because we know the order of the
+  // values is the same order we'll store them in.
+  auto iter = x->begin();
+  if (iter == x->end()) return;
+  insert_multi(maybe_move_from_iterator(iter));
+  ++iter;
+  for (; iter != x->end(); ++iter) {
+    // If the btree is not empty, we can just insert the new value at the end
+    // of the tree.
+    internal_emplace(end(), maybe_move_from_iterator(iter));
+  }
+}
+
+template <typename P>
+constexpr bool btree<P>::static_assert_validation() {
+  static_assert(std::is_nothrow_copy_constructible<key_compare>::value,
+                "Key comparison must be nothrow copy constructible");
+  static_assert(std::is_nothrow_copy_constructible<allocator_type>::value,
+                "Allocator must be nothrow copy constructible");
+  static_assert(type_traits_internal::is_trivially_copyable<iterator>::value,
+                "iterator not trivially copyable.");
+
+  // Note: We assert that kTargetValues, which is computed from
+  // Params::kTargetNodeSize, must fit the node_type::field_type.
+  static_assert(
+      kNodeValues < (1 << (8 * sizeof(typename node_type::field_type))),
+      "target node size too large");
+
+  // Verify that key_compare returns an absl::{weak,strong}_ordering or bool.
+  using compare_result_type =
+      absl::result_of_t<key_compare(key_type, key_type)>;
+  static_assert(
+      std::is_same<compare_result_type, bool>::value ||
+          std::is_convertible<compare_result_type, absl::weak_ordering>::value,
+      "key comparison function must return absl::{weak,strong}_ordering or "
+      "bool.");
+
+  // Test the assumption made in setting kNodeValueSpace.
+  static_assert(node_type::MinimumOverhead() >= sizeof(void *) + 4,
+                "node space assumption incorrect");
+
+  return true;
+}
+
+template <typename P>
+btree<P>::btree(const key_compare &comp, const allocator_type &alloc)
+    : root_(comp, alloc, EmptyNode()), rightmost_(EmptyNode()), size_(0) {}
+
+template <typename P>
+btree<P>::btree(const btree &x) : btree(x.key_comp(), x.allocator()) {
+  copy_or_move_values_in_order(&x);
+}
+
+template <typename P>
+template <typename... Args>
+auto btree<P>::insert_unique(const key_type &key, Args &&... args)
+    -> std::pair<iterator, bool> {
+  if (empty()) {
+    mutable_root() = rightmost_ = new_leaf_root_node(1);
+  }
+
+  auto res = internal_locate(key);
+  iterator &iter = res.value;
+
+  if (res.HasMatch()) {
+    if (res.IsEq()) {
+      // The key already exists in the tree, do nothing.
+      return {iter, false};
+    }
+  } else {
+    iterator last = internal_last(iter);
+    if (last.node && !compare_keys(key, last.key())) {
+      // The key already exists in the tree, do nothing.
+      return {last, false};
+    }
+  }
+  return {internal_emplace(iter, std::forward<Args>(args)...), true};
+}
+
+template <typename P>
+template <typename... Args>
+inline auto btree<P>::insert_hint_unique(iterator position, const key_type &key,
+                                         Args &&... args)
+    -> std::pair<iterator, bool> {
+  if (!empty()) {
+    if (position == end() || compare_keys(key, position.key())) {
+      if (position == begin() || compare_keys(std::prev(position).key(), key)) {
+        // prev.key() < key < position.key()
+        return {internal_emplace(position, std::forward<Args>(args)...), true};
+      }
+    } else if (compare_keys(position.key(), key)) {
+      ++position;
+      if (position == end() || compare_keys(key, position.key())) {
+        // {original `position`}.key() < key < {current `position`}.key()
+        return {internal_emplace(position, std::forward<Args>(args)...), true};
+      }
+    } else {
+      // position.key() == key
+      return {position, false};
+    }
+  }
+  return insert_unique(key, std::forward<Args>(args)...);
+}
+
+template <typename P>
+template <typename InputIterator>
+void btree<P>::insert_iterator_unique(InputIterator b, InputIterator e) {
+  for (; b != e; ++b) {
+    insert_hint_unique(end(), params_type::key(*b), *b);
+  }
+}
+
+template <typename P>
+template <typename ValueType>
+auto btree<P>::insert_multi(const key_type &key, ValueType &&v) -> iterator {
+  if (empty()) {
+    mutable_root() = rightmost_ = new_leaf_root_node(1);
+  }
+
+  iterator iter = internal_upper_bound(key);
+  if (iter.node == nullptr) {
+    iter = end();
+  }
+  return internal_emplace(iter, std::forward<ValueType>(v));
+}
+
+template <typename P>
+template <typename ValueType>
+auto btree<P>::insert_hint_multi(iterator position, ValueType &&v) -> iterator {
+  if (!empty()) {
+    const key_type &key = params_type::key(v);
+    if (position == end() || !compare_keys(position.key(), key)) {
+      if (position == begin() ||
+          !compare_keys(key, std::prev(position).key())) {
+        // prev.key() <= key <= position.key()
+        return internal_emplace(position, std::forward<ValueType>(v));
+      }
+    } else {
+      ++position;
+      if (position == end() || !compare_keys(position.key(), key)) {
+        // {original `position`}.key() < key < {current `position`}.key()
+        return internal_emplace(position, std::forward<ValueType>(v));
+      }
+    }
+  }
+  return insert_multi(std::forward<ValueType>(v));
+}
+
+template <typename P>
+template <typename InputIterator>
+void btree<P>::insert_iterator_multi(InputIterator b, InputIterator e) {
+  for (; b != e; ++b) {
+    insert_hint_multi(end(), *b);
+  }
+}
+
+template <typename P>
+auto btree<P>::operator=(const btree &x) -> btree & {
+  if (this != &x) {
+    clear();
+
+    *mutable_key_comp() = x.key_comp();
+    if (absl::allocator_traits<
+            allocator_type>::propagate_on_container_copy_assignment::value) {
+      *mutable_allocator() = x.allocator();
+    }
+
+    copy_or_move_values_in_order(&x);
+  }
+  return *this;
+}
+
+template <typename P>
+auto btree<P>::operator=(btree &&x) noexcept -> btree & {
+  if (this != &x) {
+    clear();
+
+    using std::swap;
+    if (absl::allocator_traits<
+            allocator_type>::propagate_on_container_copy_assignment::value) {
+      // Note: `root_` also contains the allocator and the key comparator.
+      swap(root_, x.root_);
+      swap(rightmost_, x.rightmost_);
+      swap(size_, x.size_);
+    } else {
+      if (allocator() == x.allocator()) {
+        swap(mutable_root(), x.mutable_root());
+        swap(*mutable_key_comp(), *x.mutable_key_comp());
+        swap(rightmost_, x.rightmost_);
+        swap(size_, x.size_);
+      } else {
+        // We aren't allowed to propagate the allocator and the allocator is
+        // different so we can't take over its memory. We must move each element
+        // individually. We need both `x` and `this` to have `x`s key comparator
+        // while moving the values so we can't swap the key comparators.
+        *mutable_key_comp() = x.key_comp();
+        copy_or_move_values_in_order(&x);
+      }
+    }
+  }
+  return *this;
+}
+
+template <typename P>
+auto btree<P>::erase(iterator iter) -> iterator {
+  bool internal_delete = false;
+  if (!iter.node->leaf()) {
+    // Deletion of a value on an internal node. First, move the largest value
+    // from our left child here, then delete that position (in remove_value()
+    // below). We can get to the largest value from our left child by
+    // decrementing iter.
+    iterator internal_iter(iter);
+    --iter;
+    assert(iter.node->leaf());
+    params_type::move(mutable_allocator(), iter.node->slot(iter.position),
+                      internal_iter.node->slot(internal_iter.position));
+    internal_delete = true;
+  }
+
+  // Delete the key from the leaf.
+  iter.node->remove_value(iter.position, mutable_allocator());
+  --size_;
+
+  // We want to return the next value after the one we just erased. If we
+  // erased from an internal node (internal_delete == true), then the next
+  // value is ++(++iter). If we erased from a leaf node (internal_delete ==
+  // false) then the next value is ++iter. Note that ++iter may point to an
+  // internal node and the value in the internal node may move to a leaf node
+  // (iter.node) when rebalancing is performed at the leaf level.
+
+  iterator res = rebalance_after_delete(iter);
+
+  // If we erased from an internal node, advance the iterator.
+  if (internal_delete) {
+    ++res;
+  }
+  return res;
+}
+
+template <typename P>
+auto btree<P>::rebalance_after_delete(iterator iter) -> iterator {
+  // Merge/rebalance as we walk back up the tree.
+  iterator res(iter);
+  bool first_iteration = true;
+  for (;;) {
+    if (iter.node == root()) {
+      try_shrink();
+      if (empty()) {
+        return end();
+      }
+      break;
+    }
+    if (iter.node->count() >= kMinNodeValues) {
+      break;
+    }
+    bool merged = try_merge_or_rebalance(&iter);
+    // On the first iteration, we should update `res` with `iter` because `res`
+    // may have been invalidated.
+    if (first_iteration) {
+      res = iter;
+      first_iteration = false;
+    }
+    if (!merged) {
+      break;
+    }
+    iter.position = iter.node->position();
+    iter.node = iter.node->parent();
+  }
+
+  // Adjust our return value. If we're pointing at the end of a node, advance
+  // the iterator.
+  if (res.position == res.node->finish()) {
+    res.position = res.node->finish() - 1;
+    ++res;
+  }
+
+  return res;
+}
+
+template <typename P>
+auto btree<P>::erase_range(iterator begin, iterator end)
+    -> std::pair<size_type, iterator> {
+  difference_type count = std::distance(begin, end);
+  assert(count >= 0);
+
+  if (count == 0) {
+    return {0, begin};
+  }
+
+  if (count == size_) {
+    clear();
+    return {count, this->end()};
+  }
+
+  if (begin.node == end.node) {
+    erase_same_node(begin, end);
+    size_ -= count;
+    return {count, rebalance_after_delete(begin)};
+  }
+
+  const size_type target_size = size_ - count;
+  while (size_ > target_size) {
+    if (begin.node->leaf()) {
+      const size_type remaining_to_erase = size_ - target_size;
+      const size_type remaining_in_node = begin.node->finish() - begin.position;
+      begin = erase_from_leaf_node(
+          begin, (std::min)(remaining_to_erase, remaining_in_node));
+    } else {
+      begin = erase(begin);
+    }
+  }
+  return {count, begin};
+}
+
+template <typename P>
+void btree<P>::erase_same_node(iterator begin, iterator end) {
+  assert(begin.node == end.node);
+  assert(end.position > begin.position);
+
+  node_type *node = begin.node;
+  size_type to_erase = end.position - begin.position;
+  if (!node->leaf()) {
+    // Delete all children between begin and end.
+    for (size_type i = 0; i < to_erase; ++i) {
+      internal_clear(node->child(begin.position + i + 1));
+    }
+    // Rotate children after end into new positions.
+    for (size_type i = begin.position + to_erase + 1; i <= node->finish();
+         ++i) {
+      node->set_child(i - to_erase, node->child(i));
+      node->clear_child(i);
+    }
+  }
+  node->remove_values_ignore_children(begin.position, to_erase,
+                                      mutable_allocator());
+
+  // Do not need to update rightmost_, because
+  // * either end == this->end(), and therefore node == rightmost_, and still
+  //   exists
+  // * or end != this->end(), and therefore rightmost_ hasn't been erased, since
+  //   it wasn't covered in [begin, end)
+}
+
+template <typename P>
+auto btree<P>::erase_from_leaf_node(iterator begin, size_type to_erase)
+    -> iterator {
+  node_type *node = begin.node;
+  assert(node->leaf());
+  assert(node->finish() > begin.position);
+  assert(begin.position + to_erase <= node->finish());
+
+  node->remove_values_ignore_children(begin.position, to_erase,
+                                      mutable_allocator());
+
+  size_ -= to_erase;
+
+  return rebalance_after_delete(begin);
+}
+
+template <typename P>
+template <typename K>
+auto btree<P>::erase_unique(const K &key) -> size_type {
+  const iterator iter = internal_find(key);
+  if (iter.node == nullptr) {
+    // The key doesn't exist in the tree, return nothing done.
+    return 0;
+  }
+  erase(iter);
+  return 1;
+}
+
+template <typename P>
+template <typename K>
+auto btree<P>::erase_multi(const K &key) -> size_type {
+  const iterator begin = internal_lower_bound(key);
+  if (begin.node == nullptr) {
+    // The key doesn't exist in the tree, return nothing done.
+    return 0;
+  }
+  // Delete all of the keys between begin and upper_bound(key).
+  const iterator end = internal_end(internal_upper_bound(key));
+  return erase_range(begin, end).first;
+}
+
+template <typename P>
+void btree<P>::clear() {
+  if (!empty()) {
+    internal_clear(root());
+  }
+  mutable_root() = EmptyNode();
+  rightmost_ = EmptyNode();
+  size_ = 0;
+}
+
+template <typename P>
+void btree<P>::swap(btree &x) {
+  using std::swap;
+  if (absl::allocator_traits<
+          allocator_type>::propagate_on_container_swap::value) {
+    // Note: `root_` also contains the allocator and the key comparator.
+    swap(root_, x.root_);
+  } else {
+    // It's undefined behavior if the allocators are unequal here.
+    assert(allocator() == x.allocator());
+    swap(mutable_root(), x.mutable_root());
+    swap(*mutable_key_comp(), *x.mutable_key_comp());
+  }
+  swap(rightmost_, x.rightmost_);
+  swap(size_, x.size_);
+}
+
+template <typename P>
+void btree<P>::verify() const {
+  assert(root() != nullptr);
+  assert(leftmost() != nullptr);
+  assert(rightmost_ != nullptr);
+  assert(empty() || size() == internal_verify(root(), nullptr, nullptr));
+  assert(leftmost() == (++const_iterator(root(), -1)).node);
+  assert(rightmost_ == (--const_iterator(root(), root()->finish())).node);
+  assert(leftmost()->leaf());
+  assert(rightmost_->leaf());
+}
+
+template <typename P>
+void btree<P>::rebalance_or_split(iterator *iter) {
+  node_type *&node = iter->node;
+  int &insert_position = iter->position;
+  assert(node->count() == node->max_count());
+  assert(kNodeValues == node->max_count());
+
+  // First try to make room on the node by rebalancing.
+  node_type *parent = node->parent();
+  if (node != root()) {
+    if (node->position() > parent->start()) {
+      // Try rebalancing with our left sibling.
+      node_type *left = parent->child(node->position() - 1);
+      assert(left->max_count() == kNodeValues);
+      if (left->count() < kNodeValues) {
+        // We bias rebalancing based on the position being inserted. If we're
+        // inserting at the end of the right node then we bias rebalancing to
+        // fill up the left node.
+        int to_move = (kNodeValues - left->count()) /
+                      (1 + (insert_position < kNodeValues));
+        to_move = (std::max)(1, to_move);
+
+        if (insert_position - to_move >= node->start() ||
+            left->count() + to_move < kNodeValues) {
+          left->rebalance_right_to_left(to_move, node, mutable_allocator());
+
+          assert(node->max_count() - node->count() == to_move);
+          insert_position = insert_position - to_move;
+          if (insert_position < node->start()) {
+            insert_position = insert_position + left->count() + 1;
+            node = left;
+          }
+
+          assert(node->count() < node->max_count());
+          return;
+        }
+      }
+    }
+
+    if (node->position() < parent->finish()) {
+      // Try rebalancing with our right sibling.
+      node_type *right = parent->child(node->position() + 1);
+      assert(right->max_count() == kNodeValues);
+      if (right->count() < kNodeValues) {
+        // We bias rebalancing based on the position being inserted. If we're
+        // inserting at the beginning of the left node then we bias rebalancing
+        // to fill up the right node.
+        int to_move = (kNodeValues - right->count()) /
+                      (1 + (insert_position > node->start()));
+        to_move = (std::max)(1, to_move);
+
+        if (insert_position <= node->finish() - to_move ||
+            right->count() + to_move < kNodeValues) {
+          node->rebalance_left_to_right(to_move, right, mutable_allocator());
+
+          if (insert_position > node->finish()) {
+            insert_position = insert_position - node->count() - 1;
+            node = right;
+          }
+
+          assert(node->count() < node->max_count());
+          return;
+        }
+      }
+    }
+
+    // Rebalancing failed, make sure there is room on the parent node for a new
+    // value.
+    assert(parent->max_count() == kNodeValues);
+    if (parent->count() == kNodeValues) {
+      iterator parent_iter(node->parent(), node->position());
+      rebalance_or_split(&parent_iter);
+    }
+  } else {
+    // Rebalancing not possible because this is the root node.
+    // Create a new root node and set the current root node as the child of the
+    // new root.
+    parent = new_internal_node(parent);
+    parent->init_child(parent->start(), root());
+    mutable_root() = parent;
+    // If the former root was a leaf node, then it's now the rightmost node.
+    assert(!parent->start_child()->leaf() ||
+           parent->start_child() == rightmost_);
+  }
+
+  // Split the node.
+  node_type *split_node;
+  if (node->leaf()) {
+    split_node = new_leaf_node(parent);
+    node->split(insert_position, split_node, mutable_allocator());
+    if (rightmost_ == node) rightmost_ = split_node;
+  } else {
+    split_node = new_internal_node(parent);
+    node->split(insert_position, split_node, mutable_allocator());
+  }
+
+  if (insert_position > node->finish()) {
+    insert_position = insert_position - node->count() - 1;
+    node = split_node;
+  }
+}
+
+template <typename P>
+void btree<P>::merge_nodes(node_type *left, node_type *right) {
+  left->merge(right, mutable_allocator());
+  if (right->leaf()) {
+    if (rightmost_ == right) rightmost_ = left;
+    delete_leaf_node(right);
+  } else {
+    delete_internal_node(right);
+  }
+}
+
+template <typename P>
+bool btree<P>::try_merge_or_rebalance(iterator *iter) {
+  node_type *parent = iter->node->parent();
+  if (iter->node->position() > parent->start()) {
+    // Try merging with our left sibling.
+    node_type *left = parent->child(iter->node->position() - 1);
+    assert(left->max_count() == kNodeValues);
+    if (1 + left->count() + iter->node->count() <= kNodeValues) {
+      iter->position += 1 + left->count();
+      merge_nodes(left, iter->node);
+      iter->node = left;
+      return true;
+    }
+  }
+  if (iter->node->position() < parent->finish()) {
+    // Try merging with our right sibling.
+    node_type *right = parent->child(iter->node->position() + 1);
+    assert(right->max_count() == kNodeValues);
+    if (1 + iter->node->count() + right->count() <= kNodeValues) {
+      merge_nodes(iter->node, right);
+      return true;
+    }
+    // Try rebalancing with our right sibling. We don't perform rebalancing if
+    // we deleted the first element from iter->node and the node is not
+    // empty. This is a small optimization for the common pattern of deleting
+    // from the front of the tree.
+    if (right->count() > kMinNodeValues &&
+        (iter->node->count() == 0 || iter->position > iter->node->start())) {
+      int to_move = (right->count() - iter->node->count()) / 2;
+      to_move = (std::min)(to_move, right->count() - 1);
+      iter->node->rebalance_right_to_left(to_move, right, mutable_allocator());
+      return false;
+    }
+  }
+  if (iter->node->position() > parent->start()) {
+    // Try rebalancing with our left sibling. We don't perform rebalancing if
+    // we deleted the last element from iter->node and the node is not
+    // empty. This is a small optimization for the common pattern of deleting
+    // from the back of the tree.
+    node_type *left = parent->child(iter->node->position() - 1);
+    if (left->count() > kMinNodeValues &&
+        (iter->node->count() == 0 || iter->position < iter->node->finish())) {
+      int to_move = (left->count() - iter->node->count()) / 2;
+      to_move = (std::min)(to_move, left->count() - 1);
+      left->rebalance_left_to_right(to_move, iter->node, mutable_allocator());
+      iter->position += to_move;
+      return false;
+    }
+  }
+  return false;
+}
+
+template <typename P>
+void btree<P>::try_shrink() {
+  if (root()->count() > 0) {
+    return;
+  }
+  // Deleted the last item on the root node, shrink the height of the tree.
+  if (root()->leaf()) {
+    assert(size() == 0);
+    delete_leaf_node(root());
+    mutable_root() = EmptyNode();
+    rightmost_ = EmptyNode();
+  } else {
+    node_type *child = root()->start_child();
+    child->make_root();
+    delete_internal_node(root());
+    mutable_root() = child;
+  }
+}
+
+template <typename P>
+template <typename IterType>
+inline IterType btree<P>::internal_last(IterType iter) {
+  assert(iter.node != nullptr);
+  while (iter.position == iter.node->finish()) {
+    iter.position = iter.node->position();
+    iter.node = iter.node->parent();
+    if (iter.node->leaf()) {
+      iter.node = nullptr;
+      break;
+    }
+  }
+  return iter;
+}
+
+template <typename P>
+template <typename... Args>
+inline auto btree<P>::internal_emplace(iterator iter, Args &&... args)
+    -> iterator {
+  if (!iter.node->leaf()) {
+    // We can't insert on an internal node. Instead, we'll insert after the
+    // previous value which is guaranteed to be on a leaf node.
+    --iter;
+    ++iter.position;
+  }
+  const int max_count = iter.node->max_count();
+  if (iter.node->count() == max_count) {
+    // Make room in the leaf for the new item.
+    if (max_count < kNodeValues) {
+      // Insertion into the root where the root is smaller than the full node
+      // size. Simply grow the size of the root node.
+      assert(iter.node == root());
+      iter.node =
+          new_leaf_root_node((std::min<int>)(kNodeValues, 2 * max_count));
+      iter.node->swap(root(), mutable_allocator());
+      delete_leaf_node(root());
+      mutable_root() = iter.node;
+      rightmost_ = iter.node;
+    } else {
+      rebalance_or_split(&iter);
+    }
+  }
+  iter.node->emplace_value(iter.position, mutable_allocator(),
+                           std::forward<Args>(args)...);
+  ++size_;
+  return iter;
+}
+
+template <typename P>
+template <typename K>
+inline auto btree<P>::internal_locate(const K &key) const
+    -> SearchResult<iterator, is_key_compare_to::value> {
+  return internal_locate_impl(key, is_key_compare_to());
+}
+
+template <typename P>
+template <typename K>
+inline auto btree<P>::internal_locate_impl(
+    const K &key, std::false_type /* IsCompareTo */) const
+    -> SearchResult<iterator, false> {
+  iterator iter(const_cast<node_type *>(root()));
+  for (;;) {
+    iter.position = iter.node->lower_bound(key, key_comp()).value;
+    // NOTE: we don't need to walk all the way down the tree if the keys are
+    // equal, but determining equality would require doing an extra comparison
+    // on each node on the way down, and we will need to go all the way to the
+    // leaf node in the expected case.
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return {iter};
+}
+
+template <typename P>
+template <typename K>
+inline auto btree<P>::internal_locate_impl(
+    const K &key, std::true_type /* IsCompareTo */) const
+    -> SearchResult<iterator, true> {
+  iterator iter(const_cast<node_type *>(root()));
+  for (;;) {
+    SearchResult<int, true> res = iter.node->lower_bound(key, key_comp());
+    iter.position = res.value;
+    if (res.match == MatchKind::kEq) {
+      return {iter, MatchKind::kEq};
+    }
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return {iter, MatchKind::kNe};
+}
+
+template <typename P>
+template <typename K>
+auto btree<P>::internal_lower_bound(const K &key) const -> iterator {
+  iterator iter(const_cast<node_type *>(root()));
+  for (;;) {
+    iter.position = iter.node->lower_bound(key, key_comp()).value;
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return internal_last(iter);
+}
+
+template <typename P>
+template <typename K>
+auto btree<P>::internal_upper_bound(const K &key) const -> iterator {
+  iterator iter(const_cast<node_type *>(root()));
+  for (;;) {
+    iter.position = iter.node->upper_bound(key, key_comp());
+    if (iter.node->leaf()) {
+      break;
+    }
+    iter.node = iter.node->child(iter.position);
+  }
+  return internal_last(iter);
+}
+
+template <typename P>
+template <typename K>
+auto btree<P>::internal_find(const K &key) const -> iterator {
+  auto res = internal_locate(key);
+  if (res.HasMatch()) {
+    if (res.IsEq()) {
+      return res.value;
+    }
+  } else {
+    const iterator iter = internal_last(res.value);
+    if (iter.node != nullptr && !compare_keys(key, iter.key())) {
+      return iter;
+    }
+  }
+  return {nullptr, 0};
+}
+
+template <typename P>
+void btree<P>::internal_clear(node_type *node) {
+  if (!node->leaf()) {
+    for (int i = node->start(); i <= node->finish(); ++i) {
+      internal_clear(node->child(i));
+    }
+    delete_internal_node(node);
+  } else {
+    delete_leaf_node(node);
+  }
+}
+
+template <typename P>
+int btree<P>::internal_verify(const node_type *node, const key_type *lo,
+                              const key_type *hi) const {
+  assert(node->count() > 0);
+  assert(node->count() <= node->max_count());
+  if (lo) {
+    assert(!compare_keys(node->key(node->start()), *lo));
+  }
+  if (hi) {
+    assert(!compare_keys(*hi, node->key(node->finish() - 1)));
+  }
+  for (int i = node->start() + 1; i < node->finish(); ++i) {
+    assert(!compare_keys(node->key(i), node->key(i - 1)));
+  }
+  int count = node->count();
+  if (!node->leaf()) {
+    for (int i = node->start(); i <= node->finish(); ++i) {
+      assert(node->child(i) != nullptr);
+      assert(node->child(i)->parent() == node);
+      assert(node->child(i)->position() == i);
+      count += internal_verify(node->child(i),
+                               i == node->start() ? lo : &node->key(i - 1),
+                               i == node->finish() ? hi : &node->key(i));
+    }
+  }
+  return count;
+}
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_BTREE_H_
diff --git a/absl/container/internal/btree_container.h b/absl/container/internal/btree_container.h
new file mode 100644
index 0000000..f2e4c3a
--- /dev/null
+++ b/absl/container/internal/btree_container.h
@@ -0,0 +1,672 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_CONTAINER_INTERNAL_BTREE_CONTAINER_H_
+#define ABSL_CONTAINER_INTERNAL_BTREE_CONTAINER_H_
+
+#include <algorithm>
+#include <initializer_list>
+#include <iterator>
+#include <utility>
+
+#include "absl/base/internal/throw_delegate.h"
+#include "absl/container/internal/btree.h"  // IWYU pragma: export
+#include "absl/container/internal/common.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace container_internal {
+
+// A common base class for btree_set, btree_map, btree_multiset, and
+// btree_multimap.
+template <typename Tree>
+class btree_container {
+  using params_type = typename Tree::params_type;
+
+ protected:
+  // Alias used for heterogeneous lookup functions.
+  // `key_arg<K>` evaluates to `K` when the functors are transparent and to
+  // `key_type` otherwise. It permits template argument deduction on `K` for the
+  // transparent case.
+  template <class K>
+  using key_arg =
+      typename KeyArg<IsTransparent<typename Tree::key_compare>::value>::
+          template type<K, typename Tree::key_type>;
+
+ public:
+  using key_type = typename Tree::key_type;
+  using value_type = typename Tree::value_type;
+  using size_type = typename Tree::size_type;
+  using difference_type = typename Tree::difference_type;
+  using key_compare = typename Tree::key_compare;
+  using value_compare = typename Tree::value_compare;
+  using allocator_type = typename Tree::allocator_type;
+  using reference = typename Tree::reference;
+  using const_reference = typename Tree::const_reference;
+  using pointer = typename Tree::pointer;
+  using const_pointer = typename Tree::const_pointer;
+  using iterator = typename Tree::iterator;
+  using const_iterator = typename Tree::const_iterator;
+  using reverse_iterator = typename Tree::reverse_iterator;
+  using const_reverse_iterator = typename Tree::const_reverse_iterator;
+  using node_type = typename Tree::node_handle_type;
+
+  // Constructors/assignments.
+  btree_container() : tree_(key_compare(), allocator_type()) {}
+  explicit btree_container(const key_compare &comp,
+                           const allocator_type &alloc = allocator_type())
+      : tree_(comp, alloc) {}
+  btree_container(const btree_container &x) = default;
+  btree_container(btree_container &&x) noexcept = default;
+  btree_container &operator=(const btree_container &x) = default;
+  btree_container &operator=(btree_container &&x) noexcept(
+      std::is_nothrow_move_assignable<Tree>::value) = default;
+
+  // Iterator routines.
+  iterator begin() { return tree_.begin(); }
+  const_iterator begin() const { return tree_.begin(); }
+  const_iterator cbegin() const { return tree_.begin(); }
+  iterator end() { return tree_.end(); }
+  const_iterator end() const { return tree_.end(); }
+  const_iterator cend() const { return tree_.end(); }
+  reverse_iterator rbegin() { return tree_.rbegin(); }
+  const_reverse_iterator rbegin() const { return tree_.rbegin(); }
+  const_reverse_iterator crbegin() const { return tree_.rbegin(); }
+  reverse_iterator rend() { return tree_.rend(); }
+  const_reverse_iterator rend() const { return tree_.rend(); }
+  const_reverse_iterator crend() const { return tree_.rend(); }
+
+  // Lookup routines.
+  template <typename K = key_type>
+  iterator find(const key_arg<K> &key) {
+    return tree_.find(key);
+  }
+  template <typename K = key_type>
+  const_iterator find(const key_arg<K> &key) const {
+    return tree_.find(key);
+  }
+  template <typename K = key_type>
+  bool contains(const key_arg<K> &key) const {
+    return find(key) != end();
+  }
+  template <typename K = key_type>
+  iterator lower_bound(const key_arg<K> &key) {
+    return tree_.lower_bound(key);
+  }
+  template <typename K = key_type>
+  const_iterator lower_bound(const key_arg<K> &key) const {
+    return tree_.lower_bound(key);
+  }
+  template <typename K = key_type>
+  iterator upper_bound(const key_arg<K> &key) {
+    return tree_.upper_bound(key);
+  }
+  template <typename K = key_type>
+  const_iterator upper_bound(const key_arg<K> &key) const {
+    return tree_.upper_bound(key);
+  }
+  template <typename K = key_type>
+  std::pair<iterator, iterator> equal_range(const key_arg<K> &key) {
+    return tree_.equal_range(key);
+  }
+  template <typename K = key_type>
+  std::pair<const_iterator, const_iterator> equal_range(
+      const key_arg<K> &key) const {
+    return tree_.equal_range(key);
+  }
+
+  // Deletion routines. Note that there is also a deletion routine that is
+  // specific to btree_set_container/btree_multiset_container.
+
+  // Erase the specified iterator from the btree. The iterator must be valid
+  // (i.e. not equal to end()).  Return an iterator pointing to the node after
+  // the one that was erased (or end() if none exists).
+  iterator erase(const_iterator iter) { return tree_.erase(iterator(iter)); }
+  iterator erase(iterator iter) { return tree_.erase(iter); }
+  iterator erase(const_iterator first, const_iterator last) {
+    return tree_.erase_range(iterator(first), iterator(last)).second;
+  }
+
+  // Extract routines.
+  node_type extract(iterator position) {
+    // Use Move instead of Transfer, because the rebalancing code expects to
+    // have a valid object to scribble metadata bits on top of.
+    auto node = CommonAccess::Move<node_type>(get_allocator(), position.slot());
+    erase(position);
+    return node;
+  }
+  node_type extract(const_iterator position) {
+    return extract(iterator(position));
+  }
+
+ public:
+  // Utility routines.
+  void clear() { tree_.clear(); }
+  void swap(btree_container &x) { tree_.swap(x.tree_); }
+  void verify() const { tree_.verify(); }
+
+  // Size routines.
+  size_type size() const { return tree_.size(); }
+  size_type max_size() const { return tree_.max_size(); }
+  bool empty() const { return tree_.empty(); }
+
+  friend bool operator==(const btree_container &x, const btree_container &y) {
+    if (x.size() != y.size()) return false;
+    return std::equal(x.begin(), x.end(), y.begin());
+  }
+
+  friend bool operator!=(const btree_container &x, const btree_container &y) {
+    return !(x == y);
+  }
+
+  friend bool operator<(const btree_container &x, const btree_container &y) {
+    return std::lexicographical_compare(x.begin(), x.end(), y.begin(), y.end());
+  }
+
+  friend bool operator>(const btree_container &x, const btree_container &y) {
+    return y < x;
+  }
+
+  friend bool operator<=(const btree_container &x, const btree_container &y) {
+    return !(y < x);
+  }
+
+  friend bool operator>=(const btree_container &x, const btree_container &y) {
+    return !(x < y);
+  }
+
+  // The allocator used by the btree.
+  allocator_type get_allocator() const { return tree_.get_allocator(); }
+
+  // The key comparator used by the btree.
+  key_compare key_comp() const { return tree_.key_comp(); }
+  value_compare value_comp() const { return tree_.value_comp(); }
+
+  // Support absl::Hash.
+  template <typename State>
+  friend State AbslHashValue(State h, const btree_container &b) {
+    for (const auto &v : b) {
+      h = State::combine(std::move(h), v);
+    }
+    return State::combine(std::move(h), b.size());
+  }
+
+ protected:
+  Tree tree_;
+};
+
+// A common base class for btree_set and btree_map.
+template <typename Tree>
+class btree_set_container : public btree_container<Tree> {
+  using super_type = btree_container<Tree>;
+  using params_type = typename Tree::params_type;
+  using init_type = typename params_type::init_type;
+  using is_key_compare_to = typename params_type::is_key_compare_to;
+  friend class BtreeNodePeer;
+
+ protected:
+  template <class K>
+  using key_arg = typename super_type::template key_arg<K>;
+
+ public:
+  using key_type = typename Tree::key_type;
+  using value_type = typename Tree::value_type;
+  using size_type = typename Tree::size_type;
+  using key_compare = typename Tree::key_compare;
+  using allocator_type = typename Tree::allocator_type;
+  using iterator = typename Tree::iterator;
+  using const_iterator = typename Tree::const_iterator;
+  using node_type = typename super_type::node_type;
+  using insert_return_type = InsertReturnType<iterator, node_type>;
+
+  // Inherit constructors.
+  using super_type::super_type;
+  btree_set_container() {}
+
+  // Range constructor.
+  template <class InputIterator>
+  btree_set_container(InputIterator b, InputIterator e,
+                      const key_compare &comp = key_compare(),
+                      const allocator_type &alloc = allocator_type())
+      : super_type(comp, alloc) {
+    insert(b, e);
+  }
+
+  // Initializer list constructor.
+  btree_set_container(std::initializer_list<init_type> init,
+                      const key_compare &comp = key_compare(),
+                      const allocator_type &alloc = allocator_type())
+      : btree_set_container(init.begin(), init.end(), comp, alloc) {}
+
+  // Lookup routines.
+  template <typename K = key_type>
+  size_type count(const key_arg<K> &key) const {
+    return this->tree_.count_unique(key);
+  }
+
+  // Insertion routines.
+  std::pair<iterator, bool> insert(const value_type &x) {
+    return this->tree_.insert_unique(params_type::key(x), x);
+  }
+  std::pair<iterator, bool> insert(value_type &&x) {
+    return this->tree_.insert_unique(params_type::key(x), std::move(x));
+  }
+  template <typename... Args>
+  std::pair<iterator, bool> emplace(Args &&... args) {
+    init_type v(std::forward<Args>(args)...);
+    return this->tree_.insert_unique(params_type::key(v), std::move(v));
+  }
+  iterator insert(const_iterator position, const value_type &x) {
+    return this->tree_
+        .insert_hint_unique(iterator(position), params_type::key(x), x)
+        .first;
+  }
+  iterator insert(const_iterator position, value_type &&x) {
+    return this->tree_
+        .insert_hint_unique(iterator(position), params_type::key(x),
+                            std::move(x))
+        .first;
+  }
+  template <typename... Args>
+  iterator emplace_hint(const_iterator position, Args &&... args) {
+    init_type v(std::forward<Args>(args)...);
+    return this->tree_
+        .insert_hint_unique(iterator(position), params_type::key(v),
+                            std::move(v))
+        .first;
+  }
+  template <typename InputIterator>
+  void insert(InputIterator b, InputIterator e) {
+    this->tree_.insert_iterator_unique(b, e);
+  }
+  void insert(std::initializer_list<init_type> init) {
+    this->tree_.insert_iterator_unique(init.begin(), init.end());
+  }
+  insert_return_type insert(node_type &&node) {
+    if (!node) return {this->end(), false, node_type()};
+    std::pair<iterator, bool> res =
+        this->tree_.insert_unique(params_type::key(CommonAccess::GetSlot(node)),
+                                  CommonAccess::GetSlot(node));
+    if (res.second) {
+      CommonAccess::Destroy(&node);
+      return {res.first, true, node_type()};
+    } else {
+      return {res.first, false, std::move(node)};
+    }
+  }
+  iterator insert(const_iterator hint, node_type &&node) {
+    if (!node) return this->end();
+    std::pair<iterator, bool> res = this->tree_.insert_hint_unique(
+        iterator(hint), params_type::key(CommonAccess::GetSlot(node)),
+        CommonAccess::GetSlot(node));
+    if (res.second) CommonAccess::Destroy(&node);
+    return res.first;
+  }
+
+  // Deletion routines.
+  template <typename K = key_type>
+  size_type erase(const key_arg<K> &key) {
+    return this->tree_.erase_unique(key);
+  }
+  using super_type::erase;
+
+  // Node extraction routines.
+  template <typename K = key_type>
+  node_type extract(const key_arg<K> &key) {
+    auto it = this->find(key);
+    return it == this->end() ? node_type() : extract(it);
+  }
+  using super_type::extract;
+
+  // Merge routines.
+  // Moves elements from `src` into `this`. If the element already exists in
+  // `this`, it is left unmodified in `src`.
+  template <
+      typename T,
+      typename absl::enable_if_t<
+          absl::conjunction<
+              std::is_same<value_type, typename T::value_type>,
+              std::is_same<allocator_type, typename T::allocator_type>,
+              std::is_same<typename params_type::is_map_container,
+                           typename T::params_type::is_map_container>>::value,
+          int> = 0>
+  void merge(btree_container<T> &src) {  // NOLINT
+    for (auto src_it = src.begin(); src_it != src.end();) {
+      if (insert(std::move(*src_it)).second) {
+        src_it = src.erase(src_it);
+      } else {
+        ++src_it;
+      }
+    }
+  }
+
+  template <
+      typename T,
+      typename absl::enable_if_t<
+          absl::conjunction<
+              std::is_same<value_type, typename T::value_type>,
+              std::is_same<allocator_type, typename T::allocator_type>,
+              std::is_same<typename params_type::is_map_container,
+                           typename T::params_type::is_map_container>>::value,
+          int> = 0>
+  void merge(btree_container<T> &&src) {
+    merge(src);
+  }
+};
+
+// Base class for btree_map.
+template <typename Tree>
+class btree_map_container : public btree_set_container<Tree> {
+  using super_type = btree_set_container<Tree>;
+  using params_type = typename Tree::params_type;
+
+ private:
+  template <class K>
+  using key_arg = typename super_type::template key_arg<K>;
+
+ public:
+  using key_type = typename Tree::key_type;
+  using mapped_type = typename params_type::mapped_type;
+  using value_type = typename Tree::value_type;
+  using key_compare = typename Tree::key_compare;
+  using allocator_type = typename Tree::allocator_type;
+  using iterator = typename Tree::iterator;
+  using const_iterator = typename Tree::const_iterator;
+
+  // Inherit constructors.
+  using super_type::super_type;
+  btree_map_container() {}
+
+  // Insertion routines.
+  // Note: the nullptr template arguments and extra `const M&` overloads allow
+  // for supporting bitfield arguments.
+  // Note: when we call `std::forward<M>(obj)` twice, it's safe because
+  // insert_unique/insert_hint_unique are guaranteed to not consume `obj` when
+  // `ret.second` is false.
+  template <class M>
+  std::pair<iterator, bool> insert_or_assign(const key_type &k, const M &obj) {
+    const std::pair<iterator, bool> ret = this->tree_.insert_unique(k, k, obj);
+    if (!ret.second) ret.first->second = obj;
+    return ret;
+  }
+  template <class M, key_type * = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_type &&k, const M &obj) {
+    const std::pair<iterator, bool> ret =
+        this->tree_.insert_unique(k, std::move(k), obj);
+    if (!ret.second) ret.first->second = obj;
+    return ret;
+  }
+  template <class M, M * = nullptr>
+  std::pair<iterator, bool> insert_or_assign(const key_type &k, M &&obj) {
+    const std::pair<iterator, bool> ret =
+        this->tree_.insert_unique(k, k, std::forward<M>(obj));
+    if (!ret.second) ret.first->second = std::forward<M>(obj);
+    return ret;
+  }
+  template <class M, key_type * = nullptr, M * = nullptr>
+  std::pair<iterator, bool> insert_or_assign(key_type &&k, M &&obj) {
+    const std::pair<iterator, bool> ret =
+        this->tree_.insert_unique(k, std::move(k), std::forward<M>(obj));
+    if (!ret.second) ret.first->second = std::forward<M>(obj);
+    return ret;
+  }
+  template <class M>
+  iterator insert_or_assign(const_iterator position, const key_type &k,
+                            const M &obj) {
+    const std::pair<iterator, bool> ret =
+        this->tree_.insert_hint_unique(iterator(position), k, k, obj);
+    if (!ret.second) ret.first->second = obj;
+    return ret.first;
+  }
+  template <class M, key_type * = nullptr>
+  iterator insert_or_assign(const_iterator position, key_type &&k,
+                            const M &obj) {
+    const std::pair<iterator, bool> ret = this->tree_.insert_hint_unique(
+        iterator(position), k, std::move(k), obj);
+    if (!ret.second) ret.first->second = obj;
+    return ret.first;
+  }
+  template <class M, M * = nullptr>
+  iterator insert_or_assign(const_iterator position, const key_type &k,
+                            M &&obj) {
+    const std::pair<iterator, bool> ret = this->tree_.insert_hint_unique(
+        iterator(position), k, k, std::forward<M>(obj));
+    if (!ret.second) ret.first->second = std::forward<M>(obj);
+    return ret.first;
+  }
+  template <class M, key_type * = nullptr, M * = nullptr>
+  iterator insert_or_assign(const_iterator position, key_type &&k, M &&obj) {
+    const std::pair<iterator, bool> ret = this->tree_.insert_hint_unique(
+        iterator(position), k, std::move(k), std::forward<M>(obj));
+    if (!ret.second) ret.first->second = std::forward<M>(obj);
+    return ret.first;
+  }
+  template <typename... Args>
+  std::pair<iterator, bool> try_emplace(const key_type &k, Args &&... args) {
+    return this->tree_.insert_unique(
+        k, std::piecewise_construct, std::forward_as_tuple(k),
+        std::forward_as_tuple(std::forward<Args>(args)...));
+  }
+  template <typename... Args>
+  std::pair<iterator, bool> try_emplace(key_type &&k, Args &&... args) {
+    // Note: `key_ref` exists to avoid a ClangTidy warning about moving from `k`
+    // and then using `k` unsequenced. This is safe because the move is into a
+    // forwarding reference and insert_unique guarantees that `key` is never
+    // referenced after consuming `args`.
+    const key_type &key_ref = k;
+    return this->tree_.insert_unique(
+        key_ref, std::piecewise_construct, std::forward_as_tuple(std::move(k)),
+        std::forward_as_tuple(std::forward<Args>(args)...));
+  }
+  template <typename... Args>
+  iterator try_emplace(const_iterator hint, const key_type &k,
+                       Args &&... args) {
+    return this->tree_
+        .insert_hint_unique(iterator(hint), k, std::piecewise_construct,
+                            std::forward_as_tuple(k),
+                            std::forward_as_tuple(std::forward<Args>(args)...))
+        .first;
+  }
+  template <typename... Args>
+  iterator try_emplace(const_iterator hint, key_type &&k, Args &&... args) {
+    // Note: `key_ref` exists to avoid a ClangTidy warning about moving from `k`
+    // and then using `k` unsequenced. This is safe because the move is into a
+    // forwarding reference and insert_hint_unique guarantees that `key` is
+    // never referenced after consuming `args`.
+    const key_type &key_ref = k;
+    return this->tree_
+        .insert_hint_unique(iterator(hint), key_ref, std::piecewise_construct,
+                            std::forward_as_tuple(std::move(k)),
+                            std::forward_as_tuple(std::forward<Args>(args)...))
+        .first;
+  }
+  mapped_type &operator[](const key_type &k) {
+    return try_emplace(k).first->second;
+  }
+  mapped_type &operator[](key_type &&k) {
+    return try_emplace(std::move(k)).first->second;
+  }
+
+  template <typename K = key_type>
+  mapped_type &at(const key_arg<K> &key) {
+    auto it = this->find(key);
+    if (it == this->end())
+      base_internal::ThrowStdOutOfRange("absl::btree_map::at");
+    return it->second;
+  }
+  template <typename K = key_type>
+  const mapped_type &at(const key_arg<K> &key) const {
+    auto it = this->find(key);
+    if (it == this->end())
+      base_internal::ThrowStdOutOfRange("absl::btree_map::at");
+    return it->second;
+  }
+};
+
+// A common base class for btree_multiset and btree_multimap.
+template <typename Tree>
+class btree_multiset_container : public btree_container<Tree> {
+  using super_type = btree_container<Tree>;
+  using params_type = typename Tree::params_type;
+  using init_type = typename params_type::init_type;
+  using is_key_compare_to = typename params_type::is_key_compare_to;
+
+  template <class K>
+  using key_arg = typename super_type::template key_arg<K>;
+
+ public:
+  using key_type = typename Tree::key_type;
+  using value_type = typename Tree::value_type;
+  using size_type = typename Tree::size_type;
+  using key_compare = typename Tree::key_compare;
+  using allocator_type = typename Tree::allocator_type;
+  using iterator = typename Tree::iterator;
+  using const_iterator = typename Tree::const_iterator;
+  using node_type = typename super_type::node_type;
+
+  // Inherit constructors.
+  using super_type::super_type;
+  btree_multiset_container() {}
+
+  // Range constructor.
+  template <class InputIterator>
+  btree_multiset_container(InputIterator b, InputIterator e,
+                           const key_compare &comp = key_compare(),
+                           const allocator_type &alloc = allocator_type())
+      : super_type(comp, alloc) {
+    insert(b, e);
+  }
+
+  // Initializer list constructor.
+  btree_multiset_container(std::initializer_list<init_type> init,
+                           const key_compare &comp = key_compare(),
+                           const allocator_type &alloc = allocator_type())
+      : btree_multiset_container(init.begin(), init.end(), comp, alloc) {}
+
+  // Lookup routines.
+  template <typename K = key_type>
+  size_type count(const key_arg<K> &key) const {
+    return this->tree_.count_multi(key);
+  }
+
+  // Insertion routines.
+  iterator insert(const value_type &x) { return this->tree_.insert_multi(x); }
+  iterator insert(value_type &&x) {
+    return this->tree_.insert_multi(std::move(x));
+  }
+  iterator insert(const_iterator position, const value_type &x) {
+    return this->tree_.insert_hint_multi(iterator(position), x);
+  }
+  iterator insert(const_iterator position, value_type &&x) {
+    return this->tree_.insert_hint_multi(iterator(position), std::move(x));
+  }
+  template <typename InputIterator>
+  void insert(InputIterator b, InputIterator e) {
+    this->tree_.insert_iterator_multi(b, e);
+  }
+  void insert(std::initializer_list<init_type> init) {
+    this->tree_.insert_iterator_multi(init.begin(), init.end());
+  }
+  template <typename... Args>
+  iterator emplace(Args &&... args) {
+    return this->tree_.insert_multi(init_type(std::forward<Args>(args)...));
+  }
+  template <typename... Args>
+  iterator emplace_hint(const_iterator position, Args &&... args) {
+    return this->tree_.insert_hint_multi(
+        iterator(position), init_type(std::forward<Args>(args)...));
+  }
+  iterator insert(node_type &&node) {
+    if (!node) return this->end();
+    iterator res =
+        this->tree_.insert_multi(params_type::key(CommonAccess::GetSlot(node)),
+                                 CommonAccess::GetSlot(node));
+    CommonAccess::Destroy(&node);
+    return res;
+  }
+  iterator insert(const_iterator hint, node_type &&node) {
+    if (!node) return this->end();
+    iterator res = this->tree_.insert_hint_multi(
+        iterator(hint),
+        std::move(params_type::element(CommonAccess::GetSlot(node))));
+    CommonAccess::Destroy(&node);
+    return res;
+  }
+
+  // Deletion routines.
+  template <typename K = key_type>
+  size_type erase(const key_arg<K> &key) {
+    return this->tree_.erase_multi(key);
+  }
+  using super_type::erase;
+
+  // Node extraction routines.
+  template <typename K = key_type>
+  node_type extract(const key_arg<K> &key) {
+    auto it = this->find(key);
+    return it == this->end() ? node_type() : extract(it);
+  }
+  using super_type::extract;
+
+  // Merge routines.
+  // Moves all elements from `src` into `this`.
+  template <
+      typename T,
+      typename absl::enable_if_t<
+          absl::conjunction<
+              std::is_same<value_type, typename T::value_type>,
+              std::is_same<allocator_type, typename T::allocator_type>,
+              std::is_same<typename params_type::is_map_container,
+                           typename T::params_type::is_map_container>>::value,
+          int> = 0>
+  void merge(btree_container<T> &src) {  // NOLINT
+    insert(std::make_move_iterator(src.begin()),
+           std::make_move_iterator(src.end()));
+    src.clear();
+  }
+
+  template <
+      typename T,
+      typename absl::enable_if_t<
+          absl::conjunction<
+              std::is_same<value_type, typename T::value_type>,
+              std::is_same<allocator_type, typename T::allocator_type>,
+              std::is_same<typename params_type::is_map_container,
+                           typename T::params_type::is_map_container>>::value,
+          int> = 0>
+  void merge(btree_container<T> &&src) {
+    merge(src);
+  }
+};
+
+// A base class for btree_multimap.
+template <typename Tree>
+class btree_multimap_container : public btree_multiset_container<Tree> {
+  using super_type = btree_multiset_container<Tree>;
+  using params_type = typename Tree::params_type;
+
+ public:
+  using mapped_type = typename params_type::mapped_type;
+
+  // Inherit constructors.
+  using super_type::super_type;
+  btree_multimap_container() {}
+};
+
+}  // namespace container_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_CONTAINER_INTERNAL_BTREE_CONTAINER_H_
diff --git a/absl/container/internal/common.h b/absl/container/internal/common.h
index a02cd5c..5037d80 100644
--- a/absl/container/internal/common.h
+++ b/absl/container/internal/common.h
@@ -22,7 +22,7 @@
 #include "absl/types/optional.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class, class = void>
@@ -56,7 +56,7 @@
  public:
   using allocator_type = Alloc;
 
-  constexpr node_handle_base() {}
+  constexpr node_handle_base() = default;
   node_handle_base(node_handle_base&& other) noexcept {
     *this = std::move(other);
   }
@@ -109,16 +109,15 @@
   allocator_type* alloc() { return std::addressof(*alloc_); }
 
  private:
-  absl::optional<allocator_type> alloc_;
-  mutable absl::aligned_storage_t<sizeof(slot_type), alignof(slot_type)>
-      slot_space_;
+  absl::optional<allocator_type> alloc_ = {};
+  alignas(slot_type) mutable unsigned char slot_space_[sizeof(slot_type)] = {};
 };
 
 // For sets.
 template <typename Policy, typename PolicyTraits, typename Alloc,
           typename = void>
 class node_handle : public node_handle_base<PolicyTraits, Alloc> {
-  using Base = typename node_handle::node_handle_base;
+  using Base = node_handle_base<PolicyTraits, Alloc>;
 
  public:
   using value_type = typename PolicyTraits::value_type;
@@ -138,7 +137,7 @@
 class node_handle<Policy, PolicyTraits, Alloc,
                   absl::void_t<typename Policy::mapped_type>>
     : public node_handle_base<PolicyTraits, Alloc> {
-  using Base = typename node_handle::node_handle_base;
+  using Base = node_handle_base<PolicyTraits, Alloc>;
 
  public:
   using key_type = typename Policy::key_type;
@@ -168,6 +167,11 @@
   }
 
   template <typename Node>
+  static void Destroy(Node* node) {
+    node->destroy();
+  }
+
+  template <typename Node>
   static void Reset(Node* node) {
     node->reset();
   }
@@ -192,7 +196,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_H_
diff --git a/absl/container/internal/compressed_tuple.h b/absl/container/internal/compressed_tuple.h
index fbace49..4bfe92f 100644
--- a/absl/container/internal/compressed_tuple.h
+++ b/absl/container/internal/compressed_tuple.h
@@ -48,7 +48,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <typename... Ts>
@@ -257,7 +257,7 @@
 class ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC CompressedTuple<> {};
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_INTERNAL_COMPRESSED_TUPLE_DECLSPEC
diff --git a/absl/container/internal/compressed_tuple_test.cc b/absl/container/internal/compressed_tuple_test.cc
index ec893b9..1dae12d 100644
--- a/absl/container/internal/compressed_tuple_test.cc
+++ b/absl/container/internal/compressed_tuple_test.cc
@@ -48,7 +48,7 @@
 
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -333,10 +333,6 @@
 
   a = 0.5f;
   EXPECT_EQ(absl::any_cast<float>(x.get<1>()), 0.5);
-
-  // Ensure copy construction work in the face of a type with a universal
-  // implicit constructor;
-  CompressedTuple<absl::any> c{}, d(c);  // NOLINT
 }
 
 TEST(CompressedTupleTest, Constexpr) {
@@ -409,5 +405,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/container_memory.h b/absl/container/internal/container_memory.h
index eb6d7eb..d24b0f8 100644
--- a/absl/container/internal/container_memory.h
+++ b/absl/container/internal/container_memory.h
@@ -34,7 +34,7 @@
 #include "absl/utility/utility.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Allocates at least n bytes aligned to the specified alignment.
@@ -434,7 +434,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_CONTAINER_MEMORY_H_
diff --git a/absl/container/internal/container_memory_test.cc b/absl/container/internal/container_memory_test.cc
index ea9568d..7942c7b 100644
--- a/absl/container/internal/container_memory_test.cc
+++ b/absl/container/internal/container_memory_test.cc
@@ -23,7 +23,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -186,5 +186,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/counting_allocator.h b/absl/container/internal/counting_allocator.h
index 94a457c..9efdc66 100644
--- a/absl/container/internal/counting_allocator.h
+++ b/absl/container/internal/counting_allocator.h
@@ -19,8 +19,10 @@
 #include <cstdint>
 #include <memory>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // This is a stateful allocator, but the state lives outside of the
@@ -75,7 +77,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_COUNTING_ALLOCATOR_H_
diff --git a/absl/container/internal/hash_function_defaults.h b/absl/container/internal/hash_function_defaults.h
index 2155076..401ddf4 100644
--- a/absl/container/internal/hash_function_defaults.h
+++ b/absl/container/internal/hash_function_defaults.h
@@ -56,7 +56,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // The hash of an object of type T is computed by using absl::Hash.
@@ -140,7 +140,7 @@
 using hash_default_eq = typename container_internal::HashEq<T>::Eq;
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_FUNCTION_DEFAULTS_H_
diff --git a/absl/container/internal/hash_function_defaults_test.cc b/absl/container/internal/hash_function_defaults_test.cc
index ce6133f..2eefc7e 100644
--- a/absl/container/internal/hash_function_defaults_test.cc
+++ b/absl/container/internal/hash_function_defaults_test.cc
@@ -22,7 +22,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -249,7 +249,7 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 enum Hash : size_t {
@@ -280,7 +280,7 @@
 }  // namespace std
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -295,5 +295,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.cc b/absl/container/internal/hash_generator_testing.cc
index 36b2571..75c4db6 100644
--- a/absl/container/internal/hash_generator_testing.cc
+++ b/absl/container/internal/hash_generator_testing.cc
@@ -17,7 +17,7 @@
 #include <deque>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hash_internal {
 namespace {
@@ -70,5 +70,5 @@
 
 }  // namespace hash_internal
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_generator_testing.h b/absl/container/internal/hash_generator_testing.h
index 27962c3..6869fe4 100644
--- a/absl/container/internal/hash_generator_testing.h
+++ b/absl/container/internal/hash_generator_testing.h
@@ -19,6 +19,7 @@
 #define ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
 
 #include <stdint.h>
+
 #include <algorithm>
 #include <iosfwd>
 #include <random>
@@ -27,11 +28,12 @@
 #include <utility>
 
 #include "absl/container/internal/hash_policy_testing.h"
+#include "absl/memory/memory.h"
 #include "absl/meta/type_traits.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hash_internal {
 namespace generator_internal {
@@ -130,6 +132,13 @@
   }
 };
 
+template <class T>
+struct Generator<std::unique_ptr<T>> {
+  std::unique_ptr<T> operator()() const {
+    return absl::make_unique<T>(Generator<T>()());
+  }
+};
+
 template <class U>
 struct Generator<U, absl::void_t<decltype(std::declval<U&>().key()),
                                 decltype(std::declval<U&>().value())>>
@@ -146,7 +155,7 @@
 
 }  // namespace hash_internal
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_GENERATOR_TESTING_H_
diff --git a/absl/container/internal/hash_policy_testing.h b/absl/container/internal/hash_policy_testing.h
index 8f0d2a5..01c40d2 100644
--- a/absl/container/internal/hash_policy_testing.h
+++ b/absl/container/internal/hash_policy_testing.h
@@ -30,7 +30,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hash_testing_internal {
 
@@ -163,7 +163,7 @@
 }
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // ABSL_UNORDERED_SUPPORTS_ALLOC_CTORS is false for glibcxx versions
diff --git a/absl/container/internal/hash_policy_testing_test.cc b/absl/container/internal/hash_policy_testing_test.cc
index 8fd1df0..f0b20fe 100644
--- a/absl/container/internal/hash_policy_testing_test.cc
+++ b/absl/container/internal/hash_policy_testing_test.cc
@@ -17,7 +17,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -41,5 +41,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hash_policy_traits.h b/absl/container/internal/hash_policy_traits.h
index 3d87e82..3e1209c 100644
--- a/absl/container/internal/hash_policy_traits.h
+++ b/absl/container/internal/hash_policy_traits.h
@@ -23,7 +23,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Defines how slots are initialized/destroyed/moved.
@@ -185,7 +185,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASH_POLICY_TRAITS_H_
diff --git a/absl/container/internal/hash_policy_traits_test.cc b/absl/container/internal/hash_policy_traits_test.cc
index edfaf63..6ef8b9e 100644
--- a/absl/container/internal/hash_policy_traits_test.cc
+++ b/absl/container/internal/hash_policy_traits_test.cc
@@ -22,7 +22,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -140,5 +140,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtable_debug.h b/absl/container/internal/hashtable_debug.h
index 1d1a9c2..19d5212 100644
--- a/absl/container/internal/hashtable_debug.h
+++ b/absl/container/internal/hashtable_debug.h
@@ -38,7 +38,7 @@
 #include "absl/container/internal/hashtable_debug_hooks.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Returns the number of probes required to lookup `key`.  Returns 0 for a
@@ -104,7 +104,7 @@
 }
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_H_
diff --git a/absl/container/internal/hashtable_debug_hooks.h b/absl/container/internal/hashtable_debug_hooks.h
index 7b95fce..3e9ea59 100644
--- a/absl/container/internal/hashtable_debug_hooks.h
+++ b/absl/container/internal/hashtable_debug_hooks.h
@@ -23,8 +23,10 @@
 #include <type_traits>
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace hashtable_debug_internal {
 
@@ -77,7 +79,7 @@
 
 }  // namespace hashtable_debug_internal
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLE_DEBUG_HOOKS_H_
diff --git a/absl/container/internal/hashtablez_sampler.cc b/absl/container/internal/hashtablez_sampler.cc
index 2338045..5644725 100644
--- a/absl/container/internal/hashtablez_sampler.cc
+++ b/absl/container/internal/hashtablez_sampler.cc
@@ -21,13 +21,14 @@
 #include <limits>
 
 #include "absl/base/attributes.h"
+#include "absl/base/internal/exponential_biased.h"
 #include "absl/container/internal/have_sse.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/memory/memory.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 constexpr int HashtablezInfo::kMaxStackDepth;
 
@@ -38,80 +39,17 @@
 ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_sample_parameter{1 << 10};
 ABSL_CONST_INIT std::atomic<int32_t> g_hashtablez_max_samples{1 << 20};
 
-// Returns the next pseudo-random value.
-// pRNG is: aX+b mod c with a = 0x5DEECE66D, b =  0xB, c = 1<<48
-// This is the lrand64 generator.
-uint64_t NextRandom(uint64_t rnd) {
-  const uint64_t prng_mult = uint64_t{0x5DEECE66D};
-  const uint64_t prng_add = 0xB;
-  const uint64_t prng_mod_power = 48;
-  const uint64_t prng_mod_mask = ~(~uint64_t{0} << prng_mod_power);
-  return (prng_mult * rnd + prng_add) & prng_mod_mask;
-}
-
-// Generates a geometric variable with the specified mean.
-// This is done by generating a random number between 0 and 1 and applying
-// the inverse cumulative distribution function for an exponential.
-// Specifically: Let m be the inverse of the sample period, then
-// the probability distribution function is m*exp(-mx) so the CDF is
-// p = 1 - exp(-mx), so
-// q = 1 - p = exp(-mx)
-// log_e(q) = -mx
-// -log_e(q)/m = x
-// log_2(q) * (-log_e(2) * 1/m) = x
-// In the code, q is actually in the range 1 to 2**26, hence the -26 below
-//
-int64_t GetGeometricVariable(int64_t mean) {
-#if ABSL_HAVE_THREAD_LOCAL
-  thread_local
-#else   // ABSL_HAVE_THREAD_LOCAL
-  // SampleSlow and hence GetGeometricVariable is guarded by a single mutex when
-  // there are not thread locals.  Thus, a single global rng is acceptable for
-  // that case.
-  static
-#endif  // ABSL_HAVE_THREAD_LOCAL
-      uint64_t rng = []() {
-        // We don't get well distributed numbers from this so we call
-        // NextRandom() a bunch to mush the bits around.  We use a global_rand
-        // to handle the case where the same thread (by memory address) gets
-        // created and destroyed repeatedly.
-        ABSL_CONST_INIT static std::atomic<uint32_t> global_rand(0);
-        uint64_t r = reinterpret_cast<uint64_t>(&rng) +
-                   global_rand.fetch_add(1, std::memory_order_relaxed);
-        for (int i = 0; i < 20; ++i) {
-          r = NextRandom(r);
-        }
-        return r;
-      }();
-
-  rng = NextRandom(rng);
-
-  // Take the top 26 bits as the random number
-  // (This plus the 1<<58 sampling bound give a max possible step of
-  // 5194297183973780480 bytes.)
-  const uint64_t prng_mod_power = 48;  // Number of bits in prng
-  // The uint32_t cast is to prevent a (hard-to-reproduce) NAN
-  // under piii debug for some binaries.
-  double q = static_cast<uint32_t>(rng >> (prng_mod_power - 26)) + 1.0;
-  // Put the computed p-value through the CDF of a geometric.
-  double interval = (log2(q) - 26) * (-std::log(2.0) * mean);
-
-  // Very large values of interval overflow int64_t. If we happen to
-  // hit such improbable condition, we simply cheat and clamp interval
-  // to largest supported value.
-  if (interval > static_cast<double>(std::numeric_limits<int64_t>::max() / 2)) {
-    return std::numeric_limits<int64_t>::max() / 2;
-  }
-
-  // Small values of interval are equivalent to just sampling next time.
-  if (interval < 1) {
-    return 1;
-  }
-  return static_cast<int64_t>(interval);
-}
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+ABSL_PER_THREAD_TLS_KEYWORD absl::base_internal::ExponentialBiased
+    g_exponential_biased_generator;
+#endif
 
 }  // namespace
 
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
 HashtablezSampler& HashtablezSampler::Global() {
   static auto* sampler = new HashtablezSampler();
   return *sampler;
@@ -229,15 +167,39 @@
   return dropped_samples_.load(std::memory_order_relaxed);
 }
 
+static bool ShouldForceSampling() {
+  enum ForceState {
+    kDontForce,
+    kForce,
+    kUninitialized
+  };
+  ABSL_CONST_INIT static std::atomic<ForceState> global_state{
+      kUninitialized};
+  ForceState state = global_state.load(std::memory_order_relaxed);
+  if (ABSL_PREDICT_TRUE(state == kDontForce)) return false;
+
+  if (state == kUninitialized) {
+    state = AbslContainerInternalSampleEverything() ? kForce : kDontForce;
+    global_state.store(state, std::memory_order_relaxed);
+  }
+  return state == kForce;
+}
+
 HashtablezInfo* SampleSlow(int64_t* next_sample) {
-  if (kAbslContainerInternalSampleEverything) {
+  if (ABSL_PREDICT_FALSE(ShouldForceSampling())) {
     *next_sample = 1;
     return HashtablezSampler::Global().Register();
   }
 
+#if !defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+  *next_sample = std::numeric_limits<int64_t>::max();
+  return nullptr;
+#else
   bool first = *next_sample < 0;
-  *next_sample = GetGeometricVariable(
+  *next_sample = g_exponential_biased_generator.GetStride(
       g_hashtablez_sample_parameter.load(std::memory_order_relaxed));
+  // Small values of interval are equivalent to just sampling next time.
+  ABSL_ASSERT(*next_sample >= 1);
 
   // g_hashtablez_enabled can be dynamically flipped, we need to set a threshold
   // low enough that we will start sampling in a reasonable time, so we just use
@@ -252,12 +214,9 @@
   }
 
   return HashtablezSampler::Global().Register();
+#endif
 }
 
-#if ABSL_PER_THREAD_TLS == 1
-ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample = 0;
-#endif  // ABSL_PER_THREAD_TLS == 1
-
 void UnsampleSlow(HashtablezInfo* info) {
   HashtablezSampler::Global().Unregister(info);
 }
@@ -306,5 +265,5 @@
 }
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtablez_sampler.h b/absl/container/internal/hashtablez_sampler.h
index f17c425..34d5e57 100644
--- a/absl/container/internal/hashtablez_sampler.h
+++ b/absl/container/internal/hashtablez_sampler.h
@@ -51,7 +51,7 @@
 #include "absl/utility/utility.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // Stores information about a sampled hashtable.  All mutations to this *must*
@@ -66,7 +66,7 @@
 
   // Puts the object into a clean state, fills in the logically `const` members,
   // blocking for any readers that are currently sampling the object.
-  void PrepareForSampling() EXCLUSIVE_LOCKS_REQUIRED(init_mu);
+  void PrepareForSampling() ABSL_EXCLUSIVE_LOCKS_REQUIRED(init_mu);
 
   // These fields are mutated by the various Record* APIs and need to be
   // thread-safe.
@@ -84,7 +84,7 @@
   // prevents races with sampling and resurrecting an object.
   absl::Mutex init_mu;
   HashtablezInfo* next;
-  HashtablezInfo* dead GUARDED_BY(init_mu);
+  HashtablezInfo* dead ABSL_GUARDED_BY(init_mu);
 
   // All of the fields below are set by `PrepareForSampling`, they must not be
   // mutated in `Record*` functions.  They are logically `const` in that sense.
@@ -180,23 +180,30 @@
   HashtablezInfo* info_;
 };
 
-#if ABSL_PER_THREAD_TLS == 1
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+#error ABSL_INTERNAL_HASHTABLEZ_SAMPLE cannot be directly set
+#endif  // defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
+
+#if (ABSL_PER_THREAD_TLS == 1) && !defined(ABSL_BUILD_DLL) && \
+    !defined(ABSL_CONSUME_DLL)
+#define ABSL_INTERNAL_HASHTABLEZ_SAMPLE
+#endif
+
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
 extern ABSL_PER_THREAD_TLS_KEYWORD int64_t global_next_sample;
 #endif  // ABSL_PER_THREAD_TLS
 
 // Returns an RAII sampling handle that manages registration and unregistation
 // with the global sampler.
 inline HashtablezInfoHandle Sample() {
-#if ABSL_PER_THREAD_TLS == 0
-  static auto* mu = new absl::Mutex;
-  static int64_t global_next_sample = 0;
-  absl::MutexLock l(mu);
-#endif  // !ABSL_HAVE_THREAD_LOCAL
-
+#if defined(ABSL_INTERNAL_HASHTABLEZ_SAMPLE)
   if (ABSL_PREDICT_TRUE(--global_next_sample > 0)) {
     return HashtablezInfoHandle(nullptr);
   }
   return HashtablezInfoHandle(SampleSlow(&global_next_sample));
+#else
+  return HashtablezInfoHandle(nullptr);
+#endif  // !ABSL_PER_THREAD_TLS
 }
 
 // Holds samples and their associated stack traces with a soft limit of
@@ -281,10 +288,10 @@
 // initialization of static storage duration objects.
 // The definition of this constant is weak, which allows us to inject a
 // different value for it at link time.
-extern "C" const bool kAbslContainerInternalSampleEverything;
+extern "C" bool AbslContainerInternalSampleEverything();
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_HASHTABLEZ_SAMPLER_H_
diff --git a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
index d3f41c7..78b9d36 100644
--- a/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
+++ b/absl/container/internal/hashtablez_sampler_force_weak_definition.cc
@@ -17,13 +17,14 @@
 #include "absl/base/attributes.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // See hashtablez_sampler.h for details.
-extern "C" ABSL_ATTRIBUTE_WEAK const bool
-    kAbslContainerInternalSampleEverything = false;
+extern "C" ABSL_ATTRIBUTE_WEAK bool AbslContainerInternalSampleEverything() {
+  return false;
+}
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/hashtablez_sampler_test.cc b/absl/container/internal/hashtablez_sampler_test.cc
index bdae75f..36f5ccd 100644
--- a/absl/container/internal/hashtablez_sampler_test.cc
+++ b/absl/container/internal/hashtablez_sampler_test.cc
@@ -36,7 +36,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 class HashtablezInfoHandlePeer {
  public:
@@ -169,6 +169,7 @@
   EXPECT_EQ(info.num_erases.load(), 0);
 }
 
+#if defined(ABSL_HASHTABLEZ_SAMPLE)
 TEST(HashtablezSamplerTest, SmallSampleParameter) {
   SetHashtablezEnabled(true);
   SetHashtablezSampleParameter(100);
@@ -212,6 +213,7 @@
   }
   EXPECT_NEAR(sample_rate, 0.01, 0.005);
 }
+#endif
 
 TEST(HashtablezSamplerTest, Handle) {
   auto& sampler = HashtablezSampler::Global();
@@ -353,5 +355,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index 123e04c..4d80b72 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -30,7 +30,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace inlined_vector_internal {
 
 template <typename Iterator>
@@ -38,16 +38,17 @@
     typename std::iterator_traits<Iterator>::iterator_category,
     std::forward_iterator_tag>;
 
-template <typename AllocatorType>
-using IsMemcpyOk = absl::conjunction<
-    std::is_same<std::allocator<typename AllocatorType::value_type>,
-                 AllocatorType>,
-    absl::is_trivially_copy_constructible<typename AllocatorType::value_type>,
-    absl::is_trivially_copy_assignable<typename AllocatorType::value_type>,
-    absl::is_trivially_destructible<typename AllocatorType::value_type>>;
+template <typename AllocatorType,
+          typename ValueType =
+              typename absl::allocator_traits<AllocatorType>::value_type>
+using IsMemcpyOk =
+    absl::conjunction<std::is_same<AllocatorType, std::allocator<ValueType>>,
+                      absl::is_trivially_copy_constructible<ValueType>,
+                      absl::is_trivially_copy_assignable<ValueType>,
+                      absl::is_trivially_destructible<ValueType>>;
 
-template <typename AllocatorType, typename ValueType, typename SizeType>
-void DestroyElements(AllocatorType* alloc_ptr, ValueType* destroy_first,
+template <typename AllocatorType, typename Pointer, typename SizeType>
+void DestroyElements(AllocatorType* alloc_ptr, Pointer destroy_first,
                      SizeType destroy_size) {
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
 
@@ -57,21 +58,26 @@
       AllocatorTraits::destroy(*alloc_ptr, destroy_first + i);
     }
 
-#ifndef NDEBUG
-    // Overwrite unused memory with `0xab` so we can catch uninitialized usage.
-    //
-    // Cast to `void*` to tell the compiler that we don't care that we might be
-    // scribbling on a vtable pointer.
-    auto* memory_ptr = static_cast<void*>(destroy_first);
-    auto memory_size = sizeof(ValueType) * destroy_size;
-    std::memset(memory_ptr, 0xab, memory_size);
-#endif  // NDEBUG
+#if !defined(NDEBUG)
+    {
+      using ValueType = typename AllocatorTraits::value_type;
+
+      // Overwrite unused memory with `0xab` so we can catch uninitialized
+      // usage.
+      //
+      // Cast to `void*` to tell the compiler that we don't care that we might
+      // be scribbling on a vtable pointer.
+      void* memory_ptr = destroy_first;
+      auto memory_size = destroy_size * sizeof(ValueType);
+      std::memset(memory_ptr, 0xab, memory_size);
+    }
+#endif  // !defined(NDEBUG)
   }
 }
 
-template <typename AllocatorType, typename ValueType, typename ValueAdapter,
+template <typename AllocatorType, typename Pointer, typename ValueAdapter,
           typename SizeType>
-void ConstructElements(AllocatorType* alloc_ptr, ValueType* construct_first,
+void ConstructElements(AllocatorType* alloc_ptr, Pointer construct_first,
                        ValueAdapter* values_ptr, SizeType construct_size) {
   for (SizeType i = 0; i < construct_size; ++i) {
     ABSL_INTERNAL_TRY {
@@ -84,8 +90,8 @@
   }
 }
 
-template <typename ValueType, typename ValueAdapter, typename SizeType>
-void AssignElements(ValueType* assign_first, ValueAdapter* values_ptr,
+template <typename Pointer, typename ValueAdapter, typename SizeType>
+void AssignElements(Pointer assign_first, ValueAdapter* values_ptr,
                     SizeType assign_size) {
   for (SizeType i = 0; i < assign_size; ++i) {
     values_ptr->AssignNext(assign_first + i);
@@ -94,28 +100,29 @@
 
 template <typename AllocatorType>
 struct StorageView {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
+  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
-  pointer data;
-  size_type size;
-  size_type capacity;
+  Pointer data;
+  SizeType size;
+  SizeType capacity;
 };
 
 template <typename AllocatorType, typename Iterator>
 class IteratorValueAdapter {
-  using pointer = typename AllocatorType::pointer;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
 
  public:
   explicit IteratorValueAdapter(const Iterator& it) : it_(it) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at, *it_);
     ++it_;
   }
 
-  void AssignNext(pointer assign_at) {
+  void AssignNext(Pointer assign_at) {
     *assign_at = *it_;
     ++it_;
   }
@@ -126,46 +133,45 @@
 
 template <typename AllocatorType>
 class CopyValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using const_pointer = typename AllocatorType::const_pointer;
-  using const_reference = typename AllocatorType::const_reference;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using ValueType = typename AllocatorTraits::value_type;
+  using Pointer = typename AllocatorTraits::pointer;
+  using ConstPointer = typename AllocatorTraits::const_pointer;
 
  public:
-  explicit CopyValueAdapter(const_reference v) : ptr_(std::addressof(v)) {}
+  explicit CopyValueAdapter(const ValueType& v) : ptr_(std::addressof(v)) {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at, *ptr_);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = *ptr_; }
+  void AssignNext(Pointer assign_at) { *assign_at = *ptr_; }
 
  private:
-  const_pointer ptr_;
+  ConstPointer ptr_;
 };
 
 template <typename AllocatorType>
 class DefaultValueAdapter {
-  using pointer = typename AllocatorType::pointer;
-  using value_type = typename AllocatorType::value_type;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using ValueType = typename AllocatorTraits::value_type;
+  using Pointer = typename AllocatorTraits::pointer;
 
  public:
   explicit DefaultValueAdapter() {}
 
-  void ConstructNext(AllocatorType* alloc_ptr, pointer construct_at) {
+  void ConstructNext(AllocatorType* alloc_ptr, Pointer construct_at) {
     AllocatorTraits::construct(*alloc_ptr, construct_at);
   }
 
-  void AssignNext(pointer assign_at) { *assign_at = value_type(); }
+  void AssignNext(Pointer assign_at) { *assign_at = ValueType(); }
 };
 
 template <typename AllocatorType>
 class AllocationTransaction {
-  using value_type = typename AllocatorType::value_type;
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
   using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
  public:
   explicit AllocationTransaction(AllocatorType* alloc_ptr)
@@ -181,25 +187,31 @@
   void operator=(const AllocationTransaction&) = delete;
 
   AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetCapacity() { return capacity_; }
+  Pointer& GetData() { return alloc_data_.template get<1>(); }
+  SizeType& GetCapacity() { return capacity_; }
 
   bool DidAllocate() { return GetData() != nullptr; }
-  pointer Allocate(size_type capacity) {
+  Pointer Allocate(SizeType capacity) {
     GetData() = AllocatorTraits::allocate(GetAllocator(), capacity);
     GetCapacity() = capacity;
     return GetData();
   }
 
+  void Reset() {
+    GetData() = nullptr;
+    GetCapacity() = 0;
+  }
+
  private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type capacity_ = 0;
+  container_internal::CompressedTuple<AllocatorType, Pointer> alloc_data_;
+  SizeType capacity_ = 0;
 };
 
 template <typename AllocatorType>
 class ConstructionTransaction {
-  using pointer = typename AllocatorType::pointer;
-  using size_type = typename AllocatorType::size_type;
+  using AllocatorTraits = absl::allocator_traits<AllocatorType>;
+  using Pointer = typename AllocatorTraits::pointer;
+  using SizeType = typename AllocatorTraits::size_type;
 
  public:
   explicit ConstructionTransaction(AllocatorType* alloc_ptr)
@@ -216,12 +228,12 @@
   void operator=(const ConstructionTransaction&) = delete;
 
   AllocatorType& GetAllocator() { return alloc_data_.template get<0>(); }
-  pointer& GetData() { return alloc_data_.template get<1>(); }
-  size_type& GetSize() { return size_; }
+  Pointer& GetData() { return alloc_data_.template get<1>(); }
+  SizeType& GetSize() { return size_; }
 
   bool DidConstruct() { return GetData() != nullptr; }
   template <typename ValueAdapter>
-  void Construct(pointer data, ValueAdapter* values_ptr, size_type size) {
+  void Construct(Pointer data, ValueAdapter* values_ptr, SizeType size) {
     inlined_vector_internal::ConstructElements(std::addressof(GetAllocator()),
                                                data, values_ptr, size);
     GetData() = data;
@@ -233,28 +245,29 @@
   }
 
  private:
-  container_internal::CompressedTuple<AllocatorType, pointer> alloc_data_;
-  size_type size_ = 0;
+  container_internal::CompressedTuple<AllocatorType, Pointer> alloc_data_;
+  SizeType size_ = 0;
 };
 
 template <typename T, size_t N, typename A>
 class Storage {
  public:
-  using allocator_type = A;
-  using value_type = typename allocator_type::value_type;
-  using pointer = typename allocator_type::pointer;
-  using const_pointer = typename allocator_type::const_pointer;
-  using reference = typename allocator_type::reference;
-  using const_reference = typename allocator_type::const_reference;
-  using rvalue_reference = typename allocator_type::value_type&&;
-  using size_type = typename allocator_type::size_type;
-  using difference_type = typename allocator_type::difference_type;
+  using AllocatorTraits = absl::allocator_traits<A>;
+  using allocator_type = typename AllocatorTraits::allocator_type;
+  using value_type = typename AllocatorTraits::value_type;
+  using pointer = typename AllocatorTraits::pointer;
+  using const_pointer = typename AllocatorTraits::const_pointer;
+  using size_type = typename AllocatorTraits::size_type;
+  using difference_type = typename AllocatorTraits::difference_type;
+
+  using reference = value_type&;
+  using const_reference = const value_type&;
+  using RValueReference = value_type&&;
   using iterator = pointer;
   using const_iterator = const_pointer;
   using reverse_iterator = std::reverse_iterator<iterator>;
   using const_reverse_iterator = std::reverse_iterator<const_iterator>;
   using MoveIterator = std::move_iterator<iterator>;
-  using AllocatorTraits = absl::allocator_traits<allocator_type>;
   using IsMemcpyOk = inlined_vector_internal::IsMemcpyOk<allocator_type>;
 
   using StorageView = inlined_vector_internal::StorageView<allocator_type>;
@@ -287,8 +300,7 @@
 
   Storage() : metadata_() {}
 
-  explicit Storage(const allocator_type& alloc)
-      : metadata_(alloc, /* empty and inlined */ 0) {}
+  explicit Storage(const allocator_type& alloc) : metadata_(alloc, {}) {}
 
   ~Storage() {
     pointer data = GetIsAllocated() ? GetAllocatedData() : GetInlinedData();
@@ -414,8 +426,8 @@
   void AcquireAllocatedData(AllocationTransaction* allocation_tx_ptr) {
     SetAllocatedData(allocation_tx_ptr->GetData(),
                      allocation_tx_ptr->GetCapacity());
-    allocation_tx_ptr->GetData() = nullptr;
-    allocation_tx_ptr->GetCapacity() = 0;
+
+    allocation_tx_ptr->Reset();
   }
 
   void MemcpyFrom(const Storage& other_storage) {
@@ -442,9 +454,7 @@
   };
 
   struct Inlined {
-    using InlinedDataElement =
-        absl::aligned_storage_t<sizeof(value_type), alignof(value_type)>;
-    InlinedDataElement inlined_data[N];
+    alignas(value_type) char inlined_data[sizeof(value_type[N])];
   };
 
   union Data {
@@ -465,18 +475,14 @@
   assert(GetSize() == 0);
 
   pointer construct_data;
-
   if (new_size > GetInlinedCapacity()) {
     // Because this is only called from the `InlinedVector` constructors, it's
     // safe to take on the allocation with size `0`. If `ConstructElements(...)`
     // throws, deallocation will be automatically handled by `~Storage()`.
     size_type new_capacity = ComputeCapacity(GetInlinedCapacity(), new_size);
-    pointer new_data = AllocatorTraits::allocate(*GetAllocPtr(), new_capacity);
-
-    SetAllocatedData(new_data, new_capacity);
+    construct_data = AllocatorTraits::allocate(*GetAllocPtr(), new_capacity);
+    SetAllocatedData(construct_data, new_capacity);
     SetIsAllocated();
-
-    construct_data = new_data;
   } else {
     construct_data = GetInlinedData();
   }
@@ -503,9 +509,7 @@
 
   if (new_size > storage_view.capacity) {
     size_type new_capacity = ComputeCapacity(storage_view.capacity, new_size);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_loop = {new_data, new_size};
+    construct_loop = {allocation_tx.Allocate(new_capacity), new_size};
     destroy_loop = {storage_view.data, storage_view.size};
   } else if (new_size > storage_view.size) {
     assign_loop = {storage_view.data, storage_view.size};
@@ -539,12 +543,12 @@
 auto Storage<T, N, A>::Resize(ValueAdapter values, size_type new_size) -> void {
   StorageView storage_view = MakeStorageView();
 
-  AllocationTransaction allocation_tx(GetAllocPtr());
-  ConstructionTransaction construction_tx(GetAllocPtr());
-
   IteratorValueAdapter<MoveIterator> move_values(
       MoveIterator(storage_view.data));
 
+  AllocationTransaction allocation_tx(GetAllocPtr());
+  ConstructionTransaction construction_tx(GetAllocPtr());
+
   absl::Span<value_type> construct_loop;
   absl::Span<value_type> move_construct_loop;
   absl::Span<value_type> destroy_loop;
@@ -687,19 +691,17 @@
       MoveIterator(storage_view.data));
 
   pointer construct_data;
-
   if (storage_view.size == storage_view.capacity) {
     size_type new_capacity = NextCapacity(storage_view.capacity);
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_data = new_data;
+    construct_data = allocation_tx.Allocate(new_capacity);
   } else {
     construct_data = storage_view.data;
   }
 
-  pointer end = construct_data + storage_view.size;
+  pointer last_ptr = construct_data + storage_view.size;
 
-  AllocatorTraits::construct(*GetAllocPtr(), end, std::forward<Args>(args)...);
+  AllocatorTraits::construct(*GetAllocPtr(), last_ptr,
+                             std::forward<Args>(args)...);
 
   if (allocation_tx.DidAllocate()) {
     ABSL_INTERNAL_TRY {
@@ -708,7 +710,7 @@
           storage_view.size);
     }
     ABSL_INTERNAL_CATCH_ANY {
-      AllocatorTraits::destroy(*GetAllocPtr(), end);
+      AllocatorTraits::destroy(*GetAllocPtr(), last_ptr);
       ABSL_INTERNAL_RETHROW;
     }
 
@@ -721,14 +723,12 @@
   }
 
   AddSize(1);
-  return *end;
+  return *last_ptr;
 }
 
 template <typename T, size_t N, typename A>
 auto Storage<T, N, A>::Erase(const_iterator from, const_iterator to)
     -> iterator {
-  assert(from != to);
-
   StorageView storage_view = MakeStorageView();
 
   size_type erase_size = std::distance(from, to);
@@ -793,12 +793,9 @@
       MoveIterator(storage_view.data));
 
   pointer construct_data;
-
   if (storage_view.size > GetInlinedCapacity()) {
     size_type new_capacity = storage_view.size;
-    pointer new_data = allocation_tx.Allocate(new_capacity);
-
-    construct_data = new_data;
+    construct_data = allocation_tx.Allocate(new_capacity);
   } else {
     construct_data = GetInlinedData();
   }
@@ -889,7 +886,7 @@
 }
 
 }  // namespace inlined_vector_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_INLINED_VECTOR_INTERNAL_H_
diff --git a/absl/container/internal/layout.h b/absl/container/internal/layout.h
index 3924b8a..69cc85d 100644
--- a/absl/container/internal/layout.h
+++ b/absl/container/internal/layout.h
@@ -188,7 +188,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // A type wrapper that instructs `Layout` to use the specific alignment for the
@@ -735,7 +735,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_LAYOUT_H_
diff --git a/absl/container/internal/layout_test.cc b/absl/container/internal/layout_test.cc
index 44d8460..8f3628a 100644
--- a/absl/container/internal/layout_test.cc
+++ b/absl/container/internal/layout_test.cc
@@ -28,7 +28,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -1563,5 +1563,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/node_hash_policy.h b/absl/container/internal/node_hash_policy.h
index d758136..4617162 100644
--- a/absl/container/internal/node_hash_policy.h
+++ b/absl/container/internal/node_hash_policy.h
@@ -39,8 +39,10 @@
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class Reference, class Policy>
@@ -84,7 +86,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_NODE_HASH_POLICY_H_
diff --git a/absl/container/internal/node_hash_policy_test.cc b/absl/container/internal/node_hash_policy_test.cc
index d53b736..84aabba 100644
--- a/absl/container/internal/node_hash_policy_test.cc
+++ b/absl/container/internal/node_hash_policy_test.cc
@@ -21,7 +21,7 @@
 #include "absl/container/internal/hash_policy_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -65,5 +65,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_map.h b/absl/container/internal/raw_hash_map.h
index 00caa37..0a02757 100644
--- a/absl/container/internal/raw_hash_map.h
+++ b/absl/container/internal/raw_hash_map.h
@@ -24,7 +24,7 @@
 #include "absl/container/internal/raw_hash_set.h"  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class Policy, class Hash, class Eq, class Alloc>
@@ -110,6 +110,9 @@
     return insert_or_assign(k, v).first;
   }
 
+  // All `try_emplace()` overloads make the same guarantees regarding rvalue
+  // arguments as `std::unordered_map::try_emplace()`, namely that these
+  // functions will not move from rvalue arguments if insertions do not happen.
   template <class K = key_type, class... Args,
             typename std::enable_if<
                 !std::is_convertible<K, const_iterator>::value, int>::type = 0,
@@ -188,7 +191,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_MAP_H_
diff --git a/absl/container/internal/raw_hash_set.cc b/absl/container/internal/raw_hash_set.cc
index 02e74e2..919ac07 100644
--- a/absl/container/internal/raw_hash_set.cc
+++ b/absl/container/internal/raw_hash_set.cc
@@ -20,7 +20,7 @@
 #include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 constexpr size_t Group::kWidth;
@@ -44,5 +44,5 @@
 }
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_set.h b/absl/container/internal/raw_hash_set.h
index 7b379d4..ca7be8d 100644
--- a/absl/container/internal/raw_hash_set.h
+++ b/absl/container/internal/raw_hash_set.h
@@ -118,7 +118,7 @@
 #include "absl/utility/utility.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <size_t Width>
@@ -615,13 +615,17 @@
     iterator() {}
 
     // PRECONDITION: not an end() iterator.
-    reference operator*() const { return PolicyTraits::element(slot_); }
+    reference operator*() const {
+      assert_is_full();
+      return PolicyTraits::element(slot_);
+    }
 
     // PRECONDITION: not an end() iterator.
     pointer operator->() const { return &operator*(); }
 
     // PRECONDITION: not an end() iterator.
     iterator& operator++() {
+      assert_is_full();
       ++ctrl_;
       ++slot_;
       skip_empty_or_deleted();
@@ -635,6 +639,8 @@
     }
 
     friend bool operator==(const iterator& a, const iterator& b) {
+      a.assert_is_valid();
+      b.assert_is_valid();
       return a.ctrl_ == b.ctrl_;
     }
     friend bool operator!=(const iterator& a, const iterator& b) {
@@ -645,6 +651,11 @@
     iterator(ctrl_t* ctrl) : ctrl_(ctrl) {}  // for end()
     iterator(ctrl_t* ctrl, slot_type* slot) : ctrl_(ctrl), slot_(slot) {}
 
+    void assert_is_full() const { assert(IsFull(*ctrl_)); }
+    void assert_is_valid() const {
+      assert(!ctrl_ || IsFull(*ctrl_) || *ctrl_ == kSentinel);
+    }
+
     void skip_empty_or_deleted() {
       while (IsEmptyOrDeleted(*ctrl_)) {
         // ctrl is not necessarily aligned to Group::kWidth. It is also likely
@@ -658,7 +669,7 @@
     }
 
     ctrl_t* ctrl_ = nullptr;
-    // To avoid uninitialized member warnigs, put slot_ in an anonymous union.
+    // To avoid uninitialized member warnings, put slot_ in an anonymous union.
     // The member is not initialized on singleton and end iterators.
     union {
       slot_type* slot_;
@@ -939,8 +950,11 @@
   //
   //   flat_hash_map<std::string, int> m;
   //   m.insert(std::make_pair("abc", 42));
+  // TODO(cheshire): A type alias T2 is introduced as a workaround for the nvcc
+  // bug.
   template <class T, RequiresInsertable<T> = 0,
-            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
+            class T2 = T,
+            typename std::enable_if<IsDecomposable<T2>::value, int>::type = 0,
             T* = nullptr>
   std::pair<iterator, bool> insert(T&& value) {
     return emplace(std::forward<T>(value));
@@ -976,8 +990,10 @@
     return emplace(std::move(value));
   }
 
-  template <class T, RequiresInsertable<T> = 0,
-            typename std::enable_if<IsDecomposable<T>::value, int>::type = 0,
+  // TODO(cheshire): A type alias T2 is introduced as a workaround for the nvcc
+  // bug.
+  template <class T, RequiresInsertable<T> = 0, class T2 = T,
+            typename std::enable_if<IsDecomposable<T2>::value, int>::type = 0,
             T* = nullptr>
   iterator insert(const_iterator, T&& value) {
     return insert(std::forward<T>(value)).first;
@@ -1051,8 +1067,7 @@
   template <class... Args, typename std::enable_if<
                                !IsDecomposable<Args...>::value, int>::type = 0>
   std::pair<iterator, bool> emplace(Args&&... args) {
-    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
-        raw;
+    alignas(slot_type) unsigned char raw[sizeof(slot_type)];
     slot_type* slot = reinterpret_cast<slot_type*>(&raw);
 
     PolicyTraits::construct(&alloc_ref(), slot, std::forward<Args>(args)...);
@@ -1068,10 +1083,15 @@
   // Extension API: support for lazy emplace.
   //
   // Looks up key in the table. If found, returns the iterator to the element.
-  // Otherwise calls f with one argument of type raw_hash_set::constructor. f
-  // MUST call raw_hash_set::constructor with arguments as if a
-  // raw_hash_set::value_type is constructed, otherwise the behavior is
-  // undefined.
+  // Otherwise calls `f` with one argument of type `raw_hash_set::constructor`.
+  //
+  // `f` must abide by several restrictions:
+  //  - it MUST call `raw_hash_set::constructor` with arguments as if a
+  //    `raw_hash_set::value_type` is constructed,
+  //  - it MUST NOT access the container before the call to
+  //    `raw_hash_set::constructor`, and
+  //  - it MUST NOT erase the lazily emplaced element.
+  // Doing any of these is undefined behavior.
   //
   // For example:
   //
@@ -1134,15 +1154,16 @@
   }
 
   // Erases the element pointed to by `it`.  Unlike `std::unordered_set::erase`,
-  // this method returns void to reduce algorithmic complexity to O(1).  In
-  // order to erase while iterating across a map, use the following idiom (which
-  // also works for standard containers):
+  // this method returns void to reduce algorithmic complexity to O(1).  The
+  // iterator is invalidated, so any increment should be done before calling
+  // erase.  In order to erase while iterating across a map, use the following
+  // idiom (which also works for standard containers):
   //
   // for (auto it = m.begin(), end = m.end(); it != end;) {
+  //   // `erase()` will invalidate `it`, so advance `it` first.
+  //   auto copy_it = it++;
   //   if (<pred>) {
-  //     m.erase(it++);
-  //   } else {
-  //     ++it;
+  //     m.erase(copy_it);
   //   }
   // }
   void erase(const_iterator cit) { erase(cit.inner_); }
@@ -1150,7 +1171,7 @@
   // This overload is necessary because otherwise erase<K>(const K&) would be
   // a better match if non-const iterator is passed as an argument.
   void erase(iterator it) {
-    assert(it != end());
+    it.assert_is_full();
     PolicyTraits::destroy(&alloc_ref(), it.slot_);
     erase_meta_only(it);
   }
@@ -1167,12 +1188,14 @@
   template <typename H, typename E>
   void merge(raw_hash_set<Policy, H, E, Alloc>& src) {  // NOLINT
     assert(this != &src);
-    for (auto it = src.begin(), e = src.end(); it != e; ++it) {
+    for (auto it = src.begin(), e = src.end(); it != e;) {
+      auto next = std::next(it);
       if (PolicyTraits::apply(InsertSlot<false>{*this, std::move(*it.slot_)},
                               PolicyTraits::element(it.slot_))
               .second) {
         src.erase_meta_only(it);
       }
+      it = next;
     }
   }
 
@@ -1182,6 +1205,7 @@
   }
 
   node_type extract(const_iterator position) {
+    position.inner_.assert_is_full();
     auto node =
         CommonAccess::Transfer<node_type>(alloc_ref(), position.inner_.slot_);
     erase_meta_only(position);
@@ -1531,8 +1555,7 @@
     //       mark target as FULL
     //       repeat procedure for current slot with moved from element (target)
     ConvertDeletedToEmptyAndFullToDeleted(ctrl_, capacity_);
-    typename std::aligned_storage<sizeof(slot_type), alignof(slot_type)>::type
-        raw;
+    alignas(slot_type) unsigned char raw[sizeof(slot_type)];
     size_t total_probe_length = 0;
     slot_type* slot = reinterpret_cast<slot_type*>(&raw);
     for (size_t i = 0; i != capacity_; ++i) {
@@ -1781,6 +1804,17 @@
       settings_{0, hasher{}, key_equal{}, allocator_type{}};
 };
 
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename P, typename H, typename E, typename A, typename Predicate>
+void EraseIf(Predicate pred, raw_hash_set<P, H, E, A>* c) {
+  for (auto it = c->begin(), last = c->end(); it != last;) {
+    auto copy_it = it++;
+    if (pred(*copy_it)) {
+      c->erase(copy_it);
+    }
+  }
+}
+
 namespace hashtable_debug_internal {
 template <typename Set>
 struct HashtableDebugAccess<Set, absl::void_t<typename Set::raw_hash_set>> {
@@ -1842,7 +1876,7 @@
 
 }  // namespace hashtable_debug_internal
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_RAW_HASH_SET_H_
diff --git a/absl/container/internal/raw_hash_set_allocator_test.cc b/absl/container/internal/raw_hash_set_allocator_test.cc
index 5188b3a..7ac4b9f 100644
--- a/absl/container/internal/raw_hash_set_allocator_test.cc
+++ b/absl/container/internal/raw_hash_set_allocator_test.cc
@@ -20,7 +20,7 @@
 #include "absl/container/internal/tracked.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -426,5 +426,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/raw_hash_set_test.cc b/absl/container/internal/raw_hash_set_test.cc
index 2783f5c..a96ae68 100644
--- a/absl/container/internal/raw_hash_set_test.cc
+++ b/absl/container/internal/raw_hash_set_test.cc
@@ -35,7 +35,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 struct RawHashSetTestOnlyAccess {
@@ -418,53 +418,6 @@
   EXPECT_TRUE(t.empty());
 }
 
-#ifdef __GNUC__
-template <class T>
-ABSL_ATTRIBUTE_ALWAYS_INLINE inline void DoNotOptimize(const T& v) {
-  asm volatile("" : : "r,m"(v) : "memory");
-}
-#endif
-
-TEST(Table, Prefetch) {
-  IntTable t;
-  t.emplace(1);
-  // Works for both present and absent keys.
-  t.prefetch(1);
-  t.prefetch(2);
-
-  // Do not run in debug mode, when prefetch is not implemented, or when
-  // sanitizers are enabled, or on WebAssembly.
-#if defined(NDEBUG) && defined(__GNUC__) && defined(__x86_64__) &&          \
-    !defined(ADDRESS_SANITIZER) && !defined(MEMORY_SANITIZER) &&            \
-    !defined(THREAD_SANITIZER) && !defined(UNDEFINED_BEHAVIOR_SANITIZER) && \
-    !defined(__EMSCRIPTEN__)
-  const auto now = [] { return absl::base_internal::CycleClock::Now(); };
-
-  // Make size enough to not fit in L2 cache (16.7 Mb)
-  static constexpr int size = 1 << 22;
-  for (int i = 0; i < size; ++i) t.insert(i);
-
-  int64_t no_prefetch = 0, prefetch = 0;
-  for (int iter = 0; iter < 10; ++iter) {
-    int64_t time = now();
-    for (int i = 0; i < size; ++i) {
-      DoNotOptimize(t.find(i));
-    }
-    no_prefetch += now() - time;
-
-    time = now();
-    for (int i = 0; i < size; ++i) {
-      t.prefetch(i + 20);
-      DoNotOptimize(t.find(i));
-    }
-    prefetch += now() - time;
-  }
-
-  // no_prefetch is at least 30% slower.
-  EXPECT_GE(1.0 * no_prefetch / prefetch, 1.3);
-#endif
-}
-
 TEST(Table, LookupEmpty) {
   IntTable t;
   auto it = t.find(0);
@@ -1838,10 +1791,11 @@
 
   IntTable t;
   // Extra simple "regexp" as regexp support is highly varied across platforms.
-  constexpr char kDeathMsg[] = "it != end";
+  constexpr char kDeathMsg[] = "IsFull";
   EXPECT_DEATH_IF_SUPPORTED(t.erase(t.end()), kDeathMsg);
 }
 
+#if defined(ABSL_HASHTABLEZ_SAMPLE)
 TEST(RawHashSamplerTest, Sample) {
   // Enable the feature even if the prod default is off.
   SetHashtablezEnabled(true);
@@ -1862,6 +1816,7 @@
   EXPECT_NEAR((end_size - start_size) / static_cast<double>(tables.size()),
               0.01, 0.005);
 }
+#endif  // ABSL_HASHTABLEZ_SAMPLER
 
 TEST(RawHashSamplerTest, DoNotSampleCustomAllocators) {
   // Enable the feature even if the prod default is off.
@@ -1912,5 +1867,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/test_instance_tracker.cc b/absl/container/internal/test_instance_tracker.cc
index f4b283f..f9947f0 100644
--- a/absl/container/internal/test_instance_tracker.cc
+++ b/absl/container/internal/test_instance_tracker.cc
@@ -15,7 +15,7 @@
 #include "absl/container/internal/test_instance_tracker.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace test_internal {
 int BaseCountedInstance::num_instances_ = 0;
 int BaseCountedInstance::num_live_instances_ = 0;
@@ -25,5 +25,5 @@
 int BaseCountedInstance::num_comparisons_ = 0;
 
 }  // namespace test_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/test_instance_tracker.h b/absl/container/internal/test_instance_tracker.h
index ab7f9f2..5ff6fd7 100644
--- a/absl/container/internal/test_instance_tracker.h
+++ b/absl/container/internal/test_instance_tracker.h
@@ -21,7 +21,7 @@
 #include "absl/types/compare.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace test_internal {
 
 // A type that counts number of occurrences of the type, the live occurrences of
@@ -268,7 +268,7 @@
 };
 
 }  // namespace test_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_TEST_INSTANCE_TRACKER_H_
diff --git a/absl/container/internal/tracked.h b/absl/container/internal/tracked.h
index e9e6b95..29f5829 100644
--- a/absl/container/internal/tracked.h
+++ b/absl/container/internal/tracked.h
@@ -16,11 +16,14 @@
 #define ABSL_CONTAINER_INTERNAL_TRACKED_H_
 
 #include <stddef.h>
+
 #include <memory>
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 // A class that tracks its copies and moves so that it can be queried in tests.
@@ -74,7 +77,7 @@
 };
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_TRACKED_H_
diff --git a/absl/container/internal/unordered_map_constructor_test.h b/absl/container/internal/unordered_map_constructor_test.h
index b64b552..76ee95e 100644
--- a/absl/container/internal/unordered_map_constructor_test.h
+++ b/absl/container/internal/unordered_map_constructor_test.h
@@ -24,7 +24,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -483,7 +483,7 @@
     AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_map_lookup_test.h b/absl/container/internal/unordered_map_lookup_test.h
index 9ad78a7..e76421e 100644
--- a/absl/container/internal/unordered_map_lookup_test.h
+++ b/absl/container/internal/unordered_map_lookup_test.h
@@ -21,7 +21,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -111,7 +111,7 @@
                            EqualRange);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_map_members_test.h b/absl/container/internal/unordered_map_members_test.h
index c460040..7d48cdb 100644
--- a/absl/container/internal/unordered_map_members_test.h
+++ b/absl/container/internal/unordered_map_members_test.h
@@ -21,7 +21,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -81,7 +81,7 @@
 REGISTER_TYPED_TEST_SUITE_P(MembersTest, Typedefs, SimpleFunctions, BeginEnd);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MEMBERS_TEST_H_
diff --git a/absl/container/internal/unordered_map_modifiers_test.h b/absl/container/internal/unordered_map_modifiers_test.h
index 89dd789..b8c513f 100644
--- a/absl/container/internal/unordered_map_modifiers_test.h
+++ b/absl/container/internal/unordered_map_modifiers_test.h
@@ -15,13 +15,15 @@
 #ifndef ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
 #define ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
 
+#include <memory>
+
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/container/internal/hash_generator_testing.h"
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -268,8 +270,47 @@
                            Emplace, EmplaceHint, TryEmplace, TryEmplaceHint,
                            Erase, EraseRange, EraseKey, Swap);
 
+template <typename Type>
+struct is_unique_ptr : std::false_type {};
+
+template <typename Type>
+struct is_unique_ptr<std::unique_ptr<Type>> : std::true_type {};
+
+template <class UnordMap>
+class UniquePtrModifiersTest : public ::testing::Test {
+ protected:
+  UniquePtrModifiersTest() {
+    static_assert(is_unique_ptr<typename UnordMap::mapped_type>::value,
+                  "UniquePtrModifiersTyest may only be called with a "
+                  "std::unique_ptr value type.");
+  }
+};
+
+TYPED_TEST_SUITE_P(UniquePtrModifiersTest);
+
+// Test that we do not move from rvalue arguments if an insertion does not
+// happen.
+TYPED_TEST_P(UniquePtrModifiersTest, TryEmplace) {
+#ifdef UNORDERED_MAP_CXX17
+  using T = hash_internal::GeneratedType<TypeParam>;
+  using V = typename TypeParam::mapped_type;
+  T val = hash_internal::Generator<T>()();
+  TypeParam m;
+  auto p = m.try_emplace(val.first, std::move(val.second));
+  EXPECT_TRUE(p.second);
+  // A moved from std::unique_ptr is guaranteed to be nullptr.
+  EXPECT_EQ(val.second, nullptr);
+  T val2 = {val.first, hash_internal::Generator<V>()()};
+  p = m.try_emplace(val2.first, std::move(val2.second));
+  EXPECT_FALSE(p.second);
+  EXPECT_NE(val2.second, nullptr);
+#endif
+}
+
+REGISTER_TYPED_TEST_SUITE_P(UniquePtrModifiersTest, TryEmplace);
+
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_MAP_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_map_test.cc b/absl/container/internal/unordered_map_test.cc
index 51a90af..9cbf512 100644
--- a/absl/container/internal/unordered_map_test.cc
+++ b/absl/container/internal/unordered_map_test.cc
@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+#include <memory>
 #include <unordered_map>
 
 #include "absl/container/internal/unordered_map_constructor_test.h"
@@ -20,7 +21,7 @@
 #include "absl/container/internal/unordered_map_modifiers_test.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -36,7 +37,14 @@
 INSTANTIATE_TYPED_TEST_SUITE_P(UnorderedMap, MembersTest, MapTypes);
 INSTANTIATE_TYPED_TEST_SUITE_P(UnorderedMap, ModifiersTest, MapTypes);
 
+using UniquePtrMapTypes = ::testing::Types<std::unordered_map<
+    int, std::unique_ptr<int>, StatefulTestingHash, StatefulTestingEqual,
+    Alloc<std::pair<const int, std::unique_ptr<int>>>>>;
+
+INSTANTIATE_TYPED_TEST_SUITE_P(UnorderedMap, UniquePtrModifiersTest,
+                               UniquePtrMapTypes);
+
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/internal/unordered_set_constructor_test.h b/absl/container/internal/unordered_set_constructor_test.h
index ac73a89..41165b0 100644
--- a/absl/container/internal/unordered_set_constructor_test.h
+++ b/absl/container/internal/unordered_set_constructor_test.h
@@ -26,7 +26,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordMap>
@@ -490,7 +490,7 @@
     AssignmentFromInitializerListOverwritesExisting, AssignmentOnSelf);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_CONSTRUCTOR_TEST_H_
diff --git a/absl/container/internal/unordered_set_lookup_test.h b/absl/container/internal/unordered_set_lookup_test.h
index 722fb1c..8f2f4b2 100644
--- a/absl/container/internal/unordered_set_lookup_test.h
+++ b/absl/container/internal/unordered_set_lookup_test.h
@@ -21,7 +21,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -85,7 +85,7 @@
 REGISTER_TYPED_TEST_CASE_P(LookupTest, Count, Find, EqualRange);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_LOOKUP_TEST_H_
diff --git a/absl/container/internal/unordered_set_members_test.h b/absl/container/internal/unordered_set_members_test.h
index 756a95c..4c5e104 100644
--- a/absl/container/internal/unordered_set_members_test.h
+++ b/absl/container/internal/unordered_set_members_test.h
@@ -21,7 +21,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -80,7 +80,7 @@
 REGISTER_TYPED_TEST_SUITE_P(MembersTest, Typedefs, SimpleFunctions, BeginEnd);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MEMBERS_TEST_H_
diff --git a/absl/container/internal/unordered_set_modifiers_test.h b/absl/container/internal/unordered_set_modifiers_test.h
index d3e534d..26be58d 100644
--- a/absl/container/internal/unordered_set_modifiers_test.h
+++ b/absl/container/internal/unordered_set_modifiers_test.h
@@ -21,7 +21,7 @@
 #include "absl/container/internal/hash_policy_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 
 template <class UnordSet>
@@ -184,7 +184,7 @@
                            EraseKey, Swap);
 
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_INTERNAL_UNORDERED_SET_MODIFIERS_TEST_H_
diff --git a/absl/container/internal/unordered_set_test.cc b/absl/container/internal/unordered_set_test.cc
index 2356e18..a134b53 100644
--- a/absl/container/internal/unordered_set_test.cc
+++ b/absl/container/internal/unordered_set_test.cc
@@ -20,7 +20,7 @@
 #include "absl/container/internal/unordered_set_modifiers_test.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
@@ -37,5 +37,5 @@
 
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/node_hash_map.h b/absl/container/node_hash_map.h
index addf120..fccea18 100644
--- a/absl/container/node_hash_map.h
+++ b/absl/container/node_hash_map.h
@@ -48,7 +48,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <class Key, class Value>
 class NodeHashMapPolicy;
@@ -352,6 +352,10 @@
   // Inserts (via copy or move) the element of the specified key into the
   // `node_hash_map` using the position of `hint` as a non-binding suggestion
   // for where to begin the insertion search.
+  //
+  // All `try_emplace()` overloads make the same guarantees regarding rvalue
+  // arguments as `std::unordered_map::try_emplace()`, namely that these
+  // functions will not move from rvalue arguments if insertions do not happen.
   using Base::try_emplace;
 
   // node_hash_map::extract()
@@ -518,6 +522,15 @@
   void resize(typename Base::size_type hint) { this->rehash(hint); }
 };
 
+// erase_if(node_hash_map<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename K, typename V, typename H, typename E, typename A,
+          typename Predicate>
+void erase_if(node_hash_map<K, V, H, E, A>& c, Predicate pred) {
+  container_internal::EraseIf(pred, &c);
+}
+
 namespace container_internal {
 
 template <class Key, class Value>
@@ -578,7 +591,7 @@
 
 }  // namespace container_algorithm_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_NODE_HASH_MAP_H_
diff --git a/absl/container/node_hash_map_test.cc b/absl/container/node_hash_map_test.cc
index 7ce7ca0..5d74b81 100644
--- a/absl/container/node_hash_map_test.cc
+++ b/absl/container/node_hash_map_test.cc
@@ -21,11 +21,12 @@
 #include "absl/container/internal/unordered_map_modifiers_test.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 
 using ::testing::Field;
+using ::testing::IsEmpty;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
@@ -216,7 +217,44 @@
   EXPECT_THAT(set2, UnorderedElementsAre(Elem(7, -70), Elem(17, 23)));
 }
 
+bool FirstIsEven(std::pair<const int, int> p) { return p.first % 2 == 0; }
+
+TEST(NodeHashMap, EraseIf) {
+  // Erase all elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, [](std::pair<const int, int>) { return true; });
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, [](std::pair<const int, int>) { return false; });
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(2, 2), Pair(3, 3),
+                                        Pair(4, 4), Pair(5, 5)));
+  }
+  // Erase specific elements.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s,
+             [](std::pair<const int, int> kvp) { return kvp.first % 2 == 1; });
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(2, 2), Pair(4, 4)));
+  }
+  // Predicate is function reference.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, FirstIsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+  // Predicate is function pointer.
+  {
+    node_hash_map<int, int> s = {{1, 1}, {2, 2}, {3, 3}, {4, 4}, {5, 5}};
+    erase_if(s, &FirstIsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(Pair(1, 1), Pair(3, 3), Pair(5, 5)));
+  }
+}
+
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/container/node_hash_set.h b/absl/container/node_hash_set.h
index 103d32d..ad54b6d 100644
--- a/absl/container/node_hash_set.h
+++ b/absl/container/node_hash_set.h
@@ -44,7 +44,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 template <typename T>
 struct NodeHashSetPolicy;
@@ -77,7 +77,7 @@
 //
 //   // Create a node hash set of three strings
 //   absl::node_hash_map<std::string, std::string> ducks =
-//     {"huey", "dewey"}, "louie"};
+//     {"huey", "dewey", "louie"};
 //
 //  // Insert a new element into the node hash map
 //  ducks.insert("donald"};
@@ -111,7 +111,7 @@
   // * Initializer List constructor
   //
   //   absl::node_hash_set<std::string> set2 =
-  //       {{"huey"}, {"dewey"}, {"louie"},};
+  //       {{"huey"}, {"dewey"}, {"louie"}};
   //
   // * Copy constructor
   //
@@ -435,6 +435,14 @@
   void resize(typename Base::size_type hint) { this->rehash(hint); }
 };
 
+// erase_if(node_hash_set<>, Pred)
+//
+// Erases all elements that satisfy the predicate `pred` from the container `c`.
+template <typename T, typename H, typename E, typename A, typename Predicate>
+void erase_if(node_hash_set<T, H, E, A>& c, Predicate pred) {
+  container_internal::EraseIf(pred, &c);
+}
+
 namespace container_internal {
 
 template <class T>
@@ -484,7 +492,7 @@
     : std::true_type {};
 
 }  // namespace container_algorithm_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_CONTAINER_NODE_HASH_SET_H_
diff --git a/absl/container/node_hash_set_test.cc b/absl/container/node_hash_set_test.cc
index 65d125e..7ddad20 100644
--- a/absl/container/node_hash_set_test.cc
+++ b/absl/container/node_hash_set_test.cc
@@ -20,11 +20,12 @@
 #include "absl/container/internal/unordered_set_modifiers_test.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace container_internal {
 namespace {
 using ::absl::container_internal::hash_internal::Enum;
 using ::absl::container_internal::hash_internal::EnumClass;
+using ::testing::IsEmpty;
 using ::testing::Pointee;
 using ::testing::UnorderedElementsAre;
 
@@ -101,7 +102,42 @@
   EXPECT_THAT(set2, UnorderedElementsAre(Pointee(7), Pointee(23)));
 }
 
+bool IsEven(int k) { return k % 2 == 0; }
+
+TEST(NodeHashSet, EraseIf) {
+  // Erase all elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int) { return true; });
+    EXPECT_THAT(s, IsEmpty());
+  }
+  // Erase no elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int) { return false; });
+    EXPECT_THAT(s, UnorderedElementsAre(1, 2, 3, 4, 5));
+  }
+  // Erase specific elements.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, [](int k) { return k % 2 == 1; });
+    EXPECT_THAT(s, UnorderedElementsAre(2, 4));
+  }
+  // Predicate is function reference.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, IsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+  // Predicate is function pointer.
+  {
+    node_hash_set<int> s = {1, 2, 3, 4, 5};
+    erase_if(s, &IsEven);
+    EXPECT_THAT(s, UnorderedElementsAre(1, 3, 5));
+  }
+}
+
 }  // namespace
 }  // namespace container_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/copts/AbseilConfigureCopts.cmake b/absl/copts/AbseilConfigureCopts.cmake
index 4ece4c6..77d4ace 100644
--- a/absl/copts/AbseilConfigureCopts.cmake
+++ b/absl/copts/AbseilConfigureCopts.cmake
@@ -5,25 +5,46 @@
 set(ABSL_HAVE_LSAN OFF)
 set(ABSL_DEFAULT_LINKOPTS "")
 
+if (BUILD_SHARED_LIBS AND MSVC)
+  set(ABSL_BUILD_DLL TRUE)
+  set(CMAKE_WINDOWS_EXPORT_ALL_SYMBOLS ON)
+else()
+  set(ABSL_BUILD_DLL FALSE)
+endif()
+
+if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "x86_64" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "AMD64")
+  if (MSVC)
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
+  else()
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
+  endif()
+elseif("${CMAKE_SYSTEM_PROCESSOR}" MATCHES "arm.*|aarch64")
+  if ("${CMAKE_SIZEOF_VOID_P}" STREQUAL "8")
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_ARM64_FLAGS}")
+  elseif("${CMAKE_SIZEOF_VOID_P}" STREQUAL "4")
+    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_ARM32_FLAGS}")
+  else()
+    message(WARNING "Value of CMAKE_SIZEOF_VOID_P (${CMAKE_SIZEOF_VOID_P}) is not supported.")
+  endif()
+else()
+  message(WARNING "Value of CMAKE_SYSTEM_PROCESSOR (${CMAKE_SYSTEM_PROCESSOR}) is unknown and cannot be used to set ABSL_RANDOM_RANDEN_COPTS")
+  set(ABSL_RANDOM_RANDEN_COPTS "")
+endif()
+
+
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
   set(ABSL_DEFAULT_COPTS "${ABSL_GCC_FLAGS}")
   set(ABSL_TEST_COPTS "${ABSL_GCC_FLAGS};${ABSL_GCC_TEST_FLAGS}")
-  set(ABSL_EXCEPTIONS_FLAG "${ABSL_GCC_EXCEPTIONS_FLAGS}")
-  set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
 elseif("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang")
   # MATCHES so we get both Clang and AppleClang
   if(MSVC)
     # clang-cl is half MSVC, half LLVM
     set(ABSL_DEFAULT_COPTS "${ABSL_CLANG_CL_FLAGS}")
     set(ABSL_TEST_COPTS "${ABSL_CLANG_CL_FLAGS};${ABSL_CLANG_CL_TEST_FLAGS}")
-    set(ABSL_EXCEPTIONS_FLAG "${ABSL_CLANG_CL_EXCEPTIONS_FLAGS}")
     set(ABSL_DEFAULT_LINKOPTS "${ABSL_MSVC_LINKOPTS}")
-    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
   else()
     set(ABSL_DEFAULT_COPTS "${ABSL_LLVM_FLAGS}")
     set(ABSL_TEST_COPTS "${ABSL_LLVM_FLAGS};${ABSL_LLVM_TEST_FLAGS}")
-    set(ABSL_EXCEPTIONS_FLAG "${ABSL_LLVM_EXCEPTIONS_FLAGS}")
-    set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")
     if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
       # AppleClang doesn't have lsan
       # https://developer.apple.com/documentation/code_diagnostics
@@ -36,20 +57,13 @@
 elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
   set(ABSL_DEFAULT_COPTS "${ABSL_MSVC_FLAGS}")
   set(ABSL_TEST_COPTS "${ABSL_MSVC_FLAGS};${ABSL_MSVC_TEST_FLAGS}")
-  set(ABSL_EXCEPTIONS_FLAG "${ABSL_MSVC_EXCEPTIONS_FLAGS}")
   set(ABSL_DEFAULT_LINKOPTS "${ABSL_MSVC_LINKOPTS}")
-  set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_MSVC_X64_FLAGS}")
 else()
   message(WARNING "Unknown compiler: ${CMAKE_CXX_COMPILER}.  Building with no default flags")
   set(ABSL_DEFAULT_COPTS "")
   set(ABSL_TEST_COPTS "")
-  set(ABSL_EXCEPTIONS_FLAG "")
-  set(ABSL_RANDOM_RANDEN_COPTS "")
 endif()
 
-# This flag is used internally for Bazel builds and is kept here for consistency
-set(ABSL_EXCEPTIONS_FLAG_LINKOPTS "")
-
 if("${CMAKE_CXX_STANDARD}" EQUAL 98)
   message(FATAL_ERROR "Abseil requires at least C++11")
 elseif(NOT "${CMAKE_CXX_STANDARD}")
diff --git a/absl/copts/GENERATED_AbseilCopts.cmake b/absl/copts/GENERATED_AbseilCopts.cmake
index 01bd40b..7ef6339 100644
--- a/absl/copts/GENERATED_AbseilCopts.cmake
+++ b/absl/copts/GENERATED_AbseilCopts.cmake
@@ -3,12 +3,6 @@
 # (1) Edit absl/copts/copts.py.
 # (2) Run `python <path_to_absl>/copts/generate_copts.py`.
 
-list(APPEND ABSL_CLANG_CL_EXCEPTIONS_FLAGS
-    "/U_HAS_EXCEPTIONS"
-    "/D_HAS_EXCEPTIONS=1"
-    "/EHsc"
-)
-
 list(APPEND ABSL_CLANG_CL_FLAGS
     "/W3"
     "-Wno-c++98-compat-pedantic"
@@ -29,7 +23,6 @@
     "-Wno-gcc-compat"
     "-Wno-global-constructors"
     "-Wno-exit-time-destructors"
-    "-Wno-nested-anon-types"
     "-Wno-non-modular-include-in-module"
     "-Wno-old-style-cast"
     "-Wno-range-loop-analysis"
@@ -80,10 +73,6 @@
     "-Wno-gnu-zero-variadic-macro-arguments"
 )
 
-list(APPEND ABSL_GCC_EXCEPTIONS_FLAGS
-    "-fexceptions"
-)
-
 list(APPEND ABSL_GCC_FLAGS
     "-Wall"
     "-Wextra"
@@ -111,10 +100,6 @@
     "-Wno-unused-private-field"
 )
 
-list(APPEND ABSL_LLVM_EXCEPTIONS_FLAGS
-    "-fexceptions"
-)
-
 list(APPEND ABSL_LLVM_FLAGS
     "-Wall"
     "-Wextra"
@@ -137,7 +122,6 @@
     "-Wno-gcc-compat"
     "-Wno-global-constructors"
     "-Wno-exit-time-destructors"
-    "-Wno-nested-anon-types"
     "-Wno-non-modular-include-in-module"
     "-Wno-old-style-cast"
     "-Wno-range-loop-analysis"
@@ -183,12 +167,6 @@
     "-Wno-gnu-zero-variadic-macro-arguments"
 )
 
-list(APPEND ABSL_MSVC_EXCEPTIONS_FLAGS
-    "/U_HAS_EXCEPTIONS"
-    "/D_HAS_EXCEPTIONS=1"
-    "/EHsc"
-)
-
 list(APPEND ABSL_MSVC_FLAGS
     "/W3"
     "/DNOMINMAX"
@@ -196,6 +174,7 @@
     "/D_CRT_SECURE_NO_WARNINGS"
     "/D_SCL_SECURE_NO_WARNINGS"
     "/D_ENABLE_EXTENDED_ALIGNED_STORAGE"
+    "/bigobj"
     "/wd4005"
     "/wd4068"
     "/wd4180"
@@ -226,8 +205,6 @@
 )
 
 list(APPEND ABSL_RANDOM_HWAES_MSVC_X64_FLAGS
-    "/O2"
-    "/Ob2"
 )
 
 list(APPEND ABSL_RANDOM_HWAES_X64_FLAGS
diff --git a/absl/copts/GENERATED_copts.bzl b/absl/copts/GENERATED_copts.bzl
index 82f332f..3cc4878 100644
--- a/absl/copts/GENERATED_copts.bzl
+++ b/absl/copts/GENERATED_copts.bzl
@@ -4,12 +4,6 @@
 (2) Run `python <path_to_absl>/copts/generate_copts.py`.
 """
 
-ABSL_CLANG_CL_EXCEPTIONS_FLAGS = [
-    "/U_HAS_EXCEPTIONS",
-    "/D_HAS_EXCEPTIONS=1",
-    "/EHsc",
-]
-
 ABSL_CLANG_CL_FLAGS = [
     "/W3",
     "-Wno-c++98-compat-pedantic",
@@ -30,7 +24,6 @@
     "-Wno-gcc-compat",
     "-Wno-global-constructors",
     "-Wno-exit-time-destructors",
-    "-Wno-nested-anon-types",
     "-Wno-non-modular-include-in-module",
     "-Wno-old-style-cast",
     "-Wno-range-loop-analysis",
@@ -81,10 +74,6 @@
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
-ABSL_GCC_EXCEPTIONS_FLAGS = [
-    "-fexceptions",
-]
-
 ABSL_GCC_FLAGS = [
     "-Wall",
     "-Wextra",
@@ -112,10 +101,6 @@
     "-Wno-unused-private-field",
 ]
 
-ABSL_LLVM_EXCEPTIONS_FLAGS = [
-    "-fexceptions",
-]
-
 ABSL_LLVM_FLAGS = [
     "-Wall",
     "-Wextra",
@@ -138,7 +123,6 @@
     "-Wno-gcc-compat",
     "-Wno-global-constructors",
     "-Wno-exit-time-destructors",
-    "-Wno-nested-anon-types",
     "-Wno-non-modular-include-in-module",
     "-Wno-old-style-cast",
     "-Wno-range-loop-analysis",
@@ -184,12 +168,6 @@
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
-ABSL_MSVC_EXCEPTIONS_FLAGS = [
-    "/U_HAS_EXCEPTIONS",
-    "/D_HAS_EXCEPTIONS=1",
-    "/EHsc",
-]
-
 ABSL_MSVC_FLAGS = [
     "/W3",
     "/DNOMINMAX",
@@ -197,6 +175,7 @@
     "/D_CRT_SECURE_NO_WARNINGS",
     "/D_SCL_SECURE_NO_WARNINGS",
     "/D_ENABLE_EXTENDED_ALIGNED_STORAGE",
+    "/bigobj",
     "/wd4005",
     "/wd4068",
     "/wd4180",
@@ -227,8 +206,6 @@
 ]
 
 ABSL_RANDOM_HWAES_MSVC_X64_FLAGS = [
-    "/O2",
-    "/Ob2",
 ]
 
 ABSL_RANDOM_HWAES_X64_FLAGS = [
diff --git a/absl/copts/configure_copts.bzl b/absl/copts/configure_copts.bzl
index 8c4efe7..9dd6bd0 100644
--- a/absl/copts/configure_copts.bzl
+++ b/absl/copts/configure_copts.bzl
@@ -6,13 +6,12 @@
 
 load(
     "//absl:copts/GENERATED_copts.bzl",
-    "ABSL_GCC_EXCEPTIONS_FLAGS",
+    "ABSL_CLANG_CL_FLAGS",
+    "ABSL_CLANG_CL_TEST_FLAGS",
     "ABSL_GCC_FLAGS",
     "ABSL_GCC_TEST_FLAGS",
-    "ABSL_LLVM_EXCEPTIONS_FLAGS",
     "ABSL_LLVM_FLAGS",
     "ABSL_LLVM_TEST_FLAGS",
-    "ABSL_MSVC_EXCEPTIONS_FLAGS",
     "ABSL_MSVC_FLAGS",
     "ABSL_MSVC_LINKOPTS",
     "ABSL_MSVC_TEST_FLAGS",
@@ -36,16 +35,6 @@
     "//conditions:default": ABSL_GCC_TEST_FLAGS,
 })
 
-ABSL_EXCEPTIONS_FLAG = select({
-    "//absl:windows": ABSL_MSVC_EXCEPTIONS_FLAGS,
-    "//absl:llvm_compiler": ABSL_LLVM_EXCEPTIONS_FLAGS,
-    "//conditions:default": ABSL_GCC_EXCEPTIONS_FLAGS,
-})
-
-ABSL_EXCEPTIONS_FLAG_LINKOPTS = select({
-    "//conditions:default": [],
-})
-
 ABSL_DEFAULT_LINKOPTS = select({
     "//absl:windows": ABSL_MSVC_LINKOPTS,
     "//conditions:default": [],
diff --git a/absl/copts/copts.py b/absl/copts/copts.py
index 068abce..704ef23 100644
--- a/absl/copts/copts.py
+++ b/absl/copts/copts.py
@@ -56,7 +56,6 @@
     "-Wno-global-constructors",
     "-Wno-exit-time-destructors",
     ###
-    "-Wno-nested-anon-types",
     "-Wno-non-modular-include-in-module",
     "-Wno-old-style-cast",
     # Warns on preferred usage of non-POD types such as string_view
@@ -109,12 +108,6 @@
     "-Wno-gnu-zero-variadic-macro-arguments",
 ]
 
-MSVC_STYLE_EXCEPTIONS_FLAGS = [
-    "/U_HAS_EXCEPTIONS",
-    "/D_HAS_EXCEPTIONS=1",
-    "/EHsc"
-]
-
 MSVC_DEFINES = [
     "/DNOMINMAX",  # Don't define min and max macros (windows.h)
     # Don't bloat namespace with incompatible winsock versions.
@@ -157,20 +150,18 @@
         "-Wno-unused-parameter",
         "-Wno-unused-private-field",
     ],
-    "ABSL_GCC_EXCEPTIONS_FLAGS": ["-fexceptions"],
     "ABSL_LLVM_FLAGS":
         LLVM_BIG_WARNING_FLAGS + LLVM_DISABLE_WARNINGS_FLAGS,
     "ABSL_LLVM_TEST_FLAGS":
         LLVM_TEST_DISABLE_WARNINGS_FLAGS,
-    "ABSL_LLVM_EXCEPTIONS_FLAGS": ["-fexceptions"],
     "ABSL_CLANG_CL_FLAGS":
         (MSVC_BIG_WARNING_FLAGS + LLVM_DISABLE_WARNINGS_FLAGS + MSVC_DEFINES),
     "ABSL_CLANG_CL_TEST_FLAGS":
         LLVM_TEST_DISABLE_WARNINGS_FLAGS,
-    "ABSL_CLANG_CL_EXCEPTIONS_FLAGS":
-        MSVC_STYLE_EXCEPTIONS_FLAGS,
     "ABSL_MSVC_FLAGS":
         MSVC_BIG_WARNING_FLAGS + MSVC_DEFINES + [
+            # Increase the number of sections available in object files
+            "/bigobj",
             "/wd4005",  # macro-redefinition
             "/wd4068",  # unknown pragma
             # qualifier applied to function type has no meaning; ignored
@@ -191,8 +182,6 @@
         "/wd4996",  # use of deprecated symbol
         "/DNOMINMAX",  # disable the min() and max() macros from <windows.h>
     ],
-    "ABSL_MSVC_EXCEPTIONS_FLAGS":
-        MSVC_STYLE_EXCEPTIONS_FLAGS,
     "ABSL_MSVC_LINKOPTS": [
         # Object file doesn't export any previously undefined symbols
         "-ignore:4221",
@@ -207,8 +196,5 @@
         "-maes",
         "-msse4.1",
     ],
-    "ABSL_RANDOM_HWAES_MSVC_X64_FLAGS": [
-        "/O2",  # Maximize speed
-        "/Ob2",  # Aggressive inlining
-    ],
+    "ABSL_RANDOM_HWAES_MSVC_X64_FLAGS": [],
 }
diff --git a/absl/debugging/BUILD.bazel b/absl/debugging/BUILD.bazel
index 913cfaf..8f521be 100644
--- a/absl/debugging/BUILD.bazel
+++ b/absl/debugging/BUILD.bazel
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -30,6 +31,14 @@
 cc_library(
     name = "stacktrace",
     srcs = [
+        "internal/stacktrace_aarch64-inl.inc",
+        "internal/stacktrace_arm-inl.inc",
+        "internal/stacktrace_config.h",
+        "internal/stacktrace_generic-inl.inc",
+        "internal/stacktrace_powerpc-inl.inc",
+        "internal/stacktrace_unimplemented-inl.inc",
+        "internal/stacktrace_win32-inl.inc",
+        "internal/stacktrace_x86-inl.inc",
         "stacktrace.cc",
     ],
     hdrs = ["stacktrace.h"],
@@ -37,7 +46,7 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":debugging_internal",
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
     ],
 )
@@ -55,27 +64,39 @@
         "symbolize.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "//absl:windows": ["-DEFAULTLIB:dbghelp.lib"],
+        "//conditions:default": [],
+    }),
     deps = [
         ":debugging_internal",
         ":demangle_internal",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
         "//absl/base:malloc_internal",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
 cc_test(
     name = "symbolize_test",
     srcs = ["symbolize_test.cc"],
-    copts = ABSL_TEST_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS + select({
+        "//absl:windows": ["/Z7"],
+        "//conditions:default": [],
+    }),
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "//absl:windows": ["/DEBUG"],
+        "//conditions:default": [],
+    }),
     deps = [
         ":stack_consumption",
         ":symbolize",
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/memory",
         "@com_google_googletest//:gtest",
     ],
@@ -95,8 +116,9 @@
     deps = [
         ":stacktrace",
         ":symbolize",
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -112,6 +134,8 @@
         "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:errno_saver",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -127,7 +151,7 @@
         ":failure_signal_handler",
         ":stacktrace",
         ":symbolize",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "@com_google_googletest//:gtest",
     ],
@@ -143,22 +167,16 @@
     hdrs = [
         "internal/address_is_readable.h",
         "internal/elf_mem_image.h",
-        "internal/stacktrace_aarch64-inl.inc",
-        "internal/stacktrace_arm-inl.inc",
-        "internal/stacktrace_config.h",
-        "internal/stacktrace_generic-inl.inc",
-        "internal/stacktrace_powerpc-inl.inc",
-        "internal/stacktrace_unimplemented-inl.inc",
-        "internal/stacktrace_win32-inl.inc",
-        "internal/stacktrace_x86-inl.inc",
         "internal/vdso_support.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
+        "//absl/base:errno_saver",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -169,6 +187,7 @@
     copts = ABSL_DEFAULT_COPTS,
     deps = [
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
     ],
 )
@@ -181,8 +200,8 @@
     deps = [
         ":demangle_internal",
         ":stack_consumption",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/memory",
         "@com_google_googletest//:gtest_main",
     ],
@@ -193,7 +212,10 @@
     srcs = ["leak_check.cc"],
     hdrs = ["leak_check.h"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = ["//absl/base:core_headers"],
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
 )
 
 # Adding a dependency to leak_check_disable will disable
@@ -204,6 +226,7 @@
     srcs = ["leak_check_disable.cc"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     linkstatic = 1,
+    deps = ["//absl/base:config"],
     alwayslink = 1,
 )
 
@@ -225,6 +248,9 @@
     }),
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_library(
@@ -235,6 +261,9 @@
     copts = ["-ULEAK_SANITIZER"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
+    deps = [
+        "//absl/base:config",
+    ],
 )
 
 cc_test(
@@ -292,8 +321,9 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
     deps = [
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -304,8 +334,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":stack_consumption",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "@com_google_googletest//:gtest_main",
     ],
 )
diff --git a/absl/debugging/CMakeLists.txt b/absl/debugging/CMakeLists.txt
index 001e272..7733615 100644
--- a/absl/debugging/CMakeLists.txt
+++ b/absl/debugging/CMakeLists.txt
@@ -19,13 +19,21 @@
     stacktrace
   HDRS
     "stacktrace.h"
+    "internal/stacktrace_aarch64-inl.inc"
+    "internal/stacktrace_arm-inl.inc"
+    "internal/stacktrace_config.h"
+    "internal/stacktrace_generic-inl.inc"
+    "internal/stacktrace_powerpc-inl.inc"
+    "internal/stacktrace_unimplemented-inl.inc"
+    "internal/stacktrace_win32-inl.inc"
+    "internal/stacktrace_x86-inl.inc"
   SRCS
     "stacktrace.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::debugging_internal
-    absl::base
+    absl::config
     absl::core_headers
   PUBLIC
 )
@@ -45,13 +53,16 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${MINGW}>:"dbghelp">
   DEPS
     absl::debugging_internal
     absl::demangle_internal
     absl::base
+    absl::config
     absl::core_headers
     absl::dynamic_annotations
     absl::malloc_internal
+    absl::raw_logging_internal
   PUBLIC
 )
 
@@ -62,12 +73,16 @@
     "symbolize_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
+    $<$<BOOL:${MSVC}>:-Z7>
+  LINKOPTS
+    $<$<BOOL:${MSVC}>:-DEBUG>
   DEPS
     absl::stack_consumption
     absl::symbolize
     absl::base
     absl::core_headers
     absl::memory
+    absl::raw_logging_internal
     gmock
 )
 
@@ -83,8 +98,9 @@
   DEPS
     absl::stacktrace
     absl::symbolize
-    absl::base
+    absl::config
     absl::core_headers
+    absl::raw_logging_internal
 )
 
 absl_cc_library(
@@ -102,6 +118,8 @@
     absl::base
     absl::config
     absl::core_headers
+    absl::errno_saver
+    absl::raw_logging_internal
   PUBLIC
 )
 
@@ -116,8 +134,8 @@
     absl::failure_signal_handler
     absl::stacktrace
     absl::symbolize
-    absl::base
     absl::strings
+    absl::raw_logging_internal
     Threads::Threads
     gmock
 )
@@ -128,14 +146,6 @@
   HDRS
     "internal/address_is_readable.h"
     "internal/elf_mem_image.h"
-    "internal/stacktrace_aarch64-inl.inc"
-    "internal/stacktrace_arm-inl.inc"
-    "internal/stacktrace_config.h"
-    "internal/stacktrace_generic-inl.inc"
-    "internal/stacktrace_powerpc-inl.inc"
-    "internal/stacktrace_unimplemented-inl.inc"
-    "internal/stacktrace_win32-inl.inc"
-    "internal/stacktrace_x86-inl.inc"
     "internal/vdso_support.h"
   SRCS
     "internal/address_is_readable.cc"
@@ -144,9 +154,11 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::base
     absl::core_headers
+    absl::config
     absl::dynamic_annotations
+    absl::errno_saver
+    absl::raw_logging_internal
 )
 
 absl_cc_library(
@@ -174,9 +186,9 @@
   DEPS
     absl::demangle_internal
     absl::stack_consumption
-    absl::base
     absl::core_headers
     absl::memory
+    absl::raw_logging_internal
     gmock_main
 )
 
@@ -190,6 +202,7 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
   PUBLIC
 )
@@ -236,7 +249,7 @@
   SRCS
     "leak_check_test.cc"
   COPTS
-    ${ABSL_DEFAULT_COPTS}
+    ${ABSL_TEST_COPTS}
     "$<$<BOOL:${ABSL_HAVE_LSAN}>:-DABSL_EXPECT_LEAK_SANITIZER>"
   LINKOPTS
     "${ABSL_LSAN_LINKOPTS}"
@@ -273,6 +286,7 @@
     absl::leak_check_api_enabled_for_testing
     absl::leak_check_disable
     absl::base
+    absl::raw_logging_internal
     gmock_main
 )
 
@@ -286,8 +300,9 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
-    absl::base
+    absl::config
     absl::core_headers
+    absl::raw_logging_internal
   TESTONLY
 )
 
@@ -300,8 +315,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::stack_consumption
-    absl::base
     absl::core_headers
+    absl::raw_logging_internal
     gmock_main
 )
 
diff --git a/absl/debugging/failure_signal_handler.cc b/absl/debugging/failure_signal_handler.cc
index af651c7..1f69bfa 100644
--- a/absl/debugging/failure_signal_handler.cc
+++ b/absl/debugging/failure_signal_handler.cc
@@ -24,6 +24,10 @@
 #include <unistd.h>
 #endif
 
+#ifdef __APPLE__
+#include <TargetConditionals.h>
+#endif
+
 #ifdef ABSL_HAVE_MMAP
 #include <sys/mman.h>
 #endif
@@ -37,6 +41,7 @@
 #include <ctime>
 
 #include "absl/base/attributes.h"
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/sysinfo.h"
 #include "absl/debugging/internal/examine_stack.h"
@@ -44,10 +49,15 @@
 
 #ifndef _WIN32
 #define ABSL_HAVE_SIGACTION
+// Apple WatchOS and TVOS don't allow sigaltstack
+#if !(defined(TARGET_OS_WATCH) && TARGET_OS_WATCH) && \
+    !(defined(TARGET_OS_TV) && TARGET_OS_TV)
+#define ABSL_HAVE_SIGALTSTACK
+#endif
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 ABSL_CONST_INIT static FailureSignalHandlerOptions fsh_options;
 
@@ -117,7 +127,7 @@
 
 }  // namespace debugging_internal
 
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
 
 static bool SetupAlternateStackOnce() {
 #if defined(__wasm__) || defined (__asjms__)
@@ -169,7 +179,7 @@
 // Returns the appropriate flag for sig_action.sa_flags
 // if the system supports using an alternate stack.
 static int MaybeSetupAlternateStack() {
-#ifndef _WIN32
+#ifdef ABSL_HAVE_SIGALTSTACK
   ABSL_ATTRIBUTE_UNUSED static const bool kOnce = SetupAlternateStackOnce();
   return SA_ONSTACK;
 #else
@@ -205,9 +215,8 @@
 #endif
 
 static void WriteToStderr(const char* data) {
-  int old_errno = errno;
+  absl::base_internal::ErrnoSaver errno_saver;
   absl::raw_logging_internal::SafeWriteToStderr(data, strlen(data));
-  errno = old_errno;
 }
 
 static void WriteSignalMessage(int signo, void (*writerfn)(const char*)) {
@@ -357,5 +366,5 @@
   }
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/failure_signal_handler.h b/absl/debugging/failure_signal_handler.h
index 87b202b..f5a8396 100644
--- a/absl/debugging/failure_signal_handler.h
+++ b/absl/debugging/failure_signal_handler.h
@@ -44,8 +44,10 @@
 #ifndef ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
 #define ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // FailureSignalHandlerOptions
 //
@@ -113,7 +115,7 @@
 const char* FailureSignalToString(int signo);
 }  // namespace debugging_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_FAILURE_SIGNAL_HANDLER_H_
diff --git a/absl/debugging/failure_signal_handler_test.cc b/absl/debugging/failure_signal_handler_test.cc
index bb2cc48..863fb51 100644
--- a/absl/debugging/failure_signal_handler_test.cc
+++ b/absl/debugging/failure_signal_handler_test.cc
@@ -23,6 +23,7 @@
 #include <fstream>
 
 #include "gtest/gtest.h"
+#include "gmock/gmock.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/debugging/stacktrace.h"
 #include "absl/debugging/symbolize.h"
@@ -31,6 +32,8 @@
 
 namespace {
 
+using testing::StartsWith;
+
 #if GTEST_HAS_DEATH_TEST
 
 // For the parameterized death tests. GetParam() returns the signal number.
@@ -113,15 +116,15 @@
   ASSERT_TRUE(error_output.is_open()) << file;
   std::string error_line;
   std::getline(error_output, error_line);
-  EXPECT_TRUE(absl::StartsWith(
+  EXPECT_THAT(
       error_line,
-      absl::StrCat("*** ",
-                   absl::debugging_internal::FailureSignalToString(signo),
-                   " received at ")));
+      StartsWith(absl::StrCat(
+          "*** ", absl::debugging_internal::FailureSignalToString(signo),
+          " received at ")));
 
   if (absl::debugging_internal::StackTraceWorksForTest()) {
     std::getline(error_output, error_line);
-    EXPECT_TRUE(absl::StartsWith(error_line, "PC: "));
+    EXPECT_THAT(error_line, StartsWith("PC: "));
   }
 }
 
diff --git a/absl/debugging/internal/address_is_readable.cc b/absl/debugging/internal/address_is_readable.cc
index 64dd285..6537606 100644
--- a/absl/debugging/internal/address_is_readable.cc
+++ b/absl/debugging/internal/address_is_readable.cc
@@ -20,14 +20,14 @@
 #if !defined(__linux__) || defined(__ANDROID__)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // On platforms other than Linux, just return true.
 bool AddressIsReadable(const void* /* addr */) { return true; }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else
@@ -35,14 +35,16 @@
 #include <fcntl.h>
 #include <sys/syscall.h>
 #include <unistd.h>
+
 #include <atomic>
 #include <cerrno>
 #include <cstdint>
 
+#include "absl/base/internal/errno_saver.h"
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Pack a pid and two file descriptors into a 64-bit word,
@@ -67,7 +69,7 @@
 // This is a namespace-scoped variable for correct zero-initialization.
 static std::atomic<uint64_t> pid_and_fds;  // initially 0, an invalid pid.
 bool AddressIsReadable(const void *addr) {
-  int save_errno = errno;
+  absl::base_internal::ErrnoSaver errno_saver;
   // We test whether a byte is readable by using write().  Normally, this would
   // be done via a cached file descriptor to /dev/null, but linux fails to
   // check whether the byte is readable when the destination is /dev/null, so
@@ -126,12 +128,11 @@
                                           std::memory_order_relaxed);
     }
   } while (errno == EBADF);
-  errno = save_errno;
   return bytes_written == 1;
 }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif
diff --git a/absl/debugging/internal/address_is_readable.h b/absl/debugging/internal/address_is_readable.h
index dc626e5..4bbaf4d 100644
--- a/absl/debugging/internal/address_is_readable.h
+++ b/absl/debugging/internal/address_is_readable.h
@@ -15,8 +15,10 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
 #define ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Return whether the byte at *addr is readable, without faulting.
@@ -24,7 +26,7 @@
 bool AddressIsReadable(const void *addr);
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_ADDRESS_IS_READABLE_H_
diff --git a/absl/debugging/internal/demangle.cc b/absl/debugging/internal/demangle.cc
index 976e47a..fc615c3 100644
--- a/absl/debugging/internal/demangle.cc
+++ b/absl/debugging/internal/demangle.cc
@@ -24,7 +24,7 @@
 #include <limits>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 typedef struct {
@@ -94,6 +94,8 @@
 };
 
 // List of builtin types from Itanium C++ ABI.
+//
+// Invariant: only one- or two-character type abbreviations here.
 static const AbbrevPair kBuiltinTypeList[] = {
     {"v", "void", 0},
     {"w", "wchar_t", 0},
@@ -116,6 +118,16 @@
     {"e", "long double", 0},
     {"g", "__float128", 0},
     {"z", "ellipsis", 0},
+
+    {"De", "decimal128", 0},      // IEEE 754r decimal floating point (128 bits)
+    {"Dd", "decimal64", 0},       // IEEE 754r decimal floating point (64 bits)
+    {"Dc", "decltype(auto)", 0},
+    {"Da", "auto", 0},
+    {"Dn", "std::nullptr_t", 0},  // i.e., decltype(nullptr)
+    {"Df", "decimal32", 0},       // IEEE 754r decimal floating point (32 bits)
+    {"Di", "char32_t", 0},
+    {"Ds", "char16_t", 0},
+    {"Dh", "float16", 0},         // IEEE 754r half-precision float (16 bits)
     {nullptr, nullptr, 0},
 };
 
@@ -1169,12 +1181,6 @@
   }
   state->parse_state = copy;
 
-  // nullptr_t, i.e. decltype(nullptr).
-  if (ParseTwoCharToken(state, "Dn")) {
-    return true;
-  }
-  state->parse_state = copy;
-
   if (ParseOneCharToken(state, 'U') && ParseSourceName(state) &&
       ParseType(state)) {
     return true;
@@ -1215,16 +1221,26 @@
   return num_cv_qualifiers > 0;
 }
 
-// <builtin-type> ::= v, etc.
+// <builtin-type> ::= v, etc.  # single-character builtin types
 //                ::= u <source-name>
+//                ::= Dd, etc.  # two-character builtin types
+//
+// Not supported:
+//                ::= DF <number> _ # _FloatN (N bits)
+//
 static bool ParseBuiltinType(State *state) {
   ComplexityGuard guard(state);
   if (guard.IsTooComplex()) return false;
   const AbbrevPair *p;
   for (p = kBuiltinTypeList; p->abbrev != nullptr; ++p) {
-    if (RemainingInput(state)[0] == p->abbrev[0]) {
+    // Guaranteed only 1- or 2-character strings in kBuiltinTypeList.
+    if (p->abbrev[1] == '\0') {
+      if (ParseOneCharToken(state, p->abbrev[0])) {
+        MaybeAppend(state, p->real_name);
+        return true;
+      }
+    } else if (p->abbrev[2] == '\0' && ParseTwoCharToken(state, p->abbrev)) {
       MaybeAppend(state, p->real_name);
-      ++state->parse_state.mangled_idx;
       return true;
     }
   }
@@ -1875,5 +1891,5 @@
 }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/demangle.h b/absl/debugging/internal/demangle.h
index 20adbe9..c314d9b 100644
--- a/absl/debugging/internal/demangle.h
+++ b/absl/debugging/internal/demangle.h
@@ -53,8 +53,10 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
 #define ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Demangle `mangled`.  On success, return true and write the
@@ -63,7 +65,7 @@
 bool Demangle(const char *mangled, char *out, int out_size);
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_DEMANGLE_H_
diff --git a/absl/debugging/internal/demangle_test.cc b/absl/debugging/internal/demangle_test.cc
index 7c50fe3..c6f1ce1 100644
--- a/absl/debugging/internal/demangle_test.cc
+++ b/absl/debugging/internal/demangle_test.cc
@@ -23,7 +23,7 @@
 #include "absl/memory/memory.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
@@ -191,5 +191,5 @@
 
 }  // namespace
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/elf_mem_image.cc b/absl/debugging/internal/elf_mem_image.cc
index a795117..24cc013 100644
--- a/absl/debugging/internal/elf_mem_image.cc
+++ b/absl/debugging/internal/elf_mem_image.cc
@@ -38,7 +38,7 @@
 #define VERSYM_VERSION 0x7fff
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 namespace {
@@ -376,7 +376,7 @@
 }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/internal/elf_mem_image.h b/absl/debugging/internal/elf_mem_image.h
index fcd32af..46bfade 100644
--- a/absl/debugging/internal/elf_mem_image.h
+++ b/absl/debugging/internal/elf_mem_image.h
@@ -23,6 +23,8 @@
 // used.
 #include <climits>
 
+#include "absl/base/config.h"
+
 // Maybe one day we can rewrite this file not to require the elf
 // symbol extensions in glibc, but for right now we need them.
 #ifdef ABSL_HAVE_ELF_MEM_IMAGE
@@ -39,7 +41,7 @@
 #include <link.h>  // for ElfW
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // An in-memory ELF image (may not exist on disk).
@@ -124,7 +126,7 @@
 };
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/internal/examine_stack.cc b/absl/debugging/internal/examine_stack.cc
index 4739fbc..a3dd893 100644
--- a/absl/debugging/internal/examine_stack.cc
+++ b/absl/debugging/internal/examine_stack.cc
@@ -30,7 +30,7 @@
 #include "absl/debugging/symbolize.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Returns the program counter from signal context, nullptr if
@@ -53,6 +53,8 @@
     return reinterpret_cast<void*>(context->uc_mcontext.gp_regs[32]);
 #elif defined(__powerpc__)
     return reinterpret_cast<void*>(context->uc_mcontext.regs->nip);
+#elif defined(__riscv)
+    return reinterpret_cast<void*>(context->uc_mcontext.__gregs[REG_PC]);
 #elif defined(__s390__) && !defined(__s390x__)
     return reinterpret_cast<void*>(context->uc_mcontext.psw.addr & 0x7fffffff);
 #elif defined(__s390__) && defined(__s390x__)
@@ -151,5 +153,5 @@
 }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/internal/examine_stack.h b/absl/debugging/internal/examine_stack.h
index 861db75..3933691 100644
--- a/absl/debugging/internal/examine_stack.h
+++ b/absl/debugging/internal/examine_stack.h
@@ -17,8 +17,10 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
 #define ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Returns the program counter from signal context, or nullptr if
@@ -34,7 +36,7 @@
     void (*writerfn)(const char*, void*), void* writerfn_arg);
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_EXAMINE_STACK_H_
diff --git a/absl/debugging/internal/stack_consumption.cc b/absl/debugging/internal/stack_consumption.cc
index 8dfd94a..875ca6d 100644
--- a/absl/debugging/internal/stack_consumption.cc
+++ b/absl/debugging/internal/stack_consumption.cc
@@ -27,7 +27,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
@@ -116,10 +116,11 @@
   // Set up the alt-signal-stack (and save the older one).
   stack_t sigstk;
   memset(&sigstk, 0, sizeof(sigstk));
-  stack_t old_sigstk;
   sigstk.ss_sp = altstack;
   sigstk.ss_size = kAlternateStackSize;
   sigstk.ss_flags = 0;
+  stack_t old_sigstk;
+  memset(&old_sigstk, 0, sizeof(old_sigstk));
   ABSL_RAW_CHECK(sigaltstack(&sigstk, &old_sigstk) == 0,
                  "sigaltstack() failed");
 
@@ -153,6 +154,15 @@
   int signal_handler_stack_consumption = GetStackConsumption(altstack);
 
   // Now restore the old alt-signal-stack and signal handlers.
+  if (old_sigstk.ss_sp == nullptr && old_sigstk.ss_size == 0 &&
+      (old_sigstk.ss_flags & SS_DISABLE)) {
+    // https://git.musl-libc.org/cgit/musl/commit/src/signal/sigaltstack.c?id=7829f42a2c8944555439380498ab8b924d0f2070
+    // The original stack has ss_size==0 and ss_flags==SS_DISABLE, but some
+    // versions of musl have a bug that rejects ss_size==0. Work around this by
+    // setting ss_size to MINSIGSTKSZ, which should be ignored by the kernel
+    // when SS_DISABLE is set.
+    old_sigstk.ss_size = MINSIGSTKSZ;
+  }
   ABSL_RAW_CHECK(sigaltstack(&old_sigstk, nullptr) == 0,
                  "sigaltstack() failed");
   ABSL_RAW_CHECK(sigaction(SIGUSR1, &old_sa1, nullptr) == 0,
@@ -168,7 +178,7 @@
 }
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stack_consumption.h b/absl/debugging/internal/stack_consumption.h
index 1eb37ee..5e60ec4 100644
--- a/absl/debugging/internal/stack_consumption.h
+++ b/absl/debugging/internal/stack_consumption.h
@@ -18,6 +18,8 @@
 #ifndef ABSL_DEBUGGING_INTERNAL_STACK_CONSUMPTION_H_
 #define ABSL_DEBUGGING_INTERNAL_STACK_CONSUMPTION_H_
 
+#include "absl/base/config.h"
+
 // The code in this module is not portable.
 // Use this feature test macro to detect its availability.
 #ifdef ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
@@ -27,7 +29,7 @@
 #define ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION 1
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Returns the stack consumption in bytes for the code exercised by
@@ -39,7 +41,7 @@
 int GetSignalHandlerStackConsumption(void (*signal_handler)(int));
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stack_consumption_test.cc b/absl/debugging/internal/stack_consumption_test.cc
index 1c82d18..80445bf 100644
--- a/absl/debugging/internal/stack_consumption_test.cc
+++ b/absl/debugging/internal/stack_consumption_test.cc
@@ -23,7 +23,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 namespace {
 
@@ -44,7 +44,7 @@
 
 }  // namespace
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_DEBUGGING_STACK_CONSUMPTION
diff --git a/absl/debugging/internal/stacktrace_aarch64-inl.inc b/absl/debugging/internal/stacktrace_aarch64-inl.inc
index 14b6e6b..411ea30 100644
--- a/absl/debugging/internal/stacktrace_aarch64-inl.inc
+++ b/absl/debugging/internal/stacktrace_aarch64-inl.inc
@@ -180,13 +180,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_AARCH64_INL_H_
diff --git a/absl/debugging/internal/stacktrace_arm-inl.inc b/absl/debugging/internal/stacktrace_arm-inl.inc
index 4f51d18..fffda96 100644
--- a/absl/debugging/internal/stacktrace_arm-inl.inc
+++ b/absl/debugging/internal/stacktrace_arm-inl.inc
@@ -113,13 +113,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_ARM_INL_H_
diff --git a/absl/debugging/internal/stacktrace_generic-inl.inc b/absl/debugging/internal/stacktrace_generic-inl.inc
index 39c4786..ac034c9 100644
--- a/absl/debugging/internal/stacktrace_generic-inl.inc
+++ b/absl/debugging/internal/stacktrace_generic-inl.inc
@@ -16,6 +16,7 @@
 #include <cstring>
 
 #include "absl/debugging/stacktrace.h"
+#include "absl/base/attributes.h"
 
 // Sometimes, we can try to get a stack trace from within a stack
 // trace, because we don't block signals inside this code (which would be too
@@ -35,7 +36,7 @@
 static std::atomic<bool> disable_stacktraces(true);  // Disabled until healthy.
 // Waiting until static initializers run seems to be late enough.
 // This file is included into stacktrace.cc so this will only run once.
-static int stacktraces_enabler = []() {
+ABSL_ATTRIBUTE_UNUSED static int stacktraces_enabler = []() {
   void* unused_stack[1];
   // Force the first backtrace to happen early to get the one-time shared lib
   // loading (allocation) out of the way. After the first call it is much safer
@@ -86,13 +87,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_GENERIC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_powerpc-inl.inc b/absl/debugging/internal/stacktrace_powerpc-inl.inc
index ee6b38f..2e7c2f4 100644
--- a/absl/debugging/internal/stacktrace_powerpc-inl.inc
+++ b/absl/debugging/internal/stacktrace_powerpc-inl.inc
@@ -236,13 +236,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_POWERPC_INL_H_
diff --git a/absl/debugging/internal/stacktrace_unimplemented-inl.inc b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
index b49a929..5b8fb19 100644
--- a/absl/debugging/internal/stacktrace_unimplemented-inl.inc
+++ b/absl/debugging/internal/stacktrace_unimplemented-inl.inc
@@ -12,13 +12,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_UNIMPLEMENTED_INL_H_
diff --git a/absl/debugging/internal/stacktrace_win32-inl.inc b/absl/debugging/internal/stacktrace_win32-inl.inc
index 0cd8c33..9c2c558 100644
--- a/absl/debugging/internal/stacktrace_win32-inl.inc
+++ b/absl/debugging/internal/stacktrace_win32-inl.inc
@@ -54,7 +54,7 @@
 
 template <bool IS_STACK_FRAMES, bool IS_WITH_CONTEXT>
 static int UnwindImpl(void** result, int* sizes, int max_depth, int skip_count,
-                      const void *ucp, int *min_dropped_frames) {
+                      const void*, int* min_dropped_frames) {
   int n = 0;
   if (!RtlCaptureStackBackTrace_fn) {
     // can't find a stacktrace with no function to call
@@ -73,13 +73,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return false;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_WIN32_INL_H_
diff --git a/absl/debugging/internal/stacktrace_x86-inl.inc b/absl/debugging/internal/stacktrace_x86-inl.inc
index ff0fd31..bc320ff 100644
--- a/absl/debugging/internal/stacktrace_x86-inl.inc
+++ b/absl/debugging/internal/stacktrace_x86-inl.inc
@@ -36,6 +36,8 @@
 
 #include "absl/base/internal/raw_logging.h"
 
+using absl::debugging_internal::AddressIsReadable;
+
 #if defined(__linux__) && defined(__i386__)
 // Count "push %reg" instructions in VDSO __kernel_vsyscall(),
 // preceeding "syscall" or "sysenter".
@@ -81,7 +83,7 @@
       // "mov reg,reg"
       if (addr[i + 1] == 0xE5) {
         // Found "mov %esp,%ebp".
-        return 0;  
+        return 0;
       }
       ++i;  // Skip register encoding byte.
     } else if (addr[i] == 0x0F &&
@@ -171,6 +173,7 @@
     static const unsigned char *kernel_rt_sigreturn_address = nullptr;
     static const unsigned char *kernel_vsyscall_address = nullptr;
     if (num_push_instructions == -1) {
+#ifdef ABSL_HAVE_VDSO_SUPPORT
       absl::debugging_internal::VDSOSupport vdso;
       if (vdso.IsPresent()) {
         absl::debugging_internal::VDSOSupport::SymbolInfo
@@ -199,6 +202,9 @@
       } else {
         num_push_instructions = 0;
       }
+#else  // ABSL_HAVE_VDSO_SUPPORT
+      num_push_instructions = 0;
+#endif  // ABSL_HAVE_VDSO_SUPPORT
     }
     if (num_push_instructions != 0 && kernel_rt_sigreturn_address != nullptr &&
         old_fp[1] == kernel_rt_sigreturn_address) {
@@ -222,7 +228,7 @@
           // "double fault" in case we hit the first fault due to e.g. stack
           // corruption.
           void *const reg_esp2 = reg_esp[num_push_instructions - 1];
-          if (absl::debugging_internal::AddressIsReadable(reg_esp2)) {
+          if (AddressIsReadable(reg_esp2)) {
             // Alleged %esp is readable, use it for further unwinding.
             new_fp = reinterpret_cast<void **>(reg_esp2);
           }
@@ -274,7 +280,7 @@
     // Note: NextStackFrame<false>() is only called while the program
     //       is already on its last leg, so it's ok to be slow here.
 
-    if (!absl::debugging_internal::AddressIsReadable(new_fp)) {
+    if (!AddressIsReadable(new_fp)) {
       return nullptr;
     }
   }
@@ -328,13 +334,13 @@
 }
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 bool StackTraceWorksForTest() {
   return true;
 }
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_STACKTRACE_X86_INL_INC_
diff --git a/absl/debugging/internal/symbolize.h b/absl/debugging/internal/symbolize.h
index 151bc77..5d0858b 100644
--- a/absl/debugging/internal/symbolize.h
+++ b/absl/debugging/internal/symbolize.h
@@ -21,6 +21,8 @@
 #include <cstddef>
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 #ifdef ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
 #error ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE cannot be directly set
 #elif defined(__ELF__) && defined(__GLIBC__) && !defined(__native_client__) && \
@@ -33,7 +35,7 @@
 #include <string>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // Iterates over all sections, invoking callback on each with the section name
@@ -52,13 +54,13 @@
                             ElfW(Shdr) *out);
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 struct SymbolDecoratorArgs {
@@ -120,7 +122,7 @@
                         const char** filename);
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_INTERNAL_SYMBOLIZE_H_
diff --git a/absl/debugging/internal/vdso_support.cc b/absl/debugging/internal/vdso_support.cc
index fa88e1d..1e8a78a 100644
--- a/absl/debugging/internal/vdso_support.cc
+++ b/absl/debugging/internal/vdso_support.cc
@@ -38,7 +38,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 ABSL_CONST_INIT
@@ -188,7 +188,7 @@
 } vdso_init_helper;
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_VDSO_SUPPORT
diff --git a/absl/debugging/internal/vdso_support.h b/absl/debugging/internal/vdso_support.h
index bc5fdb1..6562c6c 100644
--- a/absl/debugging/internal/vdso_support.h
+++ b/absl/debugging/internal/vdso_support.h
@@ -53,7 +53,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace debugging_internal {
 
 // NOTE: this class may be used from within tcmalloc, and can not
@@ -150,7 +150,7 @@
 int GetCPU();
 
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HAVE_ELF_MEM_IMAGE
diff --git a/absl/debugging/leak_check.cc b/absl/debugging/leak_check.cc
index 63e54ff..ff90495 100644
--- a/absl/debugging/leak_check.cc
+++ b/absl/debugging/leak_check.cc
@@ -21,14 +21,14 @@
 #ifndef LEAK_SANITIZER
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 bool HaveLeakSanitizer() { return false; }
 void DoIgnoreLeak(const void*) { }
 void RegisterLivePointers(const void*, size_t) { }
 void UnRegisterLivePointers(const void*, size_t) { }
 LeakCheckDisabler::LeakCheckDisabler() { }
 LeakCheckDisabler::~LeakCheckDisabler() { }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else
@@ -36,7 +36,7 @@
 #include <sanitizer/lsan_interface.h>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 bool HaveLeakSanitizer() { return true; }
 void DoIgnoreLeak(const void* ptr) { __lsan_ignore_object(ptr); }
 void RegisterLivePointers(const void* ptr, size_t size) {
@@ -47,7 +47,7 @@
 }
 LeakCheckDisabler::LeakCheckDisabler() { __lsan_disable(); }
 LeakCheckDisabler::~LeakCheckDisabler() { __lsan_enable(); }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // LEAK_SANITIZER
diff --git a/absl/debugging/leak_check.h b/absl/debugging/leak_check.h
index e1215e3..7a5a22d 100644
--- a/absl/debugging/leak_check.h
+++ b/absl/debugging/leak_check.h
@@ -32,8 +32,10 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // HaveLeakSanitizer()
 //
@@ -105,7 +107,7 @@
 // `RegisterLivePointers()`, enabling leak checking of those pointers.
 void UnRegisterLivePointers(const void* ptr, size_t size);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_LEAK_CHECK_H_
diff --git a/absl/debugging/stacktrace.cc b/absl/debugging/stacktrace.cc
index 3052fb9..1f7c7d8 100644
--- a/absl/debugging/stacktrace.cc
+++ b/absl/debugging/stacktrace.cc
@@ -57,7 +57,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 typedef int (*Unwinder)(void**, int*, int, int, const void*, int*);
@@ -136,5 +136,5 @@
   return n;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/stacktrace.h b/absl/debugging/stacktrace.h
index d7565aa..0ec0ffd 100644
--- a/absl/debugging/stacktrace.h
+++ b/absl/debugging/stacktrace.h
@@ -31,16 +31,18 @@
 #ifndef ABSL_DEBUGGING_STACKTRACE_H_
 #define ABSL_DEBUGGING_STACKTRACE_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // GetStackFrames()
 //
 // Records program counter values for up to `max_depth` frames, skipping the
-// most recent `skip_count` stack frames, and stores their corresponding values
-// and sizes in `results` and `sizes` buffers. (Note that the frame generated
-// for the `absl::GetStackFrames()` routine itself is also skipped.)
-// routine itself.
+// most recent `skip_count` stack frames, stores their corresponding values
+// and sizes in `results` and `sizes` buffers, and returns the number of frames
+// stored. (Note that the frame generated for the `absl::GetStackFrames()`
+// routine itself is also skipped.)
 //
 // Example:
 //
@@ -55,8 +57,8 @@
 // The current stack frame would consist of three function calls: `bar()`,
 // `foo()`, and then `main()`; however, since the `GetStackFrames()` call sets
 // `skip_count` to `1`, it will skip the frame for `bar()`, the most recently
-// invoked function call. It will therefore return two program counters and will
-// produce values that map to the following function calls:
+// invoked function call. It will therefore return 2 and fill `result` with
+// program counters within the following functions:
 //
 //      result[0]       foo()
 //      result[1]       main()
@@ -83,9 +85,10 @@
 //
 // Records program counter values obtained from a signal handler. Records
 // program counter values for up to `max_depth` frames, skipping the most recent
-// `skip_count` stack frames, and stores their corresponding values and sizes in
-// `results` and `sizes` buffers. (Note that the frame generated for the
-// `absl::GetStackFramesWithContext()` routine itself is also skipped.)
+// `skip_count` stack frames, stores their corresponding values and sizes in
+// `results` and `sizes` buffers, and returns the number of frames stored. (Note
+// that the frame generated for the `absl::GetStackFramesWithContext()` routine
+// itself is also skipped.)
 //
 // The `uc` parameter, if non-null, should be a pointer to a `ucontext_t` value
 // passed to a signal handler registered via the `sa_sigaction` field of a
@@ -106,8 +109,9 @@
 // GetStackTrace()
 //
 // Records program counter values for up to `max_depth` frames, skipping the
-// most recent `skip_count` stack frames, and stores their corresponding values
-// in `results`. Note that this function is similar to `absl::GetStackFrames()`
+// most recent `skip_count` stack frames, stores their corresponding values
+// in `results`, and returns the number of frames
+// stored. Note that this function is similar to `absl::GetStackFrames()`
 // except that it returns the stack trace only, and not stack frame sizes.
 //
 // Example:
@@ -132,9 +136,9 @@
 //
 // Records program counter values obtained from a signal handler. Records
 // program counter values for up to `max_depth` frames, skipping the most recent
-// `skip_count` stack frames, and stores their corresponding values in
-// `results`. (Note that the frame generated for the
-// `absl::GetStackFramesWithContext()` routine itself is also skipped.)
+// `skip_count` stack frames, stores their corresponding values in `results`,
+// and returns the number of frames stored. (Note that the frame generated for
+// the `absl::GetStackFramesWithContext()` routine itself is also skipped.)
 //
 // The `uc` parameter, if non-null, should be a pointer to a `ucontext_t` value
 // passed to a signal handler registered via the `sa_sigaction` field of a
@@ -221,7 +225,7 @@
 // working.
 extern bool StackTraceWorksForTest();
 }  // namespace debugging_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_STACKTRACE_H_
diff --git a/absl/debugging/symbolize.cc b/absl/debugging/symbolize.cc
index 24e3a7f..54ed970 100644
--- a/absl/debugging/symbolize.cc
+++ b/absl/debugging/symbolize.cc
@@ -16,12 +16,9 @@
 
 #if defined(ABSL_INTERNAL_HAVE_ELF_SYMBOLIZE)
 #include "absl/debugging/symbolize_elf.inc"
-#elif defined(_WIN32) && defined(_DEBUG)
-// The Windows Symbolizer only works in debug mode. Note that _DEBUG
-// is the macro that defines whether or not MS C-Runtime debug info is
-// available. Note that the PDB files containing the debug info must
-// also be available to the program at runtime for the symbolizer to
-// work.
+#elif defined(_WIN32)
+// The Windows Symbolizer only works if PDB files containing the debug info
+// are available to the program at runtime.
 #include "absl/debugging/symbolize_win32.inc"
 #else
 #include "absl/debugging/symbolize_unimplemented.inc"
diff --git a/absl/debugging/symbolize.h b/absl/debugging/symbolize.h
index ab5447c..43d93a8 100644
--- a/absl/debugging/symbolize.h
+++ b/absl/debugging/symbolize.h
@@ -55,7 +55,7 @@
 #include "absl/debugging/internal/symbolize.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // InitializeSymbolizer()
 //
@@ -71,7 +71,7 @@
 //   // Now you can use the symbolizer
 // }
 void InitializeSymbolizer(const char* argv0);
-
+//
 // Symbolize()
 //
 // Symbolizes a program counter (instruction pointer value) `pc` and, on
@@ -89,11 +89,11 @@
 //     if (absl::Symbolize(pc, tmp, sizeof(tmp))) {
 //       symbol = tmp;
 //     }
-//     absl::PrintF("%*p  %s\n", pc, symbol);
+//     absl::PrintF("%p  %s\n", pc, symbol);
 //  }
 bool Symbolize(const void *pc, char *out, int out_size);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_DEBUGGING_SYMBOLIZE_H_
diff --git a/absl/debugging/symbolize_elf.inc b/absl/debugging/symbolize_elf.inc
index 2bd7659..c371635 100644
--- a/absl/debugging/symbolize_elf.inc
+++ b/absl/debugging/symbolize_elf.inc
@@ -76,7 +76,7 @@
 #include "absl/debugging/internal/vdso_support.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Value of argv[0]. Used by MaybeInitializeObjFile().
 static char *argv0_value = nullptr;
@@ -625,6 +625,13 @@
   return start <= address && address < (start + size);
 }
 
+static const char *ComputeOffset(const char *base, ptrdiff_t offset) {
+  // Note: cast to uintptr_t to avoid undefined behavior when base evaluates to
+  // zero and offset is non-zero.
+  return reinterpret_cast<const char *>(
+      reinterpret_cast<uintptr_t>(base) + offset);
+}
+
 // Read a symbol table and look for the symbol containing the
 // pc. Iterate over symbols in a symbol table and look for the symbol
 // containing "pc".  If the symbol is found, and its name fits in
@@ -677,7 +684,8 @@
       // We keep the original address for opd redirection below.
       const char *const original_start_address =
           reinterpret_cast<const char *>(symbol.st_value);
-      const char *start_address = original_start_address + relocation;
+      const char *start_address =
+          ComputeOffset(original_start_address, relocation);
 
       if (deref_function_descriptor_pointer &&
           InSection(original_start_address, opd)) {
@@ -689,8 +697,7 @@
 
       // If pc is inside the .opd section, it points to a function descriptor.
       const size_t size = pc_in_opd ? kFunctionDescriptorSize : symbol.st_size;
-      const void *const end_address =
-          reinterpret_cast<const char *>(start_address) + size;
+      const void *const end_address = ComputeOffset(start_address, size);
       if (symbol.st_value != 0 &&  // Skip null value symbols.
           symbol.st_shndx != 0 &&  // Skip undefined symbols.
 #ifdef STT_TLS
@@ -1469,5 +1476,5 @@
   return ok;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/symbolize_test.cc b/absl/debugging/symbolize_test.cc
index 08068c3..a1d03aa 100644
--- a/absl/debugging/symbolize_test.cc
+++ b/absl/debugging/symbolize_test.cc
@@ -35,10 +35,29 @@
 
 using testing::Contains;
 
+#ifdef _WIN32
+#define ABSL_SYMBOLIZE_TEST_NOINLINE __declspec(noinline)
+#else
+#define ABSL_SYMBOLIZE_TEST_NOINLINE ABSL_ATTRIBUTE_NOINLINE
+#endif
+
 // Functions to symbolize. Use C linkage to avoid mangled names.
 extern "C" {
-void nonstatic_func() { ABSL_BLOCK_TAIL_CALL_OPTIMIZATION(); }
-static void static_func() { ABSL_BLOCK_TAIL_CALL_OPTIMIZATION(); }
+ABSL_SYMBOLIZE_TEST_NOINLINE void nonstatic_func() {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
+
+ABSL_SYMBOLIZE_TEST_NOINLINE static void static_func() {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
+  ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
+}
 }  // extern "C"
 
 struct Foo {
@@ -46,7 +65,11 @@
 };
 
 // A C++ method that should have a mangled name.
-void ABSL_ATTRIBUTE_NOINLINE Foo::func(int) {
+ABSL_SYMBOLIZE_TEST_NOINLINE void Foo::func(int) {
+  // The next line makes this a unique function to prevent the compiler from
+  // folding identical functions together.
+  volatile int x = __LINE__;
+  static_cast<void>(x);
   ABSL_BLOCK_TAIL_CALL_OPTIMIZATION();
 }
 
@@ -80,6 +103,7 @@
     symbolize_test_thread_big[2 * 1024 * 1024];
 #endif
 
+#if !defined(__EMSCRIPTEN__)
 // Used below to hopefully inhibit some compiler/linker optimizations
 // that may remove kHpageTextPadding, kPadding0, and kPadding1 from
 // the binary.
@@ -89,6 +113,7 @@
 static constexpr size_t kHpageSize = 1 << 21;
 const char kHpageTextPadding[kHpageSize * 4] ABSL_ATTRIBUTE_SECTION_VARIABLE(
     .text) = "";
+#endif  // !defined(__EMSCRIPTEN__)
 
 static char try_symbolize_buffer[4096];
 
@@ -447,14 +472,15 @@
 #endif
 }
 
-#elif defined(_WIN32) && defined(_DEBUG)
+#elif defined(_WIN32)
+#if !defined(ABSL_CONSUME_DLL)
 
 TEST(Symbolize, Basics) {
   EXPECT_STREQ("nonstatic_func", TrySymbolize((void *)(&nonstatic_func)));
 
   // The name of an internal linkage symbol is not specified; allow either a
   // mangled or an unmangled name here.
-  const char* static_func_symbol = TrySymbolize((void *)(&static_func));
+  const char *static_func_symbol = TrySymbolize((void *)(&static_func));
   ASSERT_TRUE(static_func_symbol != nullptr);
   EXPECT_TRUE(strstr(static_func_symbol, "static_func") != nullptr);
 
@@ -481,11 +507,12 @@
 }
 
 TEST(Symbolize, SymbolizeWithDemangling) {
-  const char* result = TrySymbolize((void *)(&Foo::func));
+  const char *result = TrySymbolize((void *)(&Foo::func));
   ASSERT_TRUE(result != nullptr);
   EXPECT_TRUE(strstr(result, "Foo::func") != nullptr) << result;
 }
 
+#endif  // !defined(ABSL_CONSUME_DLL)
 #else  // Symbolizer unimplemented
 
 TEST(Symbolize, Unimplemented) {
@@ -498,10 +525,12 @@
 #endif
 
 int main(int argc, char **argv) {
+#if !defined(__EMSCRIPTEN__)
   // Make sure kHpageTextPadding is linked into the binary.
   if (volatile_bool) {
     ABSL_RAW_LOG(INFO, "%s", kHpageTextPadding);
   }
+#endif  // !defined(__EMSCRIPTEN__)
 
 #if ABSL_PER_THREAD_TLS
   // Touch the per-thread variables.
diff --git a/absl/debugging/symbolize_unimplemented.inc b/absl/debugging/symbolize_unimplemented.inc
index 0c1c195..db24456 100644
--- a/absl/debugging/symbolize_unimplemented.inc
+++ b/absl/debugging/symbolize_unimplemented.inc
@@ -17,7 +17,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace debugging_internal {
 
@@ -36,5 +36,5 @@
 void InitializeSymbolizer(const char*) {}
 bool Symbolize(const void *, char *, int) { return false; }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/debugging/symbolize_win32.inc b/absl/debugging/symbolize_win32.inc
index 498ca2d..c3df46f 100644
--- a/absl/debugging/symbolize_win32.inc
+++ b/absl/debugging/symbolize_win32.inc
@@ -17,10 +17,10 @@
 
 #include <windows.h>
 
-// MSVC header DbgHelp.h has a warning for an ignored typedef.
+// MSVC header dbghelp.h has a warning for an ignored typedef.
 #pragma warning(push)
 #pragma warning(disable:4091)
-#include <DbgHelp.h>
+#include <dbghelp.h>
 #pragma warning(pop)
 
 #pragma comment(lib, "dbghelp.lib")
@@ -31,11 +31,11 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 static HANDLE process = NULL;
 
-void InitializeSymbolizer(const char *argv0) {
+void InitializeSymbolizer(const char*) {
   if (process != nullptr) {
     return;
   }
@@ -54,13 +54,12 @@
   }
 }
 
-bool Symbolize(const void *pc, char *out, int out_size) {
+bool Symbolize(const void* pc, char* out, int out_size) {
   if (out_size <= 0) {
     return false;
   }
-  std::aligned_storage<sizeof(SYMBOL_INFO) + MAX_SYM_NAME,
-                       alignof(SYMBOL_INFO)>::type buf;
-  SYMBOL_INFO *symbol = reinterpret_cast<SYMBOL_INFO *>(&buf);
+  alignas(SYMBOL_INFO) char buf[sizeof(SYMBOL_INFO) + MAX_SYM_NAME];
+  SYMBOL_INFO* symbol = reinterpret_cast<SYMBOL_INFO*>(buf);
   symbol->SizeOfStruct = sizeof(SYMBOL_INFO);
   symbol->MaxNameLen = MAX_SYM_NAME;
   if (!SymFromAddr(process, reinterpret_cast<DWORD64>(pc), nullptr, symbol)) {
@@ -78,5 +77,5 @@
   return true;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/BUILD.bazel b/absl/flags/BUILD.bazel
index 2fe61ea..cdb4e7e 100644
--- a/absl/flags/BUILD.bazel
+++ b/absl/flags/BUILD.bazel
@@ -1,5 +1,5 @@
 #
-#  Copyright 2019 The Abseil Authors.
+# Copyright 2019 The Abseil Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -26,12 +27,35 @@
 licenses(["notice"])  # Apache 2.0
 
 cc_library(
-    name = "internal",
+    name = "flag_internal",
+    srcs = [
+        "internal/flag.cc",
+    ],
+    hdrs = [
+        "internal/flag.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = ["//absl/base:__subpackages__"],
+    deps = [
+        ":config",
+        ":handle",
+        ":registry",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/memory",
+        "//absl/strings",
+        "//absl/synchronization",
+    ],
+)
+
+cc_library(
+    name = "program_name",
     srcs = [
         "internal/program_name.cc",
     ],
     hdrs = [
-        "internal/path_util.h",
         "internal/program_name.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
@@ -40,12 +64,31 @@
         "//absl/flags:__pkg__",
     ],
     deps = [
+        ":path_util",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/strings",
         "//absl/synchronization",
     ],
 )
 
 cc_library(
+    name = "path_util",
+    hdrs = [
+        "internal/path_util.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/flags:__pkg__",
+    ],
+    deps = [
+        "//absl/base:config",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
     name = "config",
     srcs = [
         "usage_config.cc",
@@ -57,7 +100,9 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":internal",
+        ":path_util",
+        ":program_name",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/strings",
         "//absl/synchronization",
@@ -75,7 +120,9 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:log_severity",
         "//absl/strings",
         "//absl/strings:str_format",
     ],
@@ -83,9 +130,6 @@
 
 cc_library(
     name = "handle",
-    srcs = [
-        "internal/commandlineflag.cc",
-    ],
     hdrs = [
         "internal/commandlineflag.h",
     ],
@@ -97,10 +141,9 @@
     deps = [
         ":config",
         ":marshalling",
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/strings",
-        "//absl/synchronization",
         "//absl/types:optional",
     ],
 )
@@ -123,9 +166,9 @@
     deps = [
         ":config",
         ":handle",
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
-        "//absl/base:dynamic_annotations",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -139,16 +182,17 @@
     hdrs = [
         "declare.h",
         "flag.h",
-        "internal/flag.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":config",
+        ":flag_internal",
         ":handle",
         ":marshalling",
         ":registry",
         "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/strings",
     ],
@@ -170,10 +214,14 @@
     deps = [
         ":config",
         ":flag",
+        ":flag_internal",
         ":handle",
-        ":internal",
+        ":path_util",
+        ":program_name",
+        ":registry",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/strings",
-        "//absl/synchronization",
     ],
 )
 
@@ -189,6 +237,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":usage_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/strings",
         "//absl/synchronization",
     ],
@@ -206,18 +256,21 @@
     deps = [
         ":config",
         ":flag",
+        ":flag_internal",
         ":handle",
-        ":internal",
+        ":program_name",
         ":registry",
         ":usage",
         ":usage_internal",
+        "//absl/base:config",
+        "//absl/base:core_headers",
         "//absl/strings",
         "//absl/synchronization",
     ],
 )
 
 ############################################################################
-# Unit tests in alpahabetical order.
+# Unit tests in alphabetical order.
 
 cc_test(
     name = "commandlineflag_test",
@@ -228,6 +281,7 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":config",
         ":flag",
         ":handle",
         ":registry",
@@ -261,12 +315,34 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
+        ":config",
         ":flag",
+        ":flag_internal",
+        ":handle",
+        ":registry",
+        "//absl/base:core_headers",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
 )
 
+cc_binary(
+    name = "flag_benchmark",
+    testonly = 1,
+    srcs = [
+        "flag_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":flag",
+        "//absl/time",
+        "//absl/types:optional",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
+
 cc_test(
     name = "marshalling_test",
     size = "small",
@@ -290,7 +366,7 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":internal",
+        ":path_util",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -306,7 +382,8 @@
     deps = [
         ":flag",
         ":parse",
-        "//absl/base",
+        ":registry",
+        "//absl/base:raw_logging_internal",
         "//absl/base:scoped_set_env",
         "//absl/strings",
         "//absl/types:span",
@@ -323,7 +400,7 @@
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":internal",
+        ":program_name",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
@@ -339,9 +416,10 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":flag",
+        ":handle",
+        ":marshalling",
         ":registry",
         "//absl/memory",
-        "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -356,7 +434,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":config",
-        ":internal",
+        ":path_util",
+        ":program_name",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
     ],
@@ -373,8 +452,10 @@
     deps = [
         ":config",
         ":flag",
-        ":internal",
         ":parse",
+        ":path_util",
+        ":program_name",
+        ":registry",
         ":usage",
         ":usage_internal",
         "//absl/memory",
diff --git a/absl/flags/CMakeLists.txt b/absl/flags/CMakeLists.txt
index fa1d4e1..1d25f0d 100644
--- a/absl/flags/CMakeLists.txt
+++ b/absl/flags/CMakeLists.txt
@@ -19,20 +19,60 @@
   NAME
     flags_internal
   SRCS
+    "internal/flag.cc"
+  HDRS
+    "internal/flag.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::base
+    absl::config
+    absl::flags_config
+    absl::flags_handle
+    absl::flags_registry
+    absl::synchronization
+  PUBLIC
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    flags_program_name
+  SRCS
     "internal/program_name.cc"
   HDRS
-    "internal/path_util.h"
     "internal/program_name.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::core_headers
+    absl::flags_path_util
     absl::strings
     absl::synchronization
   PUBLIC
 )
 
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    flags_path_util
+  HDRS
+    "internal/path_util.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
+    absl::strings
+  PUBLIC
+)
+
 absl_cc_library(
   NAME
     flags_config
@@ -46,7 +86,9 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::flags_internal
+    absl::config
+    absl::flags_path_util
+    absl::flags_program_name
     absl::core_headers
     absl::strings
     absl::synchronization
@@ -64,7 +106,9 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::core_headers
+    absl::log_severity
     absl::strings
     absl::str_format
 )
@@ -73,8 +117,6 @@
 absl_cc_library(
   NAME
     flags_handle
-  SRCS
-    "internal/commandlineflag.cc"
   HDRS
     "internal/commandlineflag.h"
   COPTS
@@ -82,13 +124,14 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_config
     absl::flags_marshalling
-    absl::base
     absl::core_headers
+    absl::optional
+    absl::raw_logging_internal
     absl::strings
     absl::synchronization
-    absl::optional
 )
 
 # Internal-only target, do not depend on directly.
@@ -106,11 +149,11 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_config
     absl::flags_handle
-    absl::base
     absl::core_headers
-    absl::dynamic_annotations
+    absl::raw_logging_internal
     absl::strings
     absl::synchronization
 )
@@ -123,14 +166,15 @@
   HDRS
     "declare.h"
     "flag.h"
-    "internal/flag.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_config
     absl::flags_handle
+    absl::flags_internal
     absl::flags_marshalling
     absl::flags_registry
     absl::base
@@ -151,10 +195,14 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
     absl::flags_config
     absl::flags
     absl::flags_handle
     absl::flags_internal
+    absl::flags_path_util
+    absl::flags_program_name
+    absl::flags_registry
     absl::strings
     absl::synchronization
 )
@@ -171,6 +219,8 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::core_headers
     absl::flags_usage_internal
     absl::strings
     absl::synchronization
@@ -189,10 +239,13 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
+    absl::config
+    absl::core_headers
     absl::flags_config
     absl::flags
     absl::flags_handle
     absl::flags_internal
+    absl::flags_program_name
     absl::flags_registry
     absl::flags_usage
     absl::strings
@@ -211,6 +264,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
+    absl::flags_config
     absl::flags_handle
     absl::flags_registry
     absl::memory
@@ -239,7 +293,12 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::core_headers
     absl::flags
+    absl::flags_config
+    absl::flags_handle
+    absl::flags_internal
+    absl::flags_registry
     absl::strings
     gtest_main
 )
@@ -265,8 +324,9 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
-    absl::base
     absl::flags_parse
+    absl::flags_registry
+    absl::raw_logging_internal
     absl::scoped_set_env
     absl::span
     absl::strings
@@ -281,7 +341,7 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::flags_internal
+    absl::flags_path_util
     gtest_main
 )
 
@@ -293,7 +353,7 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::flags_internal
+    absl::flags_program_name
     absl::strings
     gtest_main
 )
@@ -307,6 +367,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags
+    absl::flags_handle
+    absl::flags_marshalling
     absl::flags_registry
     absl::memory
     absl::strings
@@ -322,7 +384,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::flags_config
-    absl::flags_internal
+    absl::flags_path_util
+    absl::flags_program_name
     absl::strings
     gtest_main
 )
@@ -337,8 +400,10 @@
   DEPS
     absl::flags_config
     absl::flags
-    absl::flags_internal
+    absl::flags_path_util
+    absl::flags_program_name
     absl::flags_parse
+    absl::flags_registry
     absl::flags_usage
     absl::memory
     absl::strings
diff --git a/absl/flags/config.h b/absl/flags/config.h
index a9fd97a..001f8fe 100644
--- a/absl/flags/config.h
+++ b/absl/flags/config.h
@@ -45,4 +45,23 @@
 #define ABSL_FLAGS_STRIP_HELP ABSL_FLAGS_STRIP_NAMES
 #endif
 
+// ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD macro is used for using atomics with
+// double words, e.g. absl::Duration.
+// For reasons in bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80878, modern
+// versions of GCC do not support cmpxchg16b instruction in standard atomics.
+#ifdef ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD
+#error "ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD should not be defined."
+#elif defined(__clang__) && defined(__x86_64__) && \
+    defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16)
+#define ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD 1
+#endif
+
+// ABSL_FLAGS_INTERNAL_HAS_RTTI macro is used for selecting if we can use RTTI
+// for flag type identification.
+#ifdef ABSL_FLAGS_INTERNAL_HAS_RTTI
+#error ABSL_FLAGS_INTERNAL_HAS_RTTI cannot be directly set
+#elif !defined(__GNUC__) || defined(__GXX_RTTI)
+#define ABSL_FLAGS_INTERNAL_HAS_RTTI 1
+#endif  // !defined(__GNUC__) || defined(__GXX_RTTI)
+
 #endif  // ABSL_FLAGS_CONFIG_H_
diff --git a/absl/flags/declare.h b/absl/flags/declare.h
index 7ef1d43..0f8cc6a 100644
--- a/absl/flags/declare.h
+++ b/absl/flags/declare.h
@@ -25,10 +25,11 @@
 #ifndef ABSL_FLAGS_DECLARE_H_
 #define ABSL_FLAGS_DECLARE_H_
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // absl::Flag<T> represents a flag of type 'T' created by ABSL_FLAG.
@@ -40,10 +41,15 @@
 // Flag
 //
 // Forward declaration of the `absl::Flag` type for use in defining the macro.
+#if defined(_MSC_VER) && !defined(__clang__)
+template <typename T>
+class Flag;
+#else
 template <typename T>
 using Flag = flags_internal::Flag<T>;
+#endif
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // ABSL_DECLARE_FLAG()
diff --git a/absl/flags/flag.cc b/absl/flags/flag.cc
index 0858259..f7a457b 100644
--- a/absl/flags/flag.cc
+++ b/absl/flags/flag.cc
@@ -15,32 +15,26 @@
 
 #include "absl/flags/flag.h"
 
-#include <cstring>
+#include "absl/base/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
-// We want to validate the type mismatch between type definition and
-// declaration. The lock-free implementation does not allow us to do it,
-// so in debug builds we always use the slower implementation, which always
-// validates the type.
-#ifndef NDEBUG
-#define ABSL_FLAGS_ATOMIC_GET(T) \
-  T GetFlag(const absl::Flag<T>& flag) { return flag.Get(); }
-#else
-#define ABSL_FLAGS_ATOMIC_GET(T)         \
-  T GetFlag(const absl::Flag<T>& flag) { \
-    T result;                            \
-    if (flag.AtomicGet(&result)) {       \
-      return result;                     \
-    }                                    \
-    return flag.Get();                   \
-  }
+// This global mutex protects on-demand construction of flag objects in MSVC
+// builds.
+#if defined(_MSC_VER) && !defined(__clang__)
+
+namespace flags_internal {
+
+ABSL_CONST_INIT static absl::Mutex construction_guard(absl::kConstInit);
+
+absl::Mutex* GetGlobalConstructionGuard() { return &construction_guard; }
+
+}  // namespace flags_internal
+
 #endif
 
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_ATOMIC_GET)
-
-#undef ABSL_FLAGS_ATOMIC_GET
-
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/flag.h b/absl/flags/flag.h
index 2892592..cff02c1 100644
--- a/absl/flags/flag.h
+++ b/absl/flags/flag.h
@@ -29,16 +29,21 @@
 #ifndef ABSL_FLAGS_FLAG_H_
 #define ABSL_FLAGS_FLAG_H_
 
+#include <string>
+#include <type_traits>
+
 #include "absl/base/attributes.h"
 #include "absl/base/casts.h"
+#include "absl/base/config.h"
 #include "absl/flags/config.h"
 #include "absl/flags/declare.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/flag.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/flags/marshalling.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Flag
 //
@@ -64,8 +69,103 @@
 //      ABSL_FLAG(int, count, 0, "Count of items to process");
 //
 // No public methods of `absl::Flag<T>` are part of the Abseil Flags API.
+#if !defined(_MSC_VER) || defined(__clang__)
 template <typename T>
 using Flag = flags_internal::Flag<T>;
+#else
+// MSVC debug builds do not implement initialization with constexpr constructors
+// correctly. To work around this we add a level of indirection, so that the
+// class `absl::Flag` contains an `internal::Flag*` (instead of being an alias
+// to that class) and dynamically allocates an instance when necessary. We also
+// forward all calls to internal::Flag methods via trampoline methods. In this
+// setup the `absl::Flag` class does not have constructor and virtual methods,
+// all the data members are public and thus MSVC is able to initialize it at
+// link time. To deal with multiple threads accessing the flag for the first
+// time concurrently we use an atomic boolean indicating if flag object is
+// initialized. We also employ the double-checked locking pattern where the
+// second level of protection is a global Mutex, so if two threads attempt to
+// construct the flag concurrently only one wins.
+// This solution is based on a recomendation here:
+// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html?childToView=648454#comment-648454
+
+namespace flags_internal {
+absl::Mutex* GetGlobalConstructionGuard();
+}  // namespace flags_internal
+
+template <typename T>
+class Flag {
+ public:
+  // No constructor and destructor to ensure this is an aggregate type.
+  // Visual Studio 2015 still requires the constructor for class to be
+  // constexpr initializable.
+#if _MSC_VER <= 1900
+  constexpr Flag(const char* name, const char* filename,
+                 const flags_internal::HelpGenFunc help_gen,
+                 const flags_internal::FlagDfltGenFunc default_value_gen)
+      : name_(name),
+        filename_(filename),
+        help_gen_(help_gen),
+        default_value_gen_(default_value_gen),
+        inited_(false),
+        impl_(nullptr) {}
+#endif
+
+  flags_internal::Flag<T>* GetImpl() const {
+    if (!inited_.load(std::memory_order_acquire)) {
+      absl::MutexLock l(flags_internal::GetGlobalConstructionGuard());
+
+      if (inited_.load(std::memory_order_acquire)) {
+        return impl_;
+      }
+
+      impl_ =
+          new flags_internal::Flag<T>(name_, filename_,
+                                      {flags_internal::FlagHelpMsg(help_gen_),
+                                       flags_internal::FlagHelpKind::kGenFunc},
+                                      default_value_gen_);
+      inited_.store(true, std::memory_order_release);
+    }
+
+    return impl_;
+  }
+
+  // Public methods of `absl::Flag<T>` are NOT part of the Abseil Flags API.
+  // See https://abseil.io/docs/cpp/guides/flags
+  bool IsRetired() const { return GetImpl()->IsRetired(); }
+  bool IsAbseilFlag() const { return GetImpl()->IsAbseilFlag(); }
+  absl::string_view Name() const { return GetImpl()->Name(); }
+  std::string Help() const { return GetImpl()->Help(); }
+  bool IsModified() const { return GetImpl()->IsModified(); }
+  bool IsSpecifiedOnCommandLine() const {
+    return GetImpl()->IsSpecifiedOnCommandLine();
+  }
+  absl::string_view Typename() const { return GetImpl()->Typename(); }
+  std::string Filename() const { return GetImpl()->Filename(); }
+  std::string DefaultValue() const { return GetImpl()->DefaultValue(); }
+  std::string CurrentValue() const { return GetImpl()->CurrentValue(); }
+  template <typename U>
+  inline bool IsOfType() const {
+    return GetImpl()->template IsOfType<U>();
+  }
+  T Get() const { return GetImpl()->Get(); }
+  bool AtomicGet(T* v) const { return GetImpl()->AtomicGet(v); }
+  void Set(const T& v) { GetImpl()->Set(v); }
+  void SetCallback(const flags_internal::FlagCallbackFunc mutation_callback) {
+    GetImpl()->SetCallback(mutation_callback);
+  }
+  void InvokeCallback() { GetImpl()->InvokeCallback(); }
+
+  // The data members are logically private, but they need to be public for
+  // this to be an aggregate type.
+  const char* name_;
+  const char* filename_;
+  const flags_internal::HelpGenFunc help_gen_;
+  const flags_internal::FlagDfltGenFunc default_value_gen_;
+
+  mutable std::atomic<bool> inited_;
+  mutable flags_internal::Flag<T>* impl_;
+};
+#endif
 
 // GetFlag()
 //
@@ -84,23 +184,10 @@
 //   // FLAGS_firstname is a Flag of type `std::string`
 //   std::string first_name = absl::GetFlag(FLAGS_firstname);
 template <typename T>
-T GetFlag(const absl::Flag<T>& flag) {
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE(BIT) \
-  static_assert(                                    \
-      !std::is_same<T, BIT>::value,                 \
-      "Do not specify explicit template parameters to absl::GetFlag");
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_VALIDATE
-
+ABSL_MUST_USE_RESULT T GetFlag(const absl::Flag<T>& flag) {
   return flag.Get();
 }
 
-// Overload for `GetFlag()` for types that support lock-free reads.
-#define ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT(T) \
-  extern T GetFlag(const absl::Flag<T>& flag);
-ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT)
-#undef ABSL_FLAGS_INTERNAL_LOCK_FREE_EXPORT
-
 // SetFlag()
 //
 // Sets the value of an `absl::Flag` to the value `v`. Do not construct an
@@ -122,7 +209,7 @@
   flag->Set(value);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 
@@ -186,13 +273,23 @@
 #if ABSL_FLAGS_STRIP_NAMES
 #define ABSL_FLAG_IMPL_FLAGNAME(txt) ""
 #define ABSL_FLAG_IMPL_FILENAME() ""
+#if !defined(_MSC_VER) || defined(__clang__)
 #define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
   absl::flags_internal::FlagRegistrar<T, false>(&flag)
 #else
+#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
+  absl::flags_internal::FlagRegistrar<T, false>(flag.GetImpl())
+#endif
+#else
 #define ABSL_FLAG_IMPL_FLAGNAME(txt) txt
 #define ABSL_FLAG_IMPL_FILENAME() __FILE__
+#if !defined(_MSC_VER) || defined(__clang__)
 #define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
   absl::flags_internal::FlagRegistrar<T, true>(&flag)
+#else
+#define ABSL_FLAG_IMPL_REGISTRAR(T, flag) \
+  absl::flags_internal::FlagRegistrar<T, true>(flag.GetImpl())
+#endif
 #endif
 
 // ABSL_FLAG_IMPL macro definition conditional on ABSL_FLAGS_STRIP_HELP
@@ -203,9 +300,19 @@
 #define ABSL_FLAG_IMPL_FLAGHELP(txt) txt
 #endif
 
-#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt) \
-  static std::string AbslFlagsWrapHelp##name() {       \
-    return ABSL_FLAG_IMPL_FLAGHELP(txt);               \
+// AbslFlagHelpGenFor##name is used to encapsulate both immediate (method Const)
+// and lazy (method NonConst) evaluation of help message expression. We choose
+// between the two via the call to HelpArg in absl::Flag instantiation below.
+// If help message expression is constexpr evaluable compiler will optimize
+// away this whole struct.
+#define ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, txt)                     \
+  struct AbslFlagHelpGenFor##name {                                        \
+    template <typename T = void>                                           \
+    static constexpr const char* Const() {                                 \
+      return absl::flags_internal::HelpConstexprWrap(                      \
+          ABSL_FLAG_IMPL_FLAGHELP(txt));                                   \
+    }                                                                      \
+    static std::string NonConst() { return ABSL_FLAG_IMPL_FLAGHELP(txt); } \
   }
 
 #define ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
@@ -218,17 +325,30 @@
 // Note: Name of registrar object is not arbitrary. It is used to "grab"
 // global name for FLAGS_no<flag_name> symbol, thus preventing the possibility
 // of defining two flags with names foo and nofoo.
+#if !defined(_MSC_VER) || defined(__clang__)
 #define ABSL_FLAG_IMPL(Type, name, default_value, help)             \
   namespace absl /* block flags in namespaces */ {}                 \
   ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value) \
-  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help)                   \
-  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name(                    \
-      ABSL_FLAG_IMPL_FLAGNAME(#name), &AbslFlagsWrapHelp##name,     \
-      ABSL_FLAG_IMPL_FILENAME(),                                    \
-      &absl::flags_internal::FlagMarshallingOps<Type>,              \
-      &AbslFlagsInitFlag##name);                                    \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help);                  \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                    \
+      ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),    \
+      absl::flags_internal::HelpArg<AbslFlagHelpGenFor##name>(0),   \
+      &AbslFlagsInitFlag##name};                                    \
   extern bool FLAGS_no##name;                                       \
   bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+#else
+// MSVC version uses aggregate initialization. We also do not try to
+// optimize away help wrapper.
+#define ABSL_FLAG_IMPL(Type, name, default_value, help)               \
+  namespace absl /* block flags in namespaces */ {}                   \
+  ABSL_FLAG_IMPL_DECLARE_DEF_VAL_WRAPPER(name, Type, default_value)   \
+  ABSL_FLAG_IMPL_DECLARE_HELP_WRAPPER(name, help);                    \
+  ABSL_CONST_INIT absl::Flag<Type> FLAGS_##name{                      \
+      ABSL_FLAG_IMPL_FLAGNAME(#name), ABSL_FLAG_IMPL_FILENAME(),      \
+      &AbslFlagHelpGenFor##name::NonConst, &AbslFlagsInitFlag##name}; \
+  extern bool FLAGS_no##name;                                         \
+  bool FLAGS_no##name = ABSL_FLAG_IMPL_REGISTRAR(Type, FLAGS_##name)
+#endif
 
 // ABSL_RETIRED_FLAG
 //
diff --git a/absl/flags/flag_benchmark.cc b/absl/flags/flag_benchmark.cc
new file mode 100644
index 0000000..87f7317
--- /dev/null
+++ b/absl/flags/flag_benchmark.cc
@@ -0,0 +1,111 @@
+//
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/flag.h"
+#include "absl/time/time.h"
+#include "absl/types/optional.h"
+#include "benchmark/benchmark.h"
+
+namespace {
+using String = std::string;
+using VectorOfStrings = std::vector<std::string>;
+using AbslDuration = absl::Duration;
+
+// We do not want to take over marshalling for the types absl::optional<int>,
+// absl::optional<std::string> which we do not own. Instead we introduce unique
+// "aliases" to these types, which we do.
+using AbslOptionalInt = absl::optional<int>;
+struct OptionalInt : AbslOptionalInt {
+  using AbslOptionalInt::AbslOptionalInt;
+};
+// Next two functions represent Abseil Flags marshalling for OptionalInt.
+bool AbslParseFlag(absl::string_view src, OptionalInt* flag,
+                   std::string* error) {
+  int val;
+  if (src.empty())
+    flag->reset();
+  else if (!absl::ParseFlag(src, &val, error))
+    return false;
+  *flag = val;
+  return true;
+}
+std::string AbslUnparseFlag(const OptionalInt& flag) {
+  return !flag ? "" : absl::UnparseFlag(*flag);
+}
+
+using AbslOptionalString = absl::optional<std::string>;
+struct OptionalString : AbslOptionalString {
+  using AbslOptionalString::AbslOptionalString;
+};
+// Next two functions represent Abseil Flags marshalling for OptionalString.
+bool AbslParseFlag(absl::string_view src, OptionalString* flag,
+                   std::string* error) {
+  std::string val;
+  if (src.empty())
+    flag->reset();
+  else if (!absl::ParseFlag(src, &val, error))
+    return false;
+  *flag = val;
+  return true;
+}
+std::string AbslUnparseFlag(const OptionalString& flag) {
+  return !flag ? "" : absl::UnparseFlag(*flag);
+}
+
+struct UDT {
+  UDT() = default;
+  UDT(const UDT&) {}
+  UDT& operator=(const UDT&) { return *this; }
+};
+// Next two functions represent Abseil Flags marshalling for UDT.
+bool AbslParseFlag(absl::string_view, UDT*, std::string*) { return true; }
+std::string AbslUnparseFlag(const UDT&) { return ""; }
+
+}  // namespace
+
+#define BENCHMARKED_TYPES(A) \
+  A(bool)                    \
+  A(int16_t)                 \
+  A(uint16_t)                \
+  A(int32_t)                 \
+  A(uint32_t)                \
+  A(int64_t)                 \
+  A(uint64_t)                \
+  A(double)                  \
+  A(float)                   \
+  A(String)                  \
+  A(VectorOfStrings)         \
+  A(OptionalInt)             \
+  A(OptionalString)          \
+  A(AbslDuration)            \
+  A(UDT)
+
+#define FLAG_DEF(T) ABSL_FLAG(T, T##_flag, {}, "");
+
+BENCHMARKED_TYPES(FLAG_DEF)
+
+namespace {
+
+#define BM_GetFlag(T)                                            \
+  void BM_GetFlag_##T(benchmark::State& state) {                 \
+    for (auto _ : state) {                                       \
+      benchmark::DoNotOptimize(absl::GetFlag(FLAGS_##T##_flag)); \
+    }                                                            \
+  }                                                              \
+  BENCHMARK(BM_GetFlag_##T);
+
+BENCHMARKED_TYPES(BM_GetFlag)
+
+}  // namespace
diff --git a/absl/flags/flag_test.cc b/absl/flags/flag_test.cc
index 1bcd7e9..4984d28 100644
--- a/absl/flags/flag_test.cc
+++ b/absl/flags/flag_test.cc
@@ -15,11 +15,25 @@
 
 #include "absl/flags/flag.h"
 
+#include <stdint.h>
+
+#include <cmath>
+#include <string>
+#include <vector>
+
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
+#include "absl/flags/config.h"
+#include "absl/flags/declare.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/internal/registry.h"
+#include "absl/flags/usage_config.h"
 #include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 
 ABSL_DECLARE_FLAG(int64_t, mistyped_int_flag);
 ABSL_DECLARE_FLAG(std::vector<std::string>, mistyped_string_flag);
@@ -28,7 +42,7 @@
 
 namespace flags = absl::flags_internal;
 
-std::string TestHelpMsg() { return "help"; }
+std::string TestHelpMsg() { return "dynamic help"; }
 template <typename T>
 void* TestMakeDflt() {
   return new T{};
@@ -37,20 +51,21 @@
 
 template <typename T>
 bool TestConstructionFor() {
-  constexpr flags::Flag<T> f1("f1", &TestHelpMsg, "file",
-                              &absl::flags_internal::FlagMarshallingOps<T>,
-                              &TestMakeDflt<T>);
+  constexpr flags::FlagHelpArg help_arg{flags::FlagHelpMsg("literal help"),
+                                        flags::FlagHelpKind::kLiteral};
+  constexpr flags::Flag<T> f1("f1", "file", help_arg, &TestMakeDflt<T>);
   EXPECT_EQ(f1.Name(), "f1");
-  EXPECT_EQ(f1.Help(), "help");
+  EXPECT_EQ(f1.Help(), "literal help");
   EXPECT_EQ(f1.Filename(), "file");
 
   ABSL_CONST_INIT static flags::Flag<T> f2(
-      "f2", &TestHelpMsg, "file", &absl::flags_internal::FlagMarshallingOps<T>,
+      "f2", "file",
+      {flags::FlagHelpMsg(&TestHelpMsg), flags::FlagHelpKind::kGenFunc},
       &TestMakeDflt<T>);
   flags::FlagRegistrar<T, false>(&f2).OnUpdate(TestCallback);
 
   EXPECT_EQ(f2.Name(), "f2");
-  EXPECT_EQ(f2.Help(), "help");
+  EXPECT_EQ(f2.Help(), "dynamic help");
   EXPECT_EQ(f2.Filename(), "file");
 
   return true;
@@ -63,7 +78,27 @@
 bool AbslParseFlag(absl::string_view, UDT*, std::string*) { return true; }
 std::string AbslUnparseFlag(const UDT&) { return ""; }
 
-TEST(FlagTest, TestConstruction) {
+class FlagTest : public testing::Test {
+ protected:
+  static void SetUpTestSuite() {
+    // Install a function to normalize filenames before this test is run.
+    absl::FlagsUsageConfig default_config;
+    default_config.normalize_filename = &FlagTest::NormalizeFileName;
+    absl::SetFlagsUsageConfig(default_config);
+  }
+
+ private:
+  static std::string NormalizeFileName(absl::string_view fname) {
+#ifdef _WIN32
+    std::string normalized(fname);
+    std::replace(normalized.begin(), normalized.end(), '\\', '/');
+    fname = normalized;
+#endif
+    return std::string(fname);
+  }
+};
+
+TEST_F(FlagTest, TestConstruction) {
   TestConstructionFor<bool>();
   TestConstructionFor<int16_t>();
   TestConstructionFor<uint16_t>();
@@ -98,7 +133,7 @@
 
 #if !ABSL_FLAGS_STRIP_NAMES
 
-TEST(FlagTest, TestFlagDeclaration) {
+TEST_F(FlagTest, TestFlagDeclaration) {
   // test that we can access flag objects.
   EXPECT_EQ(FLAGS_test_flag_01.Name(), "test_flag_01");
   EXPECT_EQ(FLAGS_test_flag_02.Name(), "test_flag_02");
@@ -133,69 +168,69 @@
 namespace {
 
 #if !ABSL_FLAGS_STRIP_NAMES
-TEST(FlagTest, TestFlagDefinition) {
+TEST_F(FlagTest, TestFlagDefinition) {
   absl::string_view expected_file_name = "absl/flags/flag_test.cc";
 
   EXPECT_EQ(FLAGS_test_flag_01.Name(), "test_flag_01");
   EXPECT_EQ(FLAGS_test_flag_01.Help(), "test flag 01");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_01.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_01.Filename(), expected_file_name))
+      << FLAGS_test_flag_01.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_02.Name(), "test_flag_02");
   EXPECT_EQ(FLAGS_test_flag_02.Help(), "test flag 02");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_02.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_02.Filename(), expected_file_name))
+      << FLAGS_test_flag_02.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_03.Name(), "test_flag_03");
   EXPECT_EQ(FLAGS_test_flag_03.Help(), "test flag 03");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_03.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_03.Filename(), expected_file_name))
+      << FLAGS_test_flag_03.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_04.Name(), "test_flag_04");
   EXPECT_EQ(FLAGS_test_flag_04.Help(), "test flag 04");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_04.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_04.Filename(), expected_file_name))
+      << FLAGS_test_flag_04.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_05.Name(), "test_flag_05");
   EXPECT_EQ(FLAGS_test_flag_05.Help(), "test flag 05");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_05.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_05.Filename(), expected_file_name))
+      << FLAGS_test_flag_05.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_06.Name(), "test_flag_06");
   EXPECT_EQ(FLAGS_test_flag_06.Help(), "test flag 06");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_06.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_06.Filename(), expected_file_name))
+      << FLAGS_test_flag_06.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_07.Name(), "test_flag_07");
   EXPECT_EQ(FLAGS_test_flag_07.Help(), "test flag 07");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_07.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_07.Filename(), expected_file_name))
+      << FLAGS_test_flag_07.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_08.Name(), "test_flag_08");
   EXPECT_EQ(FLAGS_test_flag_08.Help(), "test flag 08");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_08.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_08.Filename(), expected_file_name))
+      << FLAGS_test_flag_08.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_09.Name(), "test_flag_09");
   EXPECT_EQ(FLAGS_test_flag_09.Help(), "test flag 09");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_09.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_09.Filename(), expected_file_name))
+      << FLAGS_test_flag_09.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_10.Name(), "test_flag_10");
   EXPECT_EQ(FLAGS_test_flag_10.Help(), "test flag 10");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_10.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_10.Filename(), expected_file_name))
+      << FLAGS_test_flag_10.Filename();
 
   EXPECT_EQ(FLAGS_test_flag_11.Name(), "test_flag_11");
   EXPECT_EQ(FLAGS_test_flag_11.Help(), "test flag 11");
-  EXPECT_TRUE(
-      absl::EndsWith(FLAGS_test_flag_11.Filename(), expected_file_name));
+  EXPECT_TRUE(absl::EndsWith(FLAGS_test_flag_11.Filename(), expected_file_name))
+      << FLAGS_test_flag_11.Filename();
 }
 #endif  // !ABSL_FLAGS_STRIP_NAMES
 
 // --------------------------------------------------------------------
 
-TEST(FlagTest, TestDefault) {
+TEST_F(FlagTest, TestDefault) {
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), true);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_02), 1234);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_03), -34);
@@ -211,7 +246,7 @@
 
 // --------------------------------------------------------------------
 
-TEST(FlagTest, TestGetSet) {
+TEST_F(FlagTest, TestGetSet) {
   absl::SetFlag(&FLAGS_test_flag_01, false);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_01), false);
 
@@ -258,7 +293,7 @@
 
 namespace {
 
-TEST(FlagTest, TestNonConstexprDefault) {
+TEST_F(FlagTest, TestNonConstexprDefault) {
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_12), 1);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_13), "AAABBB");
 }
@@ -272,7 +307,7 @@
 namespace {
 
 #if !ABSL_FLAGS_STRIP_HELP
-TEST(FlagTest, TestNonConstexprHelp) {
+TEST_F(FlagTest, TestNonConstexprHelp) {
   EXPECT_EQ(FLAGS_test_flag_14.Help(), "test flag 14");
 }
 #endif  //! ABSL_FLAGS_STRIP_HELP
@@ -296,7 +331,7 @@
 void TestFlagCB() { cb_test_value = absl::GetFlag(FLAGS_test_flag_with_cb); }
 
 // Tests side-effects of callback invocation.
-TEST(FlagTest, CallbackInvocation) {
+TEST_F(FlagTest, CallbackInvocation) {
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_with_cb), 100);
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_with_lambda_cb), 200);
   EXPECT_EQ(cb_test_value, 300);
@@ -343,7 +378,7 @@
 
 namespace {
 
-TEST(FlagTest, TestCustomUDT) {
+TEST_F(FlagTest, TestCustomUDT) {
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(1, 1));
   absl::SetFlag(&FLAGS_test_flag_15, CustomUDT(2, 3));
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_15), CustomUDT(2, 3));
@@ -351,19 +386,20 @@
 
 // MSVC produces link error on the type mismatch.
 // Linux does not have build errors and validations work as expected.
-#if 0  // !defined(_WIN32) && GTEST_HAS_DEATH_TEST
+#if !defined(_WIN32) && GTEST_HAS_DEATH_TEST
 
-TEST(Flagtest, TestTypeMismatchValidations) {
-  // For builtin types, GetFlag() only does validation in debug mode.
+using FlagDeathTest = FlagTest;
+
+TEST_F(FlagDeathTest, TestTypeMismatchValidations) {
   EXPECT_DEBUG_DEATH(
-      absl::GetFlag(FLAGS_mistyped_int_flag),
+      static_cast<void>(absl::GetFlag(FLAGS_mistyped_int_flag)),
       "Flag 'mistyped_int_flag' is defined as one type and declared "
       "as another");
-  EXPECT_DEATH(absl::SetFlag(&FLAGS_mistyped_int_flag, 0),
+  EXPECT_DEATH(absl::SetFlag(&FLAGS_mistyped_int_flag, 1),
                "Flag 'mistyped_int_flag' is defined as one type and declared "
                "as another");
 
-  EXPECT_DEATH(absl::GetFlag(FLAGS_mistyped_string_flag),
+  EXPECT_DEATH(static_cast<void>(absl::GetFlag(FLAGS_mistyped_string_flag)),
                "Flag 'mistyped_string_flag' is defined as one type and "
                "declared as another");
   EXPECT_DEATH(
@@ -409,7 +445,7 @@
 
 namespace {
 
-TEST(FlagTest, CanSetViaImplicitConversion) {
+TEST_F(FlagTest, CanSetViaImplicitConversion) {
   EXPECT_EQ(absl::GetFlag(FLAGS_test_flag_16).a, 10);
   absl::SetFlag(&FLAGS_test_flag_16,
                 ConversionTestVal::ViaImplicitConv::kEleven);
@@ -447,7 +483,7 @@
 
 namespace {
 
-TEST(FlagTest, TestNonDefaultConstructibleType) {
+TEST_F(FlagTest, TestNonDefaultConstructibleType) {
   EXPECT_EQ(absl::GetFlag(FLAGS_ndc_flag1).value, '1' + 100);
   EXPECT_EQ(absl::GetFlag(FLAGS_ndc_flag2).value, 0);
 
@@ -468,7 +504,7 @@
 
 namespace {
 
-TEST(FlagTest, TestRetiredFlagRegistration) {
+TEST_F(FlagTest, TestRetiredFlagRegistration) {
   bool is_bool = false;
   EXPECT_TRUE(flags::IsRetiredFlag("old_bool_flag", &is_bool));
   EXPECT_TRUE(is_bool);
diff --git a/absl/flags/flag_test_defs.cc b/absl/flags/flag_test_defs.cc
index 3366c58..49f91de 100644
--- a/absl/flags/flag_test_defs.cc
+++ b/absl/flags/flag_test_defs.cc
@@ -20,3 +20,5 @@
 
 ABSL_FLAG(int, mistyped_int_flag, 0, "");
 ABSL_FLAG(std::string, mistyped_string_flag, "", "");
+ABSL_RETIRED_FLAG(bool, old_bool_flag, true,
+                  "repetition of retired flag definition");
diff --git a/absl/flags/internal/commandlineflag.cc b/absl/flags/internal/commandlineflag.cc
deleted file mode 100644
index f964165..0000000
--- a/absl/flags/internal/commandlineflag.cc
+++ /dev/null
@@ -1,496 +0,0 @@
-//
-// Copyright 2019 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include "absl/flags/internal/commandlineflag.h"
-
-#include <cassert>
-
-#include "absl/base/internal/raw_logging.h"
-#include "absl/base/optimization.h"
-#include "absl/flags/config.h"
-#include "absl/flags/usage_config.h"
-#include "absl/strings/str_cat.h"
-#include "absl/synchronization/mutex.h"
-
-namespace absl {
-inline namespace lts_2019_08_08 {
-namespace flags_internal {
-
-// The help message indicating that the commandline flag has been
-// 'stripped'. It will not show up when doing "-help" and its
-// variants. The flag is stripped if ABSL_FLAGS_STRIP_HELP is set to 1
-// before including absl/flags/flag.h
-
-// This is used by this file, and also in commandlineflags_reporting.cc
-const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";
-
-namespace {
-
-// Currently we only validate flag values for user-defined flag types.
-bool ShouldValidateFlagValue(const CommandLineFlag& flag) {
-#define DONT_VALIDATE(T) \
-  if (flag.IsOfType<T>()) return false;
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(DONT_VALIDATE)
-  DONT_VALIDATE(std::string)
-  DONT_VALIDATE(std::vector<std::string>)
-#undef DONT_VALIDATE
-
-  return true;
-}
-
-}  // namespace
-
-absl::Mutex* InitFlag(CommandLineFlag* flag) {
-  ABSL_CONST_INIT static absl::Mutex init_lock(absl::kConstInit);
-  absl::Mutex* mu;
-
-  {
-    absl::MutexLock lock(&init_lock);
-
-    if (flag->locks == nullptr) {  // Must initialize Mutexes for this flag.
-      flag->locks = new flags_internal::CommandLineFlagLocks;
-    }
-
-    mu = &flag->locks->primary_mu;
-  }
-
-  {
-    absl::MutexLock lock(mu);
-
-    if (!flag->retired && flag->def == nullptr) {
-      // Need to initialize def and cur fields.
-      flag->def = (*flag->make_init_value)();
-      flag->cur = Clone(flag->op, flag->def);
-      UpdateCopy(flag);
-      flag->inited.store(true, std::memory_order_release);
-      flag->InvokeCallback();
-    }
-  }
-
-  flag->inited.store(true, std::memory_order_release);
-  return mu;
-}
-
-// Ensure that the lazily initialized fields of *flag have been initialized,
-// and return &flag->locks->primary_mu.
-absl::Mutex* CommandLineFlag::InitFlagIfNecessary() const
-    LOCK_RETURNED(locks->primary_mu) {
-  if (!this->inited.load(std::memory_order_acquire)) {
-    return InitFlag(const_cast<CommandLineFlag*>(this));
-  }
-
-  // All fields initialized; this->locks is therefore safe to read.
-  return &this->locks->primary_mu;
-}
-
-void CommandLineFlag::Destroy() const {
-  // Values are heap allocated for retired and Abseil Flags.
-  if (IsRetired() || IsAbseilFlag()) {
-    if (this->cur) Delete(this->op, this->cur);
-    if (this->def) Delete(this->op, this->def);
-  }
-
-  delete this->locks;
-}
-
-bool CommandLineFlag::IsModified() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-  return modified;
-}
-
-void CommandLineFlag::SetModified(bool is_modified) {
-  absl::MutexLock l(InitFlagIfNecessary());
-  modified = is_modified;
-}
-
-bool CommandLineFlag::IsSpecifiedOnCommandLine() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-  return on_command_line;
-}
-
-absl::string_view CommandLineFlag::Typename() const {
-  // We do not store/report type in Abseil Flags, so that user do not rely on in
-  // at runtime
-  if (IsAbseilFlag() || IsRetired()) return "";
-
-#define HANDLE_V1_BUILTIN_TYPE(t) \
-  if (IsOfType<t>()) {            \
-    return #t;                    \
-  }
-
-  HANDLE_V1_BUILTIN_TYPE(bool);
-  HANDLE_V1_BUILTIN_TYPE(int32_t);
-  HANDLE_V1_BUILTIN_TYPE(int64_t);
-  HANDLE_V1_BUILTIN_TYPE(uint64_t);
-  HANDLE_V1_BUILTIN_TYPE(double);
-#undef HANDLE_V1_BUILTIN_TYPE
-
-  if (IsOfType<std::string>()) {
-    return "string";
-  }
-
-  return "";
-}
-
-std::string CommandLineFlag::Filename() const {
-  return flags_internal::GetUsageConfig().normalize_filename(this->filename);
-}
-
-std::string CommandLineFlag::DefaultValue() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  return Unparse(this->marshalling_op, this->def);
-}
-
-std::string CommandLineFlag::CurrentValue() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  return Unparse(this->marshalling_op, this->cur);
-}
-
-bool CommandLineFlag::HasValidatorFn() const {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  return this->validator != nullptr;
-}
-
-bool CommandLineFlag::SetValidatorFn(FlagValidator fn) {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  // ok to register the same function over and over again
-  if (fn == this->validator) return true;
-
-  // Can't set validator to a different function, unless reset first.
-  if (fn != nullptr && this->validator != nullptr) {
-    ABSL_INTERNAL_LOG(
-        WARNING, absl::StrCat("Ignoring SetValidatorFn() for flag '", Name(),
-                              "': validate-fn already registered"));
-
-    return false;
-  }
-
-  this->validator = fn;
-  return true;
-}
-
-bool CommandLineFlag::InvokeValidator(const void* value) const
-    EXCLUSIVE_LOCKS_REQUIRED(this->locks->primary_mu) {
-  if (!this->validator) {
-    return true;
-  }
-
-  (void)value;
-
-  ABSL_INTERNAL_LOG(
-      FATAL,
-      absl::StrCat("Flag '", Name(),
-                   "' of encapsulated type should not have a validator"));
-
-  return false;
-}
-
-void CommandLineFlag::SetCallback(
-    const flags_internal::FlagCallback mutation_callback) {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  callback = mutation_callback;
-
-  InvokeCallback();
-}
-
-// If the flag has a mutation callback this function invokes it. While the
-// callback is being invoked the primary flag's mutex is unlocked and it is
-// re-locked back after call to callback is completed. Callback invocation is
-// guarded by flag's secondary mutex instead which prevents concurrent callback
-// invocation. Note that it is possible for other thread to grab the primary
-// lock and update flag's value at any time during the callback invocation.
-// This is by design. Callback can get a value of the flag if necessary, but it
-// might be different from the value initiated the callback and it also can be
-// different by the time the callback invocation is completed.
-// Requires that *primary_lock be held in exclusive mode; it may be released
-// and reacquired by the implementation.
-void CommandLineFlag::InvokeCallback()
-    EXCLUSIVE_LOCKS_REQUIRED(this->locks->primary_mu) {
-  if (!this->callback) return;
-
-  // The callback lock is guaranteed initialized, because *locks->primary_mu
-  // exists.
-  absl::Mutex* callback_mu = &this->locks->callback_mu;
-
-  // When executing the callback we need the primary flag's mutex to be unlocked
-  // so that callback can retrieve the flag's value.
-  this->locks->primary_mu.Unlock();
-
-  {
-    absl::MutexLock lock(callback_mu);
-    this->callback();
-  }
-
-  this->locks->primary_mu.Lock();
-}
-
-// Attempts to parse supplied `value` string using parsing routine in the `flag`
-// argument. If parsing is successful, it will try to validate that the parsed
-// value is valid for the specified 'flag'. Finally this function stores the
-// parsed value in 'dst' assuming it is a pointer to the flag's value type. In
-// case if any error is encountered in either step, the error message is stored
-// in 'err'
-bool TryParseLocked(CommandLineFlag* flag, void* dst, absl::string_view value,
-                    std::string* err)
-    EXCLUSIVE_LOCKS_REQUIRED(flag->locks->primary_mu) {
-  void* tentative_value = Clone(flag->op, flag->def);
-  std::string parse_err;
-  if (!Parse(flag->marshalling_op, value, tentative_value, &parse_err)) {
-    auto type_name = flag->Typename();
-    absl::string_view err_sep = parse_err.empty() ? "" : "; ";
-    absl::string_view typename_sep = type_name.empty() ? "" : " ";
-    *err = absl::StrCat("Illegal value '", value, "' specified for",
-                        typename_sep, type_name, " flag '", flag->Name(), "'",
-                        err_sep, parse_err);
-    Delete(flag->op, tentative_value);
-    return false;
-  }
-
-  if (!flag->InvokeValidator(tentative_value)) {
-    *err = absl::StrCat("Failed validation of new value '",
-                        Unparse(flag->marshalling_op, tentative_value),
-                        "' for flag '", flag->Name(), "'");
-    Delete(flag->op, tentative_value);
-    return false;
-  }
-
-  flag->counter++;
-  Copy(flag->op, tentative_value, dst);
-  Delete(flag->op, tentative_value);
-  return true;
-}
-
-// Sets the value of the flag based on specified string `value`. If the flag
-// was successfully set to new value, it returns true. Otherwise, sets `err`
-// to indicate the error, leaves the flag unchanged, and returns false. There
-// are three ways to set the flag's value:
-//  * Update the current flag value
-//  * Update the flag's default value
-//  * Update the current flag value if it was never set before
-// The mode is selected based on 'set_mode' parameter.
-bool CommandLineFlag::SetFromString(absl::string_view value,
-                                    FlagSettingMode set_mode,
-                                    ValueSource source, std::string* err) {
-  if (IsRetired()) return false;
-
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  // Direct-access flags can be modified without going through the
-  // flag API. Detect such changes and update the flag->modified bit.
-  if (!IsAbseilFlag()) {
-    if (!this->modified && ChangedDirectly(this, this->cur, this->def)) {
-      this->modified = true;
-    }
-  }
-
-  switch (set_mode) {
-    case SET_FLAGS_VALUE: {
-      // set or modify the flag's value
-      if (!TryParseLocked(this, this->cur, value, err)) return false;
-      this->modified = true;
-      UpdateCopy(this);
-      InvokeCallback();
-
-      if (source == kCommandLine) {
-        this->on_command_line = true;
-      }
-      break;
-    }
-    case SET_FLAG_IF_DEFAULT: {
-      // set the flag's value, but only if it hasn't been set by someone else
-      if (!this->modified) {
-        if (!TryParseLocked(this, this->cur, value, err)) return false;
-        this->modified = true;
-        UpdateCopy(this);
-        InvokeCallback();
-      } else {
-        // TODO(rogeeff): review and fix this semantic. Currently we do not fail
-        // in this case if flag is modified. This is misleading since the flag's
-        // value is not updated even though we return true.
-        // *err = absl::StrCat(this->Name(), " is already set to ",
-        //                     CurrentValue(), "\n");
-        // return false;
-        return true;
-      }
-      break;
-    }
-    case SET_FLAGS_DEFAULT: {
-      // modify the flag's default-value
-      if (!TryParseLocked(this, this->def, value, err)) return false;
-
-      if (!this->modified) {
-        // Need to set both defvalue *and* current, in this case
-        Copy(this->op, this->def, this->cur);
-        UpdateCopy(this);
-        InvokeCallback();
-      }
-      break;
-    }
-    default: {
-      // unknown set_mode
-      assert(false);
-      return false;
-    }
-  }
-
-  return true;
-}
-
-void CommandLineFlag::StoreAtomic(size_t size) {
-  int64_t t = 0;
-  assert(size <= sizeof(int64_t));
-  memcpy(&t, this->cur, size);
-  this->atomic.store(t, std::memory_order_release);
-}
-
-void CommandLineFlag::CheckDefaultValueParsingRoundtrip() const {
-  std::string v = DefaultValue();
-
-  absl::MutexLock lock(InitFlagIfNecessary());
-
-  void* dst = Clone(this->op, this->def);
-  std::string error;
-  if (!flags_internal::Parse(this->marshalling_op, v, dst, &error)) {
-    ABSL_INTERNAL_LOG(
-        FATAL,
-        absl::StrCat("Flag ", Name(), " (from ", Filename(),
-                     "): std::string form of default value '", v,
-                     "' could not be parsed; error=", error));
-  }
-
-  // We do not compare dst to def since parsing/unparsing may make
-  // small changes, e.g., precision loss for floating point types.
-  Delete(this->op, dst);
-}
-
-bool CommandLineFlag::ValidateDefaultValue() const {
-  absl::MutexLock lock(InitFlagIfNecessary());
-  return InvokeValidator(this->def);
-}
-
-bool CommandLineFlag::ValidateInputValue(absl::string_view value) const {
-  absl::MutexLock l(InitFlagIfNecessary());  // protect default value access
-
-  void* obj = Clone(this->op, this->def);
-  std::string ignored_error;
-  const bool result =
-      flags_internal::Parse(this->marshalling_op, value, obj, &ignored_error) &&
-      InvokeValidator(obj);
-  Delete(this->op, obj);
-  return result;
-}
-
-const int64_t CommandLineFlag::kAtomicInit;
-
-void CommandLineFlag::Read(void* dst,
-                           const flags_internal::FlagOpFn dst_op) const {
-  absl::ReaderMutexLock l(InitFlagIfNecessary());
-
-  // `dst_op` is the unmarshaling operation corresponding to the declaration
-  // visibile at the call site. `op` is the Flag's defined unmarshalling
-  // operation. They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(dst_op != op)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", name,
-                     "' is defined as one type and declared as another"));
-  }
-  CopyConstruct(op, cur, dst);
-}
-
-void CommandLineFlag::Write(const void* src,
-                            const flags_internal::FlagOpFn src_op) {
-  absl::MutexLock l(InitFlagIfNecessary());
-
-  // `src_op` is the marshalling operation corresponding to the declaration
-  // visible at the call site. `op` is the Flag's defined marshalling operation.
-  // They must match for this operation to be well-defined.
-  if (ABSL_PREDICT_FALSE(src_op != op)) {
-    ABSL_INTERNAL_LOG(
-        ERROR,
-        absl::StrCat("Flag '", name,
-                     "' is defined as one type and declared as another"));
-  }
-
-  if (ShouldValidateFlagValue(*this)) {
-    void* obj = Clone(op, src);
-    std::string ignored_error;
-    std::string src_as_str = Unparse(marshalling_op, src);
-    if (!Parse(marshalling_op, src_as_str, obj, &ignored_error) ||
-        !InvokeValidator(obj)) {
-      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", name,
-                                            "' to invalid value ", src_as_str));
-    }
-    Delete(op, obj);
-  }
-
-  modified = true;
-  counter++;
-  Copy(op, src, cur);
-
-  UpdateCopy(this);
-  InvokeCallback();
-}
-
-std::string HelpText::GetHelpText() const {
-  if (help_function_) return help_function_();
-  if (help_message_) return help_message_;
-
-  return {};
-}
-
-// Update any copy of the flag value that is stored in an atomic word.
-// In addition if flag has a mutation callback this function invokes it.
-void UpdateCopy(CommandLineFlag* flag) {
-#define STORE_ATOMIC(T)           \
-  else if (flag->IsOfType<T>()) { \
-    flag->StoreAtomic(sizeof(T)); \
-  }
-
-  if (false) {
-  }
-  ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(STORE_ATOMIC)
-#undef STORE_ATOMIC
-}
-
-// Return true iff flag value was changed via direct-access.
-bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b) {
-  if (!flag->IsAbseilFlag()) {
-// Need to compare values for direct-access flags.
-#define CHANGED_FOR_TYPE(T)                                                  \
-  if (flag->IsOfType<T>()) {                                                 \
-    return *reinterpret_cast<const T*>(a) != *reinterpret_cast<const T*>(b); \
-  }
-
-    CHANGED_FOR_TYPE(bool);
-    CHANGED_FOR_TYPE(int32_t);
-    CHANGED_FOR_TYPE(int64_t);
-    CHANGED_FOR_TYPE(uint64_t);
-    CHANGED_FOR_TYPE(double);
-    CHANGED_FOR_TYPE(std::string);
-#undef CHANGED_FOR_TYPE
-  }
-
-  return false;
-}
-
-}  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
-}  // namespace absl
diff --git a/absl/flags/internal/commandlineflag.h b/absl/flags/internal/commandlineflag.h
index 382553d..6363c66 100644
--- a/absl/flags/internal/commandlineflag.h
+++ b/absl/flags/internal/commandlineflag.h
@@ -16,30 +16,41 @@
 #ifndef ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
 #define ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
 
-#include <atomic>
+#include <stddef.h>
+#include <stdint.h>
 
+#include <memory>
+#include <string>
+#include <typeinfo>
+
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
+#include "absl/flags/config.h"
 #include "absl/flags/marshalling.h"
-#include "absl/synchronization/mutex.h"
+#include "absl/strings/string_view.h"
 #include "absl/types/optional.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// Type-specific operations, eg., parsing, copying, etc. are provided
-// by function specific to that type with a signature matching FlagOpFn.
-enum FlagOp {
-  kDelete,
-  kClone,
-  kCopy,
-  kCopyConstruct,
-  kSizeof,
-  kParse,
-  kUnparse
-};
-using FlagOpFn = void* (*)(FlagOp, const void*, void*);
-using FlagMarshallingOpFn = void* (*)(FlagOp, const void*, void*, void*);
+// An alias for flag static type id. Values of type identify the flag value type
+// simialarly to typeid(T), but without relying on RTTI being available. In most
+// cases this id is enough to uniquely identify the flag's value type. In a few
+// cases we'll have to resort to using actual RTTI implementation if it is
+// available.
+using FlagStaticTypeId = void* (*)();
+
+// Address of this function template is used in current implementation as a flag
+// static type id.
+template <typename T>
+void* FlagStaticTypeIdGen() {
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+  return const_cast<std::type_info*>(&typeid(T));
+#else
+  return nullptr;
+#endif
+}
 
 // Options that control SetCommandLineOptionWithMode.
 enum FlagSettingMode {
@@ -62,215 +73,95 @@
   kProgrammaticChange,
 };
 
-// Signature for the help generation function used as an argument for the
-// absl::Flag constructor.
-using HelpGenFunc = std::string (*)();
-
-// Signature for the function generating the initial flag value based (usually
-// based on default value supplied in flag's definition)
-using InitialValGenFunc = void* (*)();
-
-struct CommandLineFlagInfo;
-
-// Signature for the mutation callback used by watched Flags
-// The callback is noexcept.
-// TODO(rogeeff): add noexcept after C++17 support is added.
-using FlagCallback = void (*)();
-
-using FlagValidator = bool (*)();
-
-extern const char kStrippedFlagHelp[];
-
-// The per-type function
-template <typename T>
-void* FlagOps(FlagOp op, const void* v1, void* v2) {
-  switch (op) {
-    case kDelete:
-      delete static_cast<const T*>(v1);
-      return nullptr;
-    case kClone:
-      return new T(*static_cast<const T*>(v1));
-    case kCopy:
-      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
-      return nullptr;
-    case kCopyConstruct:
-      new (v2) T(*static_cast<const T*>(v1));
-      return nullptr;
-    case kSizeof:
-      return reinterpret_cast<void*>(sizeof(T));
-    default:
-      return nullptr;
-  }
-}
-
-template <typename T>
-void* FlagMarshallingOps(FlagOp op, const void* v1, void* v2, void* v3) {
-  switch (op) {
-    case kParse: {
-      // initialize the temporary instance of type T based on current value in
-      // destination (which is going to be flag's default value).
-      T temp(*static_cast<T*>(v2));
-      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
-                              static_cast<std::string*>(v3))) {
-        return nullptr;
-      }
-      *static_cast<T*>(v2) = std::move(temp);
-      return v2;
-    }
-    case kUnparse:
-      *static_cast<std::string*>(v2) =
-          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
-      return nullptr;
-    default:
-      return nullptr;
-  }
-}
-
-// Functions that invoke flag-type-specific operations.
-inline void Delete(FlagOpFn op, const void* obj) {
-  op(flags_internal::kDelete, obj, nullptr);
-}
-
-inline void* Clone(FlagOpFn op, const void* obj) {
-  return op(flags_internal::kClone, obj, nullptr);
-}
-
-inline void Copy(FlagOpFn op, const void* src, void* dst) {
-  op(flags_internal::kCopy, src, dst);
-}
-
-inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
-  op(flags_internal::kCopyConstruct, src, dst);
-}
-
-inline bool Parse(FlagMarshallingOpFn op, absl::string_view text, void* dst,
-                  std::string* error) {
-  return op(flags_internal::kParse, &text, dst, error) != nullptr;
-}
-
-inline std::string Unparse(FlagMarshallingOpFn op, const void* val) {
-  std::string result;
-  op(flags_internal::kUnparse, val, &result, nullptr);
-  return result;
-}
-
-inline size_t Sizeof(FlagOpFn op) {
-  // This sequence of casts reverses the sequence from base::internal::FlagOps()
-  return static_cast<size_t>(reinterpret_cast<intptr_t>(
-      op(flags_internal::kSizeof, nullptr, nullptr)));
-}
-
-// The following struct contains the locks in a CommandLineFlag struct.
-// They are in a separate struct that is lazily allocated to avoid problems
-// with static initialization and to avoid multiple allocations.
-struct CommandLineFlagLocks {
-  absl::Mutex primary_mu;   // protects several fields in CommandLineFlag
-  absl::Mutex callback_mu;  // used to serialize callbacks
-};
-
-// Holds either a pointer to help text or a function which produces it.  This is
-// needed for supporting both static initialization of Flags while supporting
-// the legacy registration framework.  We can't use absl::variant<const char*,
-// const char*(*)()> since anybody passing 0 or nullptr in to a CommandLineFlag
-// would find an ambiguity.
-class HelpText {
+// Handle to FlagState objects. Specific flag state objects will restore state
+// of a flag produced this flag state from method CommandLineFlag::SaveState().
+class FlagStateInterface {
  public:
-  static constexpr HelpText FromFunctionPointer(const HelpGenFunc fn) {
-    return HelpText(fn, nullptr);
-  }
-  static constexpr HelpText FromStaticCString(const char* msg) {
-    return HelpText(nullptr, msg);
-  }
+  virtual ~FlagStateInterface() {}
 
-  std::string GetHelpText() const;
-
-  HelpText() = delete;
-  HelpText(const HelpText&) = default;
-  HelpText(HelpText&&) = default;
-
- private:
-  explicit constexpr HelpText(const HelpGenFunc fn, const char* msg)
-      : help_function_(fn), help_message_(msg) {}
-
-  HelpGenFunc help_function_;
-  const char* help_message_;
+  // Restores the flag originated this object to the saved state.
+  virtual void Restore() const = 0;
 };
 
 // Holds all information for a flag.
-struct CommandLineFlag {
-  constexpr CommandLineFlag(
-      const char* name_arg, HelpText help_text, const char* filename_arg,
-      const flags_internal::FlagOpFn op_arg,
-      const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
-      const flags_internal::InitialValGenFunc initial_value_gen,
-      const bool retired_arg, void* def_arg, void* cur_arg)
-      : name(name_arg),
-        help(help_text),
-        filename(filename_arg),
-        op(op_arg),
-        marshalling_op(marshalling_op_arg),
-        make_init_value(initial_value_gen),
-        retired(retired_arg),
-        inited(false),
-        modified(false),
-        on_command_line(false),
-        validator(nullptr),
-        callback(nullptr),
-        def(def_arg),
-        cur(cur_arg),
-        counter(0),
-        atomic(kAtomicInit),
-        locks(nullptr) {}
-
-  // Revert the init routine.
-  void Destroy() const;
+class CommandLineFlag {
+ public:
+  constexpr CommandLineFlag() = default;
 
   // Not copyable/assignable.
   CommandLineFlag(const CommandLineFlag&) = delete;
   CommandLineFlag& operator=(const CommandLineFlag&) = delete;
 
-  absl::string_view Name() const { return name; }
-  std::string Help() const { return help.GetHelpText(); }
-  bool IsRetired() const { return this->retired; }
-  bool IsModified() const;
-  void SetModified(bool is_modified);
-  bool IsSpecifiedOnCommandLine() const;
-  // Returns true iff this is a handle to an Abseil Flag.
-  bool IsAbseilFlag() const {
-    // Set to null for V1 flags
-    return this->make_init_value != nullptr;
-  }
-
-  absl::string_view Typename() const;
-  std::string Filename() const;
-  std::string DefaultValue() const;
-  std::string CurrentValue() const;
-
-  bool HasValidatorFn() const;
-  bool SetValidatorFn(FlagValidator fn);
-  bool InvokeValidator(const void* value) const;
+  // Non-polymorphic access methods.
 
   // Return true iff flag has type T.
   template <typename T>
   inline bool IsOfType() const {
-    return this->op == &flags_internal::FlagOps<T>;
+    return TypeId() == &flags_internal::FlagStaticTypeIdGen<T>;
   }
 
   // Attempts to retrieve the flag value. Returns value on success,
   // absl::nullopt otherwise.
   template <typename T>
   absl::optional<T> Get() const {
-    if (IsRetired() || flags_internal::FlagOps<T> != this->op)
+    if (IsRetired() || !IsOfType<T>()) {
       return absl::nullopt;
+    }
 
-    T res;
-    Read(&res, flags_internal::FlagOps<T>);
+    // Implementation notes:
+    //
+    // We are wrapping a union around the value of `T` to serve three purposes:
+    //
+    //  1. `U.value` has correct size and alignment for a value of type `T`
+    //  2. The `U.value` constructor is not invoked since U's constructor does
+    //     not do it explicitly.
+    //  3. The `U.value` destructor is invoked since U's destructor does it
+    //     explicitly. This makes `U` a kind of RAII wrapper around non default
+    //     constructible value of T, which is destructed when we leave the
+    //     scope. We do need to destroy U.value, which is constructed by
+    //     CommandLineFlag::Read even though we left it in a moved-from state
+    //     after std::move.
+    //
+    // All of this serves to avoid requiring `T` being default constructible.
+    union U {
+      T value;
+      U() {}
+      ~U() { value.~T(); }
+    };
+    U u;
 
-    return res;
+    Read(&u.value);
+    return std::move(u.value);
   }
 
-  void SetCallback(const flags_internal::FlagCallback mutation_callback);
-  void InvokeCallback();
+  // Polymorphic access methods
+
+  // Returns name of this flag.
+  virtual absl::string_view Name() const = 0;
+  // Returns name of the file where this flag is defined.
+  virtual std::string Filename() const = 0;
+  // Returns name of the flag's value type for some built-in types or empty
+  // std::string.
+  virtual absl::string_view Typename() const = 0;
+  // Returns help message associated with this flag.
+  virtual std::string Help() const = 0;
+  // Returns true iff this object corresponds to retired flag.
+  virtual bool IsRetired() const { return false; }
+  // Returns true iff this is a handle to an Abseil Flag.
+  virtual bool IsAbseilFlag() const { return true; }
+  // Returns id of the flag's value type.
+  virtual FlagStaticTypeId TypeId() const = 0;
+  virtual bool IsModified() const = 0;
+  virtual bool IsSpecifiedOnCommandLine() const = 0;
+  virtual std::string DefaultValue() const = 0;
+  virtual std::string CurrentValue() const = 0;
+
+  // Interfaces to operate on validators.
+  virtual bool ValidateInputValue(absl::string_view value) const = 0;
+
+  // Interface to save flag to some persistent state. Returns current flag state
+  // or nullptr if flag does not support saving and restoring a state.
+  virtual std::unique_ptr<FlagStateInterface> SaveState() = 0;
 
   // Sets the value of the flag based on specified std::string `value`. If the flag
   // was successfully set to new value, it returns true. Otherwise, sets `error`
@@ -280,106 +171,43 @@
   //  * Update the flag's default value
   //  * Update the current flag value if it was never set before
   // The mode is selected based on `set_mode` parameter.
-  bool SetFromString(absl::string_view value,
-                     flags_internal::FlagSettingMode set_mode,
-                     flags_internal::ValueSource source, std::string* error);
+  virtual bool SetFromString(absl::string_view value,
+                             flags_internal::FlagSettingMode set_mode,
+                             flags_internal::ValueSource source,
+                             std::string* error) = 0;
 
-  void StoreAtomic(size_t size);
-
-  void CheckDefaultValueParsingRoundtrip() const;
-  // Invoke the flag validators for old flags.
-  // TODO(rogeeff): implement proper validators for Abseil Flags
-  bool ValidateDefaultValue() const;
-  bool ValidateInputValue(absl::string_view value) const;
-
-  // Constant configuration for a particular flag.
- private:
-  const char* const name;
-  const HelpText help;
-  const char* const filename;
+  // Checks that flags default value can be converted to std::string and back to the
+  // flag's value type.
+  virtual void CheckDefaultValueParsingRoundtrip() const = 0;
 
  protected:
-  const FlagOpFn op;                         // Type-specific handler
-  const FlagMarshallingOpFn marshalling_op;  // Marshalling ops handler
-  const InitialValGenFunc make_init_value;   // Makes initial value for the flag
-  const bool retired;                        // Is the flag retired?
-  std::atomic<bool> inited;  // fields have been lazily initialized
+  ~CommandLineFlag() = default;
 
-  // Mutable state (guarded by locks->primary_mu).
-  bool modified;            // Has flag value been modified?
-  bool on_command_line;     // Specified on command line.
-  FlagValidator validator;  // Validator function, or nullptr
-  FlagCallback callback;    // Mutation callback, or nullptr
-  void* def;                // Lazily initialized pointer to default value
-  void* cur;                // Lazily initialized pointer to current value
-  int64_t counter;            // Mutation counter
-
-  // For some types, a copy of the current value is kept in an atomically
-  // accessible field.
-  static const int64_t kAtomicInit = 0xababababababababll;
-  std::atomic<int64_t> atomic;
-
-  // Lazily initialized mutexes for this flag value.  We cannot inline a
-  // SpinLock or Mutex here because those have non-constexpr constructors and
-  // so would prevent constant initialization of this type.
-  // TODO(rogeeff): fix it once Mutex has constexpr constructor
-  struct CommandLineFlagLocks* locks;  // locks, laziliy allocated.
-
-  // Ensure that the lazily initialized fields of *flag have been initialized,
-  // and return the lock which should be locked when flag's state is mutated.
-  absl::Mutex* InitFlagIfNecessary() const;
-
-  // copy construct new value of flag's type in a memory referenced by dst
-  // based on current flag's value
-  void Read(void* dst, const flags_internal::FlagOpFn dst_op) const;
-  // updates flag's value to *src (locked)
-  void Write(const void* src, const flags_internal::FlagOpFn src_op);
-
-  friend class FlagRegistry;
-  friend class FlagPtrMap;
-  friend class FlagSaverImpl;
-  friend void FillCommandLineFlagInfo(CommandLineFlag* flag,
-                                      CommandLineFlagInfo* result);
-  friend bool TryParseLocked(CommandLineFlag* flag, void* dst,
-                             absl::string_view value, std::string* err);
-  friend absl::Mutex* InitFlag(CommandLineFlag* flag);
+ private:
+  // Copy-construct a new value of the flag's type in a memory referenced by
+  // the dst based on the current flag's value.
+  virtual void Read(void* dst) const = 0;
 };
 
-// Update any copy of the flag value that is stored in an atomic word.
-// In addition if flag has a mutation callback this function invokes it. While
-// callback is being invoked the primary flag's mutex is unlocked and it is
-// re-locked back after call to callback is completed. Callback invocation is
-// guarded by flag's secondary mutex instead which prevents concurrent callback
-// invocation. Note that it is possible for other thread to grab the primary
-// lock and update flag's value at any time during the callback invocation.
-// This is by design. Callback can get a value of the flag if necessary, but it
-// might be different from the value initiated the callback and it also can be
-// different by the time the callback invocation is completed.
-// Requires that *primary_lock be held in exclusive mode; it may be released
-// and reacquired by the implementation.
-void UpdateCopy(CommandLineFlag* flag);
-// Return true iff flag value was changed via direct-access.
-bool ChangedDirectly(CommandLineFlag* flag, const void* a, const void* b);
-
-// This macro is the "source of truth" for the list of supported flag types we
-// expect to perform lock free operations on. Specifically it generates code,
-// a one argument macro operating on a type, supplied as a macro argument, for
-// each type in the list.
-#define ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(A) \
-  A(bool)                                         \
-  A(short)                                        \
-  A(unsigned short)                               \
-  A(int)                                          \
-  A(unsigned int)                                 \
-  A(long)                                         \
-  A(unsigned long)                                \
-  A(long long)                                    \
-  A(unsigned long long)                           \
-  A(double)                                       \
-  A(float)
+// This macro is the "source of truth" for the list of supported flag built-in
+// types.
+#define ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(A) \
+  A(bool)                                    \
+  A(short)                                   \
+  A(unsigned short)                          \
+  A(int)                                     \
+  A(unsigned int)                            \
+  A(long)                                    \
+  A(unsigned long)                           \
+  A(long long)                               \
+  A(unsigned long long)                      \
+  A(double)                                  \
+  A(float)                                   \
+  A(std::string)                             \
+  A(std::vector<std::string>)
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_COMMANDLINEFLAG_H_
diff --git a/absl/flags/internal/commandlineflag_test.cc b/absl/flags/internal/commandlineflag_test.cc
index f0d57ad..0e8bc31 100644
--- a/absl/flags/internal/commandlineflag_test.cc
+++ b/absl/flags/internal/commandlineflag_test.cc
@@ -15,12 +15,17 @@
 
 #include "absl/flags/internal/commandlineflag.h"
 
+#include <memory>
+#include <string>
+
 #include "gtest/gtest.h"
 #include "absl/flags/flag.h"
 #include "absl/flags/internal/registry.h"
+#include "absl/flags/usage_config.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 ABSL_FLAG(int, int_flag, 201, "int_flag help");
 ABSL_FLAG(std::string, string_flag, "dflt",
@@ -33,10 +38,26 @@
 
 class CommandLineFlagTest : public testing::Test {
  protected:
+  static void SetUpTestSuite() {
+    // Install a function to normalize filenames before this test is run.
+    absl::FlagsUsageConfig default_config;
+    default_config.normalize_filename = &CommandLineFlagTest::NormalizeFileName;
+    absl::SetFlagsUsageConfig(default_config);
+  }
+
   void SetUp() override { flag_saver_ = absl::make_unique<flags::FlagSaver>(); }
   void TearDown() override { flag_saver_.reset(); }
 
  private:
+  static std::string NormalizeFileName(absl::string_view fname) {
+#ifdef _WIN32
+    std::string normalized(fname);
+    std::replace(normalized.begin(), normalized.end(), '\\', '/');
+    fname = normalized;
+#endif
+    return std::string(fname);
+  }
+
   std::unique_ptr<flags::FlagSaver> flag_saver_;
 };
 
@@ -49,9 +70,10 @@
   EXPECT_EQ(flag_01->Typename(), "");
   EXPECT_TRUE(!flag_01->IsRetired());
   EXPECT_TRUE(flag_01->IsOfType<int>());
-  EXPECT_TRUE(absl::EndsWith(
-      flag_01->Filename(),
-      "absl/flags/internal/commandlineflag_test.cc"));
+  EXPECT_TRUE(
+      absl::EndsWith(flag_01->Filename(),
+                     "absl/flags/internal/commandlineflag_test.cc"))
+      << flag_01->Filename();
 
   auto* flag_02 = flags::FindCommandLineFlag("string_flag");
 
@@ -61,9 +83,10 @@
   EXPECT_EQ(flag_02->Typename(), "");
   EXPECT_TRUE(!flag_02->IsRetired());
   EXPECT_TRUE(flag_02->IsOfType<std::string>());
-  EXPECT_TRUE(absl::EndsWith(
-      flag_02->Filename(),
-      "absl/flags/internal/commandlineflag_test.cc"));
+  EXPECT_TRUE(
+      absl::EndsWith(flag_02->Filename(),
+                     "absl/flags/internal/commandlineflag_test.cc"))
+      << flag_02->Filename();
 
   auto* flag_03 = flags::FindRetiredFlag("bool_retired_flag");
 
diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
new file mode 100644
index 0000000..5a921e2
--- /dev/null
+++ b/absl/flags/internal/flag.cc
@@ -0,0 +1,382 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/flags/internal/flag.h"
+
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <atomic>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/optimization.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/usage_config.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace flags_internal {
+
+// The help message indicating that the commandline flag has been
+// 'stripped'. It will not show up when doing "-help" and its
+// variants. The flag is stripped if ABSL_FLAGS_STRIP_HELP is set to 1
+// before including absl/flags/flag.h
+const char kStrippedFlagHelp[] = "\001\002\003\004 (unknown) \004\003\002\001";
+
+namespace {
+
+// Currently we only validate flag values for user-defined flag types.
+bool ShouldValidateFlagValue(FlagStaticTypeId flag_type_id) {
+#define DONT_VALIDATE(T) \
+  if (flag_type_id == &FlagStaticTypeIdGen<T>) return false;
+  ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(DONT_VALIDATE)
+#undef DONT_VALIDATE
+
+  return true;
+}
+
+// RAII helper used to temporarily unlock and relock `absl::Mutex`.
+// This is used when we need to ensure that locks are released while
+// invoking user supplied callbacks and then reacquired, since callbacks may
+// need to acquire these locks themselves.
+class MutexRelock {
+ public:
+  explicit MutexRelock(absl::Mutex* mu) : mu_(mu) { mu_->Unlock(); }
+  ~MutexRelock() { mu_->Lock(); }
+
+  MutexRelock(const MutexRelock&) = delete;
+  MutexRelock& operator=(const MutexRelock&) = delete;
+
+ private:
+  absl::Mutex* mu_;
+};
+
+}  // namespace
+
+void FlagImpl::Init() {
+  new (&data_guard_) absl::Mutex;
+
+  absl::MutexLock lock(reinterpret_cast<absl::Mutex*>(&data_guard_));
+
+  value_.dynamic = MakeInitValue().release();
+  StoreAtomic();
+}
+
+// Ensures that the lazily initialized data is initialized,
+// and returns pointer to the mutex guarding flags data.
+absl::Mutex* FlagImpl::DataGuard() const {
+  absl::call_once(const_cast<FlagImpl*>(this)->init_control_, &FlagImpl::Init,
+                  const_cast<FlagImpl*>(this));
+
+  // data_guard_ is initialized.
+  return reinterpret_cast<absl::Mutex*>(&data_guard_);
+}
+
+void FlagImpl::AssertValidType(FlagStaticTypeId type_id) const {
+  FlagStaticTypeId this_type_id = flags_internal::StaticTypeId(op_);
+
+  // `type_id` is the type id corresponding to the declaration visibile at the
+  // call site. `this_type_id` is the type id corresponding to the type stored
+  // during flag definition. They must match for this operation to be
+  // well-defined.
+  if (ABSL_PREDICT_TRUE(type_id == this_type_id)) return;
+
+  void* lhs_runtime_type_id = type_id();
+  void* rhs_runtime_type_id = this_type_id();
+
+  if (lhs_runtime_type_id == rhs_runtime_type_id) return;
+
+#if defined(ABSL_FLAGS_INTERNAL_HAS_RTTI)
+  if (*reinterpret_cast<std::type_info*>(lhs_runtime_type_id) ==
+      *reinterpret_cast<std::type_info*>(rhs_runtime_type_id))
+    return;
+#endif
+
+  ABSL_INTERNAL_LOG(
+      FATAL, absl::StrCat("Flag '", Name(),
+                          "' is defined as one type and declared as another"));
+}
+
+std::unique_ptr<void, DynValueDeleter> FlagImpl::MakeInitValue() const {
+  void* res = nullptr;
+  if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
+    res = flags_internal::Clone(op_, default_value_.dynamic_value);
+  } else {
+    res = (*default_value_.gen_func)();
+  }
+  return {res, DynValueDeleter{op_}};
+}
+
+void FlagImpl::StoreValue(const void* src) {
+  flags_internal::Copy(op_, src, value_.dynamic);
+  StoreAtomic();
+  modified_ = true;
+  ++counter_;
+  InvokeCallback();
+}
+
+absl::string_view FlagImpl::Name() const { return name_; }
+
+std::string FlagImpl::Filename() const {
+  return flags_internal::GetUsageConfig().normalize_filename(filename_);
+}
+
+std::string FlagImpl::Help() const {
+  return HelpSourceKind() == FlagHelpKind::kLiteral ? help_.literal
+                                                    : help_.gen_func();
+}
+
+bool FlagImpl::IsModified() const {
+  absl::MutexLock l(DataGuard());
+  return modified_;
+}
+
+bool FlagImpl::IsSpecifiedOnCommandLine() const {
+  absl::MutexLock l(DataGuard());
+  return on_command_line_;
+}
+
+std::string FlagImpl::DefaultValue() const {
+  absl::MutexLock l(DataGuard());
+
+  auto obj = MakeInitValue();
+  return flags_internal::Unparse(op_, obj.get());
+}
+
+std::string FlagImpl::CurrentValue() const {
+  absl::MutexLock l(DataGuard());
+
+  return flags_internal::Unparse(op_, value_.dynamic);
+}
+
+void FlagImpl::SetCallback(const FlagCallbackFunc mutation_callback) {
+  absl::MutexLock l(DataGuard());
+
+  if (callback_ == nullptr) {
+    callback_ = new FlagCallback;
+  }
+  callback_->func = mutation_callback;
+
+  InvokeCallback();
+}
+
+void FlagImpl::InvokeCallback() const {
+  if (!callback_) return;
+
+  // Make a copy of the C-style function pointer that we are about to invoke
+  // before we release the lock guarding it.
+  FlagCallbackFunc cb = callback_->func;
+
+  // If the flag has a mutation callback this function invokes it. While the
+  // callback is being invoked the primary flag's mutex is unlocked and it is
+  // re-locked back after call to callback is completed. Callback invocation is
+  // guarded by flag's secondary mutex instead which prevents concurrent
+  // callback invocation. Note that it is possible for other thread to grab the
+  // primary lock and update flag's value at any time during the callback
+  // invocation. This is by design. Callback can get a value of the flag if
+  // necessary, but it might be different from the value initiated the callback
+  // and it also can be different by the time the callback invocation is
+  // completed. Requires that *primary_lock be held in exclusive mode; it may be
+  // released and reacquired by the implementation.
+  MutexRelock relock(DataGuard());
+  absl::MutexLock lock(&callback_->guard);
+  cb();
+}
+
+bool FlagImpl::RestoreState(const void* value, bool modified,
+                            bool on_command_line, int64_t counter) {
+  {
+    absl::MutexLock l(DataGuard());
+
+    if (counter_ == counter) return false;
+  }
+
+  Write(value);
+
+  {
+    absl::MutexLock l(DataGuard());
+
+    modified_ = modified;
+    on_command_line_ = on_command_line;
+  }
+
+  return true;
+}
+
+// Attempts to parse supplied `value` string using parsing routine in the `flag`
+// argument. If parsing successful, this function replaces the dst with newly
+// parsed value. In case if any error is encountered in either step, the error
+// message is stored in 'err'
+std::unique_ptr<void, DynValueDeleter> FlagImpl::TryParse(
+    absl::string_view value, std::string* err) const {
+  std::unique_ptr<void, DynValueDeleter> tentative_value = MakeInitValue();
+
+  std::string parse_err;
+  if (!flags_internal::Parse(op_, value, tentative_value.get(), &parse_err)) {
+    absl::string_view err_sep = parse_err.empty() ? "" : "; ";
+    *err = absl::StrCat("Illegal value '", value, "' specified for flag '",
+                        Name(), "'", err_sep, parse_err);
+    return nullptr;
+  }
+
+  return tentative_value;
+}
+
+void FlagImpl::Read(void* dst) const {
+  absl::ReaderMutexLock l(DataGuard());
+
+  flags_internal::CopyConstruct(op_, value_.dynamic, dst);
+}
+
+void FlagImpl::StoreAtomic() {
+  size_t data_size = flags_internal::Sizeof(op_);
+
+  if (data_size <= sizeof(int64_t)) {
+    int64_t t = 0;
+    std::memcpy(&t, value_.dynamic, data_size);
+    value_.atomics.small_atomic.store(t, std::memory_order_release);
+  }
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+  else if (data_size <= sizeof(FlagsInternalTwoWordsType)) {
+    FlagsInternalTwoWordsType t{0, 0};
+    std::memcpy(&t, value_.dynamic, data_size);
+    value_.atomics.big_atomic.store(t, std::memory_order_release);
+  }
+#endif
+}
+
+void FlagImpl::Write(const void* src) {
+  absl::MutexLock l(DataGuard());
+
+  if (ShouldValidateFlagValue(flags_internal::StaticTypeId(op_))) {
+    std::unique_ptr<void, DynValueDeleter> obj{flags_internal::Clone(op_, src),
+                                               DynValueDeleter{op_}};
+    std::string ignored_error;
+    std::string src_as_str = flags_internal::Unparse(op_, src);
+    if (!flags_internal::Parse(op_, src_as_str, obj.get(), &ignored_error)) {
+      ABSL_INTERNAL_LOG(ERROR, absl::StrCat("Attempt to set flag '", Name(),
+                                            "' to invalid value ", src_as_str));
+    }
+  }
+
+  StoreValue(src);
+}
+
+// Sets the value of the flag based on specified string `value`. If the flag
+// was successfully set to new value, it returns true. Otherwise, sets `err`
+// to indicate the error, leaves the flag unchanged, and returns false. There
+// are three ways to set the flag's value:
+//  * Update the current flag value
+//  * Update the flag's default value
+//  * Update the current flag value if it was never set before
+// The mode is selected based on 'set_mode' parameter.
+bool FlagImpl::SetFromString(absl::string_view value, FlagSettingMode set_mode,
+                             ValueSource source, std::string* err) {
+  absl::MutexLock l(DataGuard());
+
+  switch (set_mode) {
+    case SET_FLAGS_VALUE: {
+      // set or modify the flag's value
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get());
+
+      if (source == kCommandLine) {
+        on_command_line_ = true;
+      }
+      break;
+    }
+    case SET_FLAG_IF_DEFAULT: {
+      // set the flag's value, but only if it hasn't been set by someone else
+      if (modified_) {
+        // TODO(rogeeff): review and fix this semantic. Currently we do not fail
+        // in this case if flag is modified. This is misleading since the flag's
+        // value is not updated even though we return true.
+        // *err = absl::StrCat(Name(), " is already set to ",
+        //                     CurrentValue(), "\n");
+        // return false;
+        return true;
+      }
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      StoreValue(tentative_value.get());
+      break;
+    }
+    case SET_FLAGS_DEFAULT: {
+      auto tentative_value = TryParse(value, err);
+      if (!tentative_value) return false;
+
+      if (DefaultKind() == FlagDefaultKind::kDynamicValue) {
+        void* old_value = default_value_.dynamic_value;
+        default_value_.dynamic_value = tentative_value.release();
+        tentative_value.reset(old_value);
+      } else {
+        default_value_.dynamic_value = tentative_value.release();
+        def_kind_ = static_cast<uint8_t>(FlagDefaultKind::kDynamicValue);
+      }
+
+      if (!modified_) {
+        // Need to set both default value *and* current, in this case
+        StoreValue(default_value_.dynamic_value);
+        modified_ = false;
+      }
+      break;
+    }
+  }
+
+  return true;
+}
+
+void FlagImpl::CheckDefaultValueParsingRoundtrip() const {
+  std::string v = DefaultValue();
+
+  absl::MutexLock lock(DataGuard());
+
+  auto dst = MakeInitValue();
+  std::string error;
+  if (!flags_internal::Parse(op_, v, dst.get(), &error)) {
+    ABSL_INTERNAL_LOG(
+        FATAL,
+        absl::StrCat("Flag ", Name(), " (from ", Filename(),
+                     "): std::string form of default value '", v,
+                     "' could not be parsed; error=", error));
+  }
+
+  // We do not compare dst to def since parsing/unparsing may make
+  // small changes, e.g., precision loss for floating point types.
+}
+
+bool FlagImpl::ValidateInputValue(absl::string_view value) const {
+  absl::MutexLock l(DataGuard());
+
+  auto obj = MakeInitValue();
+  std::string ignored_error;
+  return flags_internal::Parse(op_, value, obj.get(), &ignored_error);
+}
+
+}  // namespace flags_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index 9a5d9b4..35a148c 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -16,45 +16,547 @@
 #ifndef ABSL_FLAGS_INTERNAL_FLAG_H_
 #define ABSL_FLAGS_INTERNAL_FLAG_H_
 
+#include <stdint.h>
+
+#include <atomic>
+#include <cstring>
+#include <memory>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/call_once.h"
+#include "absl/base/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/registry.h"
+#include "absl/memory/memory.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+#include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// This is "unspecified" implementation of absl::Flag<T> type.
 template <typename T>
-class Flag : public flags_internal::CommandLineFlag {
+class Flag;
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag value type operations, eg., parsing, copying, etc. are provided
+// by function specific to that type with a signature matching FlagOpFn.
+
+enum class FlagOp {
+  kDelete,
+  kClone,
+  kCopy,
+  kCopyConstruct,
+  kSizeof,
+  kStaticTypeId,
+  kParse,
+  kUnparse,
+};
+using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*);
+
+// Flag value specific operations routine.
+template <typename T>
+void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) {
+  switch (op) {
+    case FlagOp::kDelete:
+      delete static_cast<const T*>(v1);
+      return nullptr;
+    case FlagOp::kClone:
+      return new T(*static_cast<const T*>(v1));
+    case FlagOp::kCopy:
+      *static_cast<T*>(v2) = *static_cast<const T*>(v1);
+      return nullptr;
+    case FlagOp::kCopyConstruct:
+      new (v2) T(*static_cast<const T*>(v1));
+      return nullptr;
+    case FlagOp::kSizeof:
+      return reinterpret_cast<void*>(sizeof(T));
+    case FlagOp::kStaticTypeId:
+      return reinterpret_cast<void*>(&FlagStaticTypeIdGen<T>);
+    case FlagOp::kParse: {
+      // Initialize the temporary instance of type T based on current value in
+      // destination (which is going to be flag's default value).
+      T temp(*static_cast<T*>(v2));
+      if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp,
+                              static_cast<std::string*>(v3))) {
+        return nullptr;
+      }
+      *static_cast<T*>(v2) = std::move(temp);
+      return v2;
+    }
+    case FlagOp::kUnparse:
+      *static_cast<std::string*>(v2) =
+          absl::UnparseFlag<T>(*static_cast<const T*>(v1));
+      return nullptr;
+    default:
+      return nullptr;
+  }
+}
+
+// Deletes memory interpreting obj as flag value type pointer.
+inline void Delete(FlagOpFn op, const void* obj) {
+  op(FlagOp::kDelete, obj, nullptr, nullptr);
+}
+// Makes a copy of flag value pointed by obj.
+inline void* Clone(FlagOpFn op, const void* obj) {
+  return op(FlagOp::kClone, obj, nullptr, nullptr);
+}
+// Copies src to dst interpreting as flag value type pointers.
+inline void Copy(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopy, src, dst, nullptr);
+}
+// Construct a copy of flag value in a location pointed by dst
+// based on src - pointer to the flag's value.
+inline void CopyConstruct(FlagOpFn op, const void* src, void* dst) {
+  op(FlagOp::kCopyConstruct, src, dst, nullptr);
+}
+// Returns true if parsing of input text is successfull.
+inline bool Parse(FlagOpFn op, absl::string_view text, void* dst,
+                  std::string* error) {
+  return op(FlagOp::kParse, &text, dst, error) != nullptr;
+}
+// Returns string representing supplied value.
+inline std::string Unparse(FlagOpFn op, const void* val) {
+  std::string result;
+  op(FlagOp::kUnparse, val, &result, nullptr);
+  return result;
+}
+// Returns size of flag value type.
+inline size_t Sizeof(FlagOpFn op) {
+  // This sequence of casts reverses the sequence from
+  // `flags_internal::FlagOps()`
+  return static_cast<size_t>(reinterpret_cast<intptr_t>(
+      op(FlagOp::kSizeof, nullptr, nullptr, nullptr)));
+}
+// Returns static type id coresponding to the value type.
+inline FlagStaticTypeId StaticTypeId(FlagOpFn op) {
+  return reinterpret_cast<FlagStaticTypeId>(
+      op(FlagOp::kStaticTypeId, nullptr, nullptr, nullptr));
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Persistent state of the flag data.
+
+template <typename T>
+class FlagState : public flags_internal::FlagStateInterface {
  public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
-                 const char* filename,
-                 const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
-                 const flags_internal::InitialValGenFunc initial_value_gen)
-      : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-            filename, &flags_internal::FlagOps<T>, marshalling_op_arg,
-            initial_value_gen,
-            /*retired_arg=*/false, /*def_arg=*/nullptr,
-            /*cur_arg=*/nullptr) {}
+  FlagState(Flag<T>* flag, T&& cur, bool modified, bool on_command_line,
+            int64_t counter)
+      : flag_(flag),
+        cur_value_(std::move(cur)),
+        modified_(modified),
+        on_command_line_(on_command_line),
+        counter_(counter) {}
+
+  ~FlagState() override = default;
+
+ private:
+  friend class Flag<T>;
+
+  // Restores the flag to the saved state.
+  void Restore() const override;
+
+  // Flag and saved flag data.
+  Flag<T>* flag_;
+  T cur_value_;
+  bool modified_;
+  bool on_command_line_;
+  int64_t counter_;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag help auxiliary structs.
+
+// This is help argument for absl::Flag encapsulating the string literal pointer
+// or pointer to function generating it as well as enum descriminating two
+// cases.
+using HelpGenFunc = std::string (*)();
+
+union FlagHelpMsg {
+  constexpr explicit FlagHelpMsg(const char* help_msg) : literal(help_msg) {}
+  constexpr explicit FlagHelpMsg(HelpGenFunc help_gen) : gen_func(help_gen) {}
+
+  const char* literal;
+  HelpGenFunc gen_func;
+};
+
+enum class FlagHelpKind : uint8_t { kLiteral = 0, kGenFunc = 1 };
+
+struct FlagHelpArg {
+  FlagHelpMsg source;
+  FlagHelpKind kind;
+};
+
+extern const char kStrippedFlagHelp[];
+
+// HelpConstexprWrap is used by struct AbslFlagHelpGenFor##name generated by
+// ABSL_FLAG macro. It is only used to silence the compiler in the case where
+// help message expression is not constexpr and does not have type const char*.
+// If help message expression is indeed constexpr const char* HelpConstexprWrap
+// is just a trivial identity function.
+template <typename T>
+const char* HelpConstexprWrap(const T&) {
+  return nullptr;
+}
+constexpr const char* HelpConstexprWrap(const char* p) { return p; }
+constexpr const char* HelpConstexprWrap(char* p) { return p; }
+
+// These two HelpArg overloads allows us to select at compile time one of two
+// way to pass Help argument to absl::Flag. We'll be passing
+// AbslFlagHelpGenFor##name as T and integer 0 as a single argument to prefer
+// first overload if possible. If T::Const is evaluatable on constexpr
+// context (see non template int parameter below) we'll choose first overload.
+// In this case the help message expression is immediately evaluated and is used
+// to construct the absl::Flag. No additionl code is generated by ABSL_FLAG.
+// Otherwise SFINAE kicks in and first overload is dropped from the
+// consideration, in which case the second overload will be used. The second
+// overload does not attempt to evaluate the help message expression
+// immediately and instead delays the evaluation by returing the function
+// pointer (&T::NonConst) genering the help message when necessary. This is
+// evaluatable in constexpr context, but the cost is an extra function being
+// generated in the ABSL_FLAG code.
+template <typename T, int = (T::Const(), 1)>
+constexpr FlagHelpArg HelpArg(int) {
+  return {FlagHelpMsg(T::Const()), FlagHelpKind::kLiteral};
+}
+
+template <typename T>
+constexpr FlagHelpArg HelpArg(char) {
+  return {FlagHelpMsg(&T::NonConst), FlagHelpKind::kGenFunc};
+}
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag default value auxiliary structs.
+
+// Signature for the function generating the initial flag value (usually
+// based on default value supplied in flag's definition)
+using FlagDfltGenFunc = void* (*)();
+
+union FlagDefaultSrc {
+  constexpr explicit FlagDefaultSrc(FlagDfltGenFunc gen_func_arg)
+      : gen_func(gen_func_arg) {}
+
+  void* dynamic_value;
+  FlagDfltGenFunc gen_func;
+};
+
+enum class FlagDefaultKind : uint8_t { kDynamicValue = 0, kGenFunc = 1 };
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag current value auxiliary structs.
+
+// The minimum atomic size we believe to generate lock free code, i.e. all
+// trivially copyable types not bigger this size generate lock free code.
+static constexpr int kMinLockFreeAtomicSize = 8;
+
+// The same as kMinLockFreeAtomicSize but maximum atomic size. As double words
+// might use two registers, we want to dispatch the logic for them.
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+static constexpr int kMaxLockFreeAtomicSize = 16;
+#else
+static constexpr int kMaxLockFreeAtomicSize = 8;
+#endif
+
+// We can use atomic in cases when it fits in the register, trivially copyable
+// in order to make memcpy operations.
+template <typename T>
+struct IsAtomicFlagTypeTrait {
+  static constexpr bool value =
+      (sizeof(T) <= kMaxLockFreeAtomicSize &&
+       type_traits_internal::is_trivially_copyable<T>::value);
+};
+
+// Clang does not always produce cmpxchg16b instruction when alignment of a 16
+// bytes type is not 16.
+struct alignas(16) FlagsInternalTwoWordsType {
+  int64_t first;
+  int64_t second;
+};
+
+constexpr bool operator==(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return that.first == other.first && that.second == other.second;
+}
+constexpr bool operator!=(const FlagsInternalTwoWordsType& that,
+                          const FlagsInternalTwoWordsType& other) {
+  return !(that == other);
+}
+
+constexpr int64_t SmallAtomicInit() { return 0xababababababababll; }
+
+template <typename T, typename S = void>
+struct BestAtomicType {
+  using type = int64_t;
+  static constexpr int64_t AtomicInit() { return SmallAtomicInit(); }
+};
+
+template <typename T>
+struct BestAtomicType<
+    T, typename std::enable_if<(kMinLockFreeAtomicSize < sizeof(T) &&
+                                sizeof(T) <= kMaxLockFreeAtomicSize),
+                               void>::type> {
+  using type = FlagsInternalTwoWordsType;
+  static constexpr FlagsInternalTwoWordsType AtomicInit() {
+    return {SmallAtomicInit(), SmallAtomicInit()};
+  }
+};
+
+struct FlagValue {
+  // Heap allocated value.
+  void* dynamic = nullptr;
+  // For some types, a copy of the current value is kept in an atomically
+  // accessible field.
+  union Atomics {
+    // Using small atomic for small types.
+    std::atomic<int64_t> small_atomic;
+    template <typename T,
+              typename K = typename std::enable_if<
+                  (sizeof(T) <= kMinLockFreeAtomicSize), void>::type>
+    int64_t load() const {
+      return small_atomic.load(std::memory_order_acquire);
+    }
+
+#if defined(ABSL_FLAGS_INTERNAL_ATOMIC_DOUBLE_WORD)
+    // Using big atomics for big types.
+    std::atomic<FlagsInternalTwoWordsType> big_atomic;
+    template <typename T, typename K = typename std::enable_if<
+                              (kMinLockFreeAtomicSize < sizeof(T) &&
+                               sizeof(T) <= kMaxLockFreeAtomicSize),
+                              void>::type>
+    FlagsInternalTwoWordsType load() const {
+      return big_atomic.load(std::memory_order_acquire);
+    }
+    constexpr Atomics()
+        : big_atomic{FlagsInternalTwoWordsType{SmallAtomicInit(),
+                                               SmallAtomicInit()}} {}
+#else
+    constexpr Atomics() : small_atomic{SmallAtomicInit()} {}
+#endif
+  };
+  Atomics atomics{};
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag callback auxiliary structs.
+
+// Signature for the mutation callback used by watched Flags
+// The callback is noexcept.
+// TODO(rogeeff): add noexcept after C++17 support is added.
+using FlagCallbackFunc = void (*)();
+
+struct FlagCallback {
+  FlagCallbackFunc func;
+  absl::Mutex guard;  // Guard for concurrent callback invocations.
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Flag implementation, which does not depend on flag value type.
+// The class encapsulates the Flag's data and access to it.
+
+struct DynValueDeleter {
+  explicit DynValueDeleter(FlagOpFn op_arg = nullptr) : op(op_arg) {}
+  void operator()(void* ptr) const {
+    if (op != nullptr) Delete(op, ptr);
+  }
+
+  FlagOpFn op;
+};
+
+class FlagImpl {
+ public:
+  constexpr FlagImpl(const char* name, const char* filename, FlagOpFn op,
+                     FlagHelpArg help, FlagDfltGenFunc default_value_gen)
+      : name_(name),
+        filename_(filename),
+        op_(op),
+        help_(help.source),
+        help_source_kind_(static_cast<uint8_t>(help.kind)),
+        def_kind_(static_cast<uint8_t>(FlagDefaultKind::kGenFunc)),
+        modified_(false),
+        on_command_line_(false),
+        counter_(0),
+        callback_(nullptr),
+        default_value_(default_value_gen),
+        data_guard_{} {}
+
+  // Constant access methods
+  absl::string_view Name() const;
+  std::string Filename() const;
+  std::string Help() const;
+  bool IsModified() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool IsSpecifiedOnCommandLine() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string DefaultValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  std::string CurrentValue() const ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void Read(void* dst) const ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  template <typename T, typename std::enable_if<
+                            !IsAtomicFlagTypeTrait<T>::value, int>::type = 0>
+  void Get(T* dst) const {
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+    Read(dst);
+  }
+  // Overload for `GetFlag()` for types that support lock-free reads.
+  template <typename T, typename std::enable_if<IsAtomicFlagTypeTrait<T>::value,
+                                                int>::type = 0>
+  void Get(T* dst) const {
+    // For flags of types which can be accessed "atomically" we want to avoid
+    // slowing down flag value access due to type validation. That's why
+    // this validation is hidden behind !NDEBUG
+#ifndef NDEBUG
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+#endif
+    using U = flags_internal::BestAtomicType<T>;
+    typename U::type r = value_.atomics.template load<T>();
+    if (r != U::AtomicInit()) {
+      std::memcpy(static_cast<void*>(dst), &r, sizeof(T));
+    } else {
+      Read(dst);
+    }
+  }
+  template <typename T>
+  void Set(const T& src) {
+    AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>);
+    Write(&src);
+  }
+
+  // Mutating access methods
+  void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool SetFromString(absl::string_view value, FlagSettingMode set_mode,
+                     ValueSource source, std::string* err)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  // If possible, updates copy of the Flag's value that is stored in an
+  // atomic word.
+  void StoreAtomic() ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  // Interfaces to operate on callbacks.
+  void SetCallback(const FlagCallbackFunc mutation_callback)
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  void InvokeCallback() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  // Interfaces to save/restore mutable flag data
+  template <typename T>
+  std::unique_ptr<FlagStateInterface> SaveState(Flag<T>* flag) const
+      ABSL_LOCKS_EXCLUDED(*DataGuard()) {
+    T&& cur_value = flag->Get();
+    absl::MutexLock l(DataGuard());
+
+    return absl::make_unique<FlagState<T>>(
+        flag, std::move(cur_value), modified_, on_command_line_, counter_);
+  }
+  bool RestoreState(const void* value, bool modified, bool on_command_line,
+                    int64_t counter) ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+  // Value validation interfaces.
+  void CheckDefaultValueParsingRoundtrip() const
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+  bool ValidateInputValue(absl::string_view value) const
+      ABSL_LOCKS_EXCLUDED(*DataGuard());
+
+ private:
+  // Ensures that `data_guard_` is initialized and returns it.
+  absl::Mutex* DataGuard() const ABSL_LOCK_RETURNED((absl::Mutex*)&data_guard_);
+  // Returns heap allocated value of type T initialized with default value.
+  std::unique_ptr<void, DynValueDeleter> MakeInitValue() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+  // Flag initialization called via absl::call_once.
+  void Init();
+  // Attempts to parse supplied `value` std::string. If parsing is successful,
+  // returns new value. Otherwise returns nullptr.
+  std::unique_ptr<void, DynValueDeleter> TryParse(absl::string_view value,
+                                                  std::string* err) const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+  // Stores the flag value based on the pointer to the source.
+  void StoreValue(const void* src) ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard());
+
+  FlagHelpKind HelpSourceKind() const {
+    return static_cast<FlagHelpKind>(help_source_kind_);
+  }
+  FlagDefaultKind DefaultKind() const
+      ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()) {
+    return static_cast<FlagDefaultKind>(def_kind_);
+  }
+  // Used in read/write operations to validate source/target has correct type.
+  // For example if flag is declared as absl::Flag<int> FLAGS_foo, a call to
+  // absl::GetFlag(FLAGS_foo) validates that the type of FLAGS_foo is indeed
+  // int. To do that we pass the "assumed" type id (which is deduced from type
+  // int) as an argument `op`, which is in turn is validated against the type id
+  // stored in flag object by flag definition statement.
+  void AssertValidType(FlagStaticTypeId type_id) const;
+
+  // Immutable flag's state.
+
+  // Flags name passed to ABSL_FLAG as second arg.
+  const char* const name_;
+  // The file name where ABSL_FLAG resides.
+  const char* const filename_;
+  // Type-specific operations "vtable".
+  const FlagOpFn op_;
+  // Help message literal or function to generate it.
+  const FlagHelpMsg help_;
+  // Indicates if help message was supplied as literal or generator func.
+  const uint8_t help_source_kind_ : 1;
+
+  // ------------------------------------------------------------------------
+  // The bytes containing the const bitfields must not be shared with bytes
+  // containing the mutable bitfields.
+  // ------------------------------------------------------------------------
+
+  // Unique tag for absl::call_once call to initialize this flag.
+  //
+  // The placement of this variable between the immutable and mutable bitfields
+  // is important as prevents them from occupying the same byte. If you remove
+  // this variable, make sure to maintain this property.
+  absl::once_flag init_control_;
+
+  // Mutable flag's state (guarded by `data_guard_`).
+
+  // If def_kind_ == kDynamicValue, default_value_ holds a dynamically allocated
+  // value.
+  uint8_t def_kind_ : 1 ABSL_GUARDED_BY(*DataGuard());
+  // Has this flag's value been modified?
+  bool modified_ : 1 ABSL_GUARDED_BY(*DataGuard());
+  // Has this flag been specified on command line.
+  bool on_command_line_ : 1 ABSL_GUARDED_BY(*DataGuard());
+
+  // Mutation counter
+  int64_t counter_ ABSL_GUARDED_BY(*DataGuard());
+  // Optional flag's callback and absl::Mutex to guard the invocations.
+  FlagCallback* callback_ ABSL_GUARDED_BY(*DataGuard());
+  // Either a pointer to the function generating the default value based on the
+  // value specified in ABSL_FLAG or pointer to the dynamically set default
+  // value via SetCommandLineOptionWithMode. def_kind_ is used to distinguish
+  // these two cases.
+  FlagDefaultSrc default_value_ ABSL_GUARDED_BY(*DataGuard());
+  // Current Flag Value
+  FlagValue value_;
+
+  // This is reserved space for an absl::Mutex to guard flag data. It will be
+  // initialized in FlagImpl::Init via placement new.
+  // We can't use "absl::Mutex data_guard_", since this class is not literal.
+  // We do not want to use "absl::Mutex* data_guard_", since this would require
+  // heap allocation during initialization, which is both slows program startup
+  // and can fail. Using reserved space + placement new allows us to avoid both
+  // problems.
+  alignas(absl::Mutex) mutable char data_guard_[sizeof(absl::Mutex)];
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// The Flag object parameterized by the flag's value type. This class implements
+// flag reflection handle interface.
+
+template <typename T>
+class Flag final : public flags_internal::CommandLineFlag {
+ public:
+  constexpr Flag(const char* name, const char* filename, const FlagHelpArg help,
+                 const FlagDfltGenFunc default_value_gen)
+      : impl_(name, filename, &FlagOps<T>, help, default_value_gen) {}
 
   T Get() const {
-    // Implementation notes:
-    //
-    // We are wrapping a union around the value of `T` to serve three purposes:
-    //
-    //  1. `U.value` has correct size and alignment for a value of type `T`
-    //  2. The `U.value` constructor is not invoked since U's constructor does
-    //  not
-    //     do it explicitly.
-    //  3. The `U.value` destructor is invoked since U's destructor does it
-    //     explicitly. This makes `U` a kind of RAII wrapper around non default
-    //     constructible value of T, which is destructed when we leave the
-    //     scope. We do need to destroy U.value, which is constructed by
-    //     CommandLineFlag::Read even though we left it in a moved-from state
-    //     after std::move.
-    //
-    // All of this serves to avoid requiring `T` being default constructible.
+    // See implementation notes in CommandLineFlag::Get().
     union U {
       T value;
       U() {}
@@ -62,23 +564,69 @@
     };
     U u;
 
-    this->Read(&u.value, &flags_internal::FlagOps<T>);
+    impl_.Get(&u.value);
     return std::move(u.value);
   }
-
-  bool AtomicGet(T* v) const {
-    const int64_t r = this->atomic.load(std::memory_order_acquire);
-    if (r != flags_internal::CommandLineFlag::kAtomicInit) {
-      memcpy(v, &r, sizeof(T));
-      return true;
-    }
-
-    return false;
+  void Set(const T& v) { impl_.Set(v); }
+  void SetCallback(const FlagCallbackFunc mutation_callback) {
+    impl_.SetCallback(mutation_callback);
   }
 
-  void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); }
+  // CommandLineFlag interface
+  absl::string_view Name() const override { return impl_.Name(); }
+  std::string Filename() const override { return impl_.Filename(); }
+  absl::string_view Typename() const override { return ""; }
+  std::string Help() const override { return impl_.Help(); }
+  bool IsModified() const override { return impl_.IsModified(); }
+  bool IsSpecifiedOnCommandLine() const override {
+    return impl_.IsSpecifiedOnCommandLine();
+  }
+  std::string DefaultValue() const override { return impl_.DefaultValue(); }
+  std::string CurrentValue() const override { return impl_.CurrentValue(); }
+  bool ValidateInputValue(absl::string_view value) const override {
+    return impl_.ValidateInputValue(value);
+  }
+
+  // Interfaces to save and restore flags to/from persistent state.
+  // Returns current flag state or nullptr if flag does not support
+  // saving and restoring a state.
+  std::unique_ptr<FlagStateInterface> SaveState() override {
+    return impl_.SaveState(this);
+  }
+
+  // Restores the flag state to the supplied state object. If there is
+  // nothing to restore returns false. Otherwise returns true.
+  bool RestoreState(const FlagState<T>& flag_state) {
+    return impl_.RestoreState(&flag_state.cur_value_, flag_state.modified_,
+                              flag_state.on_command_line_, flag_state.counter_);
+  }
+  bool SetFromString(absl::string_view value, FlagSettingMode set_mode,
+                     ValueSource source, std::string* error) override {
+    return impl_.SetFromString(value, set_mode, source, error);
+  }
+  void CheckDefaultValueParsingRoundtrip() const override {
+    impl_.CheckDefaultValueParsingRoundtrip();
+  }
+
+ private:
+  friend class FlagState<T>;
+
+  void Read(void* dst) const override { impl_.Read(dst); }
+  FlagStaticTypeId TypeId() const override { return &FlagStaticTypeIdGen<T>; }
+
+  // Flag's data
+  FlagImpl impl_;
 };
 
+template <typename T>
+inline void FlagState<T>::Restore() const {
+  if (flag_->RestoreState(*this)) {
+    ABSL_INTERNAL_LOG(INFO,
+                      absl::StrCat("Restore saved value of ", flag_->Name(),
+                                   " to: ", flag_->CurrentValue()));
+  }
+}
+
 // This class facilitates Flag object registration and tail expression-based
 // flag definition, for example:
 // ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher);
@@ -89,7 +637,7 @@
     if (do_register) flags_internal::RegisterCommandLineFlag(flag_);
   }
 
-  FlagRegistrar& OnUpdate(flags_internal::FlagCallback cb) && {
+  FlagRegistrar& OnUpdate(FlagCallbackFunc cb) && {
     flag_->SetCallback(cb);
     return *this;
   }
@@ -117,7 +665,7 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_FLAG_H_
diff --git a/absl/flags/internal/parse.h b/absl/flags/internal/parse.h
index 6c79dc8..03e8a07 100644
--- a/absl/flags/internal/parse.h
+++ b/absl/flags/internal/parse.h
@@ -19,6 +19,7 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/flags/declare.h"
 
 ABSL_DECLARE_FLAG(std::vector<std::string>, flagfile);
@@ -27,7 +28,7 @@
 ABSL_DECLARE_FLAG(std::vector<std::string>, undefok);
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 enum class ArgvListAction { kRemoveParsedArgs, kKeepParsedArgs };
@@ -44,7 +45,7 @@
                                         OnUndefinedFlag on_undef_flag);
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PARSE_H_
diff --git a/absl/flags/internal/path_util.h b/absl/flags/internal/path_util.h
index 623a7bc..365c830 100644
--- a/absl/flags/internal/path_util.h
+++ b/absl/flags/internal/path_util.h
@@ -16,11 +16,12 @@
 #ifndef ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
 #define ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
 
+#include "absl/base/config.h"
 #include "absl/strings/match.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // A portable interface that returns the basename of the filename passed as an
@@ -56,7 +57,7 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PATH_UTIL_H_
diff --git a/absl/flags/internal/program_name.cc b/absl/flags/internal/program_name.cc
index 28b8ed3..51d698d 100644
--- a/absl/flags/internal/program_name.cc
+++ b/absl/flags/internal/program_name.cc
@@ -17,16 +17,21 @@
 
 #include <string>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/path_util.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 ABSL_CONST_INIT static absl::Mutex program_name_guard(absl::kConstInit);
 ABSL_CONST_INIT static std::string* program_name
-    GUARDED_BY(program_name_guard) = nullptr;
+    ABSL_GUARDED_BY(program_name_guard) = nullptr;
 
 std::string ProgramInvocationName() {
   absl::MutexLock l(&program_name_guard);
@@ -51,5 +56,5 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/program_name.h b/absl/flags/internal/program_name.h
index a2f0ca1..b99b94f 100644
--- a/absl/flags/internal/program_name.h
+++ b/absl/flags/internal/program_name.h
@@ -18,13 +18,14 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Program name
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // Returns program invocation name or "UNKNOWN" if `SetProgramInvocationName()`
@@ -43,7 +44,7 @@
 void SetProgramInvocationName(absl::string_view prog_name_str);
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_PROGRAM_NAME_H_
diff --git a/absl/flags/internal/program_name_test.cc b/absl/flags/internal/program_name_test.cc
index ed69218..269142f 100644
--- a/absl/flags/internal/program_name_test.cc
+++ b/absl/flags/internal/program_name_test.cc
@@ -15,8 +15,11 @@
 
 #include "absl/flags/internal/program_name.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
diff --git a/absl/flags/internal/registry.cc b/absl/flags/internal/registry.cc
index e39264e..e434a85 100644
--- a/absl/flags/internal/registry.cc
+++ b/absl/flags/internal/registry.cc
@@ -15,9 +15,20 @@
 
 #include "absl/flags/internal/registry.h"
 
-#include "absl/base/dynamic_annotations.h"
+#include <assert.h>
+#include <stdlib.h>
+
+#include <functional>
+#include <map>
+#include <memory>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/flags/config.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/usage_config.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/string_view.h"
@@ -30,20 +41,8 @@
 //    set it.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
-namespace {
-
-void DestroyFlag(CommandLineFlag* flag) NO_THREAD_SAFETY_ANALYSIS {
-  flag->Destroy();
-
-  // CommandLineFlag handle object is heap allocated for non Abseil Flags.
-  if (!flag->IsAbseilFlag()) {
-    delete flag;
-  }
-}
-
-}  // namespace
 
 // --------------------------------------------------------------------
 // FlagRegistry
@@ -58,17 +57,13 @@
 class FlagRegistry {
  public:
   FlagRegistry() = default;
-  ~FlagRegistry() {
-    for (auto& p : flags_) {
-      DestroyFlag(p.second);
-    }
-  }
+  ~FlagRegistry() = default;
 
   // Store a flag in this registry.  Takes ownership of *flag.
   void RegisterFlag(CommandLineFlag* flag);
 
-  void Lock() EXCLUSIVE_LOCK_FUNCTION(lock_) { lock_.Lock(); }
-  void Unlock() UNLOCK_FUNCTION(lock_) { lock_.Unlock(); }
+  void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION(lock_) { lock_.Lock(); }
+  void Unlock() ABSL_UNLOCK_FUNCTION(lock_) { lock_.Unlock(); }
 
   // Returns the flag object for the specified name, or nullptr if not found.
   // Will emit a warning if a 'retired' flag is specified.
@@ -114,6 +109,7 @@
   FlagRegistry* const fr_;
 };
 
+void DestroyRetiredFlag(CommandLineFlag* flag);
 }  // namespace
 
 void FlagRegistry::RegisterFlag(CommandLineFlag* flag) {
@@ -131,7 +127,7 @@
               (flag->IsRetired() ? old_flag->Filename() : flag->Filename()),
               "'."),
           true);
-    } else if (flag->op != old_flag->op) {
+    } else if (flag->TypeId() != old_flag->TypeId()) {
       flags_internal::ReportUsageError(
           absl::StrCat("Flag '", flag->Name(),
                        "' was defined more than once but with "
@@ -141,8 +137,8 @@
                        flag->Typename(), "', respectively."),
           true);
     } else if (old_flag->IsRetired()) {
-      // Retired definitions are idempotent. Just keep the old one.
-      DestroyFlag(flag);
+      // Retired flag can just be deleted.
+      DestroyRetiredFlag(flag);
       return;
     } else if (old_flag->Filename() != flag->Filename()) {
       flags_internal::ReportUsageError(
@@ -201,112 +197,34 @@
 
 class FlagSaverImpl {
  public:
-  // Constructs an empty FlagSaverImpl object.
-  FlagSaverImpl() {}
-  ~FlagSaverImpl() {
-    // reclaim memory from each of our CommandLineFlags
-    for (const SavedFlag& src : backup_registry_) {
-      Delete(src.op, src.current);
-      Delete(src.op, src.default_value);
-    }
-  }
+  FlagSaverImpl() = default;
+  FlagSaverImpl(const FlagSaverImpl&) = delete;
+  void operator=(const FlagSaverImpl&) = delete;
 
   // Saves the flag states from the flag registry into this object.
   // It's an error to call this more than once.
-  // Must be called when the registry mutex is not held.
   void SaveFromRegistry() {
     assert(backup_registry_.empty());  // call only once!
-    SavedFlag saved;
     flags_internal::ForEachFlag([&](flags_internal::CommandLineFlag* flag) {
-      if (flag->IsRetired()) return;
-
-      saved.name = flag->Name();
-      saved.op = flag->op;
-      saved.marshalling_op = flag->marshalling_op;
-      {
-        absl::MutexLock l(flag->InitFlagIfNecessary());
-        saved.validator = flag->validator;
-        saved.modified = flag->modified;
-        saved.on_command_line = flag->on_command_line;
-        saved.current = Clone(saved.op, flag->cur);
-        saved.default_value = Clone(saved.op, flag->def);
-        saved.counter = flag->counter;
+      if (auto flag_state = flag->SaveState()) {
+        backup_registry_.emplace_back(std::move(flag_state));
       }
-      backup_registry_.push_back(saved);
     });
   }
 
-  // Restores the saved flag states into the flag registry.  We
-  // assume no flags were added or deleted from the registry since
-  // the SaveFromRegistry; if they were, that's trouble!  Must be
-  // called when the registry mutex is not held.
+  // Restores the saved flag states into the flag registry.
   void RestoreToRegistry() {
-    FlagRegistry* const global_registry = FlagRegistry::GlobalRegistry();
-    FlagRegistryLock frl(global_registry);
-    for (const SavedFlag& src : backup_registry_) {
-      CommandLineFlag* flag = global_registry->FindFlagLocked(src.name);
-      // If null, flag got deleted from registry.
-      if (!flag) continue;
-
-      bool restored = false;
-      {
-        absl::MutexLock l(flag->InitFlagIfNecessary());
-        flag->validator = src.validator;
-        flag->modified = src.modified;
-        flag->on_command_line = src.on_command_line;
-        if (flag->counter != src.counter ||
-            ChangedDirectly(flag, src.default_value, flag->def)) {
-          restored = true;
-          Copy(src.op, src.default_value, flag->def);
-        }
-        if (flag->counter != src.counter ||
-            ChangedDirectly(flag, src.current, flag->cur)) {
-          restored = true;
-          Copy(src.op, src.current, flag->cur);
-          UpdateCopy(flag);
-          flag->InvokeCallback();
-        }
-      }
-
-      if (restored) {
-        flag->counter++;
-
-        // Revalidate the flag because the validator might store state based
-        // on the flag's value, which just changed due to the restore.
-        // Failing validation is ignored because it's assumed that the flag
-        // was valid previously and there's little that can be done about it
-        // here, anyway.
-        flag->ValidateInputValue(flag->CurrentValue());
-
-        ABSL_INTERNAL_LOG(
-            INFO, absl::StrCat("Restore saved value of ", flag->Name(), ": ",
-                               Unparse(src.marshalling_op, src.current)));
-      }
+    for (const auto& flag_state : backup_registry_) {
+      flag_state->Restore();
     }
   }
 
  private:
-  struct SavedFlag {
-    absl::string_view name;
-    FlagOpFn op;
-    FlagMarshallingOpFn marshalling_op;
-    int64_t counter;
-    bool modified;
-    bool on_command_line;
-    bool (*validator)();
-    const void* current;        // nullptr after restore
-    const void* default_value;  // nullptr after restore
-  };
-
-  std::vector<SavedFlag> backup_registry_;
-
-  FlagSaverImpl(const FlagSaverImpl&);  // no copying!
-  void operator=(const FlagSaverImpl&);
+  std::vector<std::unique_ptr<flags_internal::FlagStateInterface>>
+      backup_registry_;
 };
 
-FlagSaver::FlagSaver() : impl_(new FlagSaverImpl()) {
-  impl_->SaveFromRegistry();
-}
+FlagSaver::FlagSaver() : impl_(new FlagSaverImpl) { impl_->SaveFromRegistry(); }
 
 void FlagSaver::Ignore() {
   delete impl_;
@@ -321,44 +239,6 @@
 }
 
 // --------------------------------------------------------------------
-// GetAllFlags()
-//    The main way the FlagRegistry class exposes its data.  This
-//    returns, as strings, all the info about all the flags in
-//    the main registry, sorted first by filename they are defined
-//    in, and then by flagname.
-// --------------------------------------------------------------------
-
-struct FilenameFlagnameLess {
-  bool operator()(const CommandLineFlagInfo& a,
-                  const CommandLineFlagInfo& b) const {
-    int cmp = absl::string_view(a.filename).compare(b.filename);
-    if (cmp != 0) return cmp < 0;
-    return a.name < b.name;
-  }
-};
-
-void FillCommandLineFlagInfo(CommandLineFlag* flag,
-                             CommandLineFlagInfo* result) {
-  result->name = std::string(flag->Name());
-  result->type = std::string(flag->Typename());
-  result->description = flag->Help();
-  result->filename = flag->Filename();
-
-  if (!flag->IsAbseilFlag()) {
-    if (!flag->IsModified() && ChangedDirectly(flag, flag->cur, flag->def)) {
-      flag->modified = true;
-    }
-  }
-
-  result->current_value = flag->CurrentValue();
-  result->default_value = flag->DefaultValue();
-  result->is_default = !flag->IsModified();
-  result->has_validator_fn = flag->HasValidatorFn();
-  absl::MutexLock l(flag->InitFlagIfNecessary());
-  result->flag_ptr = flag->IsAbseilFlag() ? nullptr : flag->cur;
-}
-
-// --------------------------------------------------------------------
 
 CommandLineFlag* FindCommandLineFlag(absl::string_view name) {
   if (name.empty()) return nullptr;
@@ -393,21 +273,6 @@
 
 // --------------------------------------------------------------------
 
-void GetAllFlags(std::vector<CommandLineFlagInfo>* OUTPUT) {
-  flags_internal::ForEachFlag([&](CommandLineFlag* flag) {
-    if (flag->IsRetired()) return;
-
-    CommandLineFlagInfo fi;
-    FillCommandLineFlagInfo(flag, &fi);
-    OUTPUT->push_back(fi);
-  });
-
-  // Now sort the flags, first by filename they occur in, then alphabetically
-  std::sort(OUTPUT->begin(), OUTPUT->end(), FilenameFlagnameLess());
-}
-
-// --------------------------------------------------------------------
-
 bool RegisterCommandLineFlag(CommandLineFlag* flag) {
   FlagRegistry::GlobalRegistry()->RegisterFlag(flag);
   return true;
@@ -415,14 +280,55 @@
 
 // --------------------------------------------------------------------
 
-bool Retire(FlagOpFn ops, FlagMarshallingOpFn marshalling_ops,
-            const char* name) {
-  auto* flag = new CommandLineFlag(
-      name,
-      /*help_text=*/absl::flags_internal::HelpText::FromStaticCString(nullptr),
-      /*filename_arg=*/"RETIRED", ops, marshalling_ops,
-      /*initial_value_gen=*/nullptr,
-      /*retired_arg=*/true, nullptr, nullptr);
+namespace {
+
+class RetiredFlagObj final : public flags_internal::CommandLineFlag {
+ public:
+  constexpr RetiredFlagObj(const char* name, FlagStaticTypeId type_id)
+      : name_(name), type_id_(type_id) {}
+
+ private:
+  absl::string_view Name() const override { return name_; }
+  std::string Filename() const override { return "RETIRED"; }
+  absl::string_view Typename() const override { return ""; }
+  FlagStaticTypeId TypeId() const override { return type_id_; }
+  std::string Help() const override { return ""; }
+  bool IsRetired() const override { return true; }
+  bool IsModified() const override { return false; }
+  bool IsSpecifiedOnCommandLine() const override { return false; }
+  std::string DefaultValue() const override { return ""; }
+  std::string CurrentValue() const override { return ""; }
+
+  // Any input is valid
+  bool ValidateInputValue(absl::string_view) const override { return true; }
+
+  std::unique_ptr<flags_internal::FlagStateInterface> SaveState() override {
+    return nullptr;
+  }
+
+  bool SetFromString(absl::string_view, flags_internal::FlagSettingMode,
+                     flags_internal::ValueSource, std::string*) override {
+    return false;
+  }
+
+  void CheckDefaultValueParsingRoundtrip() const override {}
+
+  void Read(void*) const override {}
+
+  // Data members
+  const char* const name_;
+  const FlagStaticTypeId type_id_;
+};
+
+void DestroyRetiredFlag(flags_internal::CommandLineFlag* flag) {
+  assert(flag->IsRetired());
+  delete static_cast<RetiredFlagObj*>(flag);
+}
+
+}  // namespace
+
+bool Retire(const char* name, FlagStaticTypeId type_id) {
+  auto* flag = new flags_internal::RetiredFlagObj(name, type_id);
   FlagRegistry::GlobalRegistry()->RegisterFlag(flag);
   return true;
 }
@@ -441,5 +347,5 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/registry.h b/absl/flags/internal/registry.h
index d851d24..69ff889 100644
--- a/absl/flags/internal/registry.h
+++ b/absl/flags/internal/registry.h
@@ -20,43 +20,18 @@
 #include <map>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/base/macros.h"
 #include "absl/flags/internal/commandlineflag.h"
+#include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Global flags registry API.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
-// CommandLineFlagInfo holds all information for a flag.
-struct CommandLineFlagInfo {
-  std::string name;           // the name of the flag
-  std::string type;           // DO NOT use. Use flag->IsOfType<T>() instead.
-  std::string description;    // the "help text" associated with the flag
-  std::string current_value;  // the current value, as a std::string
-  std::string default_value;  // the default value, as a std::string
-  std::string filename;       // 'cleaned' version of filename holding the flag
-  bool has_validator_fn;  // true if RegisterFlagValidator called on this flag
-
-  bool is_default;  // true if the flag has the default value and
-                    // has not been set explicitly from the cmdline
-                    // or via SetCommandLineOption.
-
-  // nullptr for ABSL_FLAG.  A pointer to the flag's current value
-  // otherwise.  E.g., for DEFINE_int32(foo, ...), flag_ptr will be
-  // &FLAGS_foo.
-  const void* flag_ptr;
-};
-
-//-----------------------------------------------------------------------------
-
-void FillCommandLineFlagInfo(CommandLineFlag* flag,
-                             CommandLineFlagInfo* result);
-
-//-----------------------------------------------------------------------------
-
 CommandLineFlag* FindCommandLineFlag(absl::string_view name);
 CommandLineFlag* FindRetiredFlag(absl::string_view name);
 
@@ -69,11 +44,6 @@
 
 //-----------------------------------------------------------------------------
 
-// Store the list of all flags in *OUTPUT, sorted by file.
-void GetAllFlags(std::vector<CommandLineFlagInfo>* OUTPUT);
-
-//-----------------------------------------------------------------------------
-
 bool RegisterCommandLineFlag(CommandLineFlag*);
 
 //-----------------------------------------------------------------------------
@@ -107,29 +77,14 @@
 //   4: Remove the old_lib 'retired' registration.
 //   5: Eventually delete the graveyard registration entirely.
 //
-// Returns bool to enable use in namespace-scope initializers.
-// For example:
-//
-//   static const bool dummy = base::RetiredFlag<int32_t>("myflag");
-//
-// Or to declare several at once:
-//
-//   static bool dummies[] = {
-//       base::RetiredFlag<std::string>("some_string_flag"),
-//       base::RetiredFlag<double>("some_double_flag"),
-//       base::RetiredFlag<int32_t>("some_int32_flag")
-//   };
 
 // Retire flag with name "name" and type indicated by ops.
-bool Retire(FlagOpFn ops, FlagMarshallingOpFn marshalling_ops,
-            const char* name);
+bool Retire(const char* name, FlagStaticTypeId type_id);
 
 // Registered a retired flag with name 'flag_name' and type 'T'.
 template <typename T>
 inline bool RetiredFlag(const char* flag_name) {
-  return flags_internal::Retire(flags_internal::FlagOps<T>,
-                                flags_internal::FlagMarshallingOps<T>,
-                                flag_name);
+  return flags_internal::Retire(flag_name, &FlagStaticTypeIdGen<T>);
 }
 
 // If the flag is retired, returns true and indicates in |*type_is_bool|
@@ -163,7 +118,7 @@
 };
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_REGISTRY_H_
diff --git a/absl/flags/internal/type_erased.cc b/absl/flags/internal/type_erased.cc
index 6b91b26..490bc4e 100644
--- a/absl/flags/internal/type_erased.cc
+++ b/absl/flags/internal/type_erased.cc
@@ -15,13 +15,19 @@
 
 #include "absl/flags/internal/type_erased.h"
 
+#include <assert.h>
+
+#include <string>
+
+#include "absl/base/config.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/flags/config.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/flags/usage_config.h"
-#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 bool GetCommandLineOption(absl::string_view name, std::string* value) {
@@ -37,30 +43,6 @@
   return true;
 }
 
-bool GetCommandLineFlagInfo(absl::string_view name,
-                            CommandLineFlagInfo* OUTPUT) {
-  if (name.empty()) return false;
-
-  CommandLineFlag* flag = flags_internal::FindCommandLineFlag(name);
-  if (flag == nullptr || flag->IsRetired()) {
-    return false;
-  }
-
-  assert(OUTPUT);
-  FillCommandLineFlagInfo(flag, OUTPUT);
-  return true;
-}
-
-CommandLineFlagInfo GetCommandLineFlagInfoOrDie(absl::string_view name) {
-  CommandLineFlagInfo info;
-  if (!GetCommandLineFlagInfo(name, &info)) {
-    ABSL_INTERNAL_LOG(FATAL, absl::StrCat("Flag '", name, "' does not exist"));
-  }
-  return info;
-}
-
-// --------------------------------------------------------------------
-
 bool SetCommandLineOption(absl::string_view name, absl::string_view value) {
   return SetCommandLineOptionWithMode(name, value,
                                       flags_internal::SET_FLAGS_VALUE);
@@ -104,5 +86,5 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/type_erased.h b/absl/flags/internal/type_erased.h
index c9de6de..188429c 100644
--- a/absl/flags/internal/type_erased.h
+++ b/absl/flags/internal/type_erased.h
@@ -18,14 +18,16 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/flags/internal/registry.h"
+#include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Registry interfaces operating on type erased handles.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // If a flag named "name" exists, store its current value in *OUTPUT
@@ -33,17 +35,6 @@
 // Thread-safe.
 bool GetCommandLineOption(absl::string_view name, std::string* value);
 
-// If a flag named "name" exists, store its information in *OUTPUT
-// and return true.  Else return false without changing *OUTPUT.
-// Thread-safe.
-bool GetCommandLineFlagInfo(absl::string_view name,
-                            CommandLineFlagInfo* OUTPUT);
-
-// Returns the CommandLineFlagInfo of the flagname.  exit() with an
-// error code if name not found.
-// Thread-safe.
-CommandLineFlagInfo GetCommandLineFlagInfoOrDie(absl::string_view name);
-
 // Set the value of the flag named "name" to value.  If successful,
 // returns true.  If not successful (e.g., the flag was not found or
 // the value is not a valid value), returns false.
@@ -93,7 +84,7 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_INTERNAL_TYPE_ERASED_H_
diff --git a/absl/flags/internal/type_erased_test.cc b/absl/flags/internal/type_erased_test.cc
index ac749a6..4ce5981 100644
--- a/absl/flags/internal/type_erased_test.cc
+++ b/absl/flags/internal/type_erased_test.cc
@@ -15,12 +15,15 @@
 
 #include "absl/flags/internal/type_erased.h"
 
-#include <cmath>
+#include <memory>
+#include <string>
 
 #include "gtest/gtest.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/registry.h"
+#include "absl/flags/marshalling.h"
 #include "absl/memory/memory.h"
-#include "absl/strings/str_cat.h"
 
 ABSL_FLAG(int, int_flag, 1, "int_flag help");
 ABSL_FLAG(std::string, string_flag, "dflt", "string_flag help");
@@ -116,6 +119,13 @@
   EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "101",
                                                   flags::SET_FLAGS_DEFAULT));
 
+  // Set it again to ensure that resetting logic is covered.
+  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "102",
+                                                  flags::SET_FLAGS_DEFAULT));
+
+  EXPECT_TRUE(flags::SetCommandLineOptionWithMode("int_flag", "103",
+                                                  flags::SET_FLAGS_DEFAULT));
+
   EXPECT_TRUE(flags::SetCommandLineOptionWithMode("string_flag", "asdfgh",
                                                   flags::SET_FLAGS_DEFAULT));
   EXPECT_EQ(absl::GetFlag(FLAGS_string_flag), "asdfgh");
diff --git a/absl/flags/internal/usage.cc b/absl/flags/internal/usage.cc
index a9a8a21..ff90716 100644
--- a/absl/flags/internal/usage.cc
+++ b/absl/flags/internal/usage.cc
@@ -15,18 +15,24 @@
 
 #include "absl/flags/internal/usage.h"
 
+#include <functional>
 #include <map>
+#include <ostream>
 #include <string>
+#include <utility>
+#include <vector>
 
+#include "absl/base/config.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/flags/usage_config.h"
-#include "absl/strings/ascii.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_split.h"
 #include "absl/strings/string_view.h"
-#include "absl/synchronization/mutex.h"
 
 ABSL_FLAG(bool, help, false,
           "show help on important flags for this binary [tip: all flags can "
@@ -44,10 +50,31 @@
           "show help on modules whose name contains the specified substr");
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 
+absl::string_view TypenameForHelp(const flags_internal::CommandLineFlag& flag) {
+  // Only report names of v1 built-in types
+#define HANDLE_V1_BUILTIN_TYPE(t) \
+  if (flag.IsOfType<t>()) {       \
+    return #t;                    \
+  }
+
+  HANDLE_V1_BUILTIN_TYPE(bool);
+  HANDLE_V1_BUILTIN_TYPE(int32_t);
+  HANDLE_V1_BUILTIN_TYPE(int64_t);
+  HANDLE_V1_BUILTIN_TYPE(uint64_t);
+  HANDLE_V1_BUILTIN_TYPE(double);
+#undef HANDLE_V1_BUILTIN_TYPE
+
+  if (flag.IsOfType<std::string>()) {
+    return "string";
+  }
+
+  return "";
+}
+
 // This class is used to emit an XML element with `tag` and `text`.
 // It adds opening and closing tags and escapes special characters in the text.
 // For example:
@@ -180,14 +207,14 @@
   FlagHelpPrettyPrinter printer(80, out);  // Max line length is 80.
 
   // Flag name.
-  printer.Write(absl::StrCat("-", flag.Name()));
+  printer.Write(absl::StrCat("--", flag.Name()));
 
   // Flag help.
   printer.Write(absl::StrCat("(", flag.Help(), ");"), /*wrap_line=*/true);
 
   // Flag data type (for V1 flags only).
   if (!flag.IsAbseilFlag() && !flag.IsRetired()) {
-    printer.Write(absl::StrCat("type: ", flag.Typename(), ";"));
+    printer.Write(absl::StrCat("type: ", TypenameForHelp(flag), ";"));
   }
 
   // The listed default value will be the actual default from the flag
@@ -224,6 +251,9 @@
   } else {
     // XML schema is not a part of our public API for now.
     out << "<?xml version=\"1.0\"?>\n"
+        << "<!-- This output should be used with care. We do not report type "
+           "names for flags with user defined types -->\n"
+        << "<!-- Prefer flag only_check_args for validating flag inputs -->\n"
         // The document.
         << "<AllFlags>\n"
         // The program name and usage.
@@ -381,5 +411,5 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/internal/usage.h b/absl/flags/internal/usage.h
index f3794af..6b080fd 100644
--- a/absl/flags/internal/usage.h
+++ b/absl/flags/internal/usage.h
@@ -19,6 +19,7 @@
 #include <iosfwd>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/flags/declare.h"
 #include "absl/flags/internal/commandlineflag.h"
 #include "absl/strings/string_view.h"
@@ -27,7 +28,7 @@
 // Usage reporting interfaces
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // The format to report the help messages in.
@@ -65,7 +66,7 @@
                      absl::string_view program_usage_message);
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 ABSL_DECLARE_FLAG(bool, help);
diff --git a/absl/flags/internal/usage_test.cc b/absl/flags/internal/usage_test.cc
index 781e1d2..e1e57e5 100644
--- a/absl/flags/internal/usage_test.cc
+++ b/absl/flags/internal/usage_test.cc
@@ -15,17 +15,23 @@
 
 #include "absl/flags/internal/usage.h"
 
+#include <stdint.h>
+
 #include <sstream>
+#include <string>
 
 #include "gtest/gtest.h"
+#include "absl/flags/declare.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/parse.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/flags/parse.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
 #include "absl/memory/memory.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 ABSL_FLAG(int, usage_reporting_test_flag_01, 101,
           "usage_reporting_test_flag_01 help message");
@@ -67,9 +73,9 @@
   fname = normalized;
 #endif
 
-  auto absl_pos = fname.find("/absl/");
+  auto absl_pos = fname.rfind("absl/");
   if (absl_pos != absl::string_view::npos) {
-    fname = fname.substr(absl_pos + 1);
+    fname = fname.substr(absl_pos);
   }
   return std::string(fname);
 }
@@ -111,7 +117,7 @@
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+      R"(    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
 )");
 }
@@ -123,7 +129,7 @@
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+      R"(    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
 )");
 }
@@ -135,7 +141,7 @@
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+      R"(    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
 )");
 }
@@ -147,7 +153,7 @@
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+      R"(    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
 )");
 }
@@ -159,7 +165,7 @@
   flags::FlagHelp(test_buf, *flag, flags::HelpFormat::kHumanReadable);
   EXPECT_EQ(
       test_buf.str(),
-      R"(    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+      R"(    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
 )");
 }
@@ -171,17 +177,17 @@
       R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
@@ -247,17 +253,17 @@
             R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
@@ -276,17 +282,17 @@
             R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
@@ -307,17 +313,17 @@
             R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
@@ -372,17 +378,17 @@
             R"(usage_test: Custom usage message
 
   Flags from absl/flags/internal/usage_test.cc:
-    -usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
+    --usage_reporting_test_flag_01 (usage_reporting_test_flag_01 help message);
       default: 101;
-    -usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
+    --usage_reporting_test_flag_02 (usage_reporting_test_flag_02 help message);
       default: false;
-    -usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
+    --usage_reporting_test_flag_03 (usage_reporting_test_flag_03 help message);
       default: 1.03;
-    -usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
+    --usage_reporting_test_flag_04 (usage_reporting_test_flag_04 help message);
       default: 1000000000000004;
-    -usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
+    --usage_reporting_test_flag_05 (usage_reporting_test_flag_05 help message);
       default: UDT{};
-    -usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
+    --usage_reporting_test_flag_06 (usage_reporting_test_flag_06 help message.
 
       Some more help.
       Even more long long long long long long long long long long long long help
@@ -395,7 +401,7 @@
 }  // namespace
 
 int main(int argc, char* argv[]) {
-  absl::GetFlag(FLAGS_undefok);  // Force linking of parse.cc
+  (void)absl::GetFlag(FLAGS_undefok);  // Force linking of parse.cc
   flags::SetProgramInvocationName("usage_test");
   absl::SetProgramUsageMessage(kTestUsageMessage);
   ::testing::InitGoogleTest(&argc, argv);
diff --git a/absl/flags/marshalling.cc b/absl/flags/marshalling.cc
index 71b01d7..6f2ddda 100644
--- a/absl/flags/marshalling.cc
+++ b/absl/flags/marshalling.cc
@@ -15,18 +15,28 @@
 
 #include "absl/flags/marshalling.h"
 
-#include <limits>
+#include <stddef.h>
 
+#include <cmath>
+#include <limits>
+#include <string>
+#include <type_traits>
+#include <vector>
+
+#include "absl/base/config.h"
+#include "absl/base/log_severity.h"
 #include "absl/base/macros.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/match.h"
 #include "absl/strings/numbers.h"
 #include "absl/strings/str_cat.h"
 #include "absl/strings/str_format.h"
 #include "absl/strings/str_join.h"
 #include "absl/strings/str_split.h"
+#include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // --------------------------------------------------------------------
@@ -187,5 +197,44 @@
 }
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+
+bool AbslParseFlag(absl::string_view text, absl::LogSeverity* dst,
+                   std::string* err) {
+  text = absl::StripAsciiWhitespace(text);
+  if (text.empty()) {
+    *err = "no value provided";
+    return false;
+  }
+  if (text.front() == 'k' || text.front() == 'K') text.remove_prefix(1);
+  if (absl::EqualsIgnoreCase(text, "info")) {
+    *dst = absl::LogSeverity::kInfo;
+    return true;
+  }
+  if (absl::EqualsIgnoreCase(text, "warning")) {
+    *dst = absl::LogSeverity::kWarning;
+    return true;
+  }
+  if (absl::EqualsIgnoreCase(text, "error")) {
+    *dst = absl::LogSeverity::kError;
+    return true;
+  }
+  if (absl::EqualsIgnoreCase(text, "fatal")) {
+    *dst = absl::LogSeverity::kFatal;
+    return true;
+  }
+  std::underlying_type<absl::LogSeverity>::type numeric_value;
+  if (absl::ParseFlag(text, &numeric_value, err)) {
+    *dst = static_cast<absl::LogSeverity>(numeric_value);
+    return true;
+  }
+  *err = "only integers and absl::LogSeverity enumerators are accepted";
+  return false;
+}
+
+std::string AbslUnparseFlag(absl::LogSeverity v) {
+  if (v == absl::NormalizeLogSeverity(v)) return absl::LogSeverityName(v);
+  return absl::UnparseFlag(static_cast<int>(v));
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/marshalling.h b/absl/flags/marshalling.h
index 5598d44..0b50335 100644
--- a/absl/flags/marshalling.h
+++ b/absl/flags/marshalling.h
@@ -33,15 +33,16 @@
 // * `double`
 // * `std::string`
 // * `std::vector<std::string>`
+// * `absl::LogSeverity` (provided natively for layering reasons)
 //
 // Note that support for integral types is implemented using overloads for
 // variable-width fundamental types (`short`, `int`, `long`, etc.). However,
 // you should prefer the fixed-width integral types (`int32_t`, `uint64_t`,
 // etc.) we've noted above within flag definitions.
-
 //
 // In addition, several Abseil libraries provide their own custom support for
-// Abseil flags.
+// Abseil flags. Documentation for these formats is provided in the type's
+// `AbslParseFlag()` definition.
 //
 // The Abseil time library provides the following support for civil time values:
 //
@@ -164,10 +165,11 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 // Overloads of `AbslParseFlag()` and `AbslUnparseFlag()` for fundamental types.
@@ -179,8 +181,8 @@
 bool AbslParseFlag(absl::string_view, long*, std::string*);            // NOLINT
 bool AbslParseFlag(absl::string_view, unsigned long*, std::string*);   // NOLINT
 bool AbslParseFlag(absl::string_view, long long*, std::string*);       // NOLINT
-bool AbslParseFlag(absl::string_view, unsigned long long*,
-                   std::string*);  // NOLINT
+bool AbslParseFlag(absl::string_view, unsigned long long*,             // NOLINT
+                   std::string*);
 bool AbslParseFlag(absl::string_view, float*, std::string*);
 bool AbslParseFlag(absl::string_view, double*, std::string*);
 bool AbslParseFlag(absl::string_view, std::string*, std::string*);
@@ -249,7 +251,14 @@
   return flags_internal::Unparse(v);
 }
 
-}  // inline namespace lts_2019_08_08
+// Overloads for `absl::LogSeverity` can't (easily) appear alongside that type's
+// definition because it is layered below flags.  See proper documentation in
+// base/log_severity.h.
+enum class LogSeverity : int;
+bool AbslParseFlag(absl::string_view, absl::LogSeverity*, std::string*);
+std::string AbslUnparseFlag(absl::LogSeverity);
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_MARSHALLING_H_
diff --git a/absl/flags/marshalling_test.cc b/absl/flags/marshalling_test.cc
index 37cd194..4a64ce1 100644
--- a/absl/flags/marshalling_test.cc
+++ b/absl/flags/marshalling_test.cc
@@ -15,7 +15,12 @@
 
 #include "absl/flags/marshalling.h"
 
+#include <stdint.h>
+
 #include <cmath>
+#include <limits>
+#include <string>
+#include <vector>
 
 #include "gtest/gtest.h"
 
diff --git a/absl/flags/parse.cc b/absl/flags/parse.cc
index fd80a0c..812e498 100644
--- a/absl/flags/parse.cc
+++ b/absl/flags/parse.cc
@@ -17,43 +17,58 @@
 
 #include <stdlib.h>
 
+#include <algorithm>
 #include <fstream>
 #include <iostream>
+#include <iterator>
+#include <string>
 #include <tuple>
+#include <utility>
+#include <vector>
 
 #ifdef _WIN32
 #include <windows.h>
 #endif
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/thread_annotations.h"
+#include "absl/flags/config.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/commandlineflag.h"
+#include "absl/flags/internal/flag.h"
+#include "absl/flags/internal/parse.h"
 #include "absl/flags/internal/program_name.h"
 #include "absl/flags/internal/registry.h"
 #include "absl/flags/internal/usage.h"
 #include "absl/flags/usage.h"
 #include "absl/flags/usage_config.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 #include "absl/synchronization/mutex.h"
 
 // --------------------------------------------------------------------
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 
 ABSL_CONST_INIT absl::Mutex processing_checks_guard(absl::kConstInit);
 
 ABSL_CONST_INIT bool flagfile_needs_processing
-    GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(processing_checks_guard) = false;
 ABSL_CONST_INIT bool fromenv_needs_processing
-    GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(processing_checks_guard) = false;
 ABSL_CONST_INIT bool tryfromenv_needs_processing
-    GUARDED_BY(processing_checks_guard) = false;
+    ABSL_GUARDED_BY(processing_checks_guard) = false;
 
 }  // namespace
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 ABSL_FLAG(std::vector<std::string>, flagfile, {},
@@ -111,7 +126,7 @@
           "with that name");
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 namespace {
@@ -280,9 +295,7 @@
 #define IGNORE_TYPE(T) \
   if (flag->IsOfType<T>()) return;
 
-    ABSL_FLAGS_INTERNAL_FOR_EACH_LOCK_FREE(IGNORE_TYPE)
-    IGNORE_TYPE(std::string)
-    IGNORE_TYPE(std::vector<std::string>)
+    ABSL_FLAGS_INTERNAL_BUILTIN_TYPES(IGNORE_TYPE)
 #undef IGNORE_TYPE
 
     flag->CheckDefaultValueParsingRoundtrip();
@@ -525,7 +538,7 @@
     // --my_string_var --foo=bar
     // We look for a flag of std::string type, whose value begins with a
     // dash and corresponds to known flag or standalone --.
-    if (value[0] == '-' && flag.IsOfType<std::string>()) {
+    if (!value.empty() && value[0] == '-' && flag.IsOfType<std::string>()) {
       auto maybe_flag_name = std::get<0>(SplitNameAndValue(value.substr(1)));
 
       if (maybe_flag_name.empty() ||
@@ -751,5 +764,5 @@
       flags_internal::OnUndefinedFlag::kAbortIfUndefined);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/parse.h b/absl/flags/parse.h
index 469bd50..f37b060 100644
--- a/absl/flags/parse.h
+++ b/absl/flags/parse.h
@@ -26,10 +26,11 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/flags/internal/parse.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // ParseCommandLine()
 //
@@ -54,7 +55,7 @@
 // help messages and then exits the program.
 std::vector<char*> ParseCommandLine(int argc, char* argv[]);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_PARSE_H_
diff --git a/absl/flags/parse_test.cc b/absl/flags/parse_test.cc
index 447a3bc..6f49377 100644
--- a/absl/flags/parse_test.cc
+++ b/absl/flags/parse_test.cc
@@ -15,14 +15,22 @@
 
 #include "absl/flags/parse.h"
 
+#include <stdlib.h>
+
 #include <fstream>
+#include <string>
+#include <vector>
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/scoped_set_env.h"
+#include "absl/flags/declare.h"
 #include "absl/flags/flag.h"
+#include "absl/flags/internal/parse.h"
+#include "absl/flags/internal/registry.h"
 #include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/substitute.h"
 #include "absl/types/span.h"
 
@@ -92,8 +100,11 @@
 
       auto len = GetTempPathA(MAX_PATH, temp_path_buffer);
       if (len < MAX_PATH && len != 0) {
-        std::string temp_dir_name = absl::StrCat(
-            temp_path_buffer, "\\parse_test.", GetCurrentProcessId());
+        std::string temp_dir_name = temp_path_buffer;
+        if (!absl::EndsWith(temp_dir_name, "\\")) {
+          temp_dir_name.push_back('\\');
+        }
+        absl::StrAppend(&temp_dir_name, "parse_test.", GetCurrentProcessId());
         if (CreateDirectoryA(temp_dir_name.c_str(), nullptr)) {
           *res = temp_dir_name;
         }
@@ -104,11 +115,11 @@
         *res = unique_name;
       }
 #endif
+    }
 
-      if (res->empty()) {
-        ABSL_INTERNAL_LOG(FATAL,
-                          "Failed to make temporary directory for data files");
-      }
+    if (res->empty()) {
+      ABSL_INTERNAL_LOG(FATAL,
+                        "Failed to make temporary directory for data files");
     }
 
 #ifdef _WIN32
diff --git a/absl/flags/usage.cc b/absl/flags/usage.cc
index 12c346b..452f667 100644
--- a/absl/flags/usage.cc
+++ b/absl/flags/usage.cc
@@ -14,18 +14,25 @@
 // limitations under the License.
 #include "absl/flags/usage.h"
 
+#include <stdlib.h>
+
 #include <string>
 
+#include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/usage.h"
+#include "absl/strings/string_view.h"
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 namespace {
 ABSL_CONST_INIT absl::Mutex usage_message_guard(absl::kConstInit);
 ABSL_CONST_INIT std::string* program_usage_message
-    GUARDED_BY(usage_message_guard) = nullptr;
+    ABSL_GUARDED_BY(usage_message_guard) = nullptr;
 }  // namespace
 }  // namespace flags_internal
 
@@ -54,5 +61,5 @@
              : "Warning: SetProgramUsageMessage() never called";
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/usage.h b/absl/flags/usage.h
index c232a7d..ad12ab7 100644
--- a/absl/flags/usage.h
+++ b/absl/flags/usage.h
@@ -16,13 +16,14 @@
 #ifndef ABSL_FLAGS_USAGE_H_
 #define ABSL_FLAGS_USAGE_H_
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // --------------------------------------------------------------------
 // Usage reporting interfaces
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Sets the "usage" message to be used by help reporting routines.
 // For example:
@@ -36,7 +37,7 @@
 // Returns the usage message set by SetProgramUsageMessage().
 absl::string_view ProgramUsageMessage();
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_FLAGS_USAGE_H_
diff --git a/absl/flags/usage_config.cc b/absl/flags/usage_config.cc
index a538acd..2d837ec 100644
--- a/absl/flags/usage_config.cc
+++ b/absl/flags/usage_config.cc
@@ -16,12 +16,16 @@
 #include "absl/flags/usage_config.h"
 
 #include <iostream>
-#include <memory>
+#include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
+#include "absl/base/const_init.h"
+#include "absl/base/thread_annotations.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
-#include "absl/strings/str_cat.h"
+#include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 #include "absl/strings/strip.h"
 #include "absl/synchronization/mutex.h"
 
@@ -34,7 +38,7 @@
 }  // extern "C"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace flags_internal {
 
 namespace {
@@ -95,7 +99,7 @@
 
 ABSL_CONST_INIT absl::Mutex custom_usage_config_guard(absl::kConstInit);
 ABSL_CONST_INIT FlagsUsageConfig* custom_usage_config
-    GUARDED_BY(custom_usage_config_guard) = nullptr;
+    ABSL_GUARDED_BY(custom_usage_config_guard) = nullptr;
 
 }  // namespace
 
@@ -150,5 +154,5 @@
     flags_internal::custom_usage_config = new FlagsUsageConfig(usage_config);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/flags/usage_config.h b/absl/flags/usage_config.h
index c6d34e4..0ed7e1b 100644
--- a/absl/flags/usage_config.h
+++ b/absl/flags/usage_config.h
@@ -27,6 +27,7 @@
 #include <functional>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 // -----------------------------------------------------------------------------
@@ -54,7 +55,7 @@
 //     Shows help on modules whose name contains the specified substring
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace flags_internal {
 using FlagKindFilter = std::function<bool (absl::string_view)>;
@@ -102,7 +103,7 @@
   //   normalize_filename("/my_company/some_long_path/src/project/file.cc")
   // might produce
   //   "project/file.cc".
-  std::function<std::string (absl::string_view)> normalize_filename;
+  std::function<std::string(absl::string_view)> normalize_filename;
 };
 
 // SetFlagsUsageConfig()
@@ -119,7 +120,7 @@
 void ReportUsageError(absl::string_view msg, bool is_fatal);
 
 }  // namespace flags_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
diff --git a/absl/flags/usage_config_test.cc b/absl/flags/usage_config_test.cc
index 3bde13a..70eca30 100644
--- a/absl/flags/usage_config_test.cc
+++ b/absl/flags/usage_config_test.cc
@@ -15,10 +15,13 @@
 
 #include "absl/flags/usage_config.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
 #include "absl/flags/internal/path_util.h"
 #include "absl/flags/internal/program_name.h"
 #include "absl/strings/match.h"
+#include "absl/strings/string_view.h"
 
 namespace {
 
diff --git a/absl/functional/BUILD.bazel b/absl/functional/BUILD.bazel
new file mode 100644
index 0000000..432546c
--- /dev/null
+++ b/absl/functional/BUILD.bazel
@@ -0,0 +1,93 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_DEFAULT_LINKOPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "bind_front",
+    srcs = ["internal/front_binder.h"],
+    hdrs = ["bind_front.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:base_internal",
+        "//absl/container:compressed_tuple",
+        "//absl/meta:type_traits",
+        "//absl/utility",
+    ],
+)
+
+cc_test(
+    name = "bind_front_test",
+    srcs = ["bind_front_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bind_front",
+        "//absl/memory",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
+    name = "function_ref",
+    srcs = ["internal/function_ref.h"],
+    hdrs = ["function_ref.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:base_internal",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_test(
+    name = "function_ref_test",
+    size = "small",
+    srcs = ["function_ref_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":function_ref",
+        "//absl/container:test_instance_tracker",
+        "//absl/memory",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "function_ref_benchmark",
+    srcs = [
+        "function_ref_benchmark.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    tags = ["benchmark"],
+    visibility = ["//visibility:private"],
+    deps = [
+        ":function_ref",
+        "//absl/base:core_headers",
+        "@com_github_google_benchmark//:benchmark_main",
+    ],
+)
diff --git a/absl/functional/CMakeLists.txt b/absl/functional/CMakeLists.txt
new file mode 100644
index 0000000..cda914f
--- /dev/null
+++ b/absl/functional/CMakeLists.txt
@@ -0,0 +1,72 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+absl_cc_library(
+  NAME
+    bind_front
+  SRCS
+    "internal/front_binder.h"
+  HDRS
+    "bind_front.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base_internal
+    absl::compressed_tuple
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    bind_front_test
+  SRCS
+    "bind_front_test.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::bind_front
+    absl::memory
+    gmock_main
+)
+
+absl_cc_library(
+  NAME
+    function_ref
+  SRCS
+    "internal/function_ref.h"
+  HDRS
+    "function_ref.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::base_internal
+    absl::meta
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    function_ref_test
+  SRCS
+    "function_ref_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::function_ref
+    absl::memory
+    absl::test_instance_tracker
+    gmock_main
+)
diff --git a/absl/functional/bind_front.h b/absl/functional/bind_front.h
new file mode 100644
index 0000000..5b47970
--- /dev/null
+++ b/absl/functional/bind_front.h
@@ -0,0 +1,184 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bind_front.h
+// -----------------------------------------------------------------------------
+//
+// `absl::bind_front()` returns a functor by binding a number of arguments to
+// the front of a provided (usually more generic) functor. Unlike `std::bind`,
+// it does not require the use of argument placeholders. The simpler syntax of
+// `absl::bind_front()` allows you to avoid known misuses with `std::bind()`.
+//
+// `absl::bind_front()` is meant as a drop-in replacement for C++20's upcoming
+// `std::bind_front()`, which similarly resolves these issues with
+// `std::bind()`. Both `bind_front()` alternatives, unlike `std::bind()`, allow
+// partial function application. (See
+// https://en.wikipedia.org/wiki/Partial_application).
+
+#ifndef ABSL_FUNCTIONAL_BIND_FRONT_H_
+#define ABSL_FUNCTIONAL_BIND_FRONT_H_
+
+#include "absl/functional/internal/front_binder.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// bind_front()
+//
+// Binds the first N arguments of an invocable object and stores them by value.
+//
+// Like `std::bind()`, `absl::bind_front()` is implicitly convertible to
+// `std::function`.  In particular, it may be used as a simpler replacement for
+// `std::bind()` in most cases, as it does not require  placeholders to be
+// specified. More importantly, it provides more reliable correctness guarantees
+// than `std::bind()`; while `std::bind()` will silently ignore passing more
+// parameters than expected, for example, `absl::bind_front()` will report such
+// mis-uses as errors.
+//
+// absl::bind_front(a...) can be seen as storing the results of
+// std::make_tuple(a...).
+//
+// Example: Binding a free function.
+//
+//   int Minus(int a, int b) { return a - b; }
+//
+//   assert(absl::bind_front(Minus)(3, 2) == 3 - 2);
+//   assert(absl::bind_front(Minus, 3)(2) == 3 - 2);
+//   assert(absl::bind_front(Minus, 3, 2)() == 3 - 2);
+//
+// Example: Binding a member function.
+//
+//   struct Math {
+//     int Double(int a) const { return 2 * a; }
+//   };
+//
+//   Math math;
+//
+//   assert(absl::bind_front(&Math::Double)(&math, 3) == 2 * 3);
+//   // Stores a pointer to math inside the functor.
+//   assert(absl::bind_front(&Math::Double, &math)(3) == 2 * 3);
+//   // Stores a copy of math inside the functor.
+//   assert(absl::bind_front(&Math::Double, math)(3) == 2 * 3);
+//   // Stores std::unique_ptr<Math> inside the functor.
+//   assert(absl::bind_front(&Math::Double,
+//                           std::unique_ptr<Math>(new Math))(3) == 2 * 3);
+//
+// Example: Using `absl::bind_front()`, instead of `std::bind()`, with
+//          `std::function`.
+//
+//   class FileReader {
+//    public:
+//     void ReadFileAsync(const std::string& filename, std::string* content,
+//                        const std::function<void()>& done) {
+//       // Calls Executor::Schedule(std::function<void()>).
+//       Executor::DefaultExecutor()->Schedule(
+//           absl::bind_front(&FileReader::BlockingRead, this,
+//                            filename, content, done));
+//     }
+//
+//    private:
+//     void BlockingRead(const std::string& filename, std::string* content,
+//                       const std::function<void()>& done) {
+//       CHECK_OK(file::GetContents(filename, content, {}));
+//       done();
+//     }
+//   };
+//
+// `absl::bind_front()` stores bound arguments explicitly using the type passed
+// rather than implicitly based on the type accepted by its functor.
+//
+// Example: Binding arguments explicitly.
+//
+//   void LogStringView(absl::string_view sv) {
+//     LOG(INFO) << sv;
+//   }
+//
+//   Executor* e = Executor::DefaultExecutor();
+//   std::string s = "hello";
+//   absl::string_view sv = s;
+//
+//   // absl::bind_front(LogStringView, arg) makes a copy of arg and stores it.
+//   e->Schedule(absl::bind_front(LogStringView, sv)); // ERROR: dangling
+//                                                     // string_view.
+//
+//   e->Schedule(absl::bind_front(LogStringView, s));  // OK: stores a copy of
+//                                                     // s.
+//
+// To store some of the arguments passed to `absl::bind_front()` by reference,
+//  use std::ref()` and `std::cref()`.
+//
+// Example: Storing some of the bound arguments by reference.
+//
+//   class Service {
+//    public:
+//     void Serve(const Request& req, std::function<void()>* done) {
+//       // The request protocol buffer won't be deleted until done is called.
+//       // It's safe to store a reference to it inside the functor.
+//       Executor::DefaultExecutor()->Schedule(
+//           absl::bind_front(&Service::BlockingServe, this, std::cref(req),
+//           done));
+//     }
+//
+//    private:
+//     void BlockingServe(const Request& req, std::function<void()>* done);
+//   };
+//
+// Example: Storing bound arguments by reference.
+//
+//   void Print(const std::string& a, const std::string& b) {
+//     std::cerr << a << b;
+//   }
+//
+//   std::string hi = "Hello, ";
+//   std::vector<std::string> names = {"Chuk", "Gek"};
+//   // Doesn't copy hi.
+//   for_each(names.begin(), names.end(),
+//            absl::bind_front(Print, std::ref(hi)));
+//
+//   // DO NOT DO THIS: the functor may outlive "hi", resulting in
+//   // dangling references.
+//   foo->DoInFuture(absl::bind_front(Print, std::ref(hi), "Guest"));  // BAD!
+//   auto f = absl::bind_front(Print, std::ref(hi), "Guest"); // BAD!
+//
+// Example: Storing reference-like types.
+//
+//   void Print(absl::string_view a, const std::string& b) {
+//     std::cerr << a << b;
+//   }
+//
+//   std::string hi = "Hello, ";
+//   // Copies "hi".
+//   absl::bind_front(Print, hi)("Chuk");
+//
+//   // Compile error: std::reference_wrapper<const string> is not implicitly
+//   // convertible to string_view.
+//   // absl::bind_front(Print, std::cref(hi))("Chuk");
+//
+//   // Doesn't copy "hi".
+//   absl::bind_front(Print, absl::string_view(hi))("Chuk");
+//
+template <class F, class... BoundArgs>
+constexpr functional_internal::bind_front_t<F, BoundArgs...> bind_front(
+    F&& func, BoundArgs&&... args) {
+  return functional_internal::bind_front_t<F, BoundArgs...>(
+      absl::in_place, absl::forward<F>(func),
+      absl::forward<BoundArgs>(args)...);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_BIND_FRONT_H_
diff --git a/absl/functional/bind_front_test.cc b/absl/functional/bind_front_test.cc
new file mode 100644
index 0000000..4801a81
--- /dev/null
+++ b/absl/functional/bind_front_test.cc
@@ -0,0 +1,231 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/bind_front.h"
+
+#include <stddef.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/memory/memory.h"
+
+namespace {
+
+char CharAt(const char* s, size_t index) { return s[index]; }
+
+TEST(BindTest, Basics) {
+  EXPECT_EQ('C', absl::bind_front(CharAt)("ABC", 2));
+  EXPECT_EQ('C', absl::bind_front(CharAt, "ABC")(2));
+  EXPECT_EQ('C', absl::bind_front(CharAt, "ABC", 2)());
+}
+
+TEST(BindTest, Lambda) {
+  auto lambda = [](int x, int y, int z) { return x + y + z; };
+  EXPECT_EQ(6, absl::bind_front(lambda)(1, 2, 3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1)(2, 3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1, 2)(3));
+  EXPECT_EQ(6, absl::bind_front(lambda, 1, 2, 3)());
+}
+
+struct Functor {
+  std::string operator()() & { return "&"; }
+  std::string operator()() const& { return "const&"; }
+  std::string operator()() && { return "&&"; }
+  std::string operator()() const&& { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfBoundArgs) {
+  auto f = absl::bind_front(Functor());
+  const auto& cf = f;
+  EXPECT_EQ("&", f());
+  EXPECT_EQ("const&", cf());
+  EXPECT_EQ("&&", std::move(f)());
+  EXPECT_EQ("const&&", std::move(cf)());
+}
+
+struct ArgDescribe {
+  std::string operator()(int&) const { return "&"; }             // NOLINT
+  std::string operator()(const int&) const { return "const&"; }  // NOLINT
+  std::string operator()(int&&) const { return "&&"; }
+  std::string operator()(const int&&) const { return "const&&"; }
+};
+
+TEST(BindTest, PerfectForwardingOfFreeArgs) {
+  ArgDescribe f;
+  int i;
+  EXPECT_EQ("&", absl::bind_front(f)(static_cast<int&>(i)));
+  EXPECT_EQ("const&", absl::bind_front(f)(static_cast<const int&>(i)));
+  EXPECT_EQ("&&", absl::bind_front(f)(static_cast<int&&>(i)));
+  EXPECT_EQ("const&&", absl::bind_front(f)(static_cast<const int&&>(i)));
+}
+
+struct NonCopyableFunctor {
+  NonCopyableFunctor() = default;
+  NonCopyableFunctor(const NonCopyableFunctor&) = delete;
+  NonCopyableFunctor& operator=(const NonCopyableFunctor&) = delete;
+  const NonCopyableFunctor* operator()() const { return this; }
+};
+
+TEST(BindTest, RefToFunctor) {
+  // It won't copy/move the functor and use the original object.
+  NonCopyableFunctor ncf;
+  auto bound_ncf = absl::bind_front(std::ref(ncf));
+  auto bound_ncf_copy = bound_ncf;
+  EXPECT_EQ(&ncf, bound_ncf_copy());
+}
+
+struct Struct {
+  std::string value;
+};
+
+TEST(BindTest, StoreByCopy) {
+  Struct s = {"hello"};
+  auto f = absl::bind_front(&Struct::value, s);
+  auto g = f;
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ("hello", g());
+  EXPECT_NE(&s.value, &f());
+  EXPECT_NE(&s.value, &g());
+  EXPECT_NE(&g(), &f());
+}
+
+struct NonCopyable {
+  explicit NonCopyable(const std::string& s) : value(s) {}
+  NonCopyable(const NonCopyable&) = delete;
+  NonCopyable& operator=(const NonCopyable&) = delete;
+
+  std::string value;
+};
+
+const std::string& GetNonCopyableValue(const NonCopyable& n) { return n.value; }
+
+TEST(BindTest, StoreByRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&GetNonCopyableValue, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByCRef) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&GetNonCopyableValue, std::cref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);  // NOLINT
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+const std::string& GetNonCopyableValueByWrapper(
+    std::reference_wrapper<NonCopyable> n) {
+  return n.get().value;
+}
+
+TEST(BindTest, StoreByRefInvokeByWrapper) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(GetNonCopyableValueByWrapper, std::ref(s));
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+  s.value = "goodbye";
+  EXPECT_EQ("goodbye", g());
+}
+
+TEST(BindTest, StoreByPointer) {
+  NonCopyable s("hello");
+  auto f = absl::bind_front(&NonCopyable::value, &s);
+  EXPECT_EQ("hello", f());
+  EXPECT_EQ(&s.value, &f());
+  auto g = std::move(f);
+  EXPECT_EQ("hello", g());
+  EXPECT_EQ(&s.value, &g());
+}
+
+int Sink(std::unique_ptr<int> p) {
+  return *p;
+}
+
+std::unique_ptr<int> Factory(int n) { return absl::make_unique<int>(n); }
+
+TEST(BindTest, NonCopyableArg) {
+  EXPECT_EQ(42, absl::bind_front(Sink)(absl::make_unique<int>(42)));
+  EXPECT_EQ(42, absl::bind_front(Sink, absl::make_unique<int>(42))());
+}
+
+TEST(BindTest, NonCopyableResult) {
+  EXPECT_THAT(absl::bind_front(Factory)(42), ::testing::Pointee(42));
+  EXPECT_THAT(absl::bind_front(Factory, 42)(), ::testing::Pointee(42));
+}
+
+// is_copy_constructible<FalseCopyable<unique_ptr<T>> is true but an attempt to
+// instantiate the copy constructor leads to a compile error. This is similar
+// to how standard containers behave.
+template <class T>
+struct FalseCopyable {
+  FalseCopyable() {}
+  FalseCopyable(const FalseCopyable& other) : m(other.m) {}
+  FalseCopyable(FalseCopyable&& other) : m(std::move(other.m)) {}
+  T m;
+};
+
+int GetMember(FalseCopyable<std::unique_ptr<int>> x) { return *x.m; }
+
+TEST(BindTest, WrappedMoveOnly) {
+  FalseCopyable<std::unique_ptr<int>> x;
+  x.m = absl::make_unique<int>(42);
+  auto f = absl::bind_front(&GetMember, std::move(x));
+  EXPECT_EQ(42, std::move(f)());
+}
+
+int Plus(int a, int b) { return a + b; }
+
+TEST(BindTest, ConstExpr) {
+  constexpr auto f = absl::bind_front(CharAt);
+  EXPECT_EQ(f("ABC", 1), 'B');
+  static constexpr int five = 5;
+  constexpr auto plus5 = absl::bind_front(Plus, five);
+  EXPECT_EQ(plus5(1), 6);
+
+  // There seems to be a bug in MSVC dealing constexpr construction of
+  // char[]. Notice 'plus5' above; 'int' works just fine.
+#if !(defined(_MSC_VER) && _MSC_VER < 1910)
+  static constexpr char data[] = "DEF";
+  constexpr auto g = absl::bind_front(CharAt, data);
+  EXPECT_EQ(g(1), 'E');
+#endif
+}
+
+struct ManglingCall {
+  int operator()(int, double, std::string) const { return 0; }
+};
+
+TEST(BindTest, Mangling) {
+  // We just want to generate a particular instantiation to see its mangling.
+  absl::bind_front(ManglingCall{}, 1, 3.3)("A");
+}
+
+}  // namespace
diff --git a/absl/functional/function_ref.h b/absl/functional/function_ref.h
new file mode 100644
index 0000000..370acc5
--- /dev/null
+++ b/absl/functional/function_ref.h
@@ -0,0 +1,139 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: function_ref.h
+// -----------------------------------------------------------------------------
+//
+// This header file defines the `absl::FunctionRef` type for holding a
+// non-owning reference to an object of any invocable type. This function
+// reference is typically most useful as a type-erased argument type for
+// accepting function types that neither take ownership nor copy the type; using
+// the reference type in this case avoids a copy and an allocation. Best
+// practices of other non-owning reference-like objects (such as
+// `absl::string_view`) apply here.
+//
+//  An `absl::FunctionRef` is similar in usage to a `std::function` but has the
+//  following differences:
+//
+//  * It doesn't own the underlying object.
+//  * It doesn't have a null or empty state.
+//  * It never performs deep copies or allocations.
+//  * It's much faster and cheaper to construct.
+//  * It's trivially copyable and destructable.
+//
+// Generally, `absl::FunctionRef` should not be used as a return value, data
+// member, or to initialize a `std::function`. Such usages will often lead to
+// problematic lifetime issues. Once you convert something to an
+// `absl::FunctionRef` you cannot make a deep copy later.
+//
+// This class is suitable for use wherever a "const std::function<>&"
+// would be used without making a copy. ForEach functions and other versions of
+// the visitor pattern are a good example of when this class should be used.
+//
+// This class is trivial to copy and should be passed by value.
+#ifndef ABSL_FUNCTIONAL_FUNCTION_REF_H_
+#define ABSL_FUNCTIONAL_FUNCTION_REF_H_
+
+#include <cassert>
+#include <functional>
+#include <type_traits>
+
+#include "absl/functional/internal/function_ref.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// FunctionRef
+//
+// Dummy class declaration to allow the partial specialization based on function
+// types below.
+template <typename T>
+class FunctionRef;
+
+// FunctionRef
+//
+// An `absl::FunctionRef` is a lightweight wrapper to any invokable object with
+// a compatible signature. Generally, an `absl::FunctionRef` should only be used
+// as an argument type and should be preferred as an argument over a const
+// reference to a `std::function`.
+//
+// Example:
+//
+//   // The following function takes a function callback by const reference
+//   bool Visitor(const std::function<void(my_proto&,
+//                                         absl::string_view)>& callback);
+//
+//   // Assuming that the function is not stored or otherwise copied, it can be
+//   // replaced by an `absl::FunctionRef`:
+//   bool Visitor(absl::FunctionRef<void(my_proto&, absl::string_view)>
+//                  callback);
+//
+// Note: the assignment operator within an `absl::FunctionRef` is intentionally
+// deleted to prevent misuse; because the `absl::FunctionRef` does not own the
+// underlying type, assignment likely indicates misuse.
+template <typename R, typename... Args>
+class FunctionRef<R(Args...)> {
+ private:
+  // Used to disable constructors for objects that are not compatible with the
+  // signature of this FunctionRef.
+  template <typename F,
+            typename FR = absl::base_internal::InvokeT<F, Args&&...>>
+  using EnableIfCompatible =
+      typename std::enable_if<std::is_void<R>::value ||
+                              std::is_convertible<FR, R>::value>::type;
+
+ public:
+  // Constructs a FunctionRef from any invokable type.
+  template <typename F, typename = EnableIfCompatible<const F&>>
+  FunctionRef(const F& f)  // NOLINT(runtime/explicit)
+      : invoker_(&absl::functional_internal::InvokeObject<F, R, Args...>) {
+    absl::functional_internal::AssertNonNull(f);
+    ptr_.obj = &f;
+  }
+
+  // Overload for function pointers. This eliminates a level of indirection that
+  // would happen if the above overload was used (it lets us store the pointer
+  // instead of a pointer to a pointer).
+  //
+  // This overload is also used for references to functions, since references to
+  // functions can decay to function pointers implicitly.
+  template <
+      typename F, typename = EnableIfCompatible<F*>,
+      absl::functional_internal::EnableIf<absl::is_function<F>::value> = 0>
+  FunctionRef(F* f)  // NOLINT(runtime/explicit)
+      : invoker_(&absl::functional_internal::InvokeFunction<F*, R, Args...>) {
+    assert(f != nullptr);
+    ptr_.fun = reinterpret_cast<decltype(ptr_.fun)>(f);
+  }
+
+  // To help prevent subtle lifetime bugs, FunctionRef is not assignable.
+  // Typically, it should only be used as an argument type.
+  FunctionRef& operator=(const FunctionRef& rhs) = delete;
+
+  // Call the underlying object.
+  R operator()(Args... args) const {
+    return invoker_(ptr_, std::forward<Args>(args)...);
+  }
+
+ private:
+  absl::functional_internal::VoidPtr ptr_;
+  absl::functional_internal::Invoker<R, Args...> invoker_;
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_FUNCTION_REF_H_
diff --git a/absl/functional/function_ref_benchmark.cc b/absl/functional/function_ref_benchmark.cc
new file mode 100644
index 0000000..045305b
--- /dev/null
+++ b/absl/functional/function_ref_benchmark.cc
@@ -0,0 +1,142 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/function_ref.h"
+
+#include <memory>
+
+#include "benchmark/benchmark.h"
+#include "absl/base/attributes.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+int dummy = 0;
+
+void FreeFunction() { benchmark::DoNotOptimize(dummy); }
+
+struct TrivialFunctor {
+  void operator()() const { benchmark::DoNotOptimize(dummy); }
+};
+
+struct LargeFunctor {
+  void operator()() const { benchmark::DoNotOptimize(this); }
+  std::string a, b, c;
+};
+
+template <typename Function, typename... Args>
+void ABSL_ATTRIBUTE_NOINLINE CallFunction(Function f, Args&&... args) {
+  f(std::forward<Args>(args)...);
+}
+
+template <typename Function, typename Callable, typename... Args>
+void ConstructAndCallFunctionBenchmark(benchmark::State& state,
+                                       const Callable& c, Args&&... args) {
+  for (auto _ : state) {
+    CallFunction<Function>(c, std::forward<Args>(args)...);
+  }
+}
+
+void BM_TrivialStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state,
+                                                           TrivialFunctor{});
+}
+BENCHMARK(BM_TrivialStdFunction);
+
+void BM_TrivialFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state,
+                                                         TrivialFunctor{});
+}
+BENCHMARK(BM_TrivialFunctionRef);
+
+void BM_LargeStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state,
+                                                           LargeFunctor{});
+}
+BENCHMARK(BM_LargeStdFunction);
+
+void BM_LargeFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state, LargeFunctor{});
+}
+BENCHMARK(BM_LargeFunctionRef);
+
+void BM_FunPtrStdFunction(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<std::function<void()>>(state, FreeFunction);
+}
+BENCHMARK(BM_FunPtrStdFunction);
+
+void BM_FunPtrFunctionRef(benchmark::State& state) {
+  ConstructAndCallFunctionBenchmark<FunctionRef<void()>>(state, FreeFunction);
+}
+BENCHMARK(BM_FunPtrFunctionRef);
+
+// Doesn't include construction or copy overhead in the loop.
+template <typename Function, typename Callable, typename... Args>
+void CallFunctionBenchmark(benchmark::State& state, const Callable& c,
+                           Args... args) {
+  Function f = c;
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(&f);
+    f(args...);
+  }
+}
+
+struct FunctorWithTrivialArgs {
+  void operator()(int a, int b, int c) const {
+    benchmark::DoNotOptimize(a);
+    benchmark::DoNotOptimize(b);
+    benchmark::DoNotOptimize(c);
+  }
+};
+
+void BM_TrivialArgsStdFunction(benchmark::State& state) {
+  CallFunctionBenchmark<std::function<void(int, int, int)>>(
+      state, FunctorWithTrivialArgs{}, 1, 2, 3);
+}
+BENCHMARK(BM_TrivialArgsStdFunction);
+
+void BM_TrivialArgsFunctionRef(benchmark::State& state) {
+  CallFunctionBenchmark<FunctionRef<void(int, int, int)>>(
+      state, FunctorWithTrivialArgs{}, 1, 2, 3);
+}
+BENCHMARK(BM_TrivialArgsFunctionRef);
+
+struct FunctorWithNonTrivialArgs {
+  void operator()(std::string a, std::string b, std::string c) const {
+    benchmark::DoNotOptimize(&a);
+    benchmark::DoNotOptimize(&b);
+    benchmark::DoNotOptimize(&c);
+  }
+};
+
+void BM_NonTrivialArgsStdFunction(benchmark::State& state) {
+  std::string a, b, c;
+  CallFunctionBenchmark<
+      std::function<void(std::string, std::string, std::string)>>(
+      state, FunctorWithNonTrivialArgs{}, a, b, c);
+}
+BENCHMARK(BM_NonTrivialArgsStdFunction);
+
+void BM_NonTrivialArgsFunctionRef(benchmark::State& state) {
+  std::string a, b, c;
+  CallFunctionBenchmark<
+      FunctionRef<void(std::string, std::string, std::string)>>(
+      state, FunctorWithNonTrivialArgs{}, a, b, c);
+}
+BENCHMARK(BM_NonTrivialArgsFunctionRef);
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/functional/function_ref_test.cc b/absl/functional/function_ref_test.cc
new file mode 100644
index 0000000..3aa5974
--- /dev/null
+++ b/absl/functional/function_ref_test.cc
@@ -0,0 +1,257 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/functional/function_ref.h"
+
+#include <memory>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/internal/test_instance_tracker.h"
+#include "absl/memory/memory.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace {
+
+void RunFun(FunctionRef<void()> f) { f(); }
+
+TEST(FunctionRefTest, Lambda) {
+  bool ran = false;
+  RunFun([&] { ran = true; });
+  EXPECT_TRUE(ran);
+}
+
+int Function() { return 1337; }
+
+TEST(FunctionRefTest, Function1) {
+  FunctionRef<int()> ref(&Function);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, Function2) {
+  FunctionRef<int()> ref(Function);
+  EXPECT_EQ(1337, ref());
+}
+
+int NoExceptFunction() noexcept { return 1337; }
+
+// TODO(jdennett): Add a test for noexcept member functions.
+TEST(FunctionRefTest, NoExceptFunction) {
+  FunctionRef<int()> ref(NoExceptFunction);
+  EXPECT_EQ(1337, ref());
+}
+
+TEST(FunctionRefTest, ForwardsArgs) {
+  auto l = [](std::unique_ptr<int> i) { return *i; };
+  FunctionRef<int(std::unique_ptr<int>)> ref(l);
+  EXPECT_EQ(42, ref(absl::make_unique<int>(42)));
+}
+
+TEST(FunctionRef, ReturnMoveOnly) {
+  auto l = [] { return absl::make_unique<int>(29); };
+  FunctionRef<std::unique_ptr<int>()> ref(l);
+  EXPECT_EQ(29, *ref());
+}
+
+TEST(FunctionRef, ManyArgs) {
+  auto l = [](int a, int b, int c) { return a + b + c; };
+  FunctionRef<int(int, int, int)> ref(l);
+  EXPECT_EQ(6, ref(1, 2, 3));
+}
+
+TEST(FunctionRef, VoidResultFromNonVoidFunctor) {
+  bool ran = false;
+  auto l = [&]() -> int {
+    ran = true;
+    return 2;
+  };
+  FunctionRef<void()> ref(l);
+  ref();
+  EXPECT_TRUE(ran);
+}
+
+TEST(FunctionRef, CastFromDerived) {
+  struct Base {};
+  struct Derived : public Base {};
+
+  Derived d;
+  auto l1 = [&](Base* b) { EXPECT_EQ(&d, b); };
+  FunctionRef<void(Derived*)> ref1(l1);
+  ref1(&d);
+
+  auto l2 = [&]() -> Derived* { return &d; };
+  FunctionRef<Base*()> ref2(l2);
+  EXPECT_EQ(&d, ref2());
+}
+
+TEST(FunctionRef, VoidResultFromNonVoidFuncton) {
+  FunctionRef<void()> ref(Function);
+  ref();
+}
+
+TEST(FunctionRef, MemberPtr) {
+  struct S {
+    int i;
+  };
+
+  S s{1100111};
+  auto mem_ptr = &S::i;
+  FunctionRef<int(const S& s)> ref(mem_ptr);
+  EXPECT_EQ(1100111, ref(s));
+}
+
+TEST(FunctionRef, MemberFun) {
+  struct S {
+    int i;
+    int get_i() const { return i; }
+  };
+
+  S s{22};
+  auto mem_fun_ptr = &S::get_i;
+  FunctionRef<int(const S& s)> ref(mem_fun_ptr);
+  EXPECT_EQ(22, ref(s));
+}
+
+TEST(FunctionRef, MemberFunRefqualified) {
+  struct S {
+    int i;
+    int get_i() && { return i; }
+  };
+  auto mem_fun_ptr = &S::get_i;
+  S s{22};
+  FunctionRef<int(S && s)> ref(mem_fun_ptr);
+  EXPECT_EQ(22, ref(std::move(s)));
+}
+
+#if !defined(_WIN32) && defined(GTEST_HAS_DEATH_TEST)
+
+TEST(FunctionRef, MemberFunRefqualifiedNull) {
+  struct S {
+    int i;
+    int get_i() && { return i; }
+  };
+  auto mem_fun_ptr = &S::get_i;
+  mem_fun_ptr = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<int(S && s)> ref(mem_fun_ptr); }, "");
+}
+
+TEST(FunctionRef, NullMemberPtrAssertFails) {
+  struct S {
+    int i;
+  };
+  using MemberPtr = int S::*;
+  MemberPtr mem_ptr = nullptr;
+  EXPECT_DEBUG_DEATH({ FunctionRef<int(const S& s)> ref(mem_ptr); }, "");
+}
+
+#endif  // GTEST_HAS_DEATH_TEST
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValue) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](absl::test_internal::CopyableMovableInstance) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(instance);
+  EXPECT_EQ(tracker.copies(), 1);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByRef) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](const absl::test_internal::CopyableMovableInstance&) {};
+  FunctionRef<void(const absl::test_internal::CopyableMovableInstance&)> ref(l);
+  ref(instance);
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 0);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValueCallByMove) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](absl::test_internal::CopyableMovableInstance) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(std::move(instance));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 2);
+}
+
+TEST(FunctionRef, CopiesAndMovesPerPassByValueToRef) {
+  absl::test_internal::InstanceTracker tracker;
+  absl::test_internal::CopyableMovableInstance instance(0);
+  auto l = [](const absl::test_internal::CopyableMovableInstance&) {};
+  FunctionRef<void(absl::test_internal::CopyableMovableInstance)> ref(l);
+  ref(std::move(instance));
+  EXPECT_EQ(tracker.copies(), 0);
+  EXPECT_EQ(tracker.moves(), 1);
+}
+
+TEST(FunctionRef, PassByValueTypes) {
+  using absl::functional_internal::Invoker;
+  using absl::functional_internal::VoidPtr;
+  using absl::test_internal::CopyableMovableInstance;
+  struct Trivial {
+    void* p[2];
+  };
+  struct LargeTrivial {
+    void* p[3];
+  };
+
+  static_assert(std::is_same<Invoker<void, int>, void (*)(VoidPtr, int)>::value,
+                "Scalar types should be passed by value");
+  static_assert(
+      std::is_same<Invoker<void, Trivial>, void (*)(VoidPtr, Trivial)>::value,
+      "Small trivial types should be passed by value");
+  static_assert(std::is_same<Invoker<void, LargeTrivial>,
+                             void (*)(VoidPtr, LargeTrivial &&)>::value,
+                "Large trivial types should be passed by rvalue reference");
+  static_assert(
+      std::is_same<Invoker<void, CopyableMovableInstance>,
+                   void (*)(VoidPtr, CopyableMovableInstance &&)>::value,
+      "Types with copy/move ctor should be passed by rvalue reference");
+
+  // References are passed as references.
+  static_assert(
+      std::is_same<Invoker<void, int&>, void (*)(VoidPtr, int&)>::value,
+      "Reference types should be preserved");
+  static_assert(
+      std::is_same<Invoker<void, CopyableMovableInstance&>,
+                   void (*)(VoidPtr, CopyableMovableInstance&)>::value,
+      "Reference types should be preserved");
+  static_assert(
+      std::is_same<Invoker<void, CopyableMovableInstance&&>,
+                   void (*)(VoidPtr, CopyableMovableInstance &&)>::value,
+      "Reference types should be preserved");
+
+  // Make sure the address of an object received by reference is the same as the
+  // addess of the object passed by the caller.
+  {
+    LargeTrivial obj;
+    auto test = [&obj](LargeTrivial& input) { ASSERT_EQ(&input, &obj); };
+    absl::FunctionRef<void(LargeTrivial&)> ref(test);
+    ref(obj);
+  }
+
+  {
+    Trivial obj;
+    auto test = [&obj](Trivial& input) { ASSERT_EQ(&input, &obj); };
+    absl::FunctionRef<void(Trivial&)> ref(test);
+    ref(obj);
+  }
+}
+
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/functional/internal/front_binder.h b/absl/functional/internal/front_binder.h
new file mode 100644
index 0000000..a4d95da
--- /dev/null
+++ b/absl/functional/internal/front_binder.h
@@ -0,0 +1,95 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Implementation details for `absl::bind_front()`.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
+#define ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
+
+#include <cstddef>
+#include <type_traits>
+#include <utility>
+
+#include "absl/base/internal/invoke.h"
+#include "absl/container/internal/compressed_tuple.h"
+#include "absl/meta/type_traits.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Invoke the method, expanding the tuple of bound arguments.
+template <class R, class Tuple, size_t... Idx, class... Args>
+R Apply(Tuple&& bound, absl::index_sequence<Idx...>, Args&&... free) {
+  return base_internal::Invoke(
+      absl::forward<Tuple>(bound).template get<Idx>()...,
+      absl::forward<Args>(free)...);
+}
+
+template <class F, class... BoundArgs>
+class FrontBinder {
+  using BoundArgsT = absl::container_internal::CompressedTuple<F, BoundArgs...>;
+  using Idx = absl::make_index_sequence<sizeof...(BoundArgs) + 1>;
+
+  BoundArgsT bound_args_;
+
+ public:
+  template <class... Ts>
+  constexpr explicit FrontBinder(absl::in_place_t, Ts&&... ts)
+      : bound_args_(absl::forward<Ts>(ts)...) {}
+
+  template <class... FreeArgs,
+            class R = base_internal::InvokeT<F&, BoundArgs&..., FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) & {
+    return functional_internal::Apply<R>(bound_args_, Idx(),
+                                         absl::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = base_internal::InvokeT<const F&, const BoundArgs&...,
+                                             FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) const& {
+    return functional_internal::Apply<R>(bound_args_, Idx(),
+                                         absl::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs, class R = base_internal::InvokeT<
+                                   F&&, BoundArgs&&..., FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) && {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::Apply<R>(absl::move(bound_args_), Idx(),
+                                         absl::forward<FreeArgs>(free_args)...);
+  }
+
+  template <class... FreeArgs,
+            class R = base_internal::InvokeT<const F&&, const BoundArgs&&...,
+                                             FreeArgs&&...>>
+  R operator()(FreeArgs&&... free_args) const&& {
+    // This overload is called when *this is an rvalue. If some of the bound
+    // arguments are stored by value or rvalue reference, we move them.
+    return functional_internal::Apply<R>(absl::move(bound_args_), Idx(),
+                                         absl::forward<FreeArgs>(free_args)...);
+  }
+};
+
+template <class F, class... BoundArgs>
+using bind_front_t = FrontBinder<decay_t<F>, absl::decay_t<BoundArgs>...>;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_FRONT_BINDER_H_
diff --git a/absl/functional/internal/function_ref.h b/absl/functional/internal/function_ref.h
new file mode 100644
index 0000000..d157505
--- /dev/null
+++ b/absl/functional/internal/function_ref.h
@@ -0,0 +1,106 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+#define ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
+
+#include <cassert>
+#include <functional>
+#include <type_traits>
+
+#include "absl/base/internal/invoke.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace functional_internal {
+
+// Like a void* that can handle function pointers as well. The standard does not
+// allow function pointers to round-trip through void*, but void(*)() is fine.
+//
+// Note: It's important that this class remains trivial and is the same size as
+// a pointer, since this allows the compiler to perform tail-call optimizations
+// when the underlying function is a callable object with a matching signature.
+union VoidPtr {
+  const void* obj;
+  void (*fun)();
+};
+
+// Chooses the best type for passing T as an argument.
+// Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
+// passed by value.
+template <typename T>
+constexpr bool PassByValue() {
+  return !std::is_lvalue_reference<T>::value &&
+         absl::is_trivially_copy_constructible<T>::value &&
+         absl::is_trivially_copy_assignable<
+             typename std::remove_cv<T>::type>::value &&
+         std::is_trivially_destructible<T>::value &&
+         sizeof(T) <= 2 * sizeof(void*);
+}
+
+template <typename T>
+struct ForwardT : std::conditional<PassByValue<T>(), T, T&&> {};
+
+// An Invoker takes a pointer to the type-erased invokable object, followed by
+// the arguments that the invokable object expects.
+//
+// Note: The order of arguments here is an optimization, since member functions
+// have an implicit "this" pointer as their first argument, putting VoidPtr
+// first allows the compiler to perform tail-call optimization in many cases.
+template <typename R, typename... Args>
+using Invoker = R (*)(VoidPtr, typename ForwardT<Args>::type...);
+
+//
+// InvokeObject and InvokeFunction provide static "Invoke" functions that can be
+// used as Invokers for objects or functions respectively.
+//
+// static_cast<R> handles the case the return type is void.
+template <typename Obj, typename R, typename... Args>
+R InvokeObject(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  auto o = static_cast<const Obj*>(ptr.obj);
+  return static_cast<R>(
+      absl::base_internal::Invoke(*o, std::forward<Args>(args)...));
+}
+
+template <typename Fun, typename R, typename... Args>
+R InvokeFunction(VoidPtr ptr, typename ForwardT<Args>::type... args) {
+  auto f = reinterpret_cast<Fun>(ptr.fun);
+  return static_cast<R>(
+      absl::base_internal::Invoke(f, std::forward<Args>(args)...));
+}
+
+template <typename Sig>
+void AssertNonNull(const std::function<Sig>& f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <typename F>
+void AssertNonNull(const F&) {}
+
+template <typename F, typename C>
+void AssertNonNull(F C::*f) {
+  assert(f != nullptr);
+  (void)f;
+}
+
+template <bool C>
+using EnableIf = typename ::std::enable_if<C, int>::type;
+
+}  // namespace functional_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_FUNCTIONAL_INTERNAL_FUNCTION_REF_H_
diff --git a/absl/hash/BUILD.bazel b/absl/hash/BUILD.bazel
index 8c2daf7..ffe8c29 100644
--- a/absl/hash/BUILD.bazel
+++ b/absl/hash/BUILD.bazel
@@ -1,5 +1,5 @@
 #
-# Copyright 2018 The Abseil Authors.
+# Copyright 2019 The Abseil Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -70,9 +71,9 @@
     deps = [
         ":hash",
         ":hash_testing",
+        ":spy_hash_state",
         "//absl/base:core_headers",
         "//absl/container:flat_hash_set",
-        "//absl/hash:spy_hash_state",
         "//absl/meta:type_traits",
         "//absl/numeric:int128",
         "@com_google_googletest//:gtest_main",
diff --git a/absl/hash/hash.h b/absl/hash/hash.h
index d36f096..23a65ea 100644
--- a/absl/hash/hash.h
+++ b/absl/hash/hash.h
@@ -73,7 +73,7 @@
 #include "absl/hash/internal/hash.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // `absl::Hash`
@@ -318,7 +318,7 @@
   void (*combine_contiguous_)(void*, const unsigned char*, size_t);
 };
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_HASH_H_
diff --git a/absl/hash/hash_test.cc b/absl/hash/hash_test.cc
index 2a4e226..f02a537 100644
--- a/absl/hash/hash_test.cc
+++ b/absl/hash/hash_test.cc
@@ -274,8 +274,8 @@
 
   const std::string small = "foo";
   const std::string dup = "foofoo";
-  const std::string large = "large";
-  const std::string huge = std::string(5000, 'a');
+  const std::string large = std::string(2048, 'x');  // multiple of chunk size
+  const std::string huge = std::string(5000, 'a');   // not a multiple
 
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
       std::string(), absl::string_view(),
@@ -300,6 +300,33 @@
             SpyHash(absl::string_view("ABC")));
 }
 
+TEST(HashValueTest, WString) {
+  EXPECT_TRUE((is_hashable<std::wstring>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::wstring(), std::wstring(L"ABC"), std::wstring(L"ABC"),
+      std::wstring(L"Some other different string"),
+      std::wstring(L"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U16String) {
+  EXPECT_TRUE((is_hashable<std::u16string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u16string(), std::u16string(u"ABC"), std::u16string(u"ABC"),
+      std::u16string(u"Some other different string"),
+      std::u16string(u"Iñtërnâtiônàlizætiøn"))));
+}
+
+TEST(HashValueTest, U32String) {
+  EXPECT_TRUE((is_hashable<std::u32string>::value));
+
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
+      std::u32string(), std::u32string(U"ABC"), std::u32string(U"ABC"),
+      std::u32string(U"Some other different string"),
+      std::u32string(U"Iñtërnâtiônàlizætiøn"))));
+}
+
 TEST(HashValueTest, StdArray) {
   EXPECT_TRUE((is_hashable<std::array<int, 3>>::value));
 
@@ -378,6 +405,116 @@
   }
 };
 
+// Test helper for combine_piecewise_buffer.  It holds a string_view to the
+// buffer-to-be-hashed.  Its AbslHashValue specialization will split up its
+// contents at the character offsets requested.
+class PiecewiseHashTester {
+ public:
+  // Create a hash view of a buffer to be hashed contiguously.
+  explicit PiecewiseHashTester(absl::string_view buf)
+      : buf_(buf), piecewise_(false), split_locations_() {}
+
+  // Create a hash view of a buffer to be hashed piecewise, with breaks at the
+  // given locations.
+  PiecewiseHashTester(absl::string_view buf, std::set<size_t> split_locations)
+      : buf_(buf),
+        piecewise_(true),
+        split_locations_(std::move(split_locations)) {}
+
+  template <typename H>
+  friend H AbslHashValue(H h, const PiecewiseHashTester& p) {
+    if (!p.piecewise_) {
+      return H::combine_contiguous(std::move(h), p.buf_.data(), p.buf_.size());
+    }
+    absl::hash_internal::PiecewiseCombiner combiner;
+    if (p.split_locations_.empty()) {
+      h = combiner.add_buffer(std::move(h), p.buf_.data(), p.buf_.size());
+      return combiner.finalize(std::move(h));
+    }
+    size_t begin = 0;
+    for (size_t next : p.split_locations_) {
+      absl::string_view chunk = p.buf_.substr(begin, next - begin);
+      h = combiner.add_buffer(std::move(h), chunk.data(), chunk.size());
+      begin = next;
+    }
+    absl::string_view last_chunk = p.buf_.substr(begin);
+    if (!last_chunk.empty()) {
+      h = combiner.add_buffer(std::move(h), last_chunk.data(),
+                              last_chunk.size());
+    }
+    return combiner.finalize(std::move(h));
+  }
+
+ private:
+  absl::string_view buf_;
+  bool piecewise_;
+  std::set<size_t> split_locations_;
+};
+
+// Dummy object that hashes as two distinct contiguous buffers, "foo" followed
+// by "bar"
+struct DummyFooBar {
+  template <typename H>
+  friend H AbslHashValue(H h, const DummyFooBar&) {
+    const char* foo = "foo";
+    const char* bar = "bar";
+    h = H::combine_contiguous(std::move(h), foo, 3);
+    h = H::combine_contiguous(std::move(h), bar, 3);
+    return h;
+  }
+};
+
+TEST(HashValueTest, CombinePiecewiseBuffer) {
+  absl::Hash<PiecewiseHashTester> hash;
+
+  // Check that hashing an empty buffer through the piecewise API works.
+  EXPECT_EQ(hash(PiecewiseHashTester("")), hash(PiecewiseHashTester("", {})));
+
+  // Similarly, small buffers should give consistent results
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {})));
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {3})));
+
+  // But hashing "foobar" in pieces gives a different answer than hashing "foo"
+  // contiguously, then "bar" contiguously.
+  EXPECT_NE(hash(PiecewiseHashTester("foobar", {3})),
+            absl::Hash<DummyFooBar>()(DummyFooBar{}));
+
+  // Test hashing a large buffer incrementally, broken up in several different
+  // ways.  Arrange for breaks on and near the stride boundaries to look for
+  // off-by-one errors in the implementation.
+  //
+  // This test is run on a buffer that is a multiple of the stride size, and one
+  // that isn't.
+  for (size_t big_buffer_size : {1024 * 2 + 512, 1024 * 3}) {
+    SCOPED_TRACE(big_buffer_size);
+    std::string big_buffer;
+    for (int i = 0; i < big_buffer_size; ++i) {
+      // Arbitrary std::string
+      big_buffer.push_back(32 + (i * (i / 3)) % 64);
+    }
+    auto big_buffer_hash = hash(PiecewiseHashTester(big_buffer));
+
+    const int possible_breaks = 9;
+    size_t breaks[possible_breaks] = {1,    512,  1023, 1024, 1025,
+                                      1536, 2047, 2048, 2049};
+    for (unsigned test_mask = 0; test_mask < (1u << possible_breaks);
+         ++test_mask) {
+      SCOPED_TRACE(test_mask);
+      std::set<size_t> break_locations;
+      for (int j = 0; j < possible_breaks; ++j) {
+        if (test_mask & (1u << j)) {
+          break_locations.insert(breaks[j]);
+        }
+      }
+      EXPECT_EQ(
+          hash(PiecewiseHashTester(big_buffer, std::move(break_locations))),
+          big_buffer_hash);
+    }
+  }
+}
+
 TEST(HashValueTest, PrivateSanity) {
   // Sanity check that Private is working as the tests below expect it to work.
   EXPECT_TRUE(is_hashable<Private>::value);
@@ -458,7 +595,10 @@
   EXPECT_FALSE(absl::is_copy_assignable<absl::Hash<X>>::value);
   EXPECT_FALSE(absl::is_move_assignable<absl::Hash<X>>::value);
   EXPECT_FALSE(IsHashCallable<X>::value);
+#if !defined(__GNUC__) || __GNUC__ < 9
+  // This doesn't compile on GCC 9.
   EXPECT_FALSE(IsAggregateInitializable<absl::Hash<X>>::value);
+#endif
 }
 #endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
 
@@ -544,7 +684,7 @@
 }  // namespace
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 template <InvokeTag... Tags>
 struct is_uniquely_represented<
@@ -552,7 +692,7 @@
     typename EnableIfContained<InvokeTag::kUniquelyRepresented, Tags...>::type>
     : std::true_type {};
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #if ABSL_HASH_INTERNAL_SUPPORT_LEGACY_HASH_
diff --git a/absl/hash/hash_testing.h b/absl/hash/hash_testing.h
index 6e39028..1e1c574 100644
--- a/absl/hash/hash_testing.h
+++ b/absl/hash/hash_testing.h
@@ -28,7 +28,7 @@
 #include "absl/types/variant.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Run the absl::Hash algorithm over all the elements passed in and verify that
 // their hash expansion is congruent with their `==` operator.
@@ -372,7 +372,7 @@
       equals);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_HASH_TESTING_H_
diff --git a/absl/hash/internal/city.cc b/absl/hash/internal/city.cc
index c7ad159..e122c18 100644
--- a/absl/hash/internal/city.cc
+++ b/absl/hash/internal/city.cc
@@ -30,7 +30,7 @@
 #include "absl/base/optimization.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 #ifdef ABSL_IS_BIG_ENDIAN
@@ -342,5 +342,5 @@
 }
 
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/city.h b/absl/hash/internal/city.h
index 7586ba0..161c774 100644
--- a/absl/hash/internal/city.h
+++ b/absl/hash/internal/city.h
@@ -47,10 +47,13 @@
 
 #include <stdint.h>
 #include <stdlib.h>  // for size_t.
+
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 typedef std::pair<uint64_t, uint64_t> uint128;
@@ -87,7 +90,7 @@
 }
 
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_CITY_H_
diff --git a/absl/hash/internal/city_test.cc b/absl/hash/internal/city_test.cc
index 1b9373c..251d381 100644
--- a/absl/hash/internal/city_test.cc
+++ b/absl/hash/internal/city_test.cc
@@ -20,7 +20,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 static const uint64_t k0 = 0xc3a5c85c97cb3127ULL;
@@ -591,5 +591,5 @@
 }
 
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/hash.cc b/absl/hash/internal/hash.cc
index 087b389..b44ecb3 100644
--- a/absl/hash/internal/hash.cc
+++ b/absl/hash/internal/hash.cc
@@ -15,11 +15,41 @@
 #include "absl/hash/internal/hash.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
+uint64_t CityHashState::CombineLargeContiguousImpl32(uint64_t state,
+                                                     const unsigned char* first,
+                                                     size_t len) {
+  while (len >= PiecewiseChunkSize()) {
+    state =
+        Mix(state, absl::hash_internal::CityHash32(reinterpret_cast<const char*>(first),
+                                         PiecewiseChunkSize()));
+    len -= PiecewiseChunkSize();
+    first += PiecewiseChunkSize();
+  }
+  // Handle the remainder.
+  return CombineContiguousImpl(state, first, len,
+                               std::integral_constant<int, 4>{});
+}
+
+uint64_t CityHashState::CombineLargeContiguousImpl64(uint64_t state,
+                                                     const unsigned char* first,
+                                                     size_t len) {
+  while (len >= PiecewiseChunkSize()) {
+    state =
+        Mix(state, absl::hash_internal::CityHash64(reinterpret_cast<const char*>(first),
+                                         PiecewiseChunkSize()));
+    len -= PiecewiseChunkSize();
+    first += PiecewiseChunkSize();
+  }
+  // Handle the remainder.
+  return CombineContiguousImpl(state, first, len,
+                               std::integral_constant<int, 8>{});
+}
+
 ABSL_CONST_INIT const void* const CityHashState::kSeed = &kSeed;
 
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index 81f1edf..ae7a60c 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -50,9 +50,15 @@
 #include "absl/hash/internal/city.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
+class PiecewiseCombiner;
+
+// Internal detail: Large buffers are hashed in smaller chunks.  This function
+// returns the size of these chunks.
+constexpr size_t PiecewiseChunkSize() { return 1024; }
+
 // HashStateBase
 //
 // A hash state object represents an intermediate state in the computation
@@ -69,7 +75,7 @@
 //
 //    `static H combine_contiguous(H state, const unsigned char*, size_t)`.
 //
-// `HashStateBase` will provide a complete implementations for a hash state
+// `HashStateBase` will provide a complete implementation for a hash state
 // object in terms of this method.
 //
 // Example:
@@ -118,6 +124,9 @@
   // for-loop instead.
   template <typename T>
   static H combine_contiguous(H state, const T* data, size_t size);
+
+ private:
+  friend class PiecewiseCombiner;
 };
 
 // is_uniquely_represented
@@ -188,6 +197,61 @@
   return H::combine_contiguous(std::move(hash_state), start, sizeof(value));
 }
 
+// PiecewiseCombiner
+//
+// PiecewiseCombiner is an internal-only helper class for hashing a piecewise
+// buffer of `char` or `unsigned char` as though it were contiguous.  This class
+// provides two methods:
+//
+//   H add_buffer(state, data, size)
+//   H finalize(state)
+//
+// `add_buffer` can be called zero or more times, followed by a single call to
+// `finalize`.  This will produce the same hash expansion as concatenating each
+// buffer piece into a single contiguous buffer, and passing this to
+// `H::combine_contiguous`.
+//
+//  Example usage:
+//    PiecewiseCombiner combiner;
+//    for (const auto& piece : pieces) {
+//      state = combiner.add_buffer(std::move(state), piece.data, piece.size);
+//    }
+//    return combiner.finalize(std::move(state));
+class PiecewiseCombiner {
+ public:
+  PiecewiseCombiner() : position_(0) {}
+  PiecewiseCombiner(const PiecewiseCombiner&) = delete;
+  PiecewiseCombiner& operator=(const PiecewiseCombiner&) = delete;
+
+  // PiecewiseCombiner::add_buffer()
+  //
+  // Appends the given range of bytes to the sequence to be hashed, which may
+  // modify the provided hash state.
+  template <typename H>
+  H add_buffer(H state, const unsigned char* data, size_t size);
+  template <typename H>
+  H add_buffer(H state, const char* data, size_t size) {
+    return add_buffer(std::move(state),
+                      reinterpret_cast<const unsigned char*>(data), size);
+  }
+
+  // PiecewiseCombiner::finalize()
+  //
+  // Finishes combining the hash sequence, which may may modify the provided
+  // hash state.
+  //
+  // Once finalize() is called, add_buffer() may no longer be called. The
+  // resulting hash state will be the same as if the pieces passed to
+  // add_buffer() were concatenated into a single flat buffer, and then provided
+  // to H::combine_contiguous().
+  template <typename H>
+  H finalize(H state);
+
+ private:
+  unsigned char buf_[PiecewiseChunkSize()];
+  size_t position_;
+};
+
 // -----------------------------------------------------------------------------
 // AbslHashValue for Basic Types
 // -----------------------------------------------------------------------------
@@ -364,6 +428,19 @@
       str.size());
 }
 
+// Support std::wstring, std::u16string and std::u32string.
+template <typename Char, typename Alloc, typename H,
+          typename = absl::enable_if_t<std::is_same<Char, wchar_t>::value ||
+                                       std::is_same<Char, char16_t>::value ||
+                                       std::is_same<Char, char32_t>::value>>
+H AbslHashValue(
+    H hash_state,
+    const std::basic_string<Char, std::char_traits<Char>, Alloc>& str) {
+  return H::combine(
+      H::combine_contiguous(std::move(hash_state), str.data(), str.size()),
+      str.size());
+}
+
 // -----------------------------------------------------------------------------
 // AbslHashValue for Sequence Containers
 // -----------------------------------------------------------------------------
@@ -631,7 +708,8 @@
     : std::integral_constant<bool, HashSelect::template Apply<T>::value> {};
 
 // CityHashState
-class CityHashState : public HashStateBase<CityHashState> {
+class ABSL_DLL CityHashState
+    : public HashStateBase<CityHashState> {
   // absl::uint128 is not an alias or a thin wrapper around the intrinsic.
   // We use the intrinsic when available to improve performance.
 #ifdef ABSL_HAVE_INTRINSIC_INT128
@@ -710,6 +788,16 @@
                                         std::integral_constant<int, 8>
                                         /* sizeof_size_t*/);
 
+  // Slow dispatch path for calls to CombineContiguousImpl with a size argument
+  // larger than PiecewiseChunkSize().  Has the same effect as calling
+  // CombineContiguousImpl() repeatedly with the chunk stride size.
+  static uint64_t CombineLargeContiguousImpl32(uint64_t state,
+                                               const unsigned char* first,
+                                               size_t len);
+  static uint64_t CombineLargeContiguousImpl64(uint64_t state,
+                                               const unsigned char* first,
+                                               size_t len);
+
   // Reads 9 to 16 bytes from p.
   // The first 8 bytes are in .first, the rest (zero padded) bytes are in
   // .second.
@@ -777,6 +865,9 @@
   // multiplicative hash.
   uint64_t v;
   if (len > 8) {
+    if (ABSL_PREDICT_FALSE(len > PiecewiseChunkSize())) {
+      return CombineLargeContiguousImpl32(state, first, len);
+    }
     v = absl::hash_internal::CityHash32(reinterpret_cast<const char*>(first), len);
   } else if (len >= 4) {
     v = Read4To8(first, len);
@@ -797,6 +888,9 @@
   // multiplicative hash.
   uint64_t v;
   if (len > 16) {
+    if (ABSL_PREDICT_FALSE(len > PiecewiseChunkSize())) {
+      return CombineLargeContiguousImpl64(state, first, len);
+    }
     v = absl::hash_internal::CityHash64(reinterpret_cast<const char*>(first), len);
   } else if (len > 8) {
     auto p = Read9To16(first, len);
@@ -813,7 +907,6 @@
   return Mix(state, v);
 }
 
-
 struct AggregateBarrier {};
 
 // HashImpl
@@ -850,8 +943,46 @@
 H HashStateBase<H>::combine_contiguous(H state, const T* data, size_t size) {
   return hash_internal::hash_range_or_bytes(std::move(state), data, size);
 }
+
+// HashStateBase::PiecewiseCombiner::add_buffer()
+template <typename H>
+H PiecewiseCombiner::add_buffer(H state, const unsigned char* data,
+                                size_t size) {
+  if (position_ + size < PiecewiseChunkSize()) {
+    // This partial chunk does not fill our existing buffer
+    memcpy(buf_ + position_, data, size);
+    position_ += size;
+    return state;
+  }
+
+  // Complete the buffer and hash it
+  const size_t bytes_needed = PiecewiseChunkSize() - position_;
+  memcpy(buf_ + position_, data, bytes_needed);
+  state = H::combine_contiguous(std::move(state), buf_, PiecewiseChunkSize());
+  data += bytes_needed;
+  size -= bytes_needed;
+
+  // Hash whatever chunks we can without copying
+  while (size >= PiecewiseChunkSize()) {
+    state = H::combine_contiguous(std::move(state), data, PiecewiseChunkSize());
+    data += PiecewiseChunkSize();
+    size -= PiecewiseChunkSize();
+  }
+  // Fill the buffer with the remainder
+  memcpy(buf_, data, size);
+  position_ = size;
+  return state;
+}
+
+// HashStateBase::PiecewiseCombiner::finalize()
+template <typename H>
+H PiecewiseCombiner::finalize(H state) {
+  // Hash the remainder left in the buffer, which may be empty
+  return H::combine_contiguous(std::move(state), buf_, position_);
+}
+
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_HASH_H_
diff --git a/absl/hash/internal/spy_hash_state.h b/absl/hash/internal/spy_hash_state.h
index 57cd70b..c083120 100644
--- a/absl/hash/internal/spy_hash_state.h
+++ b/absl/hash/internal/spy_hash_state.h
@@ -25,7 +25,7 @@
 #include "absl/strings/str_join.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace hash_internal {
 
 // SpyHashState is an implementation of the HashState API that simply
@@ -147,6 +147,19 @@
   static SpyHashStateImpl combine_contiguous(SpyHashStateImpl hash_state,
                                              const unsigned char* begin,
                                              size_t size) {
+    const size_t large_chunk_stride = PiecewiseChunkSize();
+    if (size > large_chunk_stride) {
+      // Combining a large contiguous buffer must have the same effect as
+      // doing it piecewise by the stride length, followed by the (possibly
+      // empty) remainder.
+      while (size >= large_chunk_stride) {
+        hash_state = SpyHashStateImpl::combine_contiguous(
+            std::move(hash_state), begin, large_chunk_stride);
+        begin += large_chunk_stride;
+        size -= large_chunk_stride;
+      }
+    }
+
     hash_state.hash_representation_.emplace_back(
         reinterpret_cast<const char*>(begin), size);
     return hash_state;
@@ -212,7 +225,7 @@
 using SpyHashState = SpyHashStateImpl<void>;
 
 }  // namespace hash_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_HASH_INTERNAL_SPY_HASH_STATE_H_
diff --git a/absl/memory/BUILD.bazel b/absl/memory/BUILD.bazel
index f815ef9..2ba9d7c 100644
--- a/absl/memory/BUILD.bazel
+++ b/absl/memory/BUILD.bazel
@@ -1,5 +1,5 @@
 #
-# Copyright 2017 The Abseil Authors.
+# Copyright 2019 The Abseil Authors.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,13 +14,12 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
     "ABSL_TEST_COPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
 )
 
 package(default_visibility = ["//visibility:public"])
@@ -45,7 +44,6 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":memory",
-        "//absl/base",
         "//absl/base:core_headers",
         "@com_google_googletest//:gtest_main",
     ],
@@ -56,10 +54,11 @@
     srcs = [
         "memory_exception_safety_test.cc",
     ],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":memory",
+        "//absl/base:config",
         "//absl/base:exception_safety_testing",
         "@com_google_googletest//:gtest_main",
     ],
diff --git a/absl/memory/CMakeLists.txt b/absl/memory/CMakeLists.txt
index 0a81220..78fb7e1 100644
--- a/absl/memory/CMakeLists.txt
+++ b/absl/memory/CMakeLists.txt
@@ -36,7 +36,6 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::memory
-    absl::base
     absl::core_headers
     gmock_main
 )
@@ -48,11 +47,9 @@
     "memory_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::memory
+    absl::config
     absl::exception_safety_testing
     gmock_main
 )
diff --git a/absl/memory/memory.h b/absl/memory/memory.h
index 7f525e4..513f710 100644
--- a/absl/memory/memory.h
+++ b/absl/memory/memory.h
@@ -34,7 +34,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Function Template: WrapUnique()
@@ -93,11 +93,12 @@
 
 }  // namespace memory_internal
 
-// gcc 4.8 has __cplusplus at 201301 but doesn't define make_unique.  Other
-// supported compilers either just define __cplusplus as 201103 but have
-// make_unique (msvc), or have make_unique whenever __cplusplus > 201103 (clang)
+// gcc 4.8 has __cplusplus at 201301 but the libstdc++ shipped with it doesn't
+// define make_unique.  Other supported compilers either just define __cplusplus
+// as 201103 but have make_unique (msvc), or have make_unique whenever
+// __cplusplus > 201103 (clang).
 #if (__cplusplus > 201103L || defined(_MSC_VER)) && \
-    !(defined(__GNUC__) && __GNUC__ == 4 && __GNUC_MINOR__ == 8)
+    !(defined(__GLIBCXX__) && !defined(__cpp_lib_make_unique))
 using std::make_unique;
 #else
 // -----------------------------------------------------------------------------
@@ -688,7 +689,7 @@
   }
 }
 }  // namespace memory_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_MEMORY_MEMORY_H_
diff --git a/absl/memory/memory_exception_safety_test.cc b/absl/memory/memory_exception_safety_test.cc
index f5b39b3..1df7261 100644
--- a/absl/memory/memory_exception_safety_test.cc
+++ b/absl/memory/memory_exception_safety_test.cc
@@ -14,18 +14,19 @@
 
 #include "absl/memory/memory.h"
 
+#include "absl/base/config.h"
+
+#ifdef ABSL_HAVE_EXCEPTIONS
+
 #include "gtest/gtest.h"
 #include "absl/base/internal/exception_safety_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 constexpr int kLength = 50;
 using Thrower = testing::ThrowingValue<testing::TypeSpec::kEverythingThrows>;
-using ThrowerStorage =
-    absl::aligned_storage_t<sizeof(Thrower), alignof(Thrower)>;
-using ThrowerList = std::array<ThrowerStorage, kLength>;
 
 TEST(MakeUnique, CheckForLeaks) {
   constexpr int kValue = 321;
@@ -50,5 +51,7 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
+
+#endif  // ABSL_HAVE_EXCEPTIONS
diff --git a/absl/meta/BUILD.bazel b/absl/meta/BUILD.bazel
index e004b50..c06d2d9 100644
--- a/absl/meta/BUILD.bazel
+++ b/absl/meta/BUILD.bazel
@@ -1,3 +1,20 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index ae7130d..ba87d2f 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -41,8 +41,18 @@
 
 #include "absl/base/config.h"
 
+// MSVC constructibility traits do not detect destructor properties and so our
+// implementations should not use them as a source-of-truth.
+#if defined(_MSC_VER) && !defined(__clang__) && !defined(__GNUC__)
+#define ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION 1
+#endif
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
+
+// Defined and documented later on in this file.
+template <typename T>
+struct is_trivially_destructible;
 
 // Defined and documented later on in this file.
 template <typename T>
@@ -67,6 +77,20 @@
 #endif  // defined(_MSC_VER) && !defined(__GNUC__)
 
 template <class T>
+struct IsTriviallyMoveConstructibleObject
+    : std::integral_constant<
+          bool, std::is_move_constructible<
+                    type_traits_internal::SingleMemberUnion<T>>::value &&
+                    absl::is_trivially_destructible<T>::value> {};
+
+template <class T>
+struct IsTriviallyCopyConstructibleObject
+    : std::integral_constant<
+          bool, std::is_copy_constructible<
+                    type_traits_internal::SingleMemberUnion<T>>::value &&
+                    absl::is_trivially_destructible<T>::value> {};
+
+template <class T>
 struct IsTriviallyMoveAssignableReference : std::false_type {};
 
 template <class T>
@@ -146,6 +170,18 @@
 
 }  // namespace type_traits_internal
 
+// MSVC 19.20 has a regression that causes our workarounds to fail, but their
+// std forms now appear to be compliant.
+#if defined(_MSC_VER) && !defined(__clang__) && (_MSC_VER >= 1920)
+
+template <typename T>
+using is_copy_assignable = std::is_copy_assignable<T>;
+
+template <typename T>
+using is_move_assignable = std::is_move_assignable<T>;
+
+#else
+
 template <typename T>
 struct is_copy_assignable : type_traits_internal::is_detected<
                                 type_traits_internal::IsCopyAssignableImpl, T> {
@@ -156,6 +192,8 @@
                                 type_traits_internal::IsMoveAssignableImpl, T> {
 };
 
+#endif
+
 // void_t()
 //
 // Ignores the type of any its arguments and returns `void`. In general, this
@@ -244,7 +282,7 @@
 
 // is_trivially_destructible()
 //
-// Determines whether the passed type `T` is trivially destructable.
+// Determines whether the passed type `T` is trivially destructible.
 //
 // This metafunction is designed to be a drop-in replacement for the C++11
 // `std::is_trivially_destructible()` metafunction for platforms that have
@@ -310,7 +348,9 @@
     : std::integral_constant<bool, __has_trivial_constructor(T) &&
                                    std::is_default_constructible<T>::value &&
                                    is_trivially_destructible<T>::value> {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
+#if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \
+    !defined(                                            \
+        ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION)
  private:
   static constexpr bool compliant =
       std::is_trivially_default_constructible<T>::value ==
@@ -341,10 +381,11 @@
 struct is_trivially_move_constructible
     : std::conditional<
           std::is_object<T>::value && !std::is_array<T>::value,
-          std::is_move_constructible<
-              type_traits_internal::SingleMemberUnion<T>>,
+          type_traits_internal::IsTriviallyMoveConstructibleObject<T>,
           std::is_reference<T>>::type::type {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
+#if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \
+    !defined(                                            \
+        ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION)
  private:
   static constexpr bool compliant =
       std::is_trivially_move_constructible<T>::value ==
@@ -375,10 +416,11 @@
 struct is_trivially_copy_constructible
     : std::conditional<
           std::is_object<T>::value && !std::is_array<T>::value,
-          std::is_copy_constructible<
-              type_traits_internal::SingleMemberUnion<T>>,
+          type_traits_internal::IsTriviallyCopyConstructibleObject<T>,
           std::is_lvalue_reference<T>>::type::type {
-#ifdef ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE
+#if defined(ABSL_HAVE_STD_IS_TRIVIALLY_CONSTRUCTIBLE) && \
+    !defined(                                            \
+        ABSL_META_INTERNAL_STD_CONSTRUCTION_TRAITS_DONT_CHECK_DESTRUCTION)
  private:
   static constexpr bool compliant =
       std::is_trivially_copy_constructible<T>::value ==
@@ -410,7 +452,8 @@
 template <typename T>
 struct is_trivially_move_assignable
     : std::conditional<
-          std::is_object<T>::value && !std::is_array<T>::value,
+          std::is_object<T>::value && !std::is_array<T>::value &&
+              std::is_move_assignable<T>::value,
           std::is_move_assignable<type_traits_internal::SingleMemberUnion<T>>,
           type_traits_internal::IsTriviallyMoveAssignableReference<T>>::type::
           type {
@@ -710,7 +753,7 @@
 using swap_internal::StdSwapIsUnconstrained;
 
 }  // namespace type_traits_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_META_TYPE_TRAITS_H_
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index a7a9c5c..1aafd0d 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -347,21 +347,6 @@
   virtual ~Base() {}
 };
 
-// In GCC/Clang, std::is_trivially_constructible requires that the destructor is
-// trivial. However, MSVC doesn't require that. This results in different
-// behavior when checking is_trivially_constructible on any type with
-// nontrivial destructor. Since absl::is_trivially_default_constructible and
-// absl::is_trivially_copy_constructible both follows Clang/GCC's interpretation
-// and check is_trivially_destructible, it results in inconsistency with
-// std::is_trivially_xxx_constructible on MSVC. This macro is used to work
-// around this issue in test. In practice, a trivially constructible type
-// should also be trivially destructible.
-// GCC bug 51452: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51452
-// LWG issue 2116: http://cplusplus.github.io/LWG/lwg-active.html#2116
-#ifndef _MSC_VER
-#define ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE 1
-#endif
-
 // Old versions of libc++, around Clang 3.5 to 3.6, consider deleted destructors
 // as also being trivial. With the resolution of CWG 1928 and CWG 1734, this
 // is no longer considered true and has thus been amended.
@@ -499,11 +484,9 @@
   EXPECT_FALSE(
       absl::is_trivially_default_constructible<DeletedDefaultCtor>::value);
 
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
   // types with nontrivial destructor are nontrivial
   EXPECT_FALSE(
       absl::is_trivially_default_constructible<NontrivialDestructor>::value);
-#endif
 
   // types with vtables
   EXPECT_FALSE(absl::is_trivially_default_constructible<Base>::value);
@@ -546,6 +529,28 @@
 #endif
 }
 
+// GCC prior to 7.4 had a bug in its trivially-constructible traits
+// (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80654).
+// This test makes sure that we do not depend on the trait in these cases when
+// implementing absl triviality traits.
+
+template <class T>
+struct BadConstructors {
+  BadConstructors() { static_assert(T::value, ""); }
+
+  BadConstructors(BadConstructors&&) { static_assert(T::value, ""); }
+
+  BadConstructors(const BadConstructors&) { static_assert(T::value, ""); }
+};
+
+TEST(TypeTraitsTest, TestTrivialityBadConstructors) {
+  using BadType = BadConstructors<int>;
+
+  EXPECT_FALSE(absl::is_trivially_default_constructible<BadType>::value);
+  EXPECT_FALSE(absl::is_trivially_move_constructible<BadType>::value);
+  EXPECT_FALSE(absl::is_trivially_copy_constructible<BadType>::value);
+}
+
 TEST(TypeTraitsTest, TestTrivialMoveCtor) {
   // Verify that arithmetic types and pointers have trivial move
   // constructors.
@@ -585,11 +590,9 @@
   EXPECT_FALSE(
       absl::is_trivially_move_constructible<NonCopyableOrMovable>::value);
 
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
   // type with nontrivial destructor are nontrivial move construbtible
   EXPECT_FALSE(
       absl::is_trivially_move_constructible<NontrivialDestructor>::value);
-#endif
 
   // types with vtables
   EXPECT_FALSE(absl::is_trivially_move_constructible<Base>::value);
@@ -660,11 +663,9 @@
   EXPECT_FALSE(
       absl::is_trivially_copy_constructible<NonCopyableOrMovable>::value);
 
-#ifdef ABSL_TRIVIALLY_CONSTRUCTIBLE_VERIFY_TRIVIALLY_DESTRUCTIBLE
   // type with nontrivial destructor are nontrivial copy construbtible
   EXPECT_FALSE(
       absl::is_trivially_copy_constructible<NontrivialDestructor>::value);
-#endif
 
   // types with vtables
   EXPECT_FALSE(absl::is_trivially_copy_constructible<Base>::value);
diff --git a/absl/numeric/BUILD.bazel b/absl/numeric/BUILD.bazel
index d9b561d..e09e52d 100644
--- a/absl/numeric/BUILD.bazel
+++ b/absl/numeric/BUILD.bazel
@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
diff --git a/absl/numeric/int128.cc b/absl/numeric/int128.cc
index a22f1e3..b605a87 100644
--- a/absl/numeric/int128.cc
+++ b/absl/numeric/int128.cc
@@ -23,10 +23,10 @@
 #include <type_traits>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
-const uint128 kuint128max = MakeUint128(std::numeric_limits<uint64_t>::max(),
-                                        std::numeric_limits<uint64_t>::max());
+ABSL_DLL const uint128 kuint128max = MakeUint128(
+    std::numeric_limits<uint64_t>::max(), std::numeric_limits<uint64_t>::max());
 
 namespace {
 
@@ -245,7 +245,112 @@
   return os << rep;
 }
 
-}  // inline namespace lts_2019_08_08
+namespace {
+
+uint128 UnsignedAbsoluteValue(int128 v) {
+  // Cast to uint128 before possibly negating because -Int128Min() is undefined.
+  return Int128High64(v) < 0 ? -uint128(v) : uint128(v);
+}
+
+}  // namespace
+
+#if !defined(ABSL_HAVE_INTRINSIC_INT128)
+namespace {
+
+template <typename T>
+int128 MakeInt128FromFloat(T v) {
+  // Conversion when v is NaN or cannot fit into int128 would be undefined
+  // behavior if using an intrinsic 128-bit integer.
+  assert(std::isfinite(v) && (std::numeric_limits<T>::max_exponent <= 127 ||
+                              (v >= -std::ldexp(static_cast<T>(1), 127) &&
+                               v < std::ldexp(static_cast<T>(1), 127))));
+
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  uint128 result = v < 0 ? -MakeUint128FromFloat(-v) : MakeUint128FromFloat(v);
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(result)),
+                    Uint128Low64(result));
+}
+
+}  // namespace
+
+int128::int128(float v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(double v) : int128(MakeInt128FromFloat(v)) {}
+int128::int128(long double v) : int128(MakeInt128FromFloat(v)) {}
+
+int128 operator/(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if ((Int128High64(lhs) < 0) != (Int128High64(rhs) < 0)) quotient = -quotient;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(quotient)),
+                    Uint128Low64(quotient));
+}
+
+int128 operator%(int128 lhs, int128 rhs) {
+  assert(lhs != Int128Min() || rhs != -1);  // UB on two's complement.
+
+  uint128 quotient = 0;
+  uint128 remainder = 0;
+  DivModImpl(UnsignedAbsoluteValue(lhs), UnsignedAbsoluteValue(rhs),
+             &quotient, &remainder);
+  if (Int128High64(lhs) < 0) remainder = -remainder;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(remainder)),
+                    Uint128Low64(remainder));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+std::ostream& operator<<(std::ostream& os, int128 v) {
+  std::ios_base::fmtflags flags = os.flags();
+  std::string rep;
+
+  // Add the sign if needed.
+  bool print_as_decimal =
+      (flags & std::ios::basefield) == std::ios::dec ||
+      (flags & std::ios::basefield) == std::ios_base::fmtflags();
+  if (print_as_decimal) {
+    if (Int128High64(v) < 0) {
+      rep = "-";
+    } else if (flags & std::ios::showpos) {
+      rep = "+";
+    }
+  }
+
+  rep.append(Uint128ToFormattedString(
+      print_as_decimal ? UnsignedAbsoluteValue(v) : uint128(v), os.flags()));
+
+  // Add the requisite padding.
+  std::streamsize width = os.width(0);
+  if (static_cast<size_t>(width) > rep.size()) {
+    switch (flags & std::ios::adjustfield) {
+      case std::ios::left:
+        rep.append(width - rep.size(), os.fill());
+        break;
+      case std::ios::internal:
+        if (print_as_decimal && (rep[0] == '+' || rep[0] == '-')) {
+          rep.insert(1, width - rep.size(), os.fill());
+        } else if ((flags & std::ios::basefield) == std::ios::hex &&
+                   (flags & std::ios::showbase) && v != 0) {
+          rep.insert(2, width - rep.size(), os.fill());
+        } else {
+          rep.insert(0, width - rep.size(), os.fill());
+        }
+        break;
+      default:  // std::ios::right
+        rep.insert(0, width - rep.size(), os.fill());
+        break;
+    }
+  }
+
+  return os << rep;
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace std {
@@ -272,4 +377,28 @@
 constexpr int numeric_limits<absl::uint128>::max_exponent10;
 constexpr bool numeric_limits<absl::uint128>::traps;
 constexpr bool numeric_limits<absl::uint128>::tinyness_before;
+
+constexpr bool numeric_limits<absl::int128>::is_specialized;
+constexpr bool numeric_limits<absl::int128>::is_signed;
+constexpr bool numeric_limits<absl::int128>::is_integer;
+constexpr bool numeric_limits<absl::int128>::is_exact;
+constexpr bool numeric_limits<absl::int128>::has_infinity;
+constexpr bool numeric_limits<absl::int128>::has_quiet_NaN;
+constexpr bool numeric_limits<absl::int128>::has_signaling_NaN;
+constexpr float_denorm_style numeric_limits<absl::int128>::has_denorm;
+constexpr bool numeric_limits<absl::int128>::has_denorm_loss;
+constexpr float_round_style numeric_limits<absl::int128>::round_style;
+constexpr bool numeric_limits<absl::int128>::is_iec559;
+constexpr bool numeric_limits<absl::int128>::is_bounded;
+constexpr bool numeric_limits<absl::int128>::is_modulo;
+constexpr int numeric_limits<absl::int128>::digits;
+constexpr int numeric_limits<absl::int128>::digits10;
+constexpr int numeric_limits<absl::int128>::max_digits10;
+constexpr int numeric_limits<absl::int128>::radix;
+constexpr int numeric_limits<absl::int128>::min_exponent;
+constexpr int numeric_limits<absl::int128>::min_exponent10;
+constexpr int numeric_limits<absl::int128>::max_exponent;
+constexpr int numeric_limits<absl::int128>::max_exponent10;
+constexpr bool numeric_limits<absl::int128>::traps;
+constexpr bool numeric_limits<absl::int128>::tinyness_before;
 }  // namespace std
diff --git a/absl/numeric/int128.h b/absl/numeric/int128.h
index a9693a2..636e3a5 100644
--- a/absl/numeric/int128.h
+++ b/absl/numeric/int128.h
@@ -17,10 +17,7 @@
 // File: int128.h
 // -----------------------------------------------------------------------------
 //
-// This header file defines 128-bit integer types.
-//
-// Currently, this file defines `uint128`, an unsigned 128-bit integer;
-// a signed 128-bit integer is forthcoming.
+// This header file defines 128-bit integer types, `uint128` and `int128`.
 
 #ifndef ABSL_NUMERIC_INT128_H_
 #define ABSL_NUMERIC_INT128_H_
@@ -52,7 +49,9 @@
 #endif  // defined(_MSC_VER)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
+
+class int128;
 
 // uint128
 //
@@ -117,6 +116,7 @@
   constexpr uint128(__int128 v);           // NOLINT(runtime/explicit)
   constexpr uint128(unsigned __int128 v);  // NOLINT(runtime/explicit)
 #endif  // ABSL_HAVE_INTRINSIC_INT128
+  constexpr uint128(int128 v);  // NOLINT(runtime/explicit)
   explicit uint128(float v);
   explicit uint128(double v);
   explicit uint128(long double v);
@@ -132,6 +132,7 @@
   uint128& operator=(__int128 v);
   uint128& operator=(unsigned __int128 v);
 #endif  // ABSL_HAVE_INTRINSIC_INT128
+  uint128& operator=(int128 v);
 
   // Conversion operators to other arithmetic types
   constexpr explicit operator bool() const;
@@ -233,7 +234,7 @@
 // Prefer to use the constexpr `Uint128Max()`.
 //
 // TODO(absl-team) deprecate kuint128max once migration tool is released.
-extern const uint128 kuint128max;
+ABSL_DLL extern const uint128 kuint128max;
 
 // allow uint128 to be logged
 std::ostream& operator<<(std::ostream& os, uint128 v);
@@ -245,7 +246,7 @@
                  (std::numeric_limits<uint64_t>::max)());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // Specialized numeric_limits for uint128.
@@ -293,13 +294,246 @@
 };
 }  // namespace std
 
-// TODO(absl-team): Implement signed 128-bit type
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// int128
+//
+// A signed 128-bit integer type. The API is meant to mimic an intrinsic
+// integral type as closely as is practical, including exhibiting undefined
+// behavior in analogous cases (e.g. division by zero).
+//
+// An `int128` supports the following:
+//
+//   * Implicit construction from integral types
+//   * Explicit conversion to integral types
+//
+// However, an `int128` differs from intrinsic integral types in the following
+// ways:
+//
+//   * It is not implicitly convertible to other integral types.
+//   * Requires explicit construction from and conversion to floating point
+//     types.
+
+// Additionally, if your compiler supports `__int128`, `int128` is
+// interoperable with that type. (Abseil checks for this compatibility through
+// the `ABSL_HAVE_INTRINSIC_INT128` macro.)
+//
+// The design goal for `int128` is that it will be compatible with a future
+// `int128_t`, if that type becomes a part of the standard.
+//
+// Example:
+//
+//     float y = absl::int128(17);  // Error. int128 cannot be implicitly
+//                                  // converted to float.
+//
+//     absl::int128 v;
+//     int64_t i = v;                        // Error
+//     int64_t i = static_cast<int64_t>(v);  // OK
+//
+class int128 {
+ public:
+  int128() = default;
+
+  // Constructors from arithmetic types
+  constexpr int128(int v);                 // NOLINT(runtime/explicit)
+  constexpr int128(unsigned int v);        // NOLINT(runtime/explicit)
+  constexpr int128(long v);                // NOLINT(runtime/int)
+  constexpr int128(unsigned long v);       // NOLINT(runtime/int)
+  constexpr int128(long long v);           // NOLINT(runtime/int)
+  constexpr int128(unsigned long long v);  // NOLINT(runtime/int)
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr int128(__int128 v);  // NOLINT(runtime/explicit)
+  constexpr explicit int128(unsigned __int128 v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit int128(uint128 v);
+  explicit int128(float v);
+  explicit int128(double v);
+  explicit int128(long double v);
+
+  // Assignment operators from arithmetic types
+  int128& operator=(int v);
+  int128& operator=(unsigned int v);
+  int128& operator=(long v);                // NOLINT(runtime/int)
+  int128& operator=(unsigned long v);       // NOLINT(runtime/int)
+  int128& operator=(long long v);           // NOLINT(runtime/int)
+  int128& operator=(unsigned long long v);  // NOLINT(runtime/int)
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  int128& operator=(__int128 v);
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+  // Conversion operators to other arithmetic types
+  constexpr explicit operator bool() const;
+  constexpr explicit operator char() const;
+  constexpr explicit operator signed char() const;
+  constexpr explicit operator unsigned char() const;
+  constexpr explicit operator char16_t() const;
+  constexpr explicit operator char32_t() const;
+  constexpr explicit operator ABSL_INTERNAL_WCHAR_T() const;
+  constexpr explicit operator short() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned short() const;
+  constexpr explicit operator int() const;
+  constexpr explicit operator unsigned int() const;
+  constexpr explicit operator long() const;  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator long long() const;
+  // NOLINTNEXTLINE(runtime/int)
+  constexpr explicit operator unsigned long long() const;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  constexpr explicit operator __int128() const;
+  constexpr explicit operator unsigned __int128() const;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  explicit operator float() const;
+  explicit operator double() const;
+  explicit operator long double() const;
+
+  // Trivial copy constructor, assignment operator and destructor.
+
+  // Arithmetic operators
+  int128& operator+=(int128 other);
+  int128& operator-=(int128 other);
+  int128& operator*=(int128 other);
+  int128& operator/=(int128 other);
+  int128& operator%=(int128 other);
+  int128 operator++(int);  // postfix increment: i++
+  int128 operator--(int);  // postfix decrement: i--
+  int128& operator++();    // prefix increment:  ++i
+  int128& operator--();    // prefix decrement:  --i
+  int128& operator&=(int128 other);
+  int128& operator|=(int128 other);
+  int128& operator^=(int128 other);
+  int128& operator<<=(int amount);
+  int128& operator>>=(int amount);
+
+  // Int128Low64()
+  //
+  // Returns the lower 64-bit value of a `int128` value.
+  friend constexpr uint64_t Int128Low64(int128 v);
+
+  // Int128High64()
+  //
+  // Returns the higher 64-bit value of a `int128` value.
+  friend constexpr int64_t Int128High64(int128 v);
+
+  // MakeInt128()
+  //
+  // Constructs a `int128` numeric value from two 64-bit integers. Note that
+  // signedness is conveyed in the upper `high` value.
+  //
+  //   (absl::int128(1) << 64) * high + low
+  //
+  // Note that this factory function is the only way to construct a `int128`
+  // from integer values greater than 2^64 or less than -2^64.
+  //
+  // Example:
+  //
+  //   absl::int128 big = absl::MakeInt128(1, 0);
+  //   absl::int128 big_n = absl::MakeInt128(-1, 0);
+  friend constexpr int128 MakeInt128(int64_t high, uint64_t low);
+
+  // Int128Max()
+  //
+  // Returns the maximum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Max();
+
+  // Int128Min()
+  //
+  // Returns the minimum value for a 128-bit signed integer.
+  friend constexpr int128 Int128Min();
+
+  // Support for absl::Hash.
+  template <typename H>
+  friend H AbslHashValue(H h, int128 v) {
+    return H::combine(std::move(h), Int128High64(v), Int128Low64(v));
+  }
+
+ private:
+  constexpr int128(int64_t high, uint64_t low);
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  __int128 v_;
+#else  // ABSL_HAVE_INTRINSIC_INT128
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+  uint64_t lo_;
+  int64_t hi_;
+#elif defined(ABSL_IS_BIG_ENDIAN)
+  int64_t hi_;
+  uint64_t lo_;
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+};
+
+std::ostream& operator<<(std::ostream& os, int128 v);
+
+// TODO(absl-team) add operator>>(std::istream&, int128)
+
+constexpr int128 Int128Max() {
+  return int128((std::numeric_limits<int64_t>::max)(),
+                (std::numeric_limits<uint64_t>::max)());
+}
+
+constexpr int128 Int128Min() {
+  return int128((std::numeric_limits<int64_t>::min)(), 0);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+// Specialized numeric_limits for int128.
+namespace std {
+template <>
+class numeric_limits<absl::int128> {
+ public:
+  static constexpr bool is_specialized = true;
+  static constexpr bool is_signed = true;
+  static constexpr bool is_integer = true;
+  static constexpr bool is_exact = true;
+  static constexpr bool has_infinity = false;
+  static constexpr bool has_quiet_NaN = false;
+  static constexpr bool has_signaling_NaN = false;
+  static constexpr float_denorm_style has_denorm = denorm_absent;
+  static constexpr bool has_denorm_loss = false;
+  static constexpr float_round_style round_style = round_toward_zero;
+  static constexpr bool is_iec559 = false;
+  static constexpr bool is_bounded = true;
+  static constexpr bool is_modulo = false;
+  static constexpr int digits = 127;
+  static constexpr int digits10 = 38;
+  static constexpr int max_digits10 = 0;
+  static constexpr int radix = 2;
+  static constexpr int min_exponent = 0;
+  static constexpr int min_exponent10 = 0;
+  static constexpr int max_exponent = 0;
+  static constexpr int max_exponent10 = 0;
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<__int128>::traps;
+#else   // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool traps = numeric_limits<uint64_t>::traps;
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+  static constexpr bool tinyness_before = false;
+
+  static constexpr absl::int128 (min)() { return absl::Int128Min(); }
+  static constexpr absl::int128 lowest() { return absl::Int128Min(); }
+  static constexpr absl::int128 (max)() { return absl::Int128Max(); }
+  static constexpr absl::int128 epsilon() { return 0; }
+  static constexpr absl::int128 round_error() { return 0; }
+  static constexpr absl::int128 infinity() { return 0; }
+  static constexpr absl::int128 quiet_NaN() { return 0; }
+  static constexpr absl::int128 signaling_NaN() { return 0; }
+  static constexpr absl::int128 denorm_min() { return 0; }
+};
+}  // namespace std
 
 // --------------------------------------------------------------------------
 //                      Implementation details follow
 // --------------------------------------------------------------------------
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 constexpr uint128 MakeUint128(uint64_t high, uint64_t low) {
   return uint128(high, low);
@@ -342,6 +576,10 @@
 }
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+inline uint128& uint128::operator=(int128 v) {
+  return *this = uint128(v);
+}
+
 // Arithmetic operators.
 
 uint128 operator<<(uint128 lhs, int amount);
@@ -423,6 +661,9 @@
       hi_{static_cast<uint64_t>(v >> 64)} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : lo_{Int128Low64(v)}, hi_{static_cast<uint64_t>(Int128High64(v))} {}
+
 #elif defined(ABSL_IS_BIG_ENDIAN)
 
 constexpr uint128::uint128(uint64_t high, uint64_t low)
@@ -453,6 +694,9 @@
       lo_{static_cast<uint64_t>(v & ~uint64_t{0})} {}
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
+constexpr uint128::uint128(int128 v)
+    : hi_{static_cast<uint64_t>(Int128High64(v))}, lo_{Int128Low64(v)} {}
+
 #else  // byte order
 #error "Unsupported byte order: must be little-endian or big-endian."
 #endif  // byte order
@@ -722,13 +966,124 @@
   return *this;
 }
 
+constexpr int128 MakeInt128(int64_t high, uint64_t low) {
+  return int128(high, low);
+}
+
+// Assignment from integer types.
+inline int128& int128::operator=(int v) {
+  return *this = int128(v);
+}
+
+inline int128& int128::operator=(unsigned int v) {
+  return *this = int128(v);
+}
+
+inline int128& int128::operator=(long v) {  // NOLINT(runtime/int)
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long v) {
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(long long v) {
+  return *this = int128(v);
+}
+
+// NOLINTNEXTLINE(runtime/int)
+inline int128& int128::operator=(unsigned long long v) {
+  return *this = int128(v);
+}
+
+// Arithmetic operators.
+
+int128 operator+(int128 lhs, int128 rhs);
+int128 operator-(int128 lhs, int128 rhs);
+int128 operator*(int128 lhs, int128 rhs);
+int128 operator/(int128 lhs, int128 rhs);
+int128 operator%(int128 lhs, int128 rhs);
+int128 operator|(int128 lhs, int128 rhs);
+int128 operator&(int128 lhs, int128 rhs);
+int128 operator^(int128 lhs, int128 rhs);
+int128 operator<<(int128 lhs, int amount);
+int128 operator>>(int128 lhs, int amount);
+
+inline int128& int128::operator+=(int128 other) {
+  *this = *this + other;
+  return *this;
+}
+
+inline int128& int128::operator-=(int128 other) {
+  *this = *this - other;
+  return *this;
+}
+
+inline int128& int128::operator*=(int128 other) {
+  *this = *this * other;
+  return *this;
+}
+
+inline int128& int128::operator/=(int128 other) {
+  *this = *this / other;
+  return *this;
+}
+
+inline int128& int128::operator%=(int128 other) {
+  *this = *this % other;
+  return *this;
+}
+
+inline int128& int128::operator|=(int128 other) {
+  *this = *this | other;
+  return *this;
+}
+
+inline int128& int128::operator&=(int128 other) {
+  *this = *this & other;
+  return *this;
+}
+
+inline int128& int128::operator^=(int128 other) {
+  *this = *this ^ other;
+  return *this;
+}
+
+inline int128& int128::operator<<=(int amount) {
+  *this = *this << amount;
+  return *this;
+}
+
+inline int128& int128::operator>>=(int amount) {
+  *this = *this >> amount;
+  return *this;
+}
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr int64_t BitCastToSigned(uint64_t v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang, GCC, and
+  // MSVC optimize this to a no-op on x86-64.
+  return v & (uint64_t{1} << 63) ? ~static_cast<int64_t>(~v)
+                                 : static_cast<int64_t>(v);
+}
+
+}  // namespace int128_internal
+
 #if defined(ABSL_HAVE_INTRINSIC_INT128)
-#include "absl/numeric/int128_have_intrinsic.inc"
+#include "absl/numeric/int128_have_intrinsic.inc"  // IWYU pragma: export
 #else  // ABSL_HAVE_INTRINSIC_INT128
-#include "absl/numeric/int128_no_intrinsic.inc"
+#include "absl/numeric/int128_no_intrinsic.inc"  // IWYU pragma: export
 #endif  // ABSL_HAVE_INTRINSIC_INT128
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_INTERNAL_WCHAR_T
diff --git a/absl/numeric/int128_have_intrinsic.inc b/absl/numeric/int128_have_intrinsic.inc
index c7ea683..d6c76dd 100644
--- a/absl/numeric/int128_have_intrinsic.inc
+++ b/absl/numeric/int128_have_intrinsic.inc
@@ -16,3 +16,287 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is defined. This file is
 // included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+namespace int128_internal {
+
+// Casts from unsigned to signed while preserving the underlying binary
+// representation.
+constexpr __int128 BitCastToSigned(unsigned __int128 v) {
+  // Casting an unsigned integer to a signed integer of the same
+  // width is implementation defined behavior if the source value would not fit
+  // in the destination type. We step around it with a roundtrip bitwise not
+  // operation to make sure this function remains constexpr. Clang and GCC
+  // optimize this to a no-op on x86-64.
+  return v & (static_cast<unsigned __int128>(1) << 127)
+             ? ~static_cast<__int128>(~v)
+             : static_cast<__int128>(v);
+}
+
+}  // namespace int128_internal
+
+inline int128& int128::operator=(__int128 v) {
+  v_ = v;
+  return *this;
+}
+
+constexpr uint64_t Int128Low64(int128 v) {
+  return static_cast<uint64_t>(v.v_ & ~uint64_t{0});
+}
+
+constexpr int64_t Int128High64(int128 v) {
+  // Initially cast to unsigned to prevent a right shift on a negative value.
+  return int128_internal::BitCastToSigned(
+      static_cast<uint64_t>(static_cast<unsigned __int128>(v.v_) >> 64));
+}
+
+constexpr int128::int128(int64_t high, uint64_t low)
+    // Initially cast to unsigned to prevent a left shift that overflows.
+    : v_(int128_internal::BitCastToSigned(static_cast<unsigned __int128>(high)
+                                           << 64) |
+         low) {}
+
+
+constexpr int128::int128(int v) : v_{v} {}
+
+constexpr int128::int128(long v) : v_{v} {}       // NOLINT(runtime/int)
+
+constexpr int128::int128(long long v) : v_{v} {}  // NOLINT(runtime/int)
+
+constexpr int128::int128(__int128 v) : v_{v} {}
+
+constexpr int128::int128(unsigned int v) : v_{v} {}
+
+constexpr int128::int128(unsigned long v) : v_{v} {}  // NOLINT(runtime/int)
+
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : v_{v} {}
+
+constexpr int128::int128(unsigned __int128 v) : v_{static_cast<__int128>(v)} {}
+
+inline int128::int128(float v) {
+  v_ = static_cast<__int128>(v);
+}
+
+inline int128::int128(double v) {
+  v_ = static_cast<__int128>(v);
+}
+
+inline int128::int128(long double v) {
+  v_ = static_cast<__int128>(v);
+}
+
+constexpr int128::int128(uint128 v) : v_{static_cast<__int128>(v)} {}
+
+constexpr int128::operator bool() const { return static_cast<bool>(v_); }
+
+constexpr int128::operator char() const { return static_cast<char>(v_); }
+
+constexpr int128::operator signed char() const {
+  return static_cast<signed char>(v_);
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(v_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(v_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(v_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(v_);
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  return static_cast<short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const {
+  return static_cast<int>(v_);
+}
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(v_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  return static_cast<long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  return static_cast<long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(v_);            // NOLINT(runtime/int)
+}
+
+constexpr int128::operator __int128() const { return v_; }
+
+constexpr int128::operator unsigned __int128() const {
+  return static_cast<unsigned __int128>(v_);
+}
+
+// Clang on PowerPC sometimes produces incorrect __int128 to floating point
+// conversions. In that case, we do the conversion with a similar implementation
+// to the conversion operators in int128_no_intrinsic.inc.
+#if defined(__clang__) && !defined(__ppc64__)
+inline int128::operator float() const { return static_cast<float>(v_); }
+
+inline int128::operator double () const { return static_cast<double>(v_); }
+
+inline int128::operator long double() const {
+  return static_cast<long double>(v_);
+}
+
+#else  // Clang on PowerPC
+// Forward declaration for conversion operators to floating point types.
+int128 operator-(int128 v);
+bool operator!=(int128 lhs, int128 rhs);
+
+inline int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<float>(Int128High64(*this)), 64);
+}
+
+inline int128::operator double() const {
+  // See comment in int128::operator float() above.
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<double>(Int128High64(*this)), 64);
+}
+
+inline int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  return v_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(Int128Low64(*this)) +
+                   std::ldexp(static_cast<long double>(Int128High64(*this)),
+                              64);
+}
+#endif  // Clang on PowerPC
+
+// Comparison operators.
+
+inline bool operator==(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) == static_cast<__int128>(rhs);
+}
+
+inline bool operator!=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) != static_cast<__int128>(rhs);
+}
+
+inline bool operator<(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) < static_cast<__int128>(rhs);
+}
+
+inline bool operator>(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) > static_cast<__int128>(rhs);
+}
+
+inline bool operator<=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) <= static_cast<__int128>(rhs);
+}
+
+inline bool operator>=(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) >= static_cast<__int128>(rhs);
+}
+
+// Unary operators.
+
+inline int128 operator-(int128 v) {
+  return -static_cast<__int128>(v);
+}
+
+inline bool operator!(int128 v) {
+  return !static_cast<__int128>(v);
+}
+
+inline int128 operator~(int128 val) {
+  return ~static_cast<__int128>(val);
+}
+
+// Arithmetic operators.
+
+inline int128 operator+(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) + static_cast<__int128>(rhs);
+}
+
+inline int128 operator-(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) - static_cast<__int128>(rhs);
+}
+
+inline int128 operator*(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) * static_cast<__int128>(rhs);
+}
+
+inline int128 operator/(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) / static_cast<__int128>(rhs);
+}
+
+inline int128 operator%(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) % static_cast<__int128>(rhs);
+}
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  ++v_;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  --v_;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  ++v_;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  --v_;
+  return *this;
+}
+
+inline int128 operator|(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) | static_cast<__int128>(rhs);
+}
+
+inline int128 operator&(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) & static_cast<__int128>(rhs);
+}
+
+inline int128 operator^(int128 lhs, int128 rhs) {
+  return static_cast<__int128>(lhs) ^ static_cast<__int128>(rhs);
+}
+
+inline int128 operator<<(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) << amount;
+}
+
+inline int128 operator>>(int128 lhs, int amount) {
+  return static_cast<__int128>(lhs) >> amount;
+}
diff --git a/absl/numeric/int128_no_intrinsic.inc b/absl/numeric/int128_no_intrinsic.inc
index 046cb9b..c753771 100644
--- a/absl/numeric/int128_no_intrinsic.inc
+++ b/absl/numeric/int128_no_intrinsic.inc
@@ -16,3 +16,293 @@
 // This file contains :int128 implementation details that depend on internal
 // representation when ABSL_HAVE_INTRINSIC_INT128 is *not* defined. This file
 // is included by int128.h and relies on ABSL_INTERNAL_WCHAR_T being defined.
+
+constexpr uint64_t Int128Low64(int128 v) { return v.lo_; }
+
+constexpr int64_t Int128High64(int128 v) { return v.hi_; }
+
+#if defined(ABSL_IS_LITTLE_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) :
+    lo_(low), hi_(high) {}
+
+constexpr int128::int128(int v)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : lo_{static_cast<uint64_t>(v)}, hi_{v < 0 ? ~int64_t{0} : 0} {}
+
+constexpr int128::int128(unsigned int v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : lo_{v}, hi_{0} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : lo_{v}, hi_{0} {}
+
+constexpr int128::int128(uint128 v)
+    : lo_{Uint128Low64(v)}, hi_{static_cast<int64_t>(Uint128High64(v))} {}
+
+#elif defined(ABSL_IS_BIG_ENDIAN)
+
+constexpr int128::int128(int64_t high, uint64_t low) :
+    hi_{high}, lo_{low} {}
+
+constexpr int128::int128(int v)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+constexpr int128::int128(long long v)  // NOLINT(runtime/int)
+    : hi_{v < 0 ? ~int64_t{0} : 0}, lo_{static_cast<uint64_t>(v)} {}
+
+constexpr int128::int128(unsigned int v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long v) : hi_{0}, lo_{v} {}
+// NOLINTNEXTLINE(runtime/int)
+constexpr int128::int128(unsigned long long v) : hi_{0}, lo_{v} {}
+
+constexpr int128::int128(uint128 v)
+    : hi_{static_cast<int64_t>(Uint128High64(v))}, lo_{Uint128Low64(v)} {}
+
+#else  // byte order
+#error "Unsupported byte order: must be little-endian or big-endian."
+#endif  // byte order
+
+constexpr int128::operator bool() const { return lo_ || hi_; }
+
+constexpr int128::operator char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator signed char() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<signed char>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned char() const {
+  return static_cast<unsigned char>(lo_);
+}
+
+constexpr int128::operator char16_t() const {
+  return static_cast<char16_t>(lo_);
+}
+
+constexpr int128::operator char32_t() const {
+  return static_cast<char32_t>(lo_);
+}
+
+constexpr int128::operator ABSL_INTERNAL_WCHAR_T() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<ABSL_INTERNAL_WCHAR_T>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator short() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<short>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned short() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned short>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator int() const {
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<int>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned int() const {
+  return static_cast<unsigned int>(lo_);
+}
+
+constexpr int128::operator long() const {  // NOLINT(runtime/int)
+  // NOLINTNEXTLINE(runtime/int)
+  return static_cast<long>(static_cast<long long>(*this));
+}
+
+constexpr int128::operator unsigned long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long>(lo_);           // NOLINT(runtime/int)
+}
+
+constexpr int128::operator long long() const {  // NOLINT(runtime/int)
+  // We don't bother checking the value of hi_. If *this < 0, lo_'s high bit
+  // must be set in order for the value to fit into a long long. Conversely, if
+  // lo_'s high bit is set, *this must be < 0 for the value to fit.
+  return int128_internal::BitCastToSigned(lo_);
+}
+
+constexpr int128::operator unsigned long long() const {  // NOLINT(runtime/int)
+  return static_cast<unsigned long long>(lo_);           // NOLINT(runtime/int)
+}
+
+// Forward declaration for conversion operators to floating point types.
+int128 operator-(int128 v);
+bool operator!=(int128 lhs, int128 rhs);
+
+inline int128::operator float() const {
+  // We must convert the absolute value and then negate as needed, because
+  // floating point types are typically sign-magnitude. Otherwise, the
+  // difference between the high and low 64 bits when interpreted as two's
+  // complement overwhelms the precision of the mantissa.
+  //
+  // Also check to make sure we don't negate Int128Min()
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<float>(-*this)
+             : static_cast<float>(lo_) +
+                   std::ldexp(static_cast<float>(hi_), 64);
+}
+
+inline int128::operator double() const {
+  // See comment in int128::operator float() above.
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<double>(-*this)
+             : static_cast<double>(lo_) +
+                   std::ldexp(static_cast<double>(hi_), 64);
+}
+
+inline int128::operator long double() const {
+  // See comment in int128::operator float() above.
+  return hi_ < 0 && *this != Int128Min()
+             ? -static_cast<long double>(-*this)
+             : static_cast<long double>(lo_) +
+                   std::ldexp(static_cast<long double>(hi_), 64);
+}
+
+// Comparison operators.
+
+inline bool operator==(int128 lhs, int128 rhs) {
+  return (Int128Low64(lhs) == Int128Low64(rhs) &&
+          Int128High64(lhs) == Int128High64(rhs));
+}
+
+inline bool operator!=(int128 lhs, int128 rhs) {
+  return !(lhs == rhs);
+}
+
+inline bool operator<(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) < Int128Low64(rhs))
+             : (Int128High64(lhs) < Int128High64(rhs));
+}
+
+inline bool operator>(int128 lhs, int128 rhs) {
+  return (Int128High64(lhs) == Int128High64(rhs))
+             ? (Int128Low64(lhs) > Int128Low64(rhs))
+             : (Int128High64(lhs) > Int128High64(rhs));
+}
+
+inline bool operator<=(int128 lhs, int128 rhs) {
+  return !(lhs > rhs);
+}
+
+inline bool operator>=(int128 lhs, int128 rhs) {
+  return !(lhs < rhs);
+}
+
+// Unary operators.
+
+inline int128 operator-(int128 v) {
+  int64_t hi = ~Int128High64(v);
+  uint64_t lo = ~Int128Low64(v) + 1;
+  if (lo == 0) ++hi;  // carry
+  return MakeInt128(hi, lo);
+}
+
+inline bool operator!(int128 v) {
+  return !Int128Low64(v) && !Int128High64(v);
+}
+
+inline int128 operator~(int128 val) {
+  return MakeInt128(~Int128High64(val), ~Int128Low64(val));
+}
+
+// Arithmetic operators.
+
+inline int128 operator+(int128 lhs, int128 rhs) {
+  int128 result = MakeInt128(Int128High64(lhs) + Int128High64(rhs),
+                             Int128Low64(lhs) + Int128Low64(rhs));
+  if (Int128Low64(result) < Int128Low64(lhs)) {  // check for carry
+    return MakeInt128(Int128High64(result) + 1, Int128Low64(result));
+  }
+  return result;
+}
+
+inline int128 operator-(int128 lhs, int128 rhs) {
+  int128 result = MakeInt128(Int128High64(lhs) - Int128High64(rhs),
+                             Int128Low64(lhs) - Int128Low64(rhs));
+  if (Int128Low64(lhs) < Int128Low64(rhs)) {  // check for carry
+    return MakeInt128(Int128High64(result) - 1, Int128Low64(result));
+  }
+  return result;
+}
+
+inline int128 operator*(int128 lhs, int128 rhs) {
+  uint128 result = uint128(lhs) * rhs;
+  return MakeInt128(int128_internal::BitCastToSigned(Uint128High64(result)),
+                    Uint128Low64(result));
+}
+
+inline int128 int128::operator++(int) {
+  int128 tmp(*this);
+  *this += 1;
+  return tmp;
+}
+
+inline int128 int128::operator--(int) {
+  int128 tmp(*this);
+  *this -= 1;
+  return tmp;
+}
+
+inline int128& int128::operator++() {
+  *this += 1;
+  return *this;
+}
+
+inline int128& int128::operator--() {
+  *this -= 1;
+  return *this;
+}
+
+inline int128 operator|(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) | Int128High64(rhs),
+                    Int128Low64(lhs) | Int128Low64(rhs));
+}
+
+inline int128 operator&(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) & Int128High64(rhs),
+                    Int128Low64(lhs) & Int128Low64(rhs));
+}
+
+inline int128 operator^(int128 lhs, int128 rhs) {
+  return MakeInt128(Int128High64(lhs) ^ Int128High64(rhs),
+                    Int128Low64(lhs) ^ Int128Low64(rhs));
+}
+
+inline int128 operator<<(int128 lhs, int amount) {
+  // uint64_t shifts of >= 64 are undefined, so we need some special-casing.
+  if (amount < 64) {
+    if (amount != 0) {
+      return MakeInt128(
+          (Int128High64(lhs) << amount) |
+              static_cast<int64_t>(Int128Low64(lhs) >> (64 - amount)),
+          Int128Low64(lhs) << amount);
+    }
+    return lhs;
+  }
+  return MakeInt128(static_cast<int64_t>(Int128Low64(lhs) << (amount - 64)), 0);
+}
+
+inline int128 operator>>(int128 lhs, int amount) {
+  // uint64_t shifts of >= 64 are undefined, so we need some special-casing.
+  if (amount < 64) {
+    if (amount != 0) {
+      return MakeInt128(
+          Int128High64(lhs) >> amount,
+          (Int128Low64(lhs) >> amount) |
+              (static_cast<uint64_t>(Int128High64(lhs)) << (64 - amount)));
+    }
+    return lhs;
+  }
+  return MakeInt128(0,
+                    static_cast<uint64_t>(Int128High64(lhs) >> (amount - 64)));
+}
diff --git a/absl/numeric/int128_stream_test.cc b/absl/numeric/int128_stream_test.cc
index 3cfa9dc..479ad66 100644
--- a/absl/numeric/int128_stream_test.cc
+++ b/absl/numeric/int128_stream_test.cc
@@ -147,6 +147,735 @@
   }
 }
 
+struct Int128TestCase {
+  absl::int128 value;
+  std::ios_base::fmtflags flags;
+  std::streamsize width;
+  const char* expected;
+};
+
+void CheckInt128Case(const Int128TestCase& test_case) {
+  std::ostringstream os;
+  os.flags(test_case.flags);
+  os.width(test_case.width);
+  os.fill(kFill);
+  os << test_case.value;
+  SCOPED_TRACE(StreamFormatToString(test_case.flags, test_case.width));
+  EXPECT_EQ(test_case.expected, os.str());
+}
+
+TEST(Int128, OStreamValueTest) {
+  CheckInt128Case({1, kDec, /*width = */ 0, "1"});
+  CheckInt128Case({1, kOct, /*width = */ 0, "1"});
+  CheckInt128Case({1, kHex, /*width = */ 0, "1"});
+  CheckInt128Case({9, kDec, /*width = */ 0, "9"});
+  CheckInt128Case({9, kOct, /*width = */ 0, "11"});
+  CheckInt128Case({9, kHex, /*width = */ 0, "9"});
+  CheckInt128Case({12345, kDec, /*width = */ 0, "12345"});
+  CheckInt128Case({12345, kOct, /*width = */ 0, "30071"});
+  CheckInt128Case({12345, kHex, /*width = */ 0, "3039"});
+  CheckInt128Case(
+      {0x8000000000000000, kDec, /*width = */ 0, "9223372036854775808"});
+  CheckInt128Case(
+      {0x8000000000000000, kOct, /*width = */ 0, "1000000000000000000000"});
+  CheckInt128Case(
+      {0x8000000000000000, kHex, /*width = */ 0, "8000000000000000"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kDec,
+                   /*width = */ 0, "18446744073709551615"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kOct,
+                   /*width = */ 0, "1777777777777777777777"});
+  CheckInt128Case({std::numeric_limits<uint64_t>::max(), kHex,
+                   /*width = */ 0, "ffffffffffffffff"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kDec, /*width = */ 0, "18446744073709551616"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kOct, /*width = */ 0, "2000000000000000000000"});
+  CheckInt128Case(
+      {absl::MakeInt128(1, 0), kHex, /*width = */ 0, "10000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::dec, /*width = */ 0,
+                   "170141183460469231731687303715884105727"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::oct, /*width = */ 0,
+                   "1777777777777777777777777777777777777777777"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::max(),
+                                    std::numeric_limits<uint64_t>::max()),
+                   std::ios::hex, /*width = */ 0,
+                   "7fffffffffffffffffffffffffffffff"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::dec, /*width = */ 0,
+                   "-170141183460469231731687303715884105728"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::oct, /*width = */ 0,
+                   "2000000000000000000000000000000000000000000"});
+  CheckInt128Case({absl::MakeInt128(std::numeric_limits<int64_t>::min(), 0),
+                   std::ios::hex, /*width = */ 0,
+                   "80000000000000000000000000000000"});
+  CheckInt128Case({-1, std::ios::dec, /*width = */ 0, "-1"});
+  CheckInt128Case({-1, std::ios::oct, /*width = */ 0,
+                   "3777777777777777777777777777777777777777777"});
+  CheckInt128Case(
+      {-1, std::ios::hex, /*width = */ 0, "ffffffffffffffffffffffffffffffff"});
+  CheckInt128Case({-12345, std::ios::dec, /*width = */ 0, "-12345"});
+  CheckInt128Case({-12345, std::ios::oct, /*width = */ 0,
+                   "3777777777777777777777777777777777777747707"});
+  CheckInt128Case({-12345, std::ios::hex, /*width = */ 0,
+                   "ffffffffffffffffffffffffffffcfc7"});
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases();
+TEST(Int128, OStreamFormatTest) {
+  for (const Int128TestCase& test_case : GetInt128FormatCases()) {
+    CheckInt128Case(test_case);
+  }
+}
+
+std::vector<Int128TestCase> GetInt128FormatCases() {
+  return {
+      {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
+      {0, std::ios_base::fmtflags(), /*width = */ 6, "_____0"},
+      {0, kPos, /*width = */ 0, "+0"},
+      {0, kPos, /*width = */ 6, "____+0"},
+      {0, kBase, /*width = */ 0, "0"},
+      {0, kBase, /*width = */ 6, "_____0"},
+      {0, kBase | kPos, /*width = */ 0, "+0"},
+      {0, kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper, /*width = */ 0, "0"},
+      {0, kUpper, /*width = */ 6, "_____0"},
+      {0, kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kUpper | kBase, /*width = */ 0, "0"},
+      {0, kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kLeft, /*width = */ 0, "0"},
+      {0, kLeft, /*width = */ 6, "0_____"},
+      {0, kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper, /*width = */ 0, "0"},
+      {0, kLeft | kUpper, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kPos, /*width = */ 6, "+0____"},
+      {0, kLeft | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kLeft | kUpper | kBase, /*width = */ 6, "0_____"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kLeft | kUpper | kBase | kPos, /*width = */ 6, "+0____"},
+      {0, kInt, /*width = */ 0, "0"},
+      {0, kInt, /*width = */ 6, "_____0"},
+      {0, kInt | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kBase, /*width = */ 0, "0"},
+      {0, kInt | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper, /*width = */ 0, "0"},
+      {0, kInt | kUpper, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kPos, /*width = */ 6, "+____0"},
+      {0, kInt | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kInt | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kInt | kUpper | kBase | kPos, /*width = */ 6, "+____0"},
+      {0, kRight, /*width = */ 0, "0"},
+      {0, kRight, /*width = */ 6, "_____0"},
+      {0, kRight | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kBase, /*width = */ 0, "0"},
+      {0, kRight | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper, /*width = */ 0, "0"},
+      {0, kRight | kUpper, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kRight | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kRight | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kRight | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec, /*width = */ 0, "0"},
+      {0, kDec, /*width = */ 6, "_____0"},
+      {0, kDec | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kBase, /*width = */ 0, "0"},
+      {0, kDec | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper, /*width = */ 0, "0"},
+      {0, kDec | kUpper, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kUpper | kBase, /*width = */ 0, "0"},
+      {0, kDec | kUpper | kBase, /*width = */ 6, "_____0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kUpper | kBase | kPos, /*width = */ 6, "____+0"},
+      {0, kDec | kLeft, /*width = */ 0, "0"},
+      {0, kDec | kLeft, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kPos, /*width = */ 0, "+0"},
+      {0, kDec | kLeft | kPos, /*width = */ 6, "+0____"},
+      {0, kDec | kLeft | kBase, /*width = */ 0, "0"},
+      {0, kDec | kLeft | kBase, /*width = */ 6, "0_____"},
+      {0, kDec | kLeft | kBase | kPos, /*width = */ 0, "+0"},
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+      {42, kOct | kUpper | kBase, /*width = */ 6, "___052"},
+      {42, kOct | kUpper | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kUpper | kBase | kPos, /*width = */ 6, "___052"},
+      {42, kOct | kLeft, /*width = */ 0, "52"},
+      {42, kOct | kLeft, /*width = */ 6, "52____"},
+      {42, kOct | kLeft | kPos, /*width = */ 0, "52"},
+      {42, kOct | kLeft | kPos, /*width = */ 6, "52____"},
+      {42, kOct | kLeft | kBase, /*width = */ 0, "052"},
+      {42, kOct | kLeft | kBase, /*width = */ 6, "052___"},
+      {42, kOct | kLeft | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kLeft | kBase | kPos, /*width = */ 6, "052___"},
+      {42, kOct | kLeft | kUpper, /*width = */ 0, "52"},
+      {42, kOct | kLeft | kUpper, /*width = */ 6, "52____"},
+      {42, kOct | kLeft | kUpper | kPos, /*width = */ 0, "52"},
+      {42, kOct | kLeft | kUpper | kPos, /*width = */ 6, "52____"},
+      {42, kOct | kLeft | kUpper | kBase, /*width = */ 0, "052"},
+      {42, kOct | kLeft | kUpper | kBase, /*width = */ 6, "052___"},
+      {42, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kLeft | kUpper | kBase | kPos, /*width = */ 6, "052___"},
+      {42, kOct | kInt, /*width = */ 0, "52"},
+      {42, kOct | kInt, /*width = */ 6, "____52"},
+      {42, kOct | kInt | kPos, /*width = */ 0, "52"},
+      {42, kOct | kInt | kPos, /*width = */ 6, "____52"},
+      {42, kOct | kInt | kBase, /*width = */ 0, "052"},
+      {42, kOct | kInt | kBase, /*width = */ 6, "___052"},
+      {42, kOct | kInt | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kInt | kBase | kPos, /*width = */ 6, "___052"},
+      {42, kOct | kInt | kUpper, /*width = */ 0, "52"},
+      {42, kOct | kInt | kUpper, /*width = */ 6, "____52"},
+      {42, kOct | kInt | kUpper | kPos, /*width = */ 0, "52"},
+      {42, kOct | kInt | kUpper | kPos, /*width = */ 6, "____52"},
+      {42, kOct | kInt | kUpper | kBase, /*width = */ 0, "052"},
+      {42, kOct | kInt | kUpper | kBase, /*width = */ 6, "___052"},
+      {42, kOct | kInt | kUpper | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kInt | kUpper | kBase | kPos, /*width = */ 6, "___052"},
+      {42, kOct | kRight, /*width = */ 0, "52"},
+      {42, kOct | kRight, /*width = */ 6, "____52"},
+      {42, kOct | kRight | kPos, /*width = */ 0, "52"},
+      {42, kOct | kRight | kPos, /*width = */ 6, "____52"},
+      {42, kOct | kRight | kBase, /*width = */ 0, "052"},
+      {42, kOct | kRight | kBase, /*width = */ 6, "___052"},
+      {42, kOct | kRight | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kRight | kBase | kPos, /*width = */ 6, "___052"},
+      {42, kOct | kRight | kUpper, /*width = */ 0, "52"},
+      {42, kOct | kRight | kUpper, /*width = */ 6, "____52"},
+      {42, kOct | kRight | kUpper | kPos, /*width = */ 0, "52"},
+      {42, kOct | kRight | kUpper | kPos, /*width = */ 6, "____52"},
+      {42, kOct | kRight | kUpper | kBase, /*width = */ 0, "052"},
+      {42, kOct | kRight | kUpper | kBase, /*width = */ 6, "___052"},
+      {42, kOct | kRight | kUpper | kBase | kPos, /*width = */ 0, "052"},
+      {42, kOct | kRight | kUpper | kBase | kPos, /*width = */ 6, "___052"},
+      {42, kHex, /*width = */ 0, "2a"},
+      {42, kHex, /*width = */ 6, "____2a"},
+      {42, kHex | kPos, /*width = */ 0, "2a"},
+      {42, kHex | kPos, /*width = */ 6, "____2a"},
+      {42, kHex | kBase, /*width = */ 0, "0x2a"},
+      {42, kHex | kBase, /*width = */ 6, "__0x2a"},
+      {42, kHex | kBase | kPos, /*width = */ 0, "0x2a"},
+      {42, kHex | kBase | kPos, /*width = */ 6, "__0x2a"},
+      {42, kHex | kUpper, /*width = */ 0, "2A"},
+      {42, kHex | kUpper, /*width = */ 6, "____2A"},
+      {42, kHex | kUpper | kPos, /*width = */ 0, "2A"},
+      {42, kHex | kUpper | kPos, /*width = */ 6, "____2A"},
+      {42, kHex | kUpper | kBase, /*width = */ 0, "0X2A"},
+      {42, kHex | kUpper | kBase, /*width = */ 6, "__0X2A"},
+      {42, kHex | kUpper | kBase | kPos, /*width = */ 0, "0X2A"},
+      {42, kHex | kUpper | kBase | kPos, /*width = */ 6, "__0X2A"},
+      {42, kHex | kLeft, /*width = */ 0, "2a"},
+      {42, kHex | kLeft, /*width = */ 6, "2a____"},
+      {42, kHex | kLeft | kPos, /*width = */ 0, "2a"},
+      {42, kHex | kLeft | kPos, /*width = */ 6, "2a____"},
+      {42, kHex | kLeft | kBase, /*width = */ 0, "0x2a"},
+      {42, kHex | kLeft | kBase, /*width = */ 6, "0x2a__"},
+      {42, kHex | kLeft | kBase | kPos, /*width = */ 0, "0x2a"},
+      {42, kHex | kLeft | kBase | kPos, /*width = */ 6, "0x2a__"},
+      {42, kHex | kLeft | kUpper, /*width = */ 0, "2A"},
+      {42, kHex | kLeft | kUpper, /*width = */ 6, "2A____"},
+      {42, kHex | kLeft | kUpper | kPos, /*width = */ 0, "2A"},
+      {42, kHex | kLeft | kUpper | kPos, /*width = */ 6, "2A____"},
+      {42, kHex | kLeft | kUpper | kBase, /*width = */ 0, "0X2A"},
+      {42, kHex | kLeft | kUpper | kBase, /*width = */ 6, "0X2A__"},
+      {42, kHex | kLeft | kUpper | kBase | kPos, /*width = */ 0, "0X2A"},
+      {42, kHex | kLeft | kUpper | kBase | kPos, /*width = */ 6, "0X2A__"},
+      {42, kHex | kInt, /*width = */ 0, "2a"},
+      {42, kHex | kInt, /*width = */ 6, "____2a"},
+      {42, kHex | kInt | kPos, /*width = */ 0, "2a"},
+      {42, kHex | kInt | kPos, /*width = */ 6, "____2a"},
+      {42, kHex | kInt | kBase, /*width = */ 0, "0x2a"},
+      {42, kHex | kInt | kBase, /*width = */ 6, "0x__2a"},
+      {42, kHex | kInt | kBase | kPos, /*width = */ 0, "0x2a"},
+      {42, kHex | kInt | kBase | kPos, /*width = */ 6, "0x__2a"},
+      {42, kHex | kInt | kUpper, /*width = */ 0, "2A"},
+      {42, kHex | kInt | kUpper, /*width = */ 6, "____2A"},
+      {42, kHex | kInt | kUpper | kPos, /*width = */ 0, "2A"},
+      {42, kHex | kInt | kUpper | kPos, /*width = */ 6, "____2A"},
+      {42, kHex | kInt | kUpper | kBase, /*width = */ 0, "0X2A"},
+      {42, kHex | kInt | kUpper | kBase, /*width = */ 6, "0X__2A"},
+      {42, kHex | kInt | kUpper | kBase | kPos, /*width = */ 0, "0X2A"},
+      {42, kHex | kInt | kUpper | kBase | kPos, /*width = */ 6, "0X__2A"},
+      {42, kHex | kRight, /*width = */ 0, "2a"},
+      {42, kHex | kRight, /*width = */ 6, "____2a"},
+      {42, kHex | kRight | kPos, /*width = */ 0, "2a"},
+      {42, kHex | kRight | kPos, /*width = */ 6, "____2a"},
+      {42, kHex | kRight | kBase, /*width = */ 0, "0x2a"},
+      {42, kHex | kRight | kBase, /*width = */ 6, "__0x2a"},
+      {42, kHex | kRight | kBase | kPos, /*width = */ 0, "0x2a"},
+      {42, kHex | kRight | kBase | kPos, /*width = */ 6, "__0x2a"},
+      {42, kHex | kRight | kUpper, /*width = */ 0, "2A"},
+      {42, kHex | kRight | kUpper, /*width = */ 6, "____2A"},
+      {42, kHex | kRight | kUpper | kPos, /*width = */ 0, "2A"},
+      {42, kHex | kRight | kUpper | kPos, /*width = */ 6, "____2A"},
+      {42, kHex | kRight | kUpper | kBase, /*width = */ 0, "0X2A"},
+      {42, kHex | kRight | kUpper | kBase, /*width = */ 6, "__0X2A"},
+      {42, kHex | kRight | kUpper | kBase | kPos, /*width = */ 0, "0X2A"},
+      {42, kHex | kRight | kUpper | kBase | kPos, /*width = */ 6, "__0X2A"},
+      {-321, std::ios_base::fmtflags(), /*width = */ 0, "-321"},
+      {-321, std::ios_base::fmtflags(), /*width = */ 6, "__-321"},
+      {-321, kPos, /*width = */ 0, "-321"},
+      {-321, kPos, /*width = */ 6, "__-321"},
+      {-321, kBase, /*width = */ 0, "-321"},
+      {-321, kBase, /*width = */ 6, "__-321"},
+      {-321, kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kUpper, /*width = */ 0, "-321"},
+      {-321, kUpper, /*width = */ 6, "__-321"},
+      {-321, kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kLeft, /*width = */ 0, "-321"},
+      {-321, kLeft, /*width = */ 6, "-321__"},
+      {-321, kLeft | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kBase, /*width = */ 0, "-321"},
+      {-321, kLeft | kBase, /*width = */ 6, "-321__"},
+      {-321, kLeft | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kPos, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kBase, /*width = */ 6, "-321__"},
+      {-321, kLeft | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kLeft | kUpper | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kInt, /*width = */ 0, "-321"},
+      {-321, kInt, /*width = */ 6, "-__321"},
+      {-321, kInt | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kBase, /*width = */ 0, "-321"},
+      {-321, kInt | kBase, /*width = */ 6, "-__321"},
+      {-321, kInt | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kPos, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kBase, /*width = */ 6, "-__321"},
+      {-321, kInt | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kInt | kUpper | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kRight, /*width = */ 0, "-321"},
+      {-321, kRight, /*width = */ 6, "__-321"},
+      {-321, kRight | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kBase, /*width = */ 0, "-321"},
+      {-321, kRight | kBase, /*width = */ 6, "__-321"},
+      {-321, kRight | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kRight | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kRight | kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec, /*width = */ 0, "-321"},
+      {-321, kDec, /*width = */ 6, "__-321"},
+      {-321, kDec | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kUpper | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kLeft, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase, /*width = */ 6, "-321__"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kLeft | kUpper | kBase | kPos, /*width = */ 6, "-321__"},
+      {-321, kDec | kInt, /*width = */ 0, "-321"},
+      {-321, kDec | kInt, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase, /*width = */ 6, "-__321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kInt | kUpper | kBase | kPos, /*width = */ 6, "-__321"},
+      {-321, kDec | kRight, /*width = */ 0, "-321"},
+      {-321, kDec | kRight, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kBase | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kPos, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase, /*width = */ 6, "__-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 0, "-321"},
+      {-321, kDec | kRight | kUpper | kBase | kPos, /*width = */ 6, "__-321"}};
+}
+
 std::vector<Uint128TestCase> GetUint128FormatCases() {
   return {
       {0, std::ios_base::fmtflags(), /*width = */ 0, "0"},
diff --git a/absl/numeric/int128_test.cc b/absl/numeric/int128_test.cc
index 5e1b5ec..bc86c71 100644
--- a/absl/numeric/int128_test.cc
+++ b/absl/numeric/int128_test.cc
@@ -479,4 +479,747 @@
   }));
 }
 
+
+TEST(Int128Uint128, ConversionTest) {
+  absl::int128 nonnegative_signed_values[] = {
+      0,
+      1,
+      0xffeeddccbbaa9988,
+      absl::MakeInt128(0x7766554433221100, 0),
+      absl::MakeInt128(0x1234567890abcdef, 0xfedcba0987654321),
+      absl::Int128Max()};
+  for (absl::int128 value : nonnegative_signed_values) {
+    EXPECT_EQ(value, absl::int128(absl::uint128(value)));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(value, absl::int128(assigned_value));
+  }
+
+  absl::int128 negative_values[] = {
+      -1, -0x1234567890abcdef,
+      absl::MakeInt128(-0x5544332211ffeedd, 0),
+      -absl::MakeInt128(0x76543210fedcba98, 0xabcdef0123456789)};
+  for (absl::int128 value : negative_values) {
+    EXPECT_EQ(absl::uint128(-value), -absl::uint128(value));
+
+    absl::uint128 assigned_value;
+    assigned_value = value;
+    EXPECT_EQ(absl::uint128(-value), -assigned_value);
+  }
+}
+
+template <typename T>
+class Int128IntegerTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerTraitsTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible<absl::int128, TypeParam>::value,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(std::is_assignable<absl::int128&, TypeParam>::value,
+                "absl::int128 must be assignable from TypeParam");
+  static_assert(!std::is_assignable<TypeParam&, absl::int128>::value,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+template <typename T>
+class Int128FloatTraitsTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatTraitsTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatTraitsTest, ConstructAssignTest) {
+  static_assert(std::is_constructible<absl::int128, TypeParam>::value,
+                "absl::int128 must be constructible from TypeParam");
+  static_assert(!std::is_assignable<absl::int128&, TypeParam>::value,
+                "absl::int128 must not be assignable from TypeParam");
+  static_assert(!std::is_assignable<TypeParam&, absl::int128>::value,
+                "TypeParam must not be assignable from absl::int128");
+}
+
+#ifdef ABSL_HAVE_INTRINSIC_INT128
+// These type traits done separately as TYPED_TEST requires typeinfo, and not
+// all platforms have this for __int128 even though they define the type.
+TEST(Int128, IntrinsicTypeTraitsTest) {
+  static_assert(std::is_constructible<absl::int128, __int128>::value,
+                "absl::int128 must be constructible from __int128");
+  static_assert(std::is_assignable<absl::int128&, __int128>::value,
+                "absl::int128 must be assignable from __int128");
+  static_assert(!std::is_assignable<__int128&, absl::int128>::value,
+                "__int128 must not be assignable from absl::int128");
+
+  static_assert(std::is_constructible<absl::int128, unsigned __int128>::value,
+                "absl::int128 must be constructible from unsigned __int128");
+  static_assert(!std::is_assignable<absl::int128&, unsigned __int128>::value,
+                "absl::int128 must be assignable from unsigned __int128");
+  static_assert(!std::is_assignable<unsigned __int128&, absl::int128>::value,
+                "unsigned __int128 must not be assignable from absl::int128");
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, TrivialTraitsTest) {
+  static_assert(absl::is_trivially_default_constructible<absl::int128>::value,
+                "");
+  static_assert(absl::is_trivially_copy_constructible<absl::int128>::value, "");
+  static_assert(absl::is_trivially_copy_assignable<absl::int128>::value, "");
+  static_assert(std::is_trivially_destructible<absl::int128>::value, "");
+}
+
+TEST(Int128, BoolConversionTest) {
+  EXPECT_FALSE(absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_TRUE(absl::MakeInt128(int64_t{1} << i, 0));
+  }
+  EXPECT_TRUE(absl::Int128Min());
+
+  EXPECT_EQ(absl::int128(1), absl::int128(true));
+  EXPECT_EQ(absl::int128(0), absl::int128(false));
+}
+
+template <typename T>
+class Int128IntegerConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128IntegerConversionTest, IntegerTypes);
+
+TYPED_TEST(Int128IntegerConversionTest, RoundTripTest) {
+  EXPECT_EQ(TypeParam{0}, static_cast<TypeParam>(absl::int128(0)));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::min(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::min())));
+  EXPECT_EQ(std::numeric_limits<TypeParam>::max(),
+            static_cast<TypeParam>(
+                absl::int128(std::numeric_limits<TypeParam>::max())));
+}
+
+template <typename T>
+class Int128FloatConversionTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE(Int128FloatConversionTest, FloatingPointTypes);
+
+TYPED_TEST(Int128FloatConversionTest, ConstructAndCastTest) {
+  // Conversions where the floating point values should be exactly the same.
+  // 0x9f5b is a randomly chosen small value.
+  for (int i = 0; i < 110; ++i) {  // 110 = 126 - #bits in 0x9f5b
+    SCOPED_TRACE(::testing::Message() << "i = " << i);
+
+    TypeParam float_value = std::ldexp(static_cast<TypeParam>(0x9f5b), i);
+    absl::int128 int_value = absl::int128(0x9f5b) << i;
+
+    EXPECT_EQ(float_value, static_cast<TypeParam>(int_value));
+    EXPECT_EQ(-float_value, static_cast<TypeParam>(-int_value));
+    EXPECT_EQ(int_value, absl::int128(float_value));
+    EXPECT_EQ(-int_value, absl::int128(-float_value));
+  }
+
+  // Round trip conversions with a small sample of randomly generated uint64_t
+  // values (less than int64_t max so that value * 2^64 fits into int128).
+  uint64_t values[] = {0x6d4492c24fb86199, 0x26ead65e4cb359b5,
+                       0x2c43407433ba3fd1, 0x3b574ec668df6b55,
+                       0x1c750e55a29f4f0f};
+  for (uint64_t value : values) {
+    for (int i = 0; i <= 64; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+
+      TypeParam fvalue = std::ldexp(static_cast<TypeParam>(value), i);
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(-absl::int128(fvalue)));
+      EXPECT_DOUBLE_EQ(-fvalue, static_cast<TypeParam>(absl::int128(-fvalue)));
+      EXPECT_DOUBLE_EQ(fvalue, static_cast<TypeParam>(-absl::int128(-fvalue)));
+    }
+  }
+
+  // Round trip conversions with a small sample of random large positive values.
+  absl::int128 large_values[] = {
+      absl::MakeInt128(0x5b0640d96c7b3d9f, 0xb7a7189e51d18622),
+      absl::MakeInt128(0x34bed042c6f65270, 0x73b236570669a089),
+      absl::MakeInt128(0x43deba9e6da12724, 0xf7f0f83da686797d),
+      absl::MakeInt128(0x71e8d383be4e5589, 0x75c3f96fb00752b6)};
+  for (absl::int128 value : large_values) {
+    // Make value have as many significant bits as can be represented by
+    // the mantissa, also making sure the highest and lowest bit in the range
+    // are set.
+    value >>= (127 - std::numeric_limits<TypeParam>::digits);
+    value |= absl::int128(1) << (std::numeric_limits<TypeParam>::digits - 1);
+    value |= 1;
+    for (int i = 0; i < 127 - std::numeric_limits<TypeParam>::digits; ++i) {
+      absl::int128 int_value = value << i;
+      EXPECT_EQ(int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(int_value)));
+      EXPECT_EQ(-int_value,
+                static_cast<absl::int128>(static_cast<TypeParam>(-int_value)));
+    }
+  }
+
+  // Small sample of checks that rounding is toward zero
+  EXPECT_EQ(0, absl::int128(TypeParam(0.1)));
+  EXPECT_EQ(17, absl::int128(TypeParam(17.8)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.8)));
+  EXPECT_EQ(-53, absl::int128(TypeParam(-53.1)));
+  EXPECT_EQ(0, absl::int128(TypeParam(0.5)));
+  EXPECT_EQ(0, absl::int128(TypeParam(-0.5)));
+  TypeParam just_lt_one = std::nexttoward(TypeParam(1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_lt_one));
+  TypeParam just_gt_minus_one = std::nexttoward(TypeParam(-1), TypeParam(0));
+  EXPECT_EQ(0, absl::int128(just_gt_minus_one));
+
+  // Check limits
+  EXPECT_DOUBLE_EQ(std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Max()));
+  EXPECT_DOUBLE_EQ(-std::ldexp(static_cast<TypeParam>(1), 127),
+                   static_cast<TypeParam>(absl::Int128Min()));
+}
+
+TEST(Int128, FactoryTest) {
+  EXPECT_EQ(absl::int128(-1), absl::MakeInt128(-1, -1));
+  EXPECT_EQ(absl::int128(-31), absl::MakeInt128(-1, -31));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::min()),
+            absl::MakeInt128(-1, std::numeric_limits<int64_t>::min()));
+  EXPECT_EQ(absl::int128(0), absl::MakeInt128(0, 0));
+  EXPECT_EQ(absl::int128(1), absl::MakeInt128(0, 1));
+  EXPECT_EQ(absl::int128(std::numeric_limits<int64_t>::max()),
+            absl::MakeInt128(0, std::numeric_limits<int64_t>::max()));
+}
+
+TEST(Int128, HighLowTest) {
+  struct HighLowPair {
+    int64_t high;
+    uint64_t low;
+  };
+  HighLowPair values[]{{0, 0}, {0, 1}, {1, 0}, {123, 456}, {-654, 321}};
+  for (const HighLowPair& pair : values) {
+    absl::int128 value = absl::MakeInt128(pair.high, pair.low);
+    EXPECT_EQ(pair.low, absl::Int128Low64(value));
+    EXPECT_EQ(pair.high, absl::Int128High64(value));
+  }
+}
+
+TEST(Int128, LimitsTest) {
+  EXPECT_EQ(absl::MakeInt128(0x7fffffffffffffff, 0xffffffffffffffff),
+            absl::Int128Max());
+  EXPECT_EQ(absl::Int128Max(), ~absl::Int128Min());
+}
+
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+TEST(Int128, IntrinsicConversionTest) {
+  __int128 intrinsic =
+      (static_cast<__int128>(0x3a5b76c209de76f6) << 64) + 0x1f25e1d63a2b46c5;
+  absl::int128 custom =
+      absl::MakeInt128(0x3a5b76c209de76f6, 0x1f25e1d63a2b46c5);
+
+  EXPECT_EQ(custom, absl::int128(intrinsic));
+  EXPECT_EQ(intrinsic, static_cast<__int128>(custom));
+}
+#endif  // ABSL_HAVE_INTRINSIC_INT128
+
+TEST(Int128, ConstexprTest) {
+  constexpr absl::int128 zero = absl::int128();
+  constexpr absl::int128 one = 1;
+  constexpr absl::int128 minus_two = -2;
+  constexpr absl::int128 min = absl::Int128Min();
+  constexpr absl::int128 max = absl::Int128Max();
+  EXPECT_EQ(zero, absl::int128(0));
+  EXPECT_EQ(one, absl::int128(1));
+  EXPECT_EQ(minus_two, absl::MakeInt128(-1, -2));
+  EXPECT_GT(max, one);
+  EXPECT_LT(min, minus_two);
+}
+
+TEST(Int128, ComparisonTest) {
+  struct TestCase {
+    absl::int128 smaller;
+    absl::int128 larger;
+  };
+  TestCase cases[] = {
+      {absl::int128(0), absl::int128(123)},
+      {absl::MakeInt128(-12, 34), absl::MakeInt128(12, 34)},
+      {absl::MakeInt128(1, 1000), absl::MakeInt128(1000, 1)},
+      {absl::MakeInt128(-1000, 1000), absl::MakeInt128(-1, 1)},
+  };
+  for (const TestCase& pair : cases) {
+    SCOPED_TRACE(::testing::Message() << "pair.smaller = " << pair.smaller
+                                      << "; pair.larger = " << pair.larger);
+
+    EXPECT_TRUE(pair.smaller == pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger == pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller == pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller != pair.larger);    // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller != pair.smaller);  // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger != pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller < pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger < pair.smaller);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger > pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller > pair.larger);  // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.smaller <= pair.larger);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.larger <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller <= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger <= pair.larger);    // NOLINT(readability/check)
+
+    EXPECT_TRUE(pair.larger >= pair.smaller);   // NOLINT(readability/check)
+    EXPECT_FALSE(pair.smaller >= pair.larger);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.smaller >= pair.smaller);  // NOLINT(readability/check)
+    EXPECT_TRUE(pair.larger >= pair.larger);    // NOLINT(readability/check)
+  }
+}
+
+TEST(Int128, UnaryNegationTest) {
+  int64_t values64[] = {0, 1, 12345, 0x4000000000000000,
+                        std::numeric_limits<int64_t>::max()};
+  for (int64_t value : values64) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(absl::int128(-value), -absl::int128(value));
+    EXPECT_EQ(absl::int128(value), -absl::int128(-value));
+    EXPECT_EQ(absl::MakeInt128(-value, 0), -absl::MakeInt128(value, 0));
+    EXPECT_EQ(absl::MakeInt128(value, 0), -absl::MakeInt128(-value, 0));
+  }
+}
+
+TEST(Int128, LogicalNotTest) {
+  EXPECT_TRUE(!absl::int128(0));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(0, uint64_t{1} << i));
+  }
+  for (int i = 0; i < 63; ++i) {
+    EXPECT_FALSE(!absl::MakeInt128(int64_t{1} << i, 0));
+  }
+}
+
+TEST(Int128, AdditionSubtractionTest) {
+  // 64 bit pairs that will not cause overflow / underflow. These test negative
+  // carry; positive carry must be checked separately.
+  std::pair<int64_t, int64_t> cases[]{
+      {0, 0},                              // 0, 0
+      {0, 2945781290834},                  // 0, +
+      {1908357619234, 0},                  // +, 0
+      {0, -1204895918245},                 // 0, -
+      {-2957928523560, 0},                 // -, 0
+      {89023982312461, 98346012567134},    // +, +
+      {-63454234568239, -23456235230773},  // -, -
+      {98263457263502, -21428561935925},   // +, -
+      {-88235237438467, 15923659234573},   // -, +
+  };
+  for (const auto& pair : cases) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first + pair.second),
+              absl::int128(pair.first) + absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second + pair.first),
+              absl::int128(pair.second) += absl::int128(pair.first));
+
+    EXPECT_EQ(absl::int128(pair.first - pair.second),
+              absl::int128(pair.first) - absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.second - pair.first),
+              absl::int128(pair.second) -= absl::int128(pair.first));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second + pair.first, 0),
+        absl::MakeInt128(pair.second, 0) + absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first + pair.second, 0),
+        absl::MakeInt128(pair.first, 0) += absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.second - pair.first, 0),
+        absl::MakeInt128(pair.second, 0) - absl::MakeInt128(pair.first, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first - pair.second, 0),
+        absl::MakeInt128(pair.first, 0) -= absl::MakeInt128(pair.second, 0));
+  }
+
+  // check positive carry
+  EXPECT_EQ(absl::MakeInt128(31, 0),
+            absl::MakeInt128(20, 1) +
+                absl::MakeInt128(10, std::numeric_limits<uint64_t>::max()));
+}
+
+TEST(Int128, IncrementDecrementTest) {
+  absl::int128 value = 0;
+  EXPECT_EQ(0, value++);
+  EXPECT_EQ(1, value);
+  EXPECT_EQ(1, value--);
+  EXPECT_EQ(0, value);
+  EXPECT_EQ(-1, --value);
+  EXPECT_EQ(-1, value);
+  EXPECT_EQ(0, ++value);
+  EXPECT_EQ(0, value);
+}
+
+TEST(Int128, MultiplicationTest) {
+  // 1 bit x 1 bit, and negative combinations
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j < 127 - i; ++j) {
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      absl::int128 a = absl::int128(1) << i;
+      absl::int128 b = absl::int128(1) << j;
+      absl::int128 c = absl::int128(1) << (i + j);
+
+      EXPECT_EQ(c, a * b);
+      EXPECT_EQ(-c, -a * b);
+      EXPECT_EQ(-c, a * -b);
+      EXPECT_EQ(c, -a * -b);
+
+      EXPECT_EQ(c, absl::int128(a) *= b);
+      EXPECT_EQ(-c, absl::int128(-a) *= b);
+      EXPECT_EQ(-c, absl::int128(a) *= -b);
+      EXPECT_EQ(c, absl::int128(-a) *= -b);
+    }
+  }
+
+  // Pairs of random values that will not overflow signed 64-bit multiplication
+  std::pair<int64_t, int64_t> small_values[] = {
+      {0x5e61, 0xf29f79ca14b4},    // +, +
+      {0x3e033b, -0x612c0ee549},   // +, -
+      {-0x052ce7e8, 0x7c728f0f},   // -, +
+      {-0x3af7054626, -0xfb1e1d},  // -, -
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) * absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first * pair.second),
+              absl::int128(pair.first) *= absl::int128(pair.second));
+
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) * absl::int128(pair.second));
+    EXPECT_EQ(absl::MakeInt128(pair.first * pair.second, 0),
+              absl::MakeInt128(pair.first, 0) *= absl::int128(pair.second));
+  }
+
+  // Pairs of positive random values that will not overflow 64-bit
+  // multiplication and can be left shifted by 32 without overflow
+  std::pair<int64_t, int64_t> small_values2[] = {
+      {0x1bb0a110, 0x31487671},
+      {0x4792784e, 0x28add7d7},
+      {0x7b66553a, 0x11dff8ef},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_values2) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 a = absl::int128(pair.first << 32);
+    absl::int128 b = absl::int128(pair.second << 32);
+    absl::int128 c = absl::MakeInt128(pair.first * pair.second, 0);
+
+    EXPECT_EQ(c, a * b);
+    EXPECT_EQ(-c, -a * b);
+    EXPECT_EQ(-c, a * -b);
+    EXPECT_EQ(c, -a * -b);
+
+    EXPECT_EQ(c, absl::int128(a) *= b);
+    EXPECT_EQ(-c, absl::int128(-a) *= b);
+    EXPECT_EQ(-c, absl::int128(a) *= -b);
+    EXPECT_EQ(c, absl::int128(-a) *= -b);
+  }
+
+  // check 0, 1, and -1 behavior with large values
+  absl::int128 large_values[] = {
+      {absl::MakeInt128(0xd66f061af02d0408, 0x727d2846cb475b53)},
+      {absl::MakeInt128(0x27b8d5ed6104452d, 0x03f8a33b0ee1df4f)},
+      {-absl::MakeInt128(0x621b6626b9e8d042, 0x27311ac99df00938)},
+      {-absl::MakeInt128(0x34e0656f1e95fb60, 0x4281cfd731257a47)},
+  };
+  for (absl::int128 value : large_values) {
+    EXPECT_EQ(0, 0 * value);
+    EXPECT_EQ(0, value * 0);
+    EXPECT_EQ(0, absl::int128(0) *= value);
+    EXPECT_EQ(0, value *= 0);
+
+    EXPECT_EQ(value, 1 * value);
+    EXPECT_EQ(value, value * 1);
+    EXPECT_EQ(value, absl::int128(1) *= value);
+    EXPECT_EQ(value, value *= 1);
+
+    EXPECT_EQ(-value, -1 * value);
+    EXPECT_EQ(-value, value * -1);
+    EXPECT_EQ(-value, absl::int128(-1) *= value);
+    EXPECT_EQ(-value, value *= -1);
+  }
+
+  // Manually calculated random large value cases
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) * 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) * 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            0xa9b98a8ddf66bc * -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            -0x3e39341147 * -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+
+  EXPECT_EQ(absl::MakeInt128(0xcd0efd3442219bb, 0xde47c05bcd9df6e1),
+            absl::MakeInt128(0x7c6448, 0x3bc4285c47a9d253) *= 0x1a6037537b);
+  EXPECT_EQ(-absl::MakeInt128(0x1f8f149850b1e5e6, 0x1e50d6b52d272c3e),
+            -absl::MakeInt128(0x23, 0x2e68a513ca1b8859) *= 0xe5a434cd14866e);
+  EXPECT_EQ(-absl::MakeInt128(0x55cae732029d1fce, 0xca6474b6423263e4),
+            absl::int128(0xa9b98a8ddf66bc) *=
+            -absl::MakeInt128(0x81, 0x672e58231e2469d7));
+  EXPECT_EQ(absl::MakeInt128(0x19c8b7620b507dc4, 0xfec042b71a5f29a4),
+            absl::int128(-0x3e39341147) *=
+            -absl::MakeInt128(0x6a14b2, 0x5ed34cca42327b3c));
+}
+
+TEST(Int128, DivisionAndModuloTest) {
+  // Check against 64 bit division and modulo operators with a sample of
+  // randomly generated pairs.
+  std::pair<int64_t, int64_t> small_pairs[] = {
+      {0x15f2a64138, 0x67da05},    {0x5e56d194af43045f, 0xcf1543fb99},
+      {0x15e61ed052036a, -0xc8e6}, {0x88125a341e85, -0xd23fb77683},
+      {-0xc06e20, 0x5a},           {-0x4f100219aea3e85d, 0xdcc56cb4efe993},
+      {-0x168d629105, -0xa7},      {-0x7b44e92f03ab2375, -0x6516},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : small_pairs) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    absl::int128 dividend = pair.first;
+    absl::int128 divisor = pair.second;
+    int64_t quotient = pair.first / pair.second;
+    int64_t remainder = pair.first % pair.second;
+
+    EXPECT_EQ(quotient, dividend / divisor);
+    EXPECT_EQ(quotient, absl::int128(dividend) /= divisor);
+    EXPECT_EQ(remainder, dividend % divisor);
+    EXPECT_EQ(remainder, absl::int128(dividend) %= divisor);
+  }
+
+  // Test behavior with 0, 1, and -1 with a sample of randomly generated large
+  // values.
+  absl::int128 values[] = {
+      absl::MakeInt128(0x63d26ee688a962b2, 0x9e1411abda5c1d70),
+      absl::MakeInt128(0x152f385159d6f986, 0xbf8d48ef63da395d),
+      -absl::MakeInt128(0x3098d7567030038c, 0x14e7a8a098dc2164),
+      -absl::MakeInt128(0x49a037aca35c809f, 0xa6a87525480ef330),
+  };
+  for (absl::int128 value : values) {
+    SCOPED_TRACE(::testing::Message() << "value = " << value);
+
+    EXPECT_EQ(0, 0 / value);
+    EXPECT_EQ(0, absl::int128(0) /= value);
+    EXPECT_EQ(0, 0 % value);
+    EXPECT_EQ(0, absl::int128(0) %= value);
+
+    EXPECT_EQ(value, value / 1);
+    EXPECT_EQ(value, absl::int128(value) /= 1);
+    EXPECT_EQ(0, value % 1);
+    EXPECT_EQ(0, absl::int128(value) %= 1);
+
+    EXPECT_EQ(-value, value / -1);
+    EXPECT_EQ(-value, absl::int128(value) /= -1);
+    EXPECT_EQ(0, value % -1);
+    EXPECT_EQ(0, absl::int128(value) %= -1);
+  }
+
+  // Min and max values
+  EXPECT_EQ(0, absl::Int128Max() / absl::Int128Min());
+  EXPECT_EQ(absl::Int128Max(), absl::Int128Max() % absl::Int128Min());
+  EXPECT_EQ(-1, absl::Int128Min() / absl::Int128Max());
+  EXPECT_EQ(-1, absl::Int128Min() % absl::Int128Max());
+
+  // Power of two division and modulo of random large dividends
+  absl::int128 positive_values[] = {
+      absl::MakeInt128(0x21e1a1cc69574620, 0xe7ac447fab2fc869),
+      absl::MakeInt128(0x32c2ff3ab89e66e8, 0x03379a613fd1ce74),
+      absl::MakeInt128(0x6f32ca786184dcaf, 0x046f9c9ecb3a9ce1),
+      absl::MakeInt128(0x1aeb469dd990e0ee, 0xda2740f243cd37eb),
+  };
+  for (absl::int128 value : positive_values) {
+    for (int i = 0; i < 127; ++i) {
+      SCOPED_TRACE(::testing::Message()
+                   << "value = " << value << "; i = " << i);
+      absl::int128 power_of_two = absl::int128(1) << i;
+
+      EXPECT_EQ(value >> i, value / power_of_two);
+      EXPECT_EQ(value >> i, absl::int128(value) /= power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1), value % power_of_two);
+      EXPECT_EQ(value & (power_of_two - 1),
+                absl::int128(value) %= power_of_two);
+    }
+  }
+
+  // Manually calculated cases with random large dividends
+  struct DivisionModCase {
+    absl::int128 dividend;
+    absl::int128 divisor;
+    absl::int128 quotient;
+    absl::int128 remainder;
+  };
+  DivisionModCase manual_cases[] = {
+      {absl::MakeInt128(0x6ada48d489007966, 0x3c9c5c98150d5d69),
+       absl::MakeInt128(0x8bc308fb, 0x8cb9cc9a3b803344), 0xc3b87e08,
+       absl::MakeInt128(0x1b7db5e1, 0xd9eca34b7af04b49)},
+      {absl::MakeInt128(0xd6946511b5b, 0x4886c5c96546bf5f),
+       -absl::MakeInt128(0x263b, 0xfd516279efcfe2dc), -0x59cbabf0,
+       absl::MakeInt128(0x622, 0xf462909155651d1f)},
+      {-absl::MakeInt128(0x33db734f9e8d1399, 0x8447ac92482bca4d), 0x37495078240,
+       -absl::MakeInt128(0xf01f1, 0xbc0368bf9a77eae8), -0x21a508f404d},
+      {-absl::MakeInt128(0x13f837b409a07e7d, 0x7fc8e248a7d73560), -0x1b9f,
+       absl::MakeInt128(0xb9157556d724, 0xb14f635714d7563e), -0x1ade},
+  };
+  for (const DivisionModCase test_case : manual_cases) {
+    EXPECT_EQ(test_case.quotient, test_case.dividend / test_case.divisor);
+    EXPECT_EQ(test_case.quotient,
+              absl::int128(test_case.dividend) /= test_case.divisor);
+    EXPECT_EQ(test_case.remainder, test_case.dividend % test_case.divisor);
+    EXPECT_EQ(test_case.remainder,
+              absl::int128(test_case.dividend) %= test_case.divisor);
+  }
+}
+
+TEST(Int128, BitwiseLogicTest) {
+  EXPECT_EQ(absl::int128(-1), ~absl::int128(0));
+
+  absl::int128 values[]{
+      0, -1, 0xde400bee05c3ff6b, absl::MakeInt128(0x7f32178dd81d634a, 0),
+      absl::MakeInt128(0xaf539057055613a9, 0x7d104d7d946c2e4d)};
+  for (absl::int128 value : values) {
+    EXPECT_EQ(value, ~~value);
+
+    EXPECT_EQ(value, value | value);
+    EXPECT_EQ(value, value & value);
+    EXPECT_EQ(0, value ^ value);
+
+    EXPECT_EQ(value, absl::int128(value) |= value);
+    EXPECT_EQ(value, absl::int128(value) &= value);
+    EXPECT_EQ(0, absl::int128(value) ^= value);
+
+    EXPECT_EQ(value, value | 0);
+    EXPECT_EQ(0, value & 0);
+    EXPECT_EQ(value, value ^ 0);
+
+    EXPECT_EQ(absl::int128(-1), value | absl::int128(-1));
+    EXPECT_EQ(value, value & absl::int128(-1));
+    EXPECT_EQ(~value, value ^ absl::int128(-1));
+  }
+
+  // small sample of randomly generated int64_t's
+  std::pair<int64_t, int64_t> pairs64[]{
+      {0x7f86797f5e991af4, 0x1ee30494fb007c97},
+      {0x0b278282bacf01af, 0x58780e0a57a49e86},
+      {0x059f266ccb93a666, 0x3d5b731bae9286f5},
+      {0x63c0c4820f12108c, 0x58166713c12e1c3a},
+      {0x381488bb2ed2a66e, 0x2220a3eb76a3698c},
+      {0x2a0a0dfb81e06f21, 0x4b60585927f5523c},
+      {0x555b1c3a03698537, 0x25478cd19d8e53cb},
+      {0x4750f6f27d779225, 0x16397553c6ff05fc},
+  };
+  for (const std::pair<int64_t, int64_t>& pair : pairs64) {
+    SCOPED_TRACE(::testing::Message()
+                 << "pair = {" << pair.first << ", " << pair.second << '}');
+
+    EXPECT_EQ(absl::MakeInt128(~pair.first, ~pair.second),
+              ~absl::MakeInt128(pair.first, pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) & absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) | absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^ absl::int128(pair.second));
+
+    EXPECT_EQ(absl::int128(pair.first & pair.second),
+              absl::int128(pair.first) &= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first | pair.second),
+              absl::int128(pair.first) |= absl::int128(pair.second));
+    EXPECT_EQ(absl::int128(pair.first ^ pair.second),
+              absl::int128(pair.first) ^= absl::int128(pair.second));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) & absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) | absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^ absl::MakeInt128(pair.second, 0));
+
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first & pair.second, 0),
+        absl::MakeInt128(pair.first, 0) &= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first | pair.second, 0),
+        absl::MakeInt128(pair.first, 0) |= absl::MakeInt128(pair.second, 0));
+    EXPECT_EQ(
+        absl::MakeInt128(pair.first ^ pair.second, 0),
+        absl::MakeInt128(pair.first, 0) ^= absl::MakeInt128(pair.second, 0));
+  }
+}
+
+TEST(Int128, BitwiseShiftTest) {
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) << (i - j));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) <<= (i - j));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = 0; j < 64; ++j) {
+      // Left shift from j-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) << (i + 64 - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::int128(uint64_t{1} << j) <<= (i + 64 - j));
+    }
+    for (int j = 0; j <= i; ++j) {
+      // Left shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) << (i - j));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) <<= (i - j));
+    }
+  }
+
+  for (int i = 0; i < 64; ++i) {
+    for (int j = i; j < 64; ++j) {
+      // Right shift from j-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >> (j - i));
+      EXPECT_EQ(uint64_t{1} << i, absl::int128(uint64_t{1} << j) >>= (j - i));
+    }
+    for (int j = 0; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to i-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j + 64 - i));
+      EXPECT_EQ(uint64_t{1} << i,
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j + 64 - i));
+    }
+  }
+  for (int i = 0; i < 63; ++i) {
+    for (int j = i; j < 63; ++j) {
+      // Right shift from (j + 64)-th bit to (i + 64)-th bit.
+      SCOPED_TRACE(::testing::Message() << "i = " << i << "; j = " << j);
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >> (j - i));
+      EXPECT_EQ(absl::MakeInt128(uint64_t{1} << i, 0),
+                absl::MakeInt128(uint64_t{1} << j, 0) >>= (j - i));
+    }
+  }
+}
+
+TEST(Int128, NumericLimitsTest) {
+  static_assert(std::numeric_limits<absl::int128>::is_specialized, "");
+  static_assert(std::numeric_limits<absl::int128>::is_signed, "");
+  static_assert(std::numeric_limits<absl::int128>::is_integer, "");
+  EXPECT_EQ(static_cast<int>(127 * std::log10(2)),
+            std::numeric_limits<absl::int128>::digits10);
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::min());
+  EXPECT_EQ(absl::Int128Min(), std::numeric_limits<absl::int128>::lowest());
+  EXPECT_EQ(absl::Int128Max(), std::numeric_limits<absl::int128>::max());
+}
+
 }  // namespace
diff --git a/absl/random/BUILD.bazel b/absl/random/BUILD.bazel
index f7587bf..f78fbc7 100644
--- a/absl/random/BUILD.bazel
+++ b/absl/random/BUILD.bazel
@@ -1,17 +1,32 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
 # ABSL random-number generation libraries.
 
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
 package(default_visibility = ["//visibility:public"])
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "random",
@@ -52,15 +67,17 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         "//absl/base:base_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/meta:type_traits",
-        "//absl/random/internal:distribution_impl",
         "//absl/random/internal:distributions",
         "//absl/random/internal:fast_uniform_bits",
         "//absl/random/internal:fastmath",
+        "//absl/random/internal:generate_real",
         "//absl/random/internal:iostream_state_saver",
         "//absl/random/internal:traits",
         "//absl/random/internal:uniform_helper",
+        "//absl/random/internal:wide_multiply",
         "//absl/strings",
         "//absl/types:span",
     ],
@@ -70,8 +87,8 @@
     name = "seed_gen_exception",
     srcs = ["seed_gen_exception.cc"],
     hdrs = ["seed_gen_exception.h"],
-    copts = ABSL_DEFAULT_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = ["//absl/base:config"],
 )
 
@@ -94,6 +111,58 @@
     ],
 )
 
+cc_library(
+    name = "bit_gen_ref",
+    hdrs = ["bit_gen_ref.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/base:core_headers",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:distribution_caller",
+        "//absl/random/internal:fast_uniform_bits",
+        "//absl/random/internal:mocking_bit_gen_base",
+    ],
+)
+
+cc_library(
+    name = "mock_distributions",
+    testonly = 1,
+    hdrs = ["mock_distributions.h"],
+    deps = [
+        ":distributions",
+        ":mocking_bit_gen",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:mock_overload_set",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_library(
+    name = "mocking_bit_gen",
+    testonly = 1,
+    srcs = [
+        "mocking_bit_gen.cc",
+    ],
+    hdrs = [
+        "mocking_bit_gen.h",
+    ],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":distributions",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:flat_hash_map",
+        "//absl/meta:type_traits",
+        "//absl/random/internal:distribution_caller",
+        "//absl/random/internal:mocking_bit_gen_base",
+        "//absl/strings",
+        "//absl/types:span",
+        "//absl/types:variant",
+        "//absl/utility",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_test(
     name = "bernoulli_distribution_test",
     size = "small",
@@ -115,11 +184,12 @@
     timeout = "eternal",  # Android can take a very long time
     srcs = ["beta_distribution_test.cc"],
     copts = ABSL_TEST_COPTS,
+    flaky = 1,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -168,8 +238,8 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -189,7 +259,7 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -214,8 +284,8 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/container:flat_hash_map",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
@@ -234,8 +304,8 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -256,8 +326,8 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -278,7 +348,7 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -302,7 +372,7 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -321,7 +391,7 @@
     deps = [
         ":distributions",
         ":random",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/random/internal:distribution_test_util",
         "//absl/random/internal:sequence_urbg",
         "//absl/strings",
@@ -330,6 +400,46 @@
 )
 
 cc_test(
+    name = "bit_gen_ref_test",
+    size = "small",
+    srcs = ["bit_gen_ref_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bit_gen_ref",
+        ":random",
+        "//absl/random/internal:sequence_urbg",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mocking_bit_gen_test",
+    size = "small",
+    srcs = ["mocking_bit_gen_test.cc"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":bit_gen_ref",
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
+    name = "mock_distributions_test",
+    size = "small",
+    srcs = ["mock_distributions_test.cc"],
+    deps = [
+        ":mock_distributions",
+        ":mocking_bit_gen",
+        ":random",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_test(
     name = "examples_test",
     size = "small",
     srcs = ["examples_test.cc"],
diff --git a/absl/random/CMakeLists.txt b/absl/random/CMakeLists.txt
index 2d5c065..efa55d8 100644
--- a/absl/random/CMakeLists.txt
+++ b/absl/random/CMakeLists.txt
@@ -34,6 +34,132 @@
 
 absl_cc_library(
   NAME
+    random_bit_gen_ref
+  HDRS
+    "bit_gen_ref.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::core_headers
+    absl::random_internal_distribution_caller
+    absl::random_internal_fast_uniform_bits
+    absl::random_internal_mocking_bit_gen_base
+    absl::type_traits
+)
+
+absl_cc_test(
+  NAME
+    random_bit_gen_ref_test
+  SRCS
+    "bit_gen_ref_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_bit_gen_ref
+    absl::random_random
+    absl::random_internal_sequence_urbg
+    gmock
+    gtest_main
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mocking_bit_gen_base
+  HDRS
+    "internal/mocking_bit_gen_base.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_random
+    absl::strings
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_mock_overload_set
+  HDRS
+    "internal/mock_overload_set.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_mocking_bit_gen
+  TESTONLY
+)
+
+absl_cc_library(
+  NAME
+    random_mocking_bit_gen
+  HDRS
+    "mock_distributions.h"
+    "mocking_bit_gen.h"
+  SRCS
+    "mocking_bit_gen.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::flat_hash_map
+    absl::raw_logging_internal
+    absl::random_distributions
+    absl::random_internal_distribution_caller
+    absl::random_internal_mocking_bit_gen_base
+    absl::random_internal_mock_overload_set
+    absl::strings
+    absl::span
+    absl::type_traits
+    absl::utility
+    absl::variant
+    gmock
+    gtest
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    random_mock_distributions_test
+  SRCS
+    "mock_distributions_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_mocking_bit_gen
+    absl::random_random
+    gmock
+    gtest_main
+)
+
+absl_cc_test(
+  NAME
+    random_mocking_bit_gen_test
+  SRCS
+    "mocking_bit_gen_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_bit_gen_ref
+    absl::random_mocking_bit_gen
+    absl::random_random
+    gmock
+    gtest_main
+)
+
+absl_cc_library(
+  NAME
     random_distributions
   SRCS
     "discrete_distribution.cc"
@@ -57,14 +183,16 @@
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::base_internal
+    absl::config
     absl::core_headers
-    absl::random_internal_distribution_impl
+    absl::random_internal_generate_real
     absl::random_internal_distributions
     absl::random_internal_fast_uniform_bits
     absl::random_internal_fastmath
     absl::random_internal_iostream_state_saver
     absl::random_internal_traits
     absl::random_internal_uniform_helper
+    absl::random_internal_wide_multiply
     absl::strings
     absl::span
     absl::type_traits
@@ -79,9 +207,7 @@
     "seed_gen_exception.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
   LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::config
@@ -137,9 +263,9 @@
   DEPS
     absl::random_distributions
     absl::random_random
-    absl::base
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     gmock
@@ -174,6 +300,7 @@
     ${ABSL_DEFAULT_LINKOPTS}
     absl::random_distributions
     absl::random_random
+    absl::raw_logging_internal
     gmock
     gtest_main
 )
@@ -187,12 +314,12 @@
     ${ABSL_TEST_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
-    absl::base
     absl::core_headers
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     gmock
@@ -209,11 +336,11 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     gmock
     gtest_main
@@ -231,11 +358,11 @@
   DEPS
     absl::random_distributions
     absl::random_random
-    absl::base
     absl::core_headers
     absl::flat_hash_map
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     gmock
@@ -252,12 +379,12 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::core_headers
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     gmock
@@ -274,12 +401,12 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::core_headers
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     gmock
@@ -296,11 +423,11 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     gmock
     gtest_main
@@ -316,7 +443,6 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
@@ -336,11 +462,11 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_distributions
     absl::random_internal_distribution_test_util
     absl::random_internal_sequence_urbg
     absl::random_random
+    absl::raw_logging_internal
     absl::strings
     gmock
     gtest_main
@@ -401,6 +527,8 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -434,6 +562,8 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -448,11 +578,12 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+    $<$<BOOL:${MINGW}>:"bcrypt">
   DEPS
-    absl::base
     absl::core_headers
     absl::optional
     absl::random_internal_fast_uniform_bits
+    absl::raw_logging_internal
     absl::span
     absl::strings
 )
@@ -478,6 +609,7 @@
     absl::random_internal_seed_material
     absl::random_internal_traits
     absl::random_seed_gen_exception
+    absl::raw_logging_internal
     absl::span
 )
 
@@ -491,6 +623,8 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
@@ -504,6 +638,8 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
@@ -543,9 +679,26 @@
 # Internal-only target, do not depend on directly.
 absl_cc_library(
   NAME
-    random_internal_distribution_impl
+    random_internal_generate_real
   HDRS
-    "internal/distribution_impl.h"
+    "internal/generate_real.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::bits
+    absl::random_internal_fastmath
+    absl::random_internal_traits
+    absl::type_traits
+)
+
+# Internal-only target, do not depend on directly.
+absl_cc_library(
+  NAME
+    random_internal_wide_multiply
+  HDRS
+    "internal/wide_multiply.h"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
@@ -554,8 +707,6 @@
     absl::bits
     absl::config
     absl::int128
-    absl::random_internal_fastmath
-    absl::random_internal_traits
 )
 
 # Internal-only target, do not depend on directly.
@@ -624,6 +775,7 @@
   DEPS
     absl::random_internal_iostream_state_saver
     absl::random_internal_randen
+    absl::raw_logging_internal
     absl::type_traits
 )
 
@@ -639,6 +791,8 @@
     ${ABSL_DEFAULT_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -654,7 +808,6 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_internal_platform
     absl::random_internal_randen_hwaes
     absl::random_internal_randen_slow
@@ -674,6 +827,7 @@
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_platform
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -693,6 +847,7 @@
   DEPS
     absl::random_internal_platform
     absl::random_internal_randen_hwaes_impl
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -709,6 +864,7 @@
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_platform
+    absl::config
 )
 
 # Internal-only target, do not depend on directly.
@@ -726,8 +882,9 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
+    absl::config
     absl::core_headers
+    absl::raw_logging_internal
     absl::strings
     absl::str_format
     absl::span
@@ -751,9 +908,9 @@
 # Internal-only target, do not depend on directly.
 absl_cc_test(
   NAME
-    random_internal_distribution_impl_test
+    random_internal_generate_real_test
   SRCS
-    "internal/distribution_impl_test.cc"
+    "internal/generate_real_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
   LINKOPTS
@@ -761,8 +918,7 @@
   DEPS
     absl::bits
     absl::flags
-    absl::int128
-    absl::random_internal_distribution_impl
+    absl::random_internal_generate_real
     gtest_main
 )
 
@@ -941,9 +1097,9 @@
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
-    absl::base
     absl::random_internal_explicit_seed_seq
     absl::random_internal_randen_engine
+    absl::raw_logging_internal
     absl::strings
     absl::time
     gmock
@@ -995,7 +1151,7 @@
     absl::random_internal_platform
     absl::random_internal_randen_hwaes
     absl::random_internal_randen_hwaes_impl
-    absl::base
+    absl::raw_logging_internal
     absl::str_format
     gmock
     gtest
@@ -1013,7 +1169,6 @@
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::core_headers
-    absl::random_internal_distribution_impl
     absl::random_internal_fast_uniform_bits
     absl::random_internal_iostream_state_saver
     absl::random_internal_traits
@@ -1026,9 +1181,28 @@
     random_internal_iostream_state_saver_test
   SRCS
     "internal/iostream_state_saver_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
   LINKOPTS
     ${ABSL_DEFAULT_LINKOPTS}
   DEPS
     absl::random_internal_iostream_state_saver
     gtest_main
 )
+
+# Internal-only target, do not depend on directly.
+absl_cc_test(
+  NAME
+    random_internal_wide_multiply_test
+  SRCS
+      internal/wide_multiply_test.cc
+  COPTS
+    ${ABSL_TEST_COPTS}
+  LINKOPTS
+    ${ABSL_DEFAULT_LINKOPTS}
+  DEPS
+    absl::random_internal_wide_multiply
+    absl::bits
+    absl::int128
+    gtest_main
+)
diff --git a/absl/random/bernoulli_distribution.h b/absl/random/bernoulli_distribution.h
index 0afc2c1..25bd0d5 100644
--- a/absl/random/bernoulli_distribution.h
+++ b/absl/random/bernoulli_distribution.h
@@ -24,7 +24,7 @@
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::bernoulli_distribution is a drop in replacement for
 // std::bernoulli_distribution. It guarantees that (given a perfect
@@ -194,7 +194,7 @@
   }
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_BERNOULLI_DISTRIBUTION_H_
diff --git a/absl/random/beta_distribution.h b/absl/random/beta_distribution.h
index ff1eba8..c154066 100644
--- a/absl/random/beta_distribution.h
+++ b/absl/random/beta_distribution.h
@@ -22,13 +22,14 @@
 #include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::beta_distribution:
 // Generate a floating-point variate conforming to a Beta distribution:
@@ -130,7 +131,7 @@
    private:
     friend class beta_distribution;
 
-#ifdef COMPILER_MSVC
+#ifdef _MSC_VER
     // MSVC does not have constexpr implementations for std::log and std::exp
     // so they are computed at runtime.
 #define ABSL_RANDOM_INTERNAL_LOG_EXP_CONSTEXPR
@@ -276,15 +277,21 @@
 beta_distribution<RealType>::AlgorithmJoehnk(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      absl::conditional_t<std::is_same<RealType, float>::value, float, double>;
+
   // Based on Joehnk, M. D. Erzeugung von betaverteilten und gammaverteilten
   // Zufallszahlen. Metrika 8.1 (1964): 5-15.
   // This method is described in Knuth, Vol 2 (Third Edition), pp 134.
-  using RandU64ToReal = typename random_internal::RandU64ToReal<result_type>;
-  using random_internal::PositiveValueT;
+
   result_type u, v, x, y, z;
   for (;;) {
-    u = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
-    v = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
+    u = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
+    v = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
 
     // Direct method. std::pow is slow for float, so rely on the optimizer to
     // remove the std::pow() path for that case.
@@ -328,12 +335,14 @@
 beta_distribution<RealType>::AlgorithmCheng(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      absl::conditional_t<std::is_same<RealType, float>::value, float, double>;
+
   // Based on Cheng, Russell CH. Generating beta variates with nonintegral
   // shape parameters. Communications of the ACM 21.4 (1978): 317-322.
   // (https://dl.acm.org/citation.cfm?id=359482).
-  using RandU64ToReal = typename random_internal::RandU64ToReal<result_type>;
-  using random_internal::PositiveValueT;
-
   static constexpr result_type kLogFour =
       result_type(1.3862943611198906188344642429163531361);  // log(4)
   static constexpr result_type kS =
@@ -342,8 +351,10 @@
   const bool use_algorithm_ba = (p.method_ == param_type::CHENG_BA);
   result_type u1, u2, v, w, z, r, s, t, bw_inv, lhs;
   for (;;) {
-    u1 = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
-    u2 = RandU64ToReal::template Value<PositiveValueT, false>(fast_u64_(g));
+    u1 = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
+    u2 = GenerateRealFromBits<real_type, GeneratePositiveTag, false>(
+        fast_u64_(g));
     v = p.y_ * std::log(u1 / (1 - u1));
     w = p.a_ * std::exp(v);
     bw_inv = result_type(1) / (p.b_ + w);
@@ -410,7 +421,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_BETA_DISTRIBUTION_H_
diff --git a/absl/random/beta_distribution_test.cc b/absl/random/beta_distribution_test.cc
index 966ad08..d0111b3 100644
--- a/absl/random/beta_distribution_test.cc
+++ b/absl/random/beta_distribution_test.cc
@@ -92,7 +92,7 @@
   for (TypeParam alpha : kValues) {
     for (TypeParam beta : kValues) {
       ABSL_INTERNAL_LOG(
-          INFO, absl::StrFormat("Smoke test for Beta(%f, %f)", alpha, beta));
+          INFO, absl::StrFormat("Smoke test for Beta(%a, %a)", alpha, beta));
 
       param_type param(alpha, beta);
       absl::beta_distribution<TypeParam> before(alpha, beta);
diff --git a/absl/random/bit_gen_ref.h b/absl/random/bit_gen_ref.h
new file mode 100644
index 0000000..e877116
--- /dev/null
+++ b/absl/random/bit_gen_ref.h
@@ -0,0 +1,153 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: bit_gen_ref.h
+// -----------------------------------------------------------------------------
+//
+// This header defines a bit generator "reference" class, for use in interfaces
+// that take both Abseil (e.g. `absl::BitGen`) and standard library (e.g.
+// `std::mt19937`) bit generators.
+
+#ifndef ABSL_RANDOM_BIT_GEN_REF_H_
+#define ABSL_RANDOM_BIT_GEN_REF_H_
+
+#include "absl/base/macros.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/distribution_caller.h"
+#include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/mocking_bit_gen_base.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+template <typename URBG, typename = void, typename = void, typename = void>
+struct is_urbg : std::false_type {};
+
+template <typename URBG>
+struct is_urbg<
+    URBG,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype((URBG::min)())>::type>::value>,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype((URBG::max)())>::type>::value>,
+    absl::enable_if_t<std::is_same<
+        typename URBG::result_type,
+        typename std::decay<decltype(std::declval<URBG>()())>::type>::value>>
+    : std::true_type {};
+
+}  // namespace random_internal
+
+// -----------------------------------------------------------------------------
+// absl::BitGenRef
+// -----------------------------------------------------------------------------
+//
+// `absl::BitGenRef` is a type-erasing class that provides a generator-agnostic
+// non-owning "reference" interface for use in place of any specific uniform
+// random bit generator (URBG). This class may be used for both Abseil
+// (e.g. `absl::BitGen`, `absl::InsecureBitGen`) and Standard library (e.g
+// `std::mt19937`, `std::minstd_rand`) bit generators.
+//
+// Like other reference classes, `absl::BitGenRef` does not own the
+// underlying bit generator, and the underlying instance must outlive the
+// `absl::BitGenRef`.
+//
+// `absl::BitGenRef` is particularly useful when used with an
+// `absl::MockingBitGen` to test specific paths in functions which use random
+// values.
+//
+// Example:
+//    void TakesBitGenRef(absl::BitGenRef gen) {
+//      int x = absl::Uniform<int>(gen, 0, 1000);
+//    }
+//
+class BitGenRef {
+ public:
+  using result_type = uint64_t;
+
+  BitGenRef(const absl::BitGenRef&) = default;
+  BitGenRef(absl::BitGenRef&&) = default;
+  BitGenRef& operator=(const absl::BitGenRef&) = default;
+  BitGenRef& operator=(absl::BitGenRef&&) = default;
+
+  template <typename URBG,
+            typename absl::enable_if_t<
+                (!std::is_same<URBG, BitGenRef>::value &&
+                 random_internal::is_urbg<URBG>::value)>* = nullptr>
+  BitGenRef(URBG& gen)  // NOLINT
+      : mocked_gen_ptr_(MakeMockPointer(&gen)),
+        t_erased_gen_ptr_(reinterpret_cast<uintptr_t>(&gen)),
+        generate_impl_fn_(ImplFn<URBG>) {
+  }
+
+  static constexpr result_type(min)() {
+    return (std::numeric_limits<result_type>::min)();
+  }
+
+  static constexpr result_type(max)() {
+    return (std::numeric_limits<result_type>::max)();
+  }
+
+  result_type operator()() { return generate_impl_fn_(t_erased_gen_ptr_); }
+
+ private:
+  friend struct absl::random_internal::DistributionCaller<absl::BitGenRef>;
+  using impl_fn = result_type (*)(uintptr_t);
+  using mocker_base_t = absl::random_internal::MockingBitGenBase;
+
+  // Convert an arbitrary URBG pointer into either a valid mocker_base_t
+  // pointer or a nullptr.
+  static inline mocker_base_t* MakeMockPointer(mocker_base_t* t) { return t; }
+  static inline mocker_base_t* MakeMockPointer(void*) { return nullptr; }
+
+  template <typename URBG>
+  static result_type ImplFn(uintptr_t ptr) {
+    // Ensure that the return values from operator() fill the entire
+    // range promised by result_type, min() and max().
+    absl::random_internal::FastUniformBits<result_type> fast_uniform_bits;
+    return fast_uniform_bits(*reinterpret_cast<URBG*>(ptr));
+  }
+
+  mocker_base_t* mocked_gen_ptr_;
+  uintptr_t t_erased_gen_ptr_;
+  impl_fn generate_impl_fn_;
+};
+
+namespace random_internal {
+
+template <>
+struct DistributionCaller<absl::BitGenRef> {
+  template <typename DistrT, typename FormatT, typename... Args>
+  static typename DistrT::result_type Call(absl::BitGenRef* gen_ref,
+                                           Args&&... args) {
+    auto* mock_ptr = gen_ref->mocked_gen_ptr_;
+    if (mock_ptr == nullptr) {
+      DistrT dist(std::forward<Args>(args)...);
+      return dist(*gen_ref);
+    } else {
+      return mock_ptr->template Call<DistrT, FormatT>(
+          std::forward<Args>(args)...);
+    }
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_BIT_GEN_REF_H_
diff --git a/absl/random/bit_gen_ref_test.cc b/absl/random/bit_gen_ref_test.cc
new file mode 100644
index 0000000..ca0e4d7
--- /dev/null
+++ b/absl/random/bit_gen_ref_test.cc
@@ -0,0 +1,101 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "absl/random/bit_gen_ref.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/random/internal/sequence_urbg.h"
+#include "absl/random/random.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class ConstBitGen : public absl::random_internal::MockingBitGenBase {
+  bool CallImpl(const std::type_info&, void*, void* result) override {
+    *static_cast<int*>(result) = 42;
+    return true;
+  }
+};
+
+namespace random_internal {
+template <>
+struct DistributionCaller<ConstBitGen> {
+  template <typename DistrT, typename FormatT, typename... Args>
+  static typename DistrT::result_type Call(ConstBitGen* gen, Args&&... args) {
+    return gen->template Call<DistrT, FormatT>(std::forward<Args>(args)...);
+  }
+};
+}  // namespace random_internal
+
+namespace {
+int FnTest(absl::BitGenRef gen_ref) { return absl::Uniform(gen_ref, 1, 7); }
+
+template <typename T>
+class BitGenRefTest : public testing::Test {};
+
+using BitGenTypes =
+    ::testing::Types<absl::BitGen, absl::InsecureBitGen, std::mt19937,
+                     std::mt19937_64, std::minstd_rand>;
+TYPED_TEST_SUITE(BitGenRefTest, BitGenTypes);
+
+TYPED_TEST(BitGenRefTest, BasicTest) {
+  TypeParam gen;
+  auto x = FnTest(gen);
+  EXPECT_NEAR(x, 4, 3);
+}
+
+TYPED_TEST(BitGenRefTest, Copyable) {
+  TypeParam gen;
+  absl::BitGenRef gen_ref(gen);
+  FnTest(gen_ref);  // Copy
+}
+
+TEST(BitGenRefTest, PassThroughEquivalence) {
+  // sequence_urbg returns 64-bit results.
+  absl::random_internal::sequence_urbg urbg(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  std::vector<uint64_t> output(12);
+
+  {
+    absl::BitGenRef view(urbg);
+    for (auto& v : output) {
+      v = view();
+    }
+  }
+
+  std::vector<uint64_t> expected(
+      {0x0003eb76f6f7f755ull, 0xFFCEA50FDB2F953Bull, 0xC332DDEFBE6C5AA5ull,
+       0x6558218568AB9702ull, 0x2AEF7DAD5B6E2F84ull, 0x1521B62829076170ull,
+       0xECDD4775619F1510ull, 0x13CCA830EB61BD96ull, 0x0334FE1EAA0363CFull,
+       0xB5735C904C70A239ull, 0xD59E9E0BCBAADE14ull, 0xEECC86BC60622CA7ull});
+
+  EXPECT_THAT(output, testing::Eq(expected));
+}
+
+TEST(BitGenRefTest, MockingBitGenBaseOverrides) {
+  ConstBitGen const_gen;
+  EXPECT_EQ(FnTest(const_gen), 42);
+
+  absl::BitGenRef gen_ref(const_gen);
+  EXPECT_EQ(FnTest(gen_ref), 42);  // Copy
+}
+}  // namespace
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/random/discrete_distribution.cc b/absl/random/discrete_distribution.cc
index 13a2dbe..081acce 100644
--- a/absl/random/discrete_distribution.cc
+++ b/absl/random/discrete_distribution.cc
@@ -15,7 +15,7 @@
 #include "absl/random/discrete_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Initializes the distribution table for Walker's Aliasing algorithm, described
@@ -94,5 +94,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/discrete_distribution.h b/absl/random/discrete_distribution.h
index 5866fb2..171aa11 100644
--- a/absl/random/discrete_distribution.h
+++ b/absl/random/discrete_distribution.h
@@ -29,7 +29,7 @@
 #include "absl/random/uniform_int_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::discrete_distribution
 //
@@ -241,7 +241,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_DISCRETE_DISTRIBUTION_H_
diff --git a/absl/random/distribution_format_traits.h b/absl/random/distribution_format_traits.h
index 838271c..22b358c 100644
--- a/absl/random/distribution_format_traits.h
+++ b/absl/random/distribution_format_traits.h
@@ -36,7 +36,13 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
+
+struct IntervalClosedClosedTag;
+struct IntervalClosedOpenTag;
+struct IntervalOpenClosedTag;
+struct IntervalOpenOpenTag;
+
 namespace random_internal {
 
 // ScalarTypeName defines a preferred hierarchy of preferred type names for
@@ -244,8 +250,29 @@
   }
 };
 
+template <typename NumType>
+struct UniformDistributionWrapper;
+
+template <typename NumType>
+struct DistributionFormatTraits<UniformDistributionWrapper<NumType>> {
+  using distribution_t = UniformDistributionWrapper<NumType>;
+  using result_t = NumType;
+
+  static constexpr const char* Name() { return "Uniform"; }
+
+  static std::string FunctionName() {
+    return absl::StrCat(Name(), "<", ScalarTypeName<NumType>(), ">");
+  }
+  static std::string FormatArgs(const distribution_t& d) {
+    return absl::StrCat((d.min)(), ", ", (d.max)());
+  }
+  static std::string FormatResults(absl::Span<const result_t> results) {
+    return absl::StrJoin(results, ", ");
+  }
+};
+
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_DISTRIBUTION_FORMAT_TRAITS_H_
diff --git a/absl/random/distributions.h b/absl/random/distributions.h
index f2ebfc2..d026d92 100644
--- a/absl/random/distributions.h
+++ b/absl/random/distributions.h
@@ -67,20 +67,15 @@
 #include "absl/random/zipf_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalClosedClosedT,
-                               IntervalClosedClosed, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalClosedClosedT,
-                               IntervalClosed, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalClosedOpenT,
-                               IntervalClosedOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalOpenOpenT,
-                               IntervalOpenOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalOpenOpenT,
-                               IntervalOpen, {});
-ABSL_INTERNAL_INLINE_CONSTEXPR(random_internal::IntervalOpenClosedT,
-                               IntervalOpenClosed, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedClosedTag, IntervalClosedClosed,
+                               {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedClosedTag, IntervalClosed, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalClosedOpenTag, IntervalClosedOpen, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenOpenTag, IntervalOpenOpen, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenOpenTag, IntervalOpen, {});
+ABSL_INTERNAL_INLINE_CONSTEXPR(IntervalOpenClosedTag, IntervalOpenClosed, {});
 
 // -----------------------------------------------------------------------------
 // absl::Uniform<T>(tag, bitgen, lo, hi)
@@ -102,7 +97,7 @@
 // the return type based on the provided endpoint arguments {A lo, B hi}.
 // Given these endpoints, one of {A, B} will be chosen as the return type, if
 // a type can be implicitly converted into the other in a lossless way. The
-// lack of any such implcit conversion between {A, B} will produce a
+// lack of any such implicit conversion between {A, B} will produce a
 // compile-time error
 //
 // See https://en.wikipedia.org/wiki/Uniform_distribution_(continuous)
@@ -130,7 +125,15 @@
         URBG&& urbg,  // NOLINT(runtime/references)
         R lo, R hi) {
   using gen_t = absl::decay_t<URBG>;
-  return random_internal::UniformImpl<R, TagType, gen_t>(tag, urbg, lo, hi);
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
+
+  auto a = random_internal::uniform_lower_bound(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound(tag, lo, hi);
+  if (a > b) return a;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t, format_t>(&urbg, tag, lo, hi);
 }
 
 // absl::Uniform<T>(bitgen, lo, hi)
@@ -141,11 +144,17 @@
 typename absl::enable_if_t<!std::is_same<R, void>::value, R>  //
 Uniform(URBG&& urbg,  // NOLINT(runtime/references)
         R lo, R hi) {
-  constexpr auto tag = absl::IntervalClosedOpen;
-  using tag_t = decltype(tag);
   using gen_t = absl::decay_t<URBG>;
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
-  return random_internal::UniformImpl<R, tag_t, gen_t>(tag, urbg, lo, hi);
+  constexpr auto tag = absl::IntervalClosedOpen;
+  auto a = random_internal::uniform_lower_bound(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound(tag, lo, hi);
+  if (a > b) return a;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t, format_t>(&urbg, lo, hi);
 }
 
 // absl::Uniform(tag, bitgen, lo, hi)
@@ -162,9 +171,16 @@
         A lo, B hi) {
   using gen_t = absl::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
+  using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
-  return random_internal::UniformImpl<return_t, TagType, gen_t>(tag, urbg, lo,
-                                                                hi);
+  auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound<return_t>(tag, lo, hi);
+  if (a > b) return a;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t, format_t>(&urbg, tag, static_cast<return_t>(lo),
+                                static_cast<return_t>(hi));
 }
 
 // absl::Uniform(bitgen, lo, hi)
@@ -177,13 +193,19 @@
                            random_internal::uniform_inferred_return_t<A, B>>
 Uniform(URBG&& urbg,  // NOLINT(runtime/references)
         A lo, B hi) {
-  constexpr auto tag = absl::IntervalClosedOpen;
-  using tag_t = decltype(tag);
   using gen_t = absl::decay_t<URBG>;
   using return_t = typename random_internal::uniform_inferred_return_t<A, B>;
+  using distribution_t = random_internal::UniformDistributionWrapper<return_t>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
-  return random_internal::UniformImpl<return_t, tag_t, gen_t>(tag, urbg, lo,
-                                                              hi);
+  constexpr auto tag = absl::IntervalClosedOpen;
+  auto a = random_internal::uniform_lower_bound<return_t>(tag, lo, hi);
+  auto b = random_internal::uniform_upper_bound<return_t>(tag, lo, hi);
+  if (a > b) return a;
+
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t, format_t>(&urbg, static_cast<return_t>(lo),
+                                static_cast<return_t>(hi));
 }
 
 // absl::Uniform<unsigned T>(bitgen)
@@ -193,13 +215,12 @@
 template <typename R, typename URBG>
 typename absl::enable_if_t<!std::is_signed<R>::value, R>  //
 Uniform(URBG&& urbg) {  // NOLINT(runtime/references)
-  constexpr auto tag = absl::IntervalClosedClosed;
-  constexpr auto lo = std::numeric_limits<R>::lowest();
-  constexpr auto hi = (std::numeric_limits<R>::max)();
-  using tag_t = decltype(tag);
   using gen_t = absl::decay_t<URBG>;
+  using distribution_t = random_internal::UniformDistributionWrapper<R>;
+  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
 
-  return random_internal::UniformImpl<R, tag_t, gen_t>(tag, urbg, lo, hi);
+  return random_internal::DistributionCaller<gen_t>::template Call<
+      distribution_t, format_t>(&urbg);
 }
 
 // -----------------------------------------------------------------------------
@@ -270,10 +291,10 @@
 // absl::Exponential<T>(bitgen, lambda = 1)
 // -----------------------------------------------------------------------------
 //
-// `absl::Exponential` produces a floating point number for discrete
-// distributions of events occurring continuously and independently at a
-// constant average rate. `T` must be a floating point type, but may be inferred
-// from the type of `lambda`.
+// `absl::Exponential` produces a floating point number representing the
+// distance (time) between two consecutive events in a point process of events
+// occurring continuously and independently at a constant average rate. `T` must
+// be a floating point type, but may be inferred from the type of `lambda`.
 //
 // See https://en.wikipedia.org/wiki/Exponential_distribution.
 //
@@ -438,7 +459,7 @@
       distribution_t, format_t>(&urbg, hi, q, v);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_DISTRIBUTIONS_H_
diff --git a/absl/random/distributions_test.cc b/absl/random/distributions_test.cc
index eb82868..2d92723 100644
--- a/absl/random/distributions_test.cc
+++ b/absl/random/distributions_test.cc
@@ -171,14 +171,11 @@
       "");
   static_assert(
       absl::conjunction<
+          std::is_same<Expect, decltype(InferredTaggedUniformReturnT<
+                                        absl::IntervalOpenOpenTag, A, B>(0))>,
           std::is_same<Expect,
                        decltype(InferredTaggedUniformReturnT<
-                                absl::random_internal::IntervalOpenOpenT, A, B>(
-                           0))>,
-          std::is_same<Expect,
-                       decltype(InferredTaggedUniformReturnT<
-                                absl::random_internal::IntervalOpenOpenT, B, A>(
-                           0))>>::value,
+                                absl::IntervalOpenOpenTag, B, A>(0))>>::value,
       "");
 }
 
@@ -218,12 +215,10 @@
       absl::conjunction<
           std::is_same<Expect,
                        decltype(ExplicitTaggedUniformReturnT<
-                                absl::random_internal::IntervalOpenOpenT, A, B,
-                                Expect>(0))>,
-          std::is_same<Expect,
-                       decltype(ExplicitTaggedUniformReturnT<
-                                absl::random_internal::IntervalOpenOpenT, B, A,
-                                Expect>(0))>>::value,
+                                absl::IntervalOpenOpenTag, A, B, Expect>(0))>,
+          std::is_same<Expect, decltype(ExplicitTaggedUniformReturnT<
+                                        absl::IntervalOpenOpenTag, B, A,
+                                        Expect>(0))>>::value,
       "");
 }
 
diff --git a/absl/random/exponential_distribution.h b/absl/random/exponential_distribution.h
index ed5551a..b5caf8a 100644
--- a/absl/random/exponential_distribution.h
+++ b/absl/random/exponential_distribution.h
@@ -21,12 +21,13 @@
 #include <limits>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::exponential_distribution:
 // Generates a number conforming to an exponential distribution and is
@@ -119,9 +120,14 @@
 exponential_distribution<RealType>::operator()(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
-  using random_internal::NegativeValueT;
-  const result_type u = random_internal::RandU64ToReal<
-      result_type>::template Value<NegativeValueT, false>(fast_u64_(g));
+  using random_internal::GenerateNegativeTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      absl::conditional_t<std::is_same<RealType, float>::value, float, double>;
+
+  const result_type u = GenerateRealFromBits<real_type, GenerateNegativeTag,
+                                             false>(fast_u64_(g));  // U(-1, 0)
+
   // log1p(-x) is mathematically equivalent to log(1 - x) but has more
   // accuracy for x near zero.
   return p.neg_inv_lambda_ * std::log1p(u);
@@ -153,7 +159,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_EXPONENTIAL_DISTRIBUTION_H_
diff --git a/absl/random/exponential_distribution_test.cc b/absl/random/exponential_distribution_test.cc
index 6f8865c..f3cfd76 100644
--- a/absl/random/exponential_distribution_test.cc
+++ b/absl/random/exponential_distribution_test.cc
@@ -46,7 +46,11 @@
 template <typename RealType>
 class ExponentialDistributionTypedTest : public ::testing::Test {};
 
+#if defined(__EMSCRIPTEN__)
+using RealTypes = ::testing::Types<float, double>;
+#else
 using RealTypes = ::testing::Types<float, double, long double>;
+#endif  // defined(__EMSCRIPTEN__)
 TYPED_TEST_CASE(ExponentialDistributionTypedTest, RealTypes);
 
 TYPED_TEST(ExponentialDistributionTypedTest, SerializeTest) {
@@ -346,7 +350,7 @@
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_CASE_P(, ExponentialDistributionTests,
+INSTANTIATE_TEST_CASE_P(All, ExponentialDistributionTests,
                         ::testing::ValuesIn(GenParams()), ParamName);
 
 // NOTE: absl::exponential_distribution is not guaranteed to be stable.
diff --git a/absl/random/gaussian_distribution.cc b/absl/random/gaussian_distribution.cc
index dbc2d84..c7a72cb 100644
--- a/absl/random/gaussian_distribution.cc
+++ b/absl/random/gaussian_distribution.cc
@@ -4,7 +4,7 @@
 #include "absl/random/gaussian_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 const gaussian_distribution_base::Tables
@@ -97,7 +97,7 @@
      0.9362826816850632339, 0.9635996931270905952, 1}};
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // clang-format on
diff --git a/absl/random/gaussian_distribution.h b/absl/random/gaussian_distribution.h
index 8ee9514..4b07a5c 100644
--- a/absl/random/gaussian_distribution.h
+++ b/absl/random/gaussian_distribution.h
@@ -28,12 +28,13 @@
 #include <limits>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/base/config.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // absl::gaussian_distribution_base implements the underlying ziggurat algorithm
@@ -43,7 +44,7 @@
 // The specific algorithm has some of the improvements suggested by the
 // 2005 paper, "An Improved Ziggurat Method to Generate Normal Random Samples",
 // Jurgen A Doornik.  (https://www.doornik.com/research/ziggurat.pdf)
-class gaussian_distribution_base {
+class ABSL_DLL gaussian_distribution_base {
  public:
   template <typename URBG>
   inline double zignor(URBG& g);  // NOLINT(runtime/references)
@@ -208,12 +209,18 @@
 
 template <typename URBG>
 inline double gaussian_distribution_base::zignor_fallback(URBG& g, bool neg) {
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+
   // This fallback path happens approximately 0.05% of the time.
   double x, y;
   do {
     // kRInv = 1/r, U(0, 1)
-    x = kRInv * std::log(RandU64ToDouble<PositiveValueT, false>(fast_u64_(g)));
-    y = -std::log(RandU64ToDouble<PositiveValueT, false>(fast_u64_(g)));
+    x = kRInv *
+        std::log(GenerateRealFromBits<double, GeneratePositiveTag, false>(
+            fast_u64_(g)));
+    y = -std::log(
+        GenerateRealFromBits<double, GeneratePositiveTag, false>(fast_u64_(g)));
   } while ((y + y) < (x * x));
   return neg ? (x - kR) : (kR - x);
 }
@@ -221,6 +228,10 @@
 template <typename URBG>
 inline double gaussian_distribution_base::zignor(
     URBG& g) {  // NOLINT(runtime/references)
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using random_internal::GenerateSignedTag;
+
   while (true) {
     // We use a single uint64_t to generate both a double and a strip.
     // These bits are unused when the generated double is > 1/2^5.
@@ -228,7 +239,8 @@
     // values (those smaller than 1/2^5, which all end up on the left tail).
     uint64_t bits = fast_u64_(g);
     int i = static_cast<int>(bits & kMask);  // pick a random strip
-    double j = RandU64ToDouble<SignedValueT, false>(bits);  // U(-1, 1)
+    double j = GenerateRealFromBits<double, GenerateSignedTag, false>(
+        bits);  // U(-1, 1)
     const double x = j * zg_.x[i];
 
     // Retangular box. Handles >97% of all cases.
@@ -245,7 +257,8 @@
     }
 
     // i > 0: Wedge samples using precomputed values.
-    double v = RandU64ToDouble<PositiveValueT, false>(fast_u64_(g));  // U(0, 1)
+    double v = GenerateRealFromBits<double, GeneratePositiveTag, false>(
+        fast_u64_(g));  // U(0, 1)
     if ((zg_.f[i + 1] + v * (zg_.f[i] - zg_.f[i + 1])) <
         std::exp(-0.5 * x * x)) {
       return x;
@@ -256,7 +269,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_GAUSSIAN_DISTRIBUTION_H_
diff --git a/absl/random/gaussian_distribution_test.cc b/absl/random/gaussian_distribution_test.cc
index 47c2989..49c0751 100644
--- a/absl/random/gaussian_distribution_test.cc
+++ b/absl/random/gaussian_distribution_test.cc
@@ -394,7 +394,7 @@
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_SUITE_P(, GaussianDistributionTests,
+INSTANTIATE_TEST_SUITE_P(All, GaussianDistributionTests,
                          ::testing::ValuesIn(GenParams()), ParamName);
 
 // NOTE: absl::gaussian_distribution is not guaranteed to be stable.
diff --git a/absl/random/internal/BUILD.bazel b/absl/random/internal/BUILD.bazel
index fd5471a..d7ad4ef 100644
--- a/absl/random/internal/BUILD.bazel
+++ b/absl/random/internal/BUILD.bazel
@@ -1,3 +1,21 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
+
 # Internal-only implementation classes for Abseil Random
 load(
     "//absl:copts/configure_copts.bzl",
@@ -33,24 +51,21 @@
     visibility = [
         "//absl/random:__pkg__",
     ],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
     name = "distributions",
-    hdrs = [
-        "distributions.h",
-    ],
+    hdrs = ["distributions.h"],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":distribution_caller",
-        ":fast_uniform_bits",
-        ":fastmath",
         ":traits",
         ":uniform_helper",
+        "//absl/base",
         "//absl/meta:type_traits",
         "//absl/strings",
-        "//absl/types:span",
     ],
 )
 
@@ -64,6 +79,7 @@
     visibility = [
         "//absl/random:__pkg__",
     ],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -75,11 +91,14 @@
         "seed_material.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_DEFAULT_LINKOPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS + select({
+        "//absl:windows": ["-DEFAULTLIB:bcrypt.lib"],
+        "//conditions:default": [],
+    }),
     deps = [
         ":fast_uniform_bits",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/types:optional",
         "//absl/types:span",
@@ -107,6 +126,7 @@
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
         "//absl/random:seed_gen_exception",
         "//absl/types:span",
     ],
@@ -120,6 +140,7 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -130,6 +151,7 @@
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -160,9 +182,9 @@
 )
 
 cc_library(
-    name = "distribution_impl",
+    name = "generate_real",
     hdrs = [
-        "distribution_impl.h",
+        "generate_real.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
@@ -170,8 +192,7 @@
         ":fastmath",
         ":traits",
         "//absl/base:bits",
-        "//absl/base:config",
-        "//absl/numeric:int128",
+        "//absl/meta:type_traits",
     ],
 )
 
@@ -186,6 +207,19 @@
 )
 
 cc_library(
+    name = "wide_multiply",
+    hdrs = ["wide_multiply.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":traits",
+        "//absl/base:bits",
+        "//absl/base:config",
+        "//absl/numeric:int128",
+    ],
+)
+
+cc_library(
     name = "nonsecure_base",
     hdrs = ["nonsecure_base.h"],
     copts = ABSL_DEFAULT_COPTS,
@@ -239,6 +273,7 @@
         "randen-keys.inc",
         "platform.h",
     ],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -255,7 +290,7 @@
         ":platform",
         ":randen_hwaes",
         ":randen_slow",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -267,6 +302,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":platform",
+        "//absl/base:config",
+        "//absl/base:core_headers",
     ],
 )
 
@@ -286,6 +323,7 @@
     deps = [
         ":platform",
         ":randen_hwaes_impl",
+        "//absl/base:config",
     ],
 )
 
@@ -305,7 +343,11 @@
     # anyway and thus there wouldn't be any gain from using it as a module.
     features = ["-header_modules"],
     linkopts = ABSL_DEFAULT_LINKOPTS,
-    deps = [":platform"],
+    deps = [
+        ":platform",
+        "//absl/base:config",
+        "//absl/base:core_headers",
+    ],
 )
 
 cc_binary(
@@ -334,8 +376,9 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        "//absl/base",
+        "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/strings:str_format",
         "//absl/types:span",
@@ -367,16 +410,17 @@
 )
 
 cc_test(
-    name = "distribution_impl_test",
+    name = "generate_real_test",
     size = "small",
-    srcs = ["distribution_impl_test.cc"],
+    srcs = [
+        "generate_real_test.cc",
+    ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":distribution_impl",
+        ":generate_real",
         "//absl/base:bits",
         "//absl/flags:flag",
-        "//absl/numeric:int128",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -459,6 +503,29 @@
     ],
 )
 
+cc_library(
+    name = "mocking_bit_gen_base",
+    hdrs = ["mocking_bit_gen_base.h"],
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/random",
+        "//absl/strings",
+    ],
+)
+
+cc_library(
+    name = "mock_overload_set",
+    testonly = 1,
+    hdrs = ["mock_overload_set.h"],
+    visibility = [
+        "//absl/random:__pkg__",
+    ],
+    deps = [
+        "//absl/random:mocking_bit_gen",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
 cc_test(
     name = "nonsecure_base_test",
     size = "small",
@@ -531,7 +598,7 @@
     deps = [
         ":explicit_seed_seq",
         ":randen_engine",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
         "//absl/time",
         "@com_google_googletest//:gtest_main",
@@ -574,12 +641,26 @@
         ":platform",
         ":randen_hwaes",
         ":randen_hwaes_impl",  # build_cleaner: keep
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/strings:str_format",
         "@com_google_googletest//:gtest",
     ],
 )
 
+cc_test(
+    name = "wide_multiply_test",
+    size = "small",
+    srcs = ["wide_multiply_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":wide_multiply",
+        "//absl/base:bits",
+        "//absl/numeric:int128",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_library(
     name = "nanobenchmark",
     srcs = ["nanobenchmark.cc"],
@@ -588,7 +669,8 @@
     deps = [
         ":platform",
         ":randen_engine",
-        "//absl/base",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -598,11 +680,6 @@
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        ":distribution_impl",
-        ":fast_uniform_bits",
-        ":iostream_state_saver",
-        ":traits",
-        "//absl/base:core_headers",
         "//absl/meta:type_traits",
     ],
 )
@@ -620,7 +697,7 @@
     ],
     deps = [
         ":nanobenchmark",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
     ],
 )
@@ -641,7 +718,7 @@
         ":randen_hwaes",
         ":randen_hwaes_impl",
         ":randen_slow",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/strings",
     ],
 )
diff --git a/absl/random/internal/chi_square.cc b/absl/random/internal/chi_square.cc
index 45671a3..640d48c 100644
--- a/absl/random/internal/chi_square.cc
+++ b/absl/random/internal/chi_square.cc
@@ -19,7 +19,7 @@
 #include "absl/random/internal/distribution_test_util.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -228,5 +228,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/chi_square.h b/absl/random/internal/chi_square.h
index 002b4c9..07f4fbe 100644
--- a/absl/random/internal/chi_square.h
+++ b/absl/random/internal/chi_square.h
@@ -26,8 +26,10 @@
 
 #include <cassert>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 constexpr const char kChiSquared[] = "chi-squared";
@@ -81,7 +83,7 @@
 double ChiSquarePValue(double chi_square, int dof);
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_CHI_SQUARE_H_
diff --git a/absl/random/internal/distribution_caller.h b/absl/random/internal/distribution_caller.h
index dd06c98..02603cf 100644
--- a/absl/random/internal/distribution_caller.h
+++ b/absl/random/internal/distribution_caller.h
@@ -19,8 +19,10 @@
 
 #include <utility>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // DistributionCaller provides an opportunity to overload the general
@@ -52,7 +54,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_CALLER_H_
diff --git a/absl/random/internal/distribution_impl.h b/absl/random/internal/distribution_impl.h
deleted file mode 100644
index a8e5d61..0000000
--- a/absl/random/internal/distribution_impl.h
+++ /dev/null
@@ -1,262 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#ifndef ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
-#define ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
-
-// This file contains some implementation details which are used by one or more
-// of the absl random number distributions.
-
-#include <cfloat>
-#include <cstddef>
-#include <cstdint>
-#include <cstring>
-#include <limits>
-#include <type_traits>
-
-#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64)
-#include <intrin.h>  // NOLINT(build/include_order)
-#pragma intrinsic(_umul128)
-#define ABSL_INTERNAL_USE_UMUL128 1
-#endif
-
-#include "absl/base/config.h"
-#include "absl/base/internal/bits.h"
-#include "absl/numeric/int128.h"
-#include "absl/random/internal/fastmath.h"
-#include "absl/random/internal/traits.h"
-
-namespace absl {
-inline namespace lts_2019_08_08 {
-namespace random_internal {
-
-// Creates a double from `bits`, with the template fields controlling the
-// output.
-//
-// RandU64To is both more efficient and generates more unique values in the
-// result interval than known implementations of std::generate_canonical().
-//
-// The `Signed` parameter controls whether positive, negative, or both are
-// returned (thus affecting the output interval).
-//   When Signed == SignedValueT, range is U(-1, 1)
-//   When Signed == NegativeValueT, range is U(-1, 0)
-//   When Signed == PositiveValueT, range is U(0, 1)
-//
-// When the `IncludeZero` parameter is true, the function may return 0 for some
-// inputs, otherwise it never returns 0.
-//
-// The `ExponentBias` parameter determines the scale of the output range by
-// adjusting the exponent.
-//
-// When a value in U(0,1) is required, use:
-//   RandU64ToDouble<PositiveValueT, true, 0>();
-//
-// When a value in U(-1,1) is required, use:
-//   RandU64ToDouble<SignedValueT, false, 0>() => U(-1, 1)
-// This generates more distinct values than the mathematically equivalent
-// expression `U(0, 1) * 2.0 - 1.0`, and is preferable.
-//
-// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
-//   RandU64ToDouble<PositiveValueT, false, 1>();  => U(0, 2)
-//   RandU64ToDouble<PositiveValueT, false, -1>();  => U(0, 0.5)
-//
-
-// Tristate types controlling the output.
-struct PositiveValueT {};
-struct NegativeValueT {};
-struct SignedValueT {};
-
-// RandU64ToDouble is the double-result variant of RandU64To, described above.
-template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-inline double RandU64ToDouble(uint64_t bits) {
-  static_assert(std::is_same<Signed, PositiveValueT>::value ||
-                    std::is_same<Signed, NegativeValueT>::value ||
-                    std::is_same<Signed, SignedValueT>::value,
-                "");
-
-  // Maybe use the left-most bit for a sign bit.
-  uint64_t sign = std::is_same<Signed, NegativeValueT>::value
-                      ? 0x8000000000000000ull
-                      : 0;  // Sign bits.
-
-  if (std::is_same<Signed, SignedValueT>::value) {
-    sign = bits & 0x8000000000000000ull;
-    bits = bits & 0x7FFFFFFFFFFFFFFFull;
-  }
-  if (IncludeZero) {
-    if (bits == 0u) return 0;
-  }
-
-  // Number of leading zeros is mapped to the exponent: 2^-clz
-  int clz = base_internal::CountLeadingZeros64(bits);
-  // Shift number left to erase leading zeros.
-  bits <<= IncludeZero ? clz : (clz & 63);
-
-  // Shift number right to remove bits that overflow double mantissa.  The
-  // direction of the shift depends on `clz`.
-  bits >>= (64 - DBL_MANT_DIG);
-
-  // Compute IEEE 754 double exponent.
-  // In the Signed case, bits is a 63-bit number with a 0 msb.  Adjust the
-  // exponent to account for that.
-  const uint64_t exp =
-      (std::is_same<Signed, SignedValueT>::value ? 1023U : 1022U) +
-      static_cast<uint64_t>(ExponentBias - clz);
-  constexpr int kExp = DBL_MANT_DIG - 1;
-  // Construct IEEE 754 double from exponent and mantissa.
-  const uint64_t val = sign | (exp << kExp) | (bits & ((1ULL << kExp) - 1U));
-
-  double res;
-  static_assert(sizeof(res) == sizeof(val), "double is not 64 bit");
-  // Memcpy value from "val" to "res" to avoid aliasing problems.  Assumes that
-  // endian-ness is same for double and uint64_t.
-  std::memcpy(&res, &val, sizeof(res));
-
-  return res;
-}
-
-// RandU64ToFloat is the float-result variant of RandU64To, described above.
-template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-inline float RandU64ToFloat(uint64_t bits) {
-  static_assert(std::is_same<Signed, PositiveValueT>::value ||
-                    std::is_same<Signed, NegativeValueT>::value ||
-                    std::is_same<Signed, SignedValueT>::value,
-                "");
-
-  // Maybe use the left-most bit for a sign bit.
-  uint64_t sign = std::is_same<Signed, NegativeValueT>::value
-                      ? 0x80000000ul
-                      : 0;  // Sign bits.
-
-  if (std::is_same<Signed, SignedValueT>::value) {
-    uint64_t a = bits & 0x8000000000000000ull;
-    sign = static_cast<uint32_t>(a >> 32);
-    bits = bits & 0x7FFFFFFFFFFFFFFFull;
-  }
-  if (IncludeZero) {
-    if (bits == 0u) return 0;
-  }
-
-  // Number of leading zeros is mapped to the exponent: 2^-clz
-  int clz = base_internal::CountLeadingZeros64(bits);
-  // Shift number left to erase leading zeros.
-  bits <<= IncludeZero ? clz : (clz & 63);
-  // Shift number right to remove bits that overflow double mantissa.  The
-  // direction of the shift depends on `clz`.
-  bits >>= (64 - FLT_MANT_DIG);
-
-  // Construct IEEE 754 float exponent.
-  // In the Signed case, bits is a 63-bit number with a 0 msb.  Adjust the
-  // exponent to account for that.
-  const uint32_t exp =
-      (std::is_same<Signed, SignedValueT>::value ? 127U : 126U) +
-      static_cast<uint32_t>(ExponentBias - clz);
-  constexpr int kExp = FLT_MANT_DIG - 1;
-  const uint32_t val = sign | (exp << kExp) | (bits & ((1U << kExp) - 1U));
-
-  float res;
-  static_assert(sizeof(res) == sizeof(val), "float is not 32 bit");
-  // Assumes that endian-ness is same for float and uint32_t.
-  std::memcpy(&res, &val, sizeof(res));
-
-  return res;
-}
-
-template <typename Result>
-struct RandU64ToReal {
-  template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-  static inline Result Value(uint64_t bits) {
-    return RandU64ToDouble<Signed, IncludeZero, ExponentBias>(bits);
-  }
-};
-
-template <>
-struct RandU64ToReal<float> {
-  template <typename Signed, bool IncludeZero, int ExponentBias = 0>
-  static inline float Value(uint64_t bits) {
-    return RandU64ToFloat<Signed, IncludeZero, ExponentBias>(bits);
-  }
-};
-
-inline uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
-#if defined(ABSL_HAVE_INTRINSIC_INT128)
-  return uint128(static_cast<__uint128_t>(a) * b);
-#elif defined(ABSL_INTERNAL_USE_UMUL128)
-  // uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC.
-  uint64_t high = 0;
-  const uint64_t low = _umul128(a, b, &high);
-  return absl::MakeUint128(high, low);
-#else
-  // uint128(a) * uint128(b) in emulated mode computes a full 128-bit x 128-bit
-  // multiply.  However there are many cases where that is not necessary, and it
-  // is only necessary to support a 64-bit x 64-bit = 128-bit multiply.  This is
-  // for those cases.
-  const uint64_t a00 = static_cast<uint32_t>(a);
-  const uint64_t a32 = a >> 32;
-  const uint64_t b00 = static_cast<uint32_t>(b);
-  const uint64_t b32 = b >> 32;
-
-  const uint64_t c00 = a00 * b00;
-  const uint64_t c32a = a00 * b32;
-  const uint64_t c32b = a32 * b00;
-  const uint64_t c64 = a32 * b32;
-
-  const uint32_t carry =
-      static_cast<uint32_t>(((c00 >> 32) + static_cast<uint32_t>(c32a) +
-                             static_cast<uint32_t>(c32b)) >>
-                            32);
-
-  return absl::MakeUint128(c64 + (c32a >> 32) + (c32b >> 32) + carry,
-                           c00 + (c32a << 32) + (c32b << 32));
-#endif
-}
-
-// wide_multiply<T> multiplies two N-bit values to a 2N-bit result.
-template <typename UIntType>
-struct wide_multiply {
-  static constexpr size_t kN = std::numeric_limits<UIntType>::digits;
-  using input_type = UIntType;
-  using result_type = typename random_internal::unsigned_bits<kN * 2>::type;
-
-  static result_type multiply(input_type a, input_type b) {
-    return static_cast<result_type>(a) * b;
-  }
-
-  static input_type hi(result_type r) { return r >> kN; }
-  static input_type lo(result_type r) { return r; }
-
-  static_assert(std::is_unsigned<UIntType>::value,
-                "Class-template wide_multiply<> argument must be unsigned.");
-};
-
-#ifndef ABSL_HAVE_INTRINSIC_INT128
-template <>
-struct wide_multiply<uint64_t> {
-  using input_type = uint64_t;
-  using result_type = uint128;
-
-  static result_type multiply(uint64_t a, uint64_t b) {
-    return MultiplyU64ToU128(a, b);
-  }
-
-  static uint64_t hi(result_type r) { return Uint128High64(r); }
-  static uint64_t lo(result_type r) { return Uint128Low64(r); }
-};
-#endif
-
-}  // namespace random_internal
-}  // inline namespace lts_2019_08_08
-}  // namespace absl
-
-#endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_IMPL_H_
diff --git a/absl/random/internal/distribution_test_util.cc b/absl/random/internal/distribution_test_util.cc
index 4fb4149..e900565 100644
--- a/absl/random/internal/distribution_test_util.cc
+++ b/absl/random/internal/distribution_test_util.cc
@@ -25,7 +25,7 @@
 #include "absl/strings/str_format.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -414,5 +414,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/distribution_test_util.h b/absl/random/internal/distribution_test_util.h
index 56dc86a..6d94cf6 100644
--- a/absl/random/internal/distribution_test_util.h
+++ b/absl/random/internal/distribution_test_util.h
@@ -26,7 +26,7 @@
 // non-test code.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // http://webspace.ship.edu/pgmarr/Geo441/Lectures/Lec%205%20-%20Normality%20Testing.pdf
@@ -107,7 +107,7 @@
 double BetaIncompleteInv(double p, double q, double alpha);
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTION_TEST_UTIL_H_
diff --git a/absl/random/internal/distributions.h b/absl/random/internal/distributions.h
index 98d4f31..d7e3c01 100644
--- a/absl/random/internal/distributions.h
+++ b/absl/random/internal/distributions.h
@@ -23,40 +23,8 @@
 #include "absl/random/internal/uniform_helper.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
-template <typename D>
-struct DistributionFormatTraits;
-
-// UniformImpl implements the core logic of the Uniform<T> call, which is to
-// select the correct distribution type, compute the bounds based on the
-// interval tag, and then generate a value.
-template <typename NumType, typename TagType, typename URBG>
-NumType UniformImpl(TagType tag,
-                    URBG& urbg,  // NOLINT(runtime/references)
-                    NumType lo, NumType hi) {
-  static_assert(
-      std::is_arithmetic<NumType>::value,
-      "absl::Uniform<T>() must use an integer or real parameter type.");
-
-  using distribution_t =
-      typename std::conditional<std::is_integral<NumType>::value,
-                                absl::uniform_int_distribution<NumType>,
-                                absl::uniform_real_distribution<NumType>>::type;
-  using format_t = random_internal::DistributionFormatTraits<distribution_t>;
-
-  auto a = random_internal::uniform_lower_bound<NumType>(tag, lo, hi);
-  auto b = random_internal::uniform_upper_bound<NumType>(tag, lo, hi);
-  // TODO(lar): it doesn't make a lot of sense to ask for a random number in an
-  // empty range.  Right now we just return a boundary--even though that
-  // boundary is not an acceptable value!  Is there something better we can do
-  // here?
-
-  using gen_t = absl::decay_t<URBG>;
-  if (a > b) return a;
-  return DistributionCaller<gen_t>::template Call<distribution_t, format_t>(
-      &urbg, a, b);
-}
 
 // In the absence of an explicitly provided return-type, the template
 // "uniform_inferred_return_t<A, B>" is used to derive a suitable type, based on
@@ -78,7 +46,7 @@
                           is_widening_convertible<A, B>::value, B, A>::type>;
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_DISTRIBUTIONS_H_
diff --git a/absl/random/internal/explicit_seed_seq.h b/absl/random/internal/explicit_seed_seq.h
index 66c35f2..6a743ea 100644
--- a/absl/random/internal/explicit_seed_seq.h
+++ b/absl/random/internal/explicit_seed_seq.h
@@ -22,8 +22,10 @@
 #include <iterator>
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // This class conforms to the C++ Standard "Seed Sequence" concept
@@ -83,7 +85,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_EXPLICIT_SEED_SEQ_H_
diff --git a/absl/random/internal/fast_uniform_bits.h b/absl/random/internal/fast_uniform_bits.h
index 5aa9de0..f13c872 100644
--- a/absl/random/internal/fast_uniform_bits.h
+++ b/absl/random/internal/fast_uniform_bits.h
@@ -20,8 +20,10 @@
 #include <limits>
 #include <type_traits>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 // Returns true if the input value is zero or a power of two. Useful for
 // determining if the range of output values in a URBG
@@ -256,7 +258,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_FAST_UNIFORM_BITS_H_
diff --git a/absl/random/internal/fast_uniform_bits_test.cc b/absl/random/internal/fast_uniform_bits_test.cc
index 35c9622..f5b837e 100644
--- a/absl/random/internal/fast_uniform_bits_test.cc
+++ b/absl/random/internal/fast_uniform_bits_test.cc
@@ -19,7 +19,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -270,5 +270,5 @@
 
 }  // namespace
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/fastmath.h b/absl/random/internal/fastmath.h
index 1dc2cd7..6baeb5a 100644
--- a/absl/random/internal/fastmath.h
+++ b/absl/random/internal/fastmath.h
@@ -25,7 +25,7 @@
 #include "absl/base/internal/bits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Returns the position of the first bit set.
@@ -68,7 +68,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_FASTMATH_H_
diff --git a/absl/random/internal/gaussian_distribution_gentables.cc b/absl/random/internal/gaussian_distribution_gentables.cc
index 6f9a28b..a2bf039 100644
--- a/absl/random/internal/gaussian_distribution_gentables.cc
+++ b/absl/random/internal/gaussian_distribution_gentables.cc
@@ -27,7 +27,7 @@
 #include "absl/base/macros.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -136,7 +136,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int, char**) {
diff --git a/absl/random/internal/generate_real.h b/absl/random/internal/generate_real.h
new file mode 100644
index 0000000..20f6d20
--- /dev/null
+++ b/absl/random/internal/generate_real.h
@@ -0,0 +1,146 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+#define ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+
+// This file contains some implementation details which are used by one or more
+// of the absl random number distributions.
+
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/internal/bits.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// Tristate tag types controlling the output of GenerateRealFromBits.
+struct GeneratePositiveTag {};
+struct GenerateNegativeTag {};
+struct GenerateSignedTag {};
+
+// GenerateRealFromBits generates a single real value from a single 64-bit
+// `bits` with template fields controlling the output.
+//
+// The `SignedTag` parameter controls whether positive, negative,
+// or either signed/unsigned may be returned.
+//   When SignedTag == GeneratePositiveTag, range is U(0, 1)
+//   When SignedTag == GenerateNegativeTag, range is U(-1, 0)
+//   When SignedTag == GenerateSignedTag, range is U(-1, 1)
+//
+// When the `IncludeZero` parameter is true, the function may return 0 for some
+// inputs, otherwise it never returns 0.
+//
+// When a value in U(0,1) is required, use:
+//   Uniform64ToReal<double, PositiveValueT, true>;
+//
+// When a value in U(-1,1) is required, use:
+//   Uniform64ToReal<double, SignedValueT, false>;
+//
+//   This generates more distinct values than the mathematical equivalent
+//   `U(0, 1) * 2.0 - 1.0`.
+//
+// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
+//   GenerateRealFromBits<double>(..., -1);  => U(0, 0.5)
+//   GenerateRealFromBits<double>(..., 1);   => U(0, 2)
+//
+template <typename RealType,  // Real type, either float or double.
+          typename SignedTag = GeneratePositiveTag,  // Whether a positive,
+                                                     // negative, or signed
+                                                     // value is generated.
+          bool IncludeZero = true>
+inline RealType GenerateRealFromBits(uint64_t bits, int exp_bias = 0) {
+  using real_type = RealType;
+  using uint_type = absl::conditional_t<std::is_same<real_type, float>::value,
+                                        uint32_t, uint64_t>;
+
+  static_assert(
+      (std::is_same<double, real_type>::value ||
+       std::is_same<float, real_type>::value),
+      "GenerateRealFromBits must be parameterized by either float or double.");
+
+  static_assert(sizeof(uint_type) == sizeof(real_type),
+                "Mismatched unsinged and real types.");
+
+  static_assert((std::numeric_limits<real_type>::is_iec559 &&
+                 std::numeric_limits<real_type>::radix == 2),
+                "RealType representation is not IEEE 754 binary.");
+
+  static_assert((std::is_same<SignedTag, GeneratePositiveTag>::value ||
+                 std::is_same<SignedTag, GenerateNegativeTag>::value ||
+                 std::is_same<SignedTag, GenerateSignedTag>::value),
+                "");
+
+  static constexpr int kExp = std::numeric_limits<real_type>::digits - 1;
+  static constexpr uint_type kMask = (static_cast<uint_type>(1) << kExp) - 1u;
+  static constexpr int kUintBits = sizeof(uint_type) * 8;
+
+  int exp = exp_bias + int{std::numeric_limits<real_type>::max_exponent - 2};
+
+  // Determine the sign bit.
+  // Depending on the SignedTag, this may use the left-most bit
+  // or it may be a constant value.
+  uint_type sign = std::is_same<SignedTag, GenerateNegativeTag>::value
+                       ? (static_cast<uint_type>(1) << (kUintBits - 1))
+                       : 0;
+  if (std::is_same<SignedTag, GenerateSignedTag>::value) {
+    if (std::is_same<uint_type, uint64_t>::value) {
+      sign = bits & uint64_t{0x8000000000000000};
+    }
+    if (std::is_same<uint_type, uint32_t>::value) {
+      const uint64_t tmp = bits & uint64_t{0x8000000000000000};
+      sign = static_cast<uint32_t>(tmp >> 32);
+    }
+    // adjust the bits and the exponent to account for removing
+    // the leading bit.
+    bits = bits & uint64_t{0x7FFFFFFFFFFFFFFF};
+    exp++;
+  }
+  if (IncludeZero) {
+    if (bits == 0u) return 0;
+  }
+
+  // Number of leading zeros is mapped to the exponent: 2^-clz
+  // bits is 0..01xxxxxx. After shifting, we're left with 1xxx...0..0
+  int clz = base_internal::CountLeadingZeros64(bits);
+  bits <<= (IncludeZero ? clz : (clz & 63));  // remove 0-bits.
+  exp -= clz;                                 // set the exponent.
+  bits >>= (63 - kExp);
+
+  // Construct the 32-bit or 64-bit IEEE 754 floating-point value from
+  // the individual fields: sign, exp, mantissa(bits).
+  uint_type val =
+      (std::is_same<SignedTag, GeneratePositiveTag>::value ? 0u : sign) |
+      (static_cast<uint_type>(exp) << kExp) |
+      (static_cast<uint_type>(bits) & kMask);
+
+  // bit_cast to the output-type
+  real_type result;
+  memcpy(static_cast<void*>(&result), static_cast<const void*>(&val),
+         sizeof(result));
+  return result;
+}
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
diff --git a/absl/random/internal/distribution_impl_test.cc b/absl/random/internal/generate_real_test.cc
similarity index 80%
rename from absl/random/internal/distribution_impl_test.cc
rename to absl/random/internal/generate_real_test.cc
index 09e7a31..aa02f0c 100644
--- a/absl/random/internal/distribution_impl_test.cc
+++ b/absl/random/internal/generate_real_test.cc
@@ -12,57 +12,74 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/random/internal/generate_real.h"
+
+#include <cfloat>
+#include <cstddef>
+#include <cstdint>
+#include <string>
 
 #include "gtest/gtest.h"
 #include "absl/base/internal/bits.h"
 #include "absl/flags/flag.h"
-#include "absl/numeric/int128.h"
 
 ABSL_FLAG(int64_t, absl_random_test_trials, 50000,
           "Number of trials for the probability tests.");
 
-using absl::random_internal::NegativeValueT;
-using absl::random_internal::PositiveValueT;
-using absl::random_internal::RandU64ToDouble;
-using absl::random_internal::RandU64ToFloat;
-using absl::random_internal::SignedValueT;
+using absl::random_internal::GenerateNegativeTag;
+using absl::random_internal::GeneratePositiveTag;
+using absl::random_internal::GenerateRealFromBits;
+using absl::random_internal::GenerateSignedTag;
 
 namespace {
 
-TEST(DistributionImplTest, U64ToFloat_Positive_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Positive_NoZero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, false>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 2.710505431e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), 0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Positive_Zero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Positive_Zero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0.0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), 0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Negative_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Negative_NoZero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<NegativeValueT, false>(a);
+    return GenerateRealFromBits<float, GenerateNegativeTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), -2.710505431e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), -5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), -0.5);
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_NoZero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Negative_Zero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, false>(a);
+    return GenerateRealFromBits<float, GenerateNegativeTag, true>(a);
+  };
+  EXPECT_EQ(ToFloat(0x0000000000000000), 0.0);
+  EXPECT_EQ(ToFloat(0x0000000000000001), -5.421010862e-20f);
+  EXPECT_EQ(ToFloat(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToFloat(0x8000000000000001), -0.5);
+  EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
+}
+
+TEST(GenerateRealTest, U64ToFloat_Signed_NoZero_Test) {
+  auto ToFloat = [](uint64_t a) {
+    return GenerateRealFromBits<float, GenerateSignedTag, false>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 5.421010862e-20f);
   EXPECT_EQ(ToFloat(0x0000000000000001), 1.084202172e-19f);
@@ -72,9 +89,9 @@
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_Zero_Test) {
+TEST(GenerateRealTest, U64ToFloat_Signed_Zero_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, true>(a);
+    return GenerateRealFromBits<float, GenerateSignedTag, true>(a);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 1.084202172e-19f);
@@ -84,9 +101,9 @@
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloat_Signed_Bias_Test) {
+TEST(GenerateRealTest, U64ToFloat_Signed_Bias_Test) {
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<SignedValueT, true, 1>(a);
+    return GenerateRealFromBits<float, GenerateSignedTag, true>(a, 1);
   };
   EXPECT_EQ(ToFloat(0x0000000000000000), 0);
   EXPECT_EQ(ToFloat(0x0000000000000001), 2 * 1.084202172e-19f);
@@ -96,9 +113,9 @@
   EXPECT_EQ(ToFloat(0xFFFFFFFFFFFFFFFF), 2 * -0.9999999404f);
 }
 
-TEST(DistributionImplTest, U64ToFloatTest) {
+TEST(GenerateRealTest, U64ToFloatTest) {
   auto ToFloat = [](uint64_t a) -> float {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToFloat(0x0000000000000000), 0.0f);
@@ -150,44 +167,60 @@
   }
 }
 
-TEST(DistributionImplTest, U64ToDouble_Positive_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Positive_NoZero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, false>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 2.710505431213761085e-20);
   EXPECT_EQ(ToDouble(0x0000000000000001), 5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x0000000000000002), 1.084202172485504434e-19);
   EXPECT_EQ(ToDouble(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), 0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), 0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Positive_Zero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Positive_Zero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, true>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x8000000000000000), 0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), 0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), 0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Negative_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Negative_NoZero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<NegativeValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateNegativeTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), -2.710505431213761085e-20);
   EXPECT_EQ(ToDouble(0x0000000000000001), -5.42101086242752217004e-20);
   EXPECT_EQ(ToDouble(0x0000000000000002), -1.084202172485504434e-19);
   EXPECT_EQ(ToDouble(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), -0.5);
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_NoZero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Negative_Zero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateNegativeTag, true>(a);
+  };
+
+  EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
+  EXPECT_EQ(ToDouble(0x0000000000000001), -5.42101086242752217004e-20);
+  EXPECT_EQ(ToDouble(0x0000000000000002), -1.084202172485504434e-19);
+  EXPECT_EQ(ToDouble(0x8000000000000000), -0.5);
+  EXPECT_EQ(ToDouble(0x8000000000000001), -0.5);
+  EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
+}
+
+TEST(GenerateRealTest, U64ToDouble_Signed_NoZero_Test) {
+  auto ToDouble = [](uint64_t a) {
+    return GenerateRealFromBits<double, GenerateSignedTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 5.42101086242752217004e-20);
@@ -198,9 +231,9 @@
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_Zero_Test) {
+TEST(GenerateRealTest, U64ToDouble_Signed_Zero_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, true>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, true>(a);
   };
   EXPECT_EQ(ToDouble(0x0000000000000000), 0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 1.084202172485504434e-19);
@@ -210,9 +243,9 @@
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978);
 }
 
-TEST(DistributionImplTest, U64ToDouble_Signed_Bias_Test) {
+TEST(GenerateRealTest, U64ToDouble_GenerateSignedTag_Bias_Test) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, true, -1>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, true>(a, -1);
   };
   EXPECT_EQ(ToDouble(0x0000000000000000), 0);
   EXPECT_EQ(ToDouble(0x0000000000000001), 1.084202172485504434e-19 / 2);
@@ -222,9 +255,9 @@
   EXPECT_EQ(ToDouble(0xFFFFFFFFFFFFFFFF), -0.999999999999999888978 / 2);
 }
 
-TEST(DistributionImplTest, U64ToDoubleTest) {
+TEST(GenerateRealTest, U64ToDoubleTest) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<PositiveValueT, true>(a);
+    return GenerateRealFromBits<double, GeneratePositiveTag, true>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 0.0);
@@ -296,9 +329,9 @@
   }
 }
 
-TEST(DistributionImplTest, U64ToDoubleSignedTest) {
+TEST(GenerateRealTest, U64ToDoubleSignedTest) {
   auto ToDouble = [](uint64_t a) {
-    return RandU64ToDouble<SignedValueT, false>(a);
+    return GenerateRealFromBits<double, GenerateSignedTag, false>(a);
   };
 
   EXPECT_EQ(ToDouble(0x0000000000000000), 5.42101086242752217004e-20);
@@ -379,10 +412,10 @@
   }
 }
 
-TEST(DistributionImplTest, ExhaustiveFloat) {
+TEST(GenerateRealTest, ExhaustiveFloat) {
   using absl::base_internal::CountLeadingZeros64;
   auto ToFloat = [](uint64_t a) {
-    return RandU64ToFloat<PositiveValueT, true>(a);
+    return GenerateRealFromBits<float, GeneratePositiveTag, true>(a);
   };
 
   // Rely on RandU64ToFloat generating values from greatest to least when
@@ -461,46 +494,4 @@
   }
 }
 
-TEST(DistributionImplTest, MultiplyU64ToU128Test) {
-  using absl::random_internal::MultiplyU64ToU128;
-  constexpr uint64_t k1 = 1;
-  constexpr uint64_t kMax = ~static_cast<uint64_t>(0);
-
-  EXPECT_EQ(absl::uint128(0), MultiplyU64ToU128(0, 0));
-
-  // Max uint64
-  EXPECT_EQ(MultiplyU64ToU128(kMax, kMax),
-            absl::MakeUint128(0xfffffffffffffffe, 0x0000000000000001));
-  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(kMax, 1));
-  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(1, kMax));
-  for (int i = 0; i < 64; ++i) {
-    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
-              MultiplyU64ToU128(kMax, k1 << i));
-    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
-              MultiplyU64ToU128(k1 << i, kMax));
-  }
-
-  // 1-bit x 1-bit.
-  for (int i = 0; i < 64; ++i) {
-    for (int j = 0; j < 64; ++j) {
-      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
-                MultiplyU64ToU128(k1 << i, k1 << j));
-      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
-                MultiplyU64ToU128(k1 << i, k1 << j));
-    }
-  }
-
-  // Verified multiplies
-  EXPECT_EQ(MultiplyU64ToU128(0xffffeeeeddddcccc, 0xbbbbaaaa99998888),
-            absl::MakeUint128(0xbbbb9e2692c5dddc, 0xc28f7531048d2c60));
-  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfedcba9876543210),
-            absl::MakeUint128(0x0121fa00ad77d742, 0x2236d88fe5618cf0));
-  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfdb97531eca86420),
-            absl::MakeUint128(0x0120ae99d26725fc, 0xce197f0ecac319e0));
-  EXPECT_EQ(MultiplyU64ToU128(0x97a87f4f261ba3f2, 0xfedcba9876543210),
-            absl::MakeUint128(0x96fbf1a8ae78d0ba, 0x5a6dd4b71f278320));
-  EXPECT_EQ(MultiplyU64ToU128(0xfedcba9876543210, 0xfdb97531eca86420),
-            absl::MakeUint128(0xfc98c6981a413e22, 0x342d0bbf48948200));
-}
-
 }  // namespace
diff --git a/absl/random/internal/iostream_state_saver.h b/absl/random/internal/iostream_state_saver.h
index b9adca8..7378829 100644
--- a/absl/random/internal/iostream_state_saver.h
+++ b/absl/random/internal/iostream_state_saver.h
@@ -24,7 +24,7 @@
 #include "absl/numeric/int128.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // The null_state_saver does nothing.
@@ -239,7 +239,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_IOSTREAM_STATE_SAVER_H_
diff --git a/absl/random/internal/iostream_state_saver_test.cc b/absl/random/internal/iostream_state_saver_test.cc
index 2ecbaac..7bb8ad9 100644
--- a/absl/random/internal/iostream_state_saver_test.cc
+++ b/absl/random/internal/iostream_state_saver_test.cc
@@ -196,8 +196,8 @@
     EXPECT_EQ(-d, StreamRoundTrip<double>(-d));
 
     // Avoid undefined behavior (overflow/underflow).
-    if (d <= std::numeric_limits<int64_t>::max() &&
-        d >= std::numeric_limits<int64_t>::lowest()) {
+    if (f <= static_cast<float>(std::numeric_limits<int64_t>::max()) &&
+        f >= static_cast<float>(std::numeric_limits<int64_t>::lowest())) {
       int64_t x = static_cast<int64_t>(f);
       EXPECT_EQ(x, StreamRoundTrip<int64_t>(x));
     }
@@ -264,14 +264,15 @@
     }
 
     // Avoid undefined behavior (overflow/underflow).
-    if (d <= std::numeric_limits<int64_t>::max() &&
-        d >= std::numeric_limits<int64_t>::lowest()) {
+    if (d <= static_cast<double>(std::numeric_limits<int64_t>::max()) &&
+        d >= static_cast<double>(std::numeric_limits<int64_t>::lowest())) {
       int64_t x = static_cast<int64_t>(d);
       EXPECT_EQ(x, StreamRoundTrip<int64_t>(x));
     }
   }
 }
 
+#if !defined(__EMSCRIPTEN__)
 TEST(IOStreamStateSaver, RoundTripLongDoubles) {
   // Technically, C++ only guarantees that long double is at least as large as a
   // double.  Practically it varies from 64-bits to 128-bits.
@@ -349,6 +350,7 @@
     }
   }
 }
+#endif  // !defined(__EMSCRIPTEN__)
 
 TEST(StrToDTest, DoubleMin) {
   const char kV[] = "2.22507385850720138e-308";
diff --git a/absl/random/internal/mock_overload_set.h b/absl/random/internal/mock_overload_set.h
new file mode 100644
index 0000000..c2a30d8
--- /dev/null
+++ b/absl/random/internal/mock_overload_set.h
@@ -0,0 +1,91 @@
+//
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
+#define ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
+
+#include <type_traits>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/random/mocking_bit_gen.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+template <typename DistrT, typename Fn>
+struct MockSingleOverload;
+
+// MockSingleOverload
+//
+// MockSingleOverload hooks in to gMock's `ON_CALL` and `EXPECT_CALL` macros.
+// EXPECT_CALL(mock_single_overload, Call(...))` will expand to a call to
+// `mock_single_overload.gmock_Call(...)`. Because expectations are stored on
+// the MockingBitGen (an argument passed inside `Call(...)`), this forwards to
+// arguments to Mocking::Register.
+template <typename DistrT, typename Ret, typename... Args>
+struct MockSingleOverload<DistrT, Ret(MockingBitGen&, Args...)> {
+  static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
+                "Overload signature must have return type matching the "
+                "distributions result type.");
+  auto gmock_Call(
+      absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
+      const ::testing::Matcher<Args>&... args)
+      -> decltype(gen.Register<DistrT, Args...>(args...)) {
+    return gen.Register<DistrT, Args...>(args...);
+  }
+};
+
+template <typename DistrT, typename Ret, typename Arg, typename... Args>
+struct MockSingleOverload<DistrT, Ret(Arg, MockingBitGen&, Args...)> {
+  static_assert(std::is_same<typename DistrT::result_type, Ret>::value,
+                "Overload signature must have return type matching the "
+                "distributions result type.");
+  auto gmock_Call(
+      const ::testing::Matcher<Arg>& arg,
+      absl::MockingBitGen& gen,  // NOLINT(google-runtime-references)
+      const ::testing::Matcher<Args>&... args)
+      -> decltype(gen.Register<DistrT, Arg, Args...>(arg, args...)) {
+    return gen.Register<DistrT, Arg, Args...>(arg, args...);
+  }
+};
+
+// MockOverloadSet
+//
+// MockOverloadSet takes a distribution and a collection of signatures and
+// performs overload resolution amongst all the overloads. This makes
+// `EXPECT_CALL(mock_overload_set, Call(...))` expand and do overload resolution
+// correctly.
+template <typename DistrT, typename... Signatures>
+struct MockOverloadSet;
+
+template <typename DistrT, typename Sig>
+struct MockOverloadSet<DistrT, Sig> : public MockSingleOverload<DistrT, Sig> {
+  using MockSingleOverload<DistrT, Sig>::gmock_Call;
+};
+
+template <typename DistrT, typename FirstSig, typename... Rest>
+struct MockOverloadSet<DistrT, FirstSig, Rest...>
+    : public MockSingleOverload<DistrT, FirstSig>,
+      public MockOverloadSet<DistrT, Rest...> {
+  using MockSingleOverload<DistrT, FirstSig>::gmock_Call;
+  using MockOverloadSet<DistrT, Rest...>::gmock_Call;
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // ABSL_RANDOM_INTERNAL_MOCK_OVERLOAD_SET_H_
diff --git a/absl/random/internal/mocking_bit_gen_base.h b/absl/random/internal/mocking_bit_gen_base.h
new file mode 100644
index 0000000..eeeae9d
--- /dev/null
+++ b/absl/random/internal/mocking_bit_gen_base.h
@@ -0,0 +1,120 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#ifndef ABSL_RANDOM_INTERNAL_MOCKING_BIT_GEN_BASE_H_
+#define ABSL_RANDOM_INTERNAL_MOCKING_BIT_GEN_BASE_H_
+
+#include <atomic>
+#include <deque>
+#include <string>
+#include <typeinfo>
+
+#include "absl/random/random.h"
+#include "absl/strings/str_cat.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// MockingBitGenExpectationFormatter is invoked to format unsatisfied mocks
+// and remaining results into a description string.
+template <typename DistrT, typename FormatT>
+struct MockingBitGenExpectationFormatter {
+  std::string operator()(absl::string_view args) {
+    return absl::StrCat(FormatT::FunctionName(), "(", args, ")");
+  }
+};
+
+// MockingBitGenCallFormatter is invoked to format each distribution call
+// into a description string for the mock log.
+template <typename DistrT, typename FormatT>
+struct MockingBitGenCallFormatter {
+  std::string operator()(const DistrT& dist,
+                         const typename DistrT::result_type& result) {
+    return absl::StrCat(
+        FormatT::FunctionName(), "(", FormatT::FormatArgs(dist), ") => {",
+        FormatT::FormatResults(absl::MakeSpan(&result, 1)), "}");
+  }
+};
+
+class MockingBitGenBase {
+  template <typename>
+  friend struct DistributionCaller;
+  using generator_type = absl::BitGen;
+
+ public:
+  // URBG interface
+  using result_type = generator_type::result_type;
+  static constexpr result_type(min)() { return (generator_type::min)(); }
+  static constexpr result_type(max)() { return (generator_type::max)(); }
+  result_type operator()() { return gen_(); }
+
+  MockingBitGenBase() : gen_(), observed_call_log_() {}
+  virtual ~MockingBitGenBase() = default;
+
+ protected:
+  const std::deque<std::string>& observed_call_log() {
+    return observed_call_log_;
+  }
+
+  // CallImpl is the type-erased virtual dispatch.
+  // The type of dist is always distribution<T>,
+  // The type of result is always distribution<T>::result_type.
+  virtual bool CallImpl(const std::type_info& distr_type, void* dist_args,
+                        void* result) = 0;
+
+  template <typename DistrT, typename ArgTupleT>
+  static const std::type_info& GetTypeId() {
+    return typeid(std::pair<absl::decay_t<DistrT>, absl::decay_t<ArgTupleT>>);
+  }
+
+  // Call the generating distribution function.
+  // Invoked by DistributionCaller<>::Call<DistT, FormatT>.
+  // DistT is the distribution type.
+  // FormatT is the distribution formatter traits type.
+  template <typename DistrT, typename FormatT, typename... Args>
+  typename DistrT::result_type Call(Args&&... args) {
+    using distr_result_type = typename DistrT::result_type;
+    using ArgTupleT = std::tuple<absl::decay_t<Args>...>;
+
+    ArgTupleT arg_tuple(std::forward<Args>(args)...);
+    auto dist = absl::make_from_tuple<DistrT>(arg_tuple);
+
+    distr_result_type result{};
+    bool found_match =
+        CallImpl(GetTypeId<DistrT, ArgTupleT>(), &arg_tuple, &result);
+
+    if (!found_match) {
+      result = dist(gen_);
+    }
+
+    // TODO(asoffer): Forwarding the args through means we no longer need to
+    // extract them from the from the distribution in formatter traits. We can
+    // just StrJoin them.
+    observed_call_log_.push_back(
+        MockingBitGenCallFormatter<DistrT, FormatT>{}(dist, result));
+    return result;
+  }
+
+ private:
+  generator_type gen_;
+  std::deque<std::string> observed_call_log_;
+};  // namespace random_internal
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_MOCKING_BIT_GEN_BASE_H_
diff --git a/absl/random/internal/nanobenchmark.cc b/absl/random/internal/nanobenchmark.cc
index 4d26469..8fee77f 100644
--- a/absl/random/internal/nanobenchmark.cc
+++ b/absl/random/internal/nanobenchmark.cc
@@ -27,6 +27,7 @@
 #include <utility>
 #include <vector>
 
+#include "absl/base/attributes.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/random/internal/platform.h"
 #include "absl/random/internal/randen_engine.h"
@@ -59,15 +60,6 @@
 #include <time.h>  // NOLINT
 #endif
 
-// ABSL_HAVE_ATTRIBUTE
-#if !defined(ABSL_HAVE_ATTRIBUTE)
-#ifdef __has_attribute
-#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
-#else
-#define ABSL_HAVE_ATTRIBUTE(x) 0
-#endif
-#endif
-
 // ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE prevents inlining of the method.
 #if ABSL_HAVE_ATTRIBUTE(noinline) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_RANDOM_INTERNAL_ATTRIBUTE_NEVER_INLINE __attribute__((noinline))
@@ -78,7 +70,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 namespace {
 
@@ -808,5 +800,5 @@
 }
 
 }  // namespace random_internal_nanobenchmark
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/nanobenchmark.h b/absl/random/internal/nanobenchmark.h
index 424d2ba..a5097ba 100644
--- a/absl/random/internal/nanobenchmark.h
+++ b/absl/random/internal/nanobenchmark.h
@@ -50,8 +50,10 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 
 // Input influencing the function being measured (e.g. number of bytes to copy).
@@ -164,7 +166,7 @@
 }
 
 }  // namespace random_internal_nanobenchmark
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_NANOBENCHMARK_H_
diff --git a/absl/random/internal/nanobenchmark_test.cc b/absl/random/internal/nanobenchmark_test.cc
index f96e0f5..ab824ef 100644
--- a/absl/random/internal/nanobenchmark_test.cc
+++ b/absl/random/internal/nanobenchmark_test.cc
@@ -18,7 +18,7 @@
 #include "absl/strings/numbers.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal_nanobenchmark {
 namespace {
 
@@ -68,7 +68,7 @@
 
 }  // namespace
 }  // namespace random_internal_nanobenchmark
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 int main(int argc, char* argv[]) {
diff --git a/absl/random/internal/nonsecure_base.h b/absl/random/internal/nonsecure_base.h
index c8af51c..730fa2e 100644
--- a/absl/random/internal/nonsecure_base.h
+++ b/absl/random/internal/nonsecure_base.h
@@ -33,7 +33,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Each instance of NonsecureURBGBase<URBG> will be seeded by variates produced
@@ -144,7 +144,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_NONSECURE_BASE_H_
diff --git a/absl/random/internal/nonsecure_base_test.cc b/absl/random/internal/nonsecure_base_test.cc
index d9de990..698027f 100644
--- a/absl/random/internal/nonsecure_base_test.cc
+++ b/absl/random/internal/nonsecure_base_test.cc
@@ -154,9 +154,10 @@
 TEST(NonsecureURBGBase, CompatibleWithStdDistributions) {
   ExampleNonsecureURBG rbg;
 
-  std::uniform_int_distribution<uint32_t>(0, 100)(rbg);
-  std::uniform_real_distribution<float>()(rbg);
-  std::bernoulli_distribution(0.2)(rbg);
+  // Cast to void to suppress [[nodiscard]] warnings
+  static_cast<void>(std::uniform_int_distribution<uint32_t>(0, 100)(rbg));
+  static_cast<void>(std::uniform_real_distribution<float>()(rbg));
+  static_cast<void>(std::bernoulli_distribution(0.2)(rbg));
 }
 
 TEST(NonsecureURBGBase, ConsecutiveDefaultInstancesYieldUniqueVariates) {
diff --git a/absl/random/internal/pcg_engine.h b/absl/random/internal/pcg_engine.h
index 607ac34..53c23fe 100644
--- a/absl/random/internal/pcg_engine.h
+++ b/absl/random/internal/pcg_engine.h
@@ -24,7 +24,7 @@
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // pcg_engine is a simplified implementation of Melissa O'Neil's PCG engine in
@@ -301,7 +301,7 @@
     random_internal::pcg_xsh_rr_64_32>;
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_PCG_ENGINE_H_
diff --git a/absl/random/internal/platform.h b/absl/random/internal/platform.h
index a5a42cb..bbdb4e6 100644
--- a/absl/random/internal/platform.h
+++ b/absl/random/internal/platform.h
@@ -162,7 +162,8 @@
 // iOS does not support dispatch, even on x86, since applications
 // should be bundled as fat binaries, with a different build tailored for
 // each specific supported platform/architecture.
-#if defined(__APPLE__) && (TARGET_OS_IPHONE || TARGET_OS_IPHONE_SIMULATOR)
+#if (defined(TARGET_OS_IPHONE) && TARGET_OS_IPHONE) || \
+    (defined(TARGET_OS_IPHONE_SIMULATOR) && TARGET_OS_IPHONE_SIMULATOR)
 #undef ABSL_RANDOM_INTERNAL_AES_DISPATCH
 #define ABSL_RANDOM_INTERNAL_AES_DISPATCH 0
 #endif
diff --git a/absl/random/internal/pool_urbg.cc b/absl/random/internal/pool_urbg.cc
index 304d9b1..5bee530 100644
--- a/absl/random/internal/pool_urbg.cc
+++ b/absl/random/internal/pool_urbg.cc
@@ -37,7 +37,7 @@
 using absl::base_internal::SpinLockHolder;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -60,13 +60,13 @@
   }
 
   // Copy bytes into out.
-  void Fill(uint8_t* out, size_t bytes) LOCKS_EXCLUDED(mu_);
+  void Fill(uint8_t* out, size_t bytes) ABSL_LOCKS_EXCLUDED(mu_);
 
   // Returns random bits from the buffer in units of T.
   template <typename T>
-  inline T Generate() LOCKS_EXCLUDED(mu_);
+  inline T Generate() ABSL_LOCKS_EXCLUDED(mu_);
 
-  inline void MaybeRefill() EXCLUSIVE_LOCKS_REQUIRED(mu_) {
+  inline void MaybeRefill() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
     if (next_ >= kState) {
       next_ = kCapacity;
       impl_.Generate(state_);
@@ -75,10 +75,10 @@
 
  private:
   // Randen URBG state.
-  uint32_t state_[kState] GUARDED_BY(mu_);  // First to satisfy alignment.
+  uint32_t state_[kState] ABSL_GUARDED_BY(mu_);  // First to satisfy alignment.
   SpinLock mu_;
   const Randen impl_;
-  size_t next_ GUARDED_BY(mu_);
+  size_t next_ ABSL_GUARDED_BY(mu_);
 };
 
 template <>
@@ -250,5 +250,5 @@
 template class RandenPool<uint64_t>;
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/pool_urbg.h b/absl/random/internal/pool_urbg.h
index eac75e2..0572192 100644
--- a/absl/random/internal/pool_urbg.h
+++ b/absl/random/internal/pool_urbg.h
@@ -22,7 +22,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RandenPool is a thread-safe random number generator [random.req.urbg] that
@@ -125,7 +125,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_POOL_URBG_H_
diff --git a/absl/random/internal/randen.cc b/absl/random/internal/randen.cc
index 3b08760..78a1e00 100644
--- a/absl/random/internal/randen.cc
+++ b/absl/random/internal/randen.cc
@@ -41,7 +41,7 @@
 // structured/low-entropy counters to digits of Pi.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -87,5 +87,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen.h b/absl/random/internal/randen.h
index ef375f9..c2834aa 100644
--- a/absl/random/internal/randen.h
+++ b/absl/random/internal/randen.h
@@ -23,7 +23,7 @@
 #include "absl/random/internal/randen_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RANDen = RANDom generator or beetroots in Swiss German.
@@ -96,7 +96,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_H_
diff --git a/absl/random/internal/randen_detect.cc b/absl/random/internal/randen_detect.cc
index 610ae31..d63230c 100644
--- a/absl/random/internal/randen_detect.cc
+++ b/absl/random/internal/randen_detect.cc
@@ -95,7 +95,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // The default return at the end of the function might be unreachable depending
@@ -217,5 +217,5 @@
 #endif
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen_detect.h b/absl/random/internal/randen_detect.h
index cb77755..f283f43 100644
--- a/absl/random/internal/randen_detect.h
+++ b/absl/random/internal/randen_detect.h
@@ -15,8 +15,10 @@
 #ifndef ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
 #define ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Returns whether the current CPU supports RandenHwAes implementation.
@@ -25,7 +27,7 @@
 bool CPUSupportsRandenHwAes();
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_DETECT_H_
diff --git a/absl/random/internal/randen_engine.h b/absl/random/internal/randen_engine.h
index e721559..6b33731 100644
--- a/absl/random/internal/randen_engine.h
+++ b/absl/random/internal/randen_engine.h
@@ -28,7 +28,7 @@
 #include "absl/random/internal/randen.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Deterministic pseudorandom byte generator with backtracking resistance
@@ -224,7 +224,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_ENGINE_H_
diff --git a/absl/random/internal/randen_hwaes.cc b/absl/random/internal/randen_hwaes.cc
index d7eed8b..e23844f 100644
--- a/absl/random/internal/randen_hwaes.cc
+++ b/absl/random/internal/randen_hwaes.cc
@@ -22,39 +22,9 @@
 #include <cstdint>
 #include <cstring>
 
+#include "absl/base/attributes.h"
 #include "absl/random/internal/platform.h"
 
-// ABSL_HAVE_ATTRIBUTE
-#if !defined(ABSL_HAVE_ATTRIBUTE)
-#ifdef __has_attribute
-#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
-#else
-#define ABSL_HAVE_ATTRIBUTE(x) 0
-#endif
-#endif
-
-#if ABSL_HAVE_ATTRIBUTE(always_inline) || \
-    (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE \
-  __attribute__((always_inline))
-#elif defined(_MSC_VER)
-// We can achieve something similar to attribute((always_inline)) with MSVC by
-// using the __forceinline keyword, however this is not perfect. MSVC is
-// much less aggressive about inlining, and even with the __forceinline keyword.
-#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE __forceinline
-#else
-#define ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE
-#endif
-
-// ABSL_ATTRIBUTE_FLATTEN enables much more aggressive inlining within
-// the indicated function.
-#undef ABSL_ATTRIBUTE_FLATTEN
-#if ABSL_HAVE_ATTRIBUTE(flatten) || (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_ATTRIBUTE_FLATTEN __attribute__((flatten))
-#else
-#define ABSL_ATTRIBUTE_FLATTEN
-#endif
-
 // ABSL_RANDEN_HWAES_IMPL indicates whether this file will contain
 // a hardware accelerated implementation of randen, or whether it
 // will contain stubs that exit the process.
@@ -105,7 +75,7 @@
 #include <cstdlib>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // No accelerated implementation.
@@ -137,7 +107,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #else  // defined(ABSL_RANDEN_HWAES_IMPL)
@@ -148,18 +118,6 @@
 
 #include "absl/random/internal/randen_traits.h"
 
-// ABSL_FUNCTION_ALIGN32 defines a 32-byte alignment attribute
-// for the functions in this file.
-//
-// NOTE: Determine whether we actually have any wins from ALIGN32
-// using microbenchmarks. If not, remove.
-#undef ABSL_FUNCTION_ALIGN32
-#if ABSL_HAVE_ATTRIBUTE(aligned) || (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_FUNCTION_ALIGN32 __attribute__((aligned(32)))
-#else
-#define ABSL_FUNCTION_ALIGN32
-#endif
-
 // TARGET_CRYPTO defines a crypto attribute for each architecture.
 //
 // NOTE: Evaluate whether we should eliminate ABSL_TARGET_CRYPTO.
@@ -193,8 +151,7 @@
 
 namespace {
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-ReverseBytes(const Vector128& v) {
+inline ABSL_TARGET_CRYPTO Vector128 ReverseBytes(const Vector128& v) {
   // Reverses the bytes of the vector.
   const __vector unsigned char perm = {15, 14, 13, 12, 11, 10, 9, 8,
                                        7,  6,  5,  4,  3,  2,  1, 0};
@@ -204,26 +161,23 @@
 // WARNING: these load/store in native byte order. It is OK to load and then
 // store an unchanged vector, but interpreting the bits as a number or input
 // to AES will have undefined results.
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return vec_vsx_ld(0, reinterpret_cast<const Vector128*>(from));
 }
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-Vector128Store(const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
   vec_vsx_st(v, 0, reinterpret_cast<Vector128*>(to));
 }
 
 // One round of AES. "round_key" is a public constant for breaking the
 // symmetry of AES (ensures previously equal columns differ afterwards).
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-AesRound(const Vector128& state, const Vector128& round_key) {
+inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
+                                             const Vector128& round_key) {
   return Vector128(__builtin_crypto_vcipher(state, round_key));
 }
 
 // Enables native loads in the round loop by pre-swapping.
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-SwapEndian(uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
+inline ABSL_TARGET_CRYPTO void SwapEndian(uint64_t* state) {
   using absl::random_internal::RandenTraits;
   constexpr size_t kLanes = 2;
   constexpr size_t kFeistelBlocks = RandenTraits::kFeistelBlocks;
@@ -275,20 +229,18 @@
 
 namespace {
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return vld1q_u8(reinterpret_cast<const uint8_t*>(from));
 }
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-Vector128Store(const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
   vst1q_u8(reinterpret_cast<uint8_t*>(to), v);
 }
 
 // One round of AES. "round_key" is a public constant for breaking the
 // symmetry of AES (ensures previously equal columns differ afterwards).
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-AesRound(const Vector128& state, const Vector128& round_key) {
+inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
+                                             const Vector128& round_key) {
   // It is important to always use the full round function - omitting the
   // final MixColumns reduces security [https://eprint.iacr.org/2010/041.pdf]
   // and does not help because we never decrypt.
@@ -299,8 +251,7 @@
   return vaesmcq_u8(vaeseq_u8(state, uint8x16_t{})) ^ round_key;
 }
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-SwapEndian(uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT) {}
+inline ABSL_TARGET_CRYPTO void SwapEndian(uint64_t*) {}
 
 }  // namespace
 
@@ -315,16 +266,11 @@
 class Vector128 {
  public:
   // Convert from/to intrinsics.
-  inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE explicit Vector128(
-      const __m128i& Vector128)
-      : data_(Vector128) {}
+  inline explicit Vector128(const __m128i& Vector128) : data_(Vector128) {}
 
-  inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE __m128i data() const {
-    return data_;
-  }
+  inline __m128i data() const { return data_; }
 
-  inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128& operator^=(
-      const Vector128& other) {
+  inline Vector128& operator^=(const Vector128& other) {
     data_ = _mm_xor_si128(data_, other.data());
     return *this;
   }
@@ -333,29 +279,25 @@
   __m128i data_;
 };
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-Vector128Load(const void* ABSL_RANDOM_INTERNAL_RESTRICT from) {
+inline ABSL_TARGET_CRYPTO Vector128 Vector128Load(const void* from) {
   return Vector128(_mm_load_si128(reinterpret_cast<const __m128i*>(from)));
 }
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-Vector128Store(const Vector128& v, void* ABSL_RANDOM_INTERNAL_RESTRICT to) {
-  _mm_store_si128(reinterpret_cast<__m128i * ABSL_RANDOM_INTERNAL_RESTRICT>(to),
-                  v.data());
+inline ABSL_TARGET_CRYPTO void Vector128Store(const Vector128& v, void* to) {
+  _mm_store_si128(reinterpret_cast<__m128i*>(to), v.data());
 }
 
 // One round of AES. "round_key" is a public constant for breaking the
 // symmetry of AES (ensures previously equal columns differ afterwards).
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE Vector128
-AesRound(const Vector128& state, const Vector128& round_key) {
+inline ABSL_TARGET_CRYPTO Vector128 AesRound(const Vector128& state,
+                                             const Vector128& round_key) {
   // It is important to always use the full round function - omitting the
   // final MixColumns reduces security [https://eprint.iacr.org/2010/041.pdf]
   // and does not help because we never decrypt.
   return Vector128(_mm_aesenc_si128(state.data(), round_key.data()));
 }
 
-inline ABSL_TARGET_CRYPTO ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE void
-SwapEndian(uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT) {}
+inline ABSL_TARGET_CRYPTO void SwapEndian(uint64_t*) {}
 
 }  // namespace
 
@@ -452,8 +394,7 @@
 
 // Block shuffles applies a shuffle to the entire state between AES rounds.
 // Improved odd-even shuffle from "New criterion for diffusion property".
-inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE ABSL_TARGET_CRYPTO void
-BlockShuffle(uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
+inline ABSL_TARGET_CRYPTO void BlockShuffle(uint64_t* state) {
   static_assert(kFeistelBlocks == 16, "Expecting 16 FeistelBlocks.");
 
   constexpr size_t shuffle[kFeistelBlocks] = {7,  2, 13, 4,  11, 8,  3, 6,
@@ -501,10 +442,8 @@
 // per 16 bytes (vs. 10 for AES-CTR). Computing eight round functions in
 // parallel hides the 7-cycle AESNI latency on HSW. Note that the Feistel
 // XORs are 'free' (included in the second AES instruction).
-inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE ABSL_TARGET_CRYPTO const
-    u64x2*
-    FeistelRound(uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state,
-                 const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
+inline ABSL_TARGET_CRYPTO const u64x2* FeistelRound(
+    uint64_t* state, const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys) {
   static_assert(kFeistelBlocks == 16, "Expecting 16 FeistelBlocks.");
 
   // MSVC does a horrible job at unrolling loops.
@@ -563,9 +502,8 @@
 // Indistinguishable from ideal by chosen-ciphertext adversaries using less than
 // 2^64 queries if the round function is a PRF. This is similar to the b=8 case
 // of Simpira v2, but more efficient than its generic construction for b=16.
-inline ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE ABSL_TARGET_CRYPTO void
-Permute(const void* ABSL_RANDOM_INTERNAL_RESTRICT keys,
-        uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state) {
+inline ABSL_TARGET_CRYPTO void Permute(
+    const void* ABSL_RANDOM_INTERNAL_RESTRICT keys, uint64_t* state) {
   const u64x2* ABSL_RANDOM_INTERNAL_RESTRICT keys128 =
       static_cast<const u64x2*>(keys);
 
@@ -582,25 +520,22 @@
 }  // namespace
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 bool HasRandenHwAesImplementation() { return true; }
 
-const void* ABSL_TARGET_CRYPTO ABSL_FUNCTION_ALIGN32 ABSL_ATTRIBUTE_FLATTEN
-RandenHwAes::GetKeys() {
+const void* ABSL_TARGET_CRYPTO RandenHwAes::GetKeys() {
   // Round keys for one AES per Feistel round and branch.
   // The canonical implementation uses first digits of Pi.
   return round_keys;
 }
 
 // NOLINTNEXTLINE
-void ABSL_TARGET_CRYPTO ABSL_FUNCTION_ALIGN32 ABSL_ATTRIBUTE_FLATTEN
-RandenHwAes::Absorb(const void* seed_void, void* state_void) {
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
-  const uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT seed =
-      reinterpret_cast<const uint64_t*>(seed_void);
+void ABSL_TARGET_CRYPTO RandenHwAes::Absorb(const void* seed_void,
+                                            void* state_void) {
+  auto* state = static_cast<uint64_t*>(state_void);
+  const auto* seed = static_cast<const uint64_t*>(seed_void);
 
   constexpr size_t kCapacityBlocks = kCapacityBytes / sizeof(Vector128);
   constexpr size_t kStateBlocks = kStateBytes / sizeof(Vector128);
@@ -672,12 +607,11 @@
 }
 
 // NOLINTNEXTLINE
-void ABSL_TARGET_CRYPTO ABSL_FUNCTION_ALIGN32 ABSL_ATTRIBUTE_FLATTEN
-RandenHwAes::Generate(const void* keys, void* state_void) {
+void ABSL_TARGET_CRYPTO RandenHwAes::Generate(const void* keys,
+                                              void* state_void) {
   static_assert(kCapacityBytes == sizeof(Vector128), "Capacity mismatch");
 
-  uint64_t* ABSL_RANDOM_INTERNAL_RESTRICT state =
-      reinterpret_cast<uint64_t*>(state_void);
+  auto* state = static_cast<uint64_t*>(state_void);
 
   const Vector128 prev_inner = Vector128Load(state);
 
@@ -698,7 +632,7 @@
 #endif
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // (ABSL_RANDEN_HWAES_IMPL)
diff --git a/absl/random/internal/randen_hwaes.h b/absl/random/internal/randen_hwaes.h
index 848bcea..bce36b5 100644
--- a/absl/random/internal/randen_hwaes.h
+++ b/absl/random/internal/randen_hwaes.h
@@ -15,13 +15,15 @@
 #ifndef ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
 #define ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
 
+#include "absl/base/config.h"
+
 // HERMETIC NOTE: The randen_hwaes target must not introduce duplicate
 // symbols from arbitrary system and other headers, since it may be built
 // with different flags from other targets, using different levels of
 // optimization, potentially introducing ODR violations.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RANDen = RANDom generator or beetroots in Swiss German.
@@ -42,7 +44,7 @@
 bool HasRandenHwAesImplementation();
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_HWAES_H_
diff --git a/absl/random/internal/randen_slow.cc b/absl/random/internal/randen_slow.cc
index e2d44f8..8d07458 100644
--- a/absl/random/internal/randen_slow.cc
+++ b/absl/random/internal/randen_slow.cc
@@ -18,17 +18,9 @@
 #include <cstdint>
 #include <cstring>
 
+#include "absl/base/attributes.h"
 #include "absl/random/internal/platform.h"
 
-// ABSL_HAVE_ATTRIBUTE
-#if !defined(ABSL_HAVE_ATTRIBUTE)
-#ifdef __has_attribute
-#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
-#else
-#define ABSL_HAVE_ATTRIBUTE(x) 0
-#endif
-#endif
-
 #if ABSL_HAVE_ATTRIBUTE(always_inline) || \
     (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_RANDOM_INTERNAL_ATTRIBUTE_ALWAYS_INLINE \
@@ -470,7 +462,7 @@
 }  // namespace
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 const void* RandenSlow::GetKeys() {
@@ -510,5 +502,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/randen_slow.h b/absl/random/internal/randen_slow.h
index 2133b37..72f92b5 100644
--- a/absl/random/internal/randen_slow.h
+++ b/absl/random/internal/randen_slow.h
@@ -17,8 +17,10 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RANDen = RANDom generator or beetroots in Swiss German.
@@ -39,7 +41,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_SLOW_H_
diff --git a/absl/random/internal/randen_traits.h b/absl/random/internal/randen_traits.h
index d256258..2b8bbe7 100644
--- a/absl/random/internal/randen_traits.h
+++ b/absl/random/internal/randen_traits.h
@@ -22,8 +22,10 @@
 
 #include <cstddef>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // RANDen = RANDom generator or beetroots in Swiss German.
@@ -55,7 +57,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_RANDEN_TRAITS_H_
diff --git a/absl/random/internal/salted_seed_seq.h b/absl/random/internal/salted_seed_seq.h
index 08bf369..5953a09 100644
--- a/absl/random/internal/salted_seed_seq.h
+++ b/absl/random/internal/salted_seed_seq.h
@@ -30,7 +30,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // This class conforms to the C++ Standard "Seed Sequence" concept
@@ -161,7 +161,7 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SALTED_SEED_SEQ_H_
diff --git a/absl/random/internal/seed_material.cc b/absl/random/internal/seed_material.cc
index dae7007..4d38a57 100644
--- a/absl/random/internal/seed_material.cc
+++ b/absl/random/internal/seed_material.cc
@@ -45,6 +45,9 @@
 #define ABSL_RANDOM_USE_BCRYPT 1
 #pragma comment(lib, "bcrypt.lib")
 
+#elif defined(__Fuchsia__)
+#include <zircon/syscalls.h>
+
 #endif
 
 #if defined(ABSL_RANDOM_USE_BCRYPT)
@@ -58,7 +61,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 namespace {
 
@@ -108,6 +111,15 @@
   return true;
 }
 
+#elif defined(__Fuchsia__)
+
+bool ReadSeedMaterialFromOSEntropyImpl(absl::Span<uint32_t> values) {
+  auto buffer = reinterpret_cast<uint8_t*>(values.data());
+  size_t buffer_size = sizeof(uint32_t) * values.size();
+  zx_cprng_draw(buffer, buffer_size);
+  return true;
+}
+
 #else
 
 // On *nix, read entropy from /dev/urandom.
@@ -203,5 +215,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/internal/seed_material.h b/absl/random/internal/seed_material.h
index 41387fe..4be10e9 100644
--- a/absl/random/internal/seed_material.h
+++ b/absl/random/internal/seed_material.h
@@ -27,7 +27,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // Returns the number of 32-bit blocks needed to contain the given number of
@@ -98,7 +98,7 @@
 absl::optional<uint32_t> GetSaltMaterial();
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SEED_MATERIAL_H_
diff --git a/absl/random/internal/seed_salting_sequence_generator.cc b/absl/random/internal/seed_salting_sequence_generator.cc
deleted file mode 100644
index 31fdcfe..0000000
--- a/absl/random/internal/seed_salting_sequence_generator.cc
+++ /dev/null
@@ -1,30 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <iostream>
-#include <random>
-
-#include "absl/random/random.h"
-
-// This program is used in integration tests.
-
-int main() {
-  std::seed_seq seed_seq{1234};
-  absl::BitGen rng(seed_seq);
-  constexpr size_t kSequenceLength = 8;
-  for (size_t i = 0; i < kSequenceLength; i++) {
-    std::cout << rng() << "\n";
-  }
-  return 0;
-}
diff --git a/absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc b/absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc
deleted file mode 100644
index 8797e2e..0000000
--- a/absl/random/internal/seed_salting_sequence_generator_empty_sequence.cc
+++ /dev/null
@@ -1,30 +0,0 @@
-// Copyright 2017 The Abseil Authors.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      https://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-#include <iostream>
-#include <random>
-
-#include "absl/random/random.h"
-
-// This program is used in integration tests.
-
-int main() {
-  std::seed_seq seed_seq{};
-  absl::BitGen rng(seed_seq);
-  constexpr size_t kSequenceLength = 8;
-  for (size_t i = 0; i < kSequenceLength; i++) {
-    std::cout << rng() << "\n";
-  }
-  return 0;
-}
diff --git a/absl/random/internal/sequence_urbg.h b/absl/random/internal/sequence_urbg.h
index cec0bf9..bc96a12 100644
--- a/absl/random/internal/sequence_urbg.h
+++ b/absl/random/internal/sequence_urbg.h
@@ -21,8 +21,10 @@
 #include <type_traits>
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // `sequence_urbg` is a simple random number generator which meets the
@@ -52,7 +54,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_SEQUENCE_URBG_H_
diff --git a/absl/random/internal/traits.h b/absl/random/internal/traits.h
index 9f7d126..75772bd 100644
--- a/absl/random/internal/traits.h
+++ b/absl/random/internal/traits.h
@@ -22,7 +22,7 @@
 #include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace random_internal {
 
 // random_internal::is_widening_convertible<A, B>
@@ -95,7 +95,7 @@
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_TRAITS_H_
diff --git a/absl/random/internal/uniform_helper.h b/absl/random/internal/uniform_helper.h
index 6af053e..663107c 100644
--- a/absl/random/internal/uniform_helper.h
+++ b/absl/random/internal/uniform_helper.h
@@ -22,7 +22,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 template <typename IntType>
 class uniform_int_distribution;
 
@@ -31,15 +31,33 @@
 
 // Interval tag types which specify whether the interval is open or closed
 // on either boundary.
+
 namespace random_internal {
-struct IntervalClosedClosedT {};
-struct IntervalClosedOpenT {};
-struct IntervalOpenClosedT {};
-struct IntervalOpenOpenT {};
+template <typename T>
+struct TagTypeCompare {};
+
+template <typename T>
+constexpr bool operator==(TagTypeCompare<T>, TagTypeCompare<T>) {
+  // Tags are mono-states. They always compare equal.
+  return true;
+}
+template <typename T>
+constexpr bool operator!=(TagTypeCompare<T>, TagTypeCompare<T>) {
+  return false;
+}
+
 }  // namespace random_internal
 
-namespace random_internal {
+struct IntervalClosedClosedTag
+    : public random_internal::TagTypeCompare<IntervalClosedClosedTag> {};
+struct IntervalClosedOpenTag
+    : public random_internal::TagTypeCompare<IntervalClosedOpenTag> {};
+struct IntervalOpenClosedTag
+    : public random_internal::TagTypeCompare<IntervalOpenClosedTag> {};
+struct IntervalOpenOpenTag
+    : public random_internal::TagTypeCompare<IntervalOpenOpenTag> {};
 
+namespace random_internal {
 // The functions
 //    uniform_lower_bound(tag, a, b)
 // and
@@ -60,8 +78,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalOpenClosedT>,
-                          std::is_same<Tag, IntervalOpenOpenT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
+                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
     IntType>
 uniform_lower_bound(Tag, IntType a, IntType) {
   return a + 1;
@@ -71,8 +89,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalOpenClosedT>,
-                          std::is_same<Tag, IntervalOpenOpenT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
+                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
     FloatType>
 uniform_lower_bound(Tag, FloatType a, FloatType b) {
   return std::nextafter(a, b);
@@ -80,8 +98,8 @@
 
 template <typename NumType, typename Tag>
 typename absl::enable_if_t<
-    absl::disjunction<std::is_same<Tag, IntervalClosedClosedT>,
-                      std::is_same<Tag, IntervalClosedOpenT>>::value,
+    absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
+                      std::is_same<Tag, IntervalClosedOpenTag>>::value,
     NumType>
 uniform_lower_bound(Tag, NumType a, NumType) {
   return a;
@@ -91,8 +109,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedOpenT>,
-                          std::is_same<Tag, IntervalOpenOpenT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
+                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
     IntType>
 uniform_upper_bound(Tag, IntType, IntType b) {
   return b - 1;
@@ -102,8 +120,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedOpenT>,
-                          std::is_same<Tag, IntervalOpenOpenT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
+                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
     FloatType>
 uniform_upper_bound(Tag, FloatType, FloatType b) {
   return b;
@@ -113,8 +131,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_integral<IntType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedClosedT>,
-                          std::is_same<Tag, IntervalOpenClosedT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
+                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
     IntType>
 uniform_upper_bound(Tag, IntType, IntType b) {
   return b;
@@ -124,8 +142,8 @@
 typename absl::enable_if_t<
     absl::conjunction<
         std::is_floating_point<FloatType>,
-        absl::disjunction<std::is_same<Tag, IntervalClosedClosedT>,
-                          std::is_same<Tag, IntervalOpenClosedT>>>::value,
+        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
+                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
     FloatType>
 uniform_upper_bound(Tag, FloatType, FloatType b) {
   return std::nextafter(b, (std::numeric_limits<FloatType>::max)());
@@ -137,16 +155,26 @@
                               absl::uniform_int_distribution<NumType>,
                               absl::uniform_real_distribution<NumType>>::type;
 
-template <typename TagType, typename NumType>
+template <typename NumType>
 struct UniformDistributionWrapper : public UniformDistribution<NumType> {
-  explicit UniformDistributionWrapper(NumType lo, NumType hi)
+  template <typename TagType>
+  explicit UniformDistributionWrapper(TagType, NumType lo, NumType hi)
       : UniformDistribution<NumType>(
             uniform_lower_bound<NumType>(TagType{}, lo, hi),
             uniform_upper_bound<NumType>(TagType{}, lo, hi)) {}
+
+  explicit UniformDistributionWrapper(NumType lo, NumType hi)
+      : UniformDistribution<NumType>(
+            uniform_lower_bound<NumType>(IntervalClosedOpenTag(), lo, hi),
+            uniform_upper_bound<NumType>(IntervalClosedOpenTag(), lo, hi)) {}
+
+  explicit UniformDistributionWrapper()
+      : UniformDistribution<NumType>(std::numeric_limits<NumType>::lowest(),
+                                     (std::numeric_limits<NumType>::max)()) {}
 };
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_INTERNAL_UNIFORM_HELPER_H_
diff --git a/absl/random/internal/wide_multiply.h b/absl/random/internal/wide_multiply.h
new file mode 100644
index 0000000..6e4cf1b
--- /dev/null
+++ b/absl/random/internal/wide_multiply.h
@@ -0,0 +1,111 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
+#define ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
+
+#include <cstdint>
+#include <limits>
+#include <type_traits>
+
+#if (defined(_WIN32) || defined(_WIN64)) && defined(_M_IA64)
+#include <intrin.h>  // NOLINT(build/include_order)
+#pragma intrinsic(_umul128)
+#define ABSL_INTERNAL_USE_UMUL128 1
+#endif
+
+#include "absl/base/config.h"
+#include "absl/base/internal/bits.h"
+#include "absl/numeric/int128.h"
+#include "absl/random/internal/traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// Helper object to multiply two 64-bit values to a 128-bit value.
+// MultiplyU64ToU128 multiplies two 64-bit values to a 128-bit value.
+// If an intrinsic is available, it is used, otherwise use native 32-bit
+// multiplies to construct the result.
+inline uint128 MultiplyU64ToU128(uint64_t a, uint64_t b) {
+#if defined(ABSL_HAVE_INTRINSIC_INT128)
+  return uint128(static_cast<__uint128_t>(a) * b);
+#elif defined(ABSL_INTERNAL_USE_UMUL128)
+  // uint64_t * uint64_t => uint128 multiply using imul intrinsic on MSVC.
+  uint64_t high = 0;
+  const uint64_t low = _umul128(a, b, &high);
+  return absl::MakeUint128(high, low);
+#else
+  // uint128(a) * uint128(b) in emulated mode computes a full 128-bit x 128-bit
+  // multiply.  However there are many cases where that is not necessary, and it
+  // is only necessary to support a 64-bit x 64-bit = 128-bit multiply.  This is
+  // for those cases.
+  const uint64_t a00 = static_cast<uint32_t>(a);
+  const uint64_t a32 = a >> 32;
+  const uint64_t b00 = static_cast<uint32_t>(b);
+  const uint64_t b32 = b >> 32;
+
+  const uint64_t c00 = a00 * b00;
+  const uint64_t c32a = a00 * b32;
+  const uint64_t c32b = a32 * b00;
+  const uint64_t c64 = a32 * b32;
+
+  const uint32_t carry =
+      static_cast<uint32_t>(((c00 >> 32) + static_cast<uint32_t>(c32a) +
+                             static_cast<uint32_t>(c32b)) >>
+                            32);
+
+  return absl::MakeUint128(c64 + (c32a >> 32) + (c32b >> 32) + carry,
+                           c00 + (c32a << 32) + (c32b << 32));
+#endif
+}
+
+// wide_multiply<T> multiplies two N-bit values to a 2N-bit result.
+template <typename UIntType>
+struct wide_multiply {
+  static constexpr size_t kN = std::numeric_limits<UIntType>::digits;
+  using input_type = UIntType;
+  using result_type = typename random_internal::unsigned_bits<kN * 2>::type;
+
+  static result_type multiply(input_type a, input_type b) {
+    return static_cast<result_type>(a) * b;
+  }
+
+  static input_type hi(result_type r) { return r >> kN; }
+  static input_type lo(result_type r) { return r; }
+
+  static_assert(std::is_unsigned<UIntType>::value,
+                "Class-template wide_multiply<> argument must be unsigned.");
+};
+
+#ifndef ABSL_HAVE_INTRINSIC_INT128
+template <>
+struct wide_multiply<uint64_t> {
+  using input_type = uint64_t;
+  using result_type = uint128;
+
+  static result_type multiply(uint64_t a, uint64_t b) {
+    return MultiplyU64ToU128(a, b);
+  }
+
+  static uint64_t hi(result_type r) { return Uint128High64(r); }
+  static uint64_t lo(result_type r) { return Uint128Low64(r); }
+};
+#endif
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_INTERNAL_WIDE_MULTIPLY_H_
diff --git a/absl/random/internal/wide_multiply_test.cc b/absl/random/internal/wide_multiply_test.cc
new file mode 100644
index 0000000..922603f
--- /dev/null
+++ b/absl/random/internal/wide_multiply_test.cc
@@ -0,0 +1,66 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/internal/wide_multiply.h"
+
+#include "gtest/gtest.h"
+#include "absl/base/internal/bits.h"
+#include "absl/numeric/int128.h"
+
+using absl::random_internal::MultiplyU64ToU128;
+
+namespace {
+
+TEST(WideMultiplyTest, MultiplyU64ToU128Test) {
+  constexpr uint64_t k1 = 1;
+  constexpr uint64_t kMax = ~static_cast<uint64_t>(0);
+
+  EXPECT_EQ(absl::uint128(0), MultiplyU64ToU128(0, 0));
+
+  // Max uint64
+  EXPECT_EQ(MultiplyU64ToU128(kMax, kMax),
+            absl::MakeUint128(0xfffffffffffffffe, 0x0000000000000001));
+  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(kMax, 1));
+  EXPECT_EQ(absl::MakeUint128(0, kMax), MultiplyU64ToU128(1, kMax));
+  for (int i = 0; i < 64; ++i) {
+    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
+              MultiplyU64ToU128(kMax, k1 << i));
+    EXPECT_EQ(absl::MakeUint128(0, kMax) << i,
+              MultiplyU64ToU128(k1 << i, kMax));
+  }
+
+  // 1-bit x 1-bit.
+  for (int i = 0; i < 64; ++i) {
+    for (int j = 0; j < 64; ++j) {
+      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
+                MultiplyU64ToU128(k1 << i, k1 << j));
+      EXPECT_EQ(absl::MakeUint128(0, 1) << (i + j),
+                MultiplyU64ToU128(k1 << i, k1 << j));
+    }
+  }
+
+  // Verified multiplies
+  EXPECT_EQ(MultiplyU64ToU128(0xffffeeeeddddcccc, 0xbbbbaaaa99998888),
+            absl::MakeUint128(0xbbbb9e2692c5dddc, 0xc28f7531048d2c60));
+  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfedcba9876543210),
+            absl::MakeUint128(0x0121fa00ad77d742, 0x2236d88fe5618cf0));
+  EXPECT_EQ(MultiplyU64ToU128(0x0123456789abcdef, 0xfdb97531eca86420),
+            absl::MakeUint128(0x0120ae99d26725fc, 0xce197f0ecac319e0));
+  EXPECT_EQ(MultiplyU64ToU128(0x97a87f4f261ba3f2, 0xfedcba9876543210),
+            absl::MakeUint128(0x96fbf1a8ae78d0ba, 0x5a6dd4b71f278320));
+  EXPECT_EQ(MultiplyU64ToU128(0xfedcba9876543210, 0xfdb97531eca86420),
+            absl::MakeUint128(0xfc98c6981a413e22, 0x342d0bbf48948200));
+}
+
+}  // namespace
diff --git a/absl/random/log_uniform_int_distribution.h b/absl/random/log_uniform_int_distribution.h
index a12fa4c..960816e 100644
--- a/absl/random/log_uniform_int_distribution.h
+++ b/absl/random/log_uniform_int_distribution.h
@@ -23,14 +23,14 @@
 #include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
 #include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/traits.h"
 #include "absl/random/uniform_int_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // log_uniform_int_distribution:
 //
@@ -193,13 +193,15 @@
     const double r = std::pow(p.base(), d);
     const double s = (r * p.base()) - 1.0;
 
-    base_e = (r > (std::numeric_limits<unsigned_type>::max)())
-                 ? (std::numeric_limits<unsigned_type>::max)()
-                 : static_cast<unsigned_type>(r);
+    base_e =
+        (r > static_cast<double>((std::numeric_limits<unsigned_type>::max)()))
+            ? (std::numeric_limits<unsigned_type>::max)()
+            : static_cast<unsigned_type>(r);
 
-    top_e = (s > (std::numeric_limits<unsigned_type>::max)())
-                ? (std::numeric_limits<unsigned_type>::max)()
-                : static_cast<unsigned_type>(s);
+    top_e =
+        (s > static_cast<double>((std::numeric_limits<unsigned_type>::max)()))
+            ? (std::numeric_limits<unsigned_type>::max)()
+            : static_cast<unsigned_type>(s);
   }
 
   const unsigned_type lo = (base_e >= p.range()) ? p.range() : base_e;
@@ -246,7 +248,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_LOG_UNIFORM_INT_DISTRIBUTION_H_
diff --git a/absl/random/log_uniform_int_distribution_test.cc b/absl/random/log_uniform_int_distribution_test.cc
index 0ff4c32..5270531 100644
--- a/absl/random/log_uniform_int_distribution_test.cc
+++ b/absl/random/log_uniform_int_distribution_test.cc
@@ -243,7 +243,7 @@
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_SUITE_P(, LogUniformIntChiSquaredTest,
+INSTANTIATE_TEST_SUITE_P(All, LogUniformIntChiSquaredTest,
                          ::testing::ValuesIn(GenParams()), ParamName);
 
 // NOTE: absl::log_uniform_int_distribution is not guaranteed to be stable.
diff --git a/absl/random/mock_distributions.h b/absl/random/mock_distributions.h
new file mode 100644
index 0000000..d36d5ba
--- /dev/null
+++ b/absl/random/mock_distributions.h
@@ -0,0 +1,261 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// File: mock_distributions.h
+// -----------------------------------------------------------------------------
+//
+// This file contains mock distribution functions for use alongside an
+// `absl::MockingBitGen` object within the Googletest testing framework. Such
+// mocks are useful to provide deterministic values as return values within
+// (otherwise random) Abseil distribution functions.
+//
+// The return type of each function is a mock expectation object which
+// is used to set the match result.
+//
+// More information about the Googletest testing framework is available at
+// https://github.com/google/googletest
+//
+// Example:
+//
+//   absl::MockingBitGen mock;
+//   EXPECT_CALL(absl::MockUniform<int>(), Call(mock, 1, 1000))
+//     .WillRepeatedly(testing::ReturnRoundRobin({20, 40}));
+//
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 20);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000), 40);
+
+#ifndef ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+#define ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
+
+#include <limits>
+#include <type_traits>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/distributions.h"
+#include "absl/random/internal/mock_overload_set.h"
+#include "absl/random/mocking_bit_gen.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// -----------------------------------------------------------------------------
+// absl::MockUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Uniform.
+//
+// `absl::MockUniform` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(mock))
+//     .WillOnce(Return(123456));
+//  auto x = absl::Uniform<uint32_t>(mock);
+//  assert(x == 123456)
+//
+template <typename R>
+using MockUniform = random_internal::MockOverloadSet<
+    random_internal::UniformDistributionWrapper<R>,
+    R(IntervalClosedOpenTag, MockingBitGen&, R, R),
+    R(IntervalClosedClosedTag, MockingBitGen&, R, R),
+    R(IntervalOpenOpenTag, MockingBitGen&, R, R),
+    R(IntervalOpenClosedTag, MockingBitGen&, R, R), R(MockingBitGen&, R, R),
+    R(MockingBitGen&)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBernoulli
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Bernoulli.
+//
+// `absl::MockBernoulli` is a class used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBernoulli(), Call(mock, testing::_))
+//     .WillOnce(Return(false));
+//  assert(absl::Bernoulli(mock, 0.5) == false);
+//
+using MockBernoulli =
+    random_internal::MockOverloadSet<absl::bernoulli_distribution,
+                                     bool(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockBeta
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Beta.
+//
+// `absl::MockBeta` is a class used in conjunction with Googletest's `ON_CALL()`
+// and `EXPECT_CALL()` macros. To use it, default-construct an instance of it
+// inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the same way one
+// would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockBeta(), Call(mock, 3.0, 2.0))
+//     .WillOnce(Return(0.567));
+//  auto x = absl::Beta<double>(mock, 3.0, 2.0);
+//  assert(x == 0.567);
+//
+template <typename RealType>
+using MockBeta =
+    random_internal::MockOverloadSet<absl::beta_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockExponential
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Exponential.
+//
+// `absl::MockExponential` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockExponential<double>(), Call(mock, 0.5))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Exponential<double>(mock, 0.5);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockExponential =
+    random_internal::MockOverloadSet<absl::exponential_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockGaussian
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Gaussian.
+//
+// `absl::MockGaussian` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockGaussian<double>(), Call(mock, 16.3, 3.3))
+//     .WillOnce(Return(12.3456789));
+//  auto x = absl::Gaussian<double>(mock, 16.3, 3.3);
+//  assert(x == 12.3456789)
+//
+template <typename RealType>
+using MockGaussian =
+    random_internal::MockOverloadSet<absl::gaussian_distribution<RealType>,
+                                     RealType(MockingBitGen&, RealType,
+                                              RealType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockLogUniform
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::LogUniform.
+//
+// `absl::MockLogUniform` is a class template used in conjunction with
+// Googletest's `ON_CALL()` and `EXPECT_CALL()` macros. To use it,
+// default-construct an instance of it inside `ON_CALL()` or `EXPECT_CALL()`,
+// and use `Call(...)` the same way one would define mocks on a
+// Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockLogUniform<int>(), Call(mock, 10, 10000, 10))
+//     .WillOnce(Return(1221));
+//  auto x = absl::LogUniform<int>(mock, 10, 10000, 10);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockLogUniform = random_internal::MockOverloadSet<
+    absl::log_uniform_int_distribution<IntType>,
+    IntType(MockingBitGen&, IntType, IntType, IntType)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockPoisson
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Poisson.
+//
+// `absl::MockPoisson` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockPoisson<int>(), Call(mock, 2.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Poisson<int>(mock, 2.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockPoisson =
+    random_internal::MockOverloadSet<absl::poisson_distribution<IntType>,
+                                     IntType(MockingBitGen&, double)>;
+
+// -----------------------------------------------------------------------------
+// absl::MockZipf
+// -----------------------------------------------------------------------------
+//
+// Matches calls to absl::Zipf.
+//
+// `absl::MockZipf` is a class template used in conjunction with Googletest's
+// `ON_CALL()` and `EXPECT_CALL()` macros. To use it, default-construct an
+// instance of it inside `ON_CALL()` or `EXPECT_CALL()`, and use `Call(...)` the
+// same way one would define mocks on a Googletest `MockFunction()`.
+//
+// Example:
+//
+//  absl::MockingBitGen mock;
+//  EXPECT_CALL(absl::MockZipf<int>(), Call(mock, 1000000, 2.0, 1.0))
+//     .WillOnce(Return(1221));
+//  auto x = absl::Zipf<int>(mock, 1000000, 2.0, 1.0);
+//  assert(x == 1221)
+//
+template <typename IntType>
+using MockZipf =
+    random_internal::MockOverloadSet<absl::zipf_distribution<IntType>,
+                                     IntType(MockingBitGen&, IntType, double,
+                                             double)>;
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCK_DISTRIBUTIONS_H_
diff --git a/absl/random/mock_distributions_test.cc b/absl/random/mock_distributions_test.cc
new file mode 100644
index 0000000..de23baf
--- /dev/null
+++ b/absl/random/mock_distributions_test.cc
@@ -0,0 +1,72 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/random/mock_distributions.h"
+
+#include "gtest/gtest.h"
+#include "absl/random/mocking_bit_gen.h"
+#include "absl/random/random.h"
+
+namespace {
+using ::testing::Return;
+
+TEST(MockDistributions, Examples) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+  EXPECT_CALL(absl::MockBeta<double>(), Call(gen, 3.0, 2.0))
+      .WillOnce(Return(0.567));
+  EXPECT_EQ(absl::Beta<double>(gen, 3.0, 2.0), 0.567);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillOnce(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+}  // namespace
diff --git a/absl/random/internal/named_generator.cc b/absl/random/mocking_bit_gen.cc
similarity index 62%
rename from absl/random/internal/named_generator.cc
rename to absl/random/mocking_bit_gen.cc
index b168a25..6bb1e41 100644
--- a/absl/random/internal/named_generator.cc
+++ b/absl/random/mocking_bit_gen.cc
@@ -1,3 +1,4 @@
+//
 // Copyright 2018 The Abseil Authors.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
@@ -11,20 +12,19 @@
 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 // See the License for the specific language governing permissions and
 // limitations under the License.
+//
+#include "absl/random/mocking_bit_gen.h"
 
-#include <cstddef>
-#include <iostream>
+#include <string>
 
-#include "absl/random/random.h"
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+MockingBitGen::~MockingBitGen() {
 
-// This program is used in integration tests.
-
-int main() {
-  auto seed_seq = absl::MakeTaggedSeedSeq("TEST_GENERATOR", std::cerr);
-  absl::BitGen rng(seed_seq);
-  constexpr size_t kSequenceLength = 8;
-  for (size_t i = 0; i < kSequenceLength; i++) {
-    std::cout << rng() << "\n";
+  for (const auto& del : deleters_) {
+    del();
   }
-  return 0;
 }
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/random/mocking_bit_gen.h b/absl/random/mocking_bit_gen.h
new file mode 100644
index 0000000..36cef91
--- /dev/null
+++ b/absl/random/mocking_bit_gen.h
@@ -0,0 +1,196 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// mocking_bit_gen.h
+// -----------------------------------------------------------------------------
+//
+// This file includes an `absl::MockingBitGen` class to use as a mock within the
+// Googletest testing framework. Such a mock is useful to provide deterministic
+// values as return values within (otherwise random) Abseil distribution
+// functions. Such determinism within a mock is useful within testing frameworks
+// to test otherwise indeterminate APIs.
+//
+// More information about the Googletest testing framework is available at
+// https://github.com/google/googletest
+
+#ifndef ABSL_RANDOM_MOCKING_BIT_GEN_H_
+#define ABSL_RANDOM_MOCKING_BIT_GEN_H_
+
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <tuple>
+#include <type_traits>
+#include <typeindex>
+#include <typeinfo>
+#include <utility>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/container/flat_hash_map.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/distributions.h"
+#include "absl/random/internal/distribution_caller.h"
+#include "absl/random/internal/mocking_bit_gen_base.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_join.h"
+#include "absl/types/span.h"
+#include "absl/types/variant.h"
+#include "absl/utility/utility.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+namespace random_internal {
+
+template <typename, typename>
+struct MockSingleOverload;
+
+}  // namespace random_internal
+
+// MockingBitGen
+//
+// `absl::MockingBitGen` is a mock Uniform Random Bit Generator (URBG) class
+// which can act in place of an `absl::BitGen` URBG within tests using the
+// Googletest testing framework.
+//
+// Usage:
+//
+// Use an `absl::MockingBitGen` along with a mock distribution object (within
+// mock_distributions.h) inside Googletest constructs such as ON_CALL(),
+// EXPECT_TRUE(), etc. to produce deterministic results conforming to the
+// distribution's API contract.
+//
+// Example:
+//
+//  // Mock a call to an `absl::Bernoulli` distribution using Googletest
+//   absl::MockingBitGen bitgen;
+//
+//   ON_CALL(absl::MockBernoulli(), Call(bitgen, 0.5))
+//       .WillByDefault(testing::Return(true));
+//   EXPECT_TRUE(absl::Bernoulli(bitgen, 0.5));
+//
+//  // Mock a call to an `absl::Uniform` distribution within Googletest
+//  absl::MockingBitGen bitgen;
+//
+//   ON_CALL(absl::MockUniform<int>(), Call(bitgen, testing::_, testing::_))
+//       .WillByDefault([] (int low, int high) {
+//           return (low + high) / 2;
+//       });
+//
+//   EXPECT_EQ(absl::Uniform<int>(gen, 0, 10), 5);
+//   EXPECT_EQ(absl::Uniform<int>(gen, 30, 40), 35);
+//
+// At this time, only mock distributions supplied within the Abseil random
+// library are officially supported.
+//
+class MockingBitGen : public absl::random_internal::MockingBitGenBase {
+ public:
+  MockingBitGen() {}
+
+  ~MockingBitGen() override;
+
+ private:
+  template <typename DistrT, typename... Args>
+  using MockFnType =
+      ::testing::MockFunction<typename DistrT::result_type(Args...)>;
+
+  // MockingBitGen::Register
+  //
+  // Register<DistrT, FormatT, ArgTupleT> is the main extension point for
+  // extending the MockingBitGen framework. It provides a mechanism to install a
+  // mock expectation for the distribution `distr_t` onto the MockingBitGen
+  // context.
+  //
+  // The returned MockFunction<...> type can be used to setup additional
+  // distribution parameters of the expectation.
+  template <typename DistrT, typename... Args, typename... Ms>
+  decltype(std::declval<MockFnType<DistrT, Args...>>().gmock_Call(
+      std::declval<Ms>()...))
+  Register(Ms&&... matchers) {
+    auto& mock =
+        mocks_[std::type_index(GetTypeId<DistrT, std::tuple<Args...>>())];
+
+    if (!mock.mock_fn) {
+      auto* mock_fn = new MockFnType<DistrT, Args...>;
+      mock.mock_fn = mock_fn;
+      mock.match_impl = &MatchImpl<DistrT, Args...>;
+      deleters_.emplace_back([mock_fn] { delete mock_fn; });
+    }
+
+    return static_cast<MockFnType<DistrT, Args...>*>(mock.mock_fn)
+        ->gmock_Call(std::forward<Ms>(matchers)...);
+  }
+
+  mutable std::vector<std::function<void()>> deleters_;
+
+  using match_impl_fn = void (*)(void* mock_fn, void* t_erased_dist_args,
+                                 void* t_erased_result);
+  struct MockData {
+    void* mock_fn = nullptr;
+    match_impl_fn match_impl = nullptr;
+  };
+
+  mutable absl::flat_hash_map<std::type_index, MockData> mocks_;
+
+  template <typename DistrT, typename... Args>
+  static void MatchImpl(void* mock_fn, void* dist_args, void* result) {
+    using result_type = typename DistrT::result_type;
+    *static_cast<result_type*>(result) = absl::apply(
+        [mock_fn](Args... args) -> result_type {
+          return (*static_cast<MockFnType<DistrT, Args...>*>(mock_fn))
+              .Call(std::move(args)...);
+        },
+        *static_cast<std::tuple<Args...>*>(dist_args));
+  }
+
+  // Looks for an appropriate mock - Returns the mocked result if one is found.
+  // Otherwise, returns a random value generated by the underlying URBG.
+  bool CallImpl(const std::type_info& key_type, void* dist_args,
+                void* result) override {
+    // Trigger a mock, if there exists one that matches `param`.
+    auto it = mocks_.find(std::type_index(key_type));
+    if (it == mocks_.end()) return false;
+    auto* mock_data = static_cast<MockData*>(&it->second);
+    mock_data->match_impl(mock_data->mock_fn, dist_args, result);
+    return true;
+  }
+
+  template <typename, typename>
+  friend struct ::absl::random_internal::MockSingleOverload;
+  friend struct ::absl::random_internal::DistributionCaller<
+      absl::MockingBitGen>;
+};
+
+// -----------------------------------------------------------------------------
+// Implementation Details Only Below
+// -----------------------------------------------------------------------------
+
+namespace random_internal {
+
+template <>
+struct DistributionCaller<absl::MockingBitGen> {
+  template <typename DistrT, typename FormatT, typename... Args>
+  static typename DistrT::result_type Call(absl::MockingBitGen* gen,
+                                           Args&&... args) {
+    return gen->template Call<DistrT, FormatT>(std::forward<Args>(args)...);
+  }
+};
+
+}  // namespace random_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_RANDOM_MOCKING_BIT_GEN_H_
diff --git a/absl/random/mocking_bit_gen_test.cc b/absl/random/mocking_bit_gen_test.cc
new file mode 100644
index 0000000..f0ffc9a
--- /dev/null
+++ b/absl/random/mocking_bit_gen_test.cc
@@ -0,0 +1,347 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+#include "absl/random/mocking_bit_gen.h"
+
+#include <numeric>
+#include <random>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest-spi.h"
+#include "gtest/gtest.h"
+#include "absl/random/bit_gen_ref.h"
+#include "absl/random/mock_distributions.h"
+#include "absl/random/random.h"
+
+namespace {
+using ::testing::Ne;
+using ::testing::Return;
+
+TEST(BasicMocking, AllDistributionsAreOverridable) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillOnce(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  EXPECT_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillOnce(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillOnce(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillOnce(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  EXPECT_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillOnce(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  EXPECT_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillOnce(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
+  EXPECT_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillOnce(Return(500000));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 500000);
+}
+
+TEST(BasicMocking, OnDistribution) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Uniform<int>(gen, 1, 1000000), 20);
+  ON_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .WillByDefault(Return(20));
+  EXPECT_EQ(absl::Uniform<int>(gen, 1, 1000000), 20);
+
+  EXPECT_NE(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+  ON_CALL(absl::MockUniform<double>(), Call(gen, 0.0, 100.0))
+      .WillByDefault(Return(5.0));
+  EXPECT_EQ(absl::Uniform<double>(gen, 0.0, 100.0), 5.0);
+
+  EXPECT_NE(absl::Exponential<double>(gen, 1.0), 42);
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(42));
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 42);
+
+  EXPECT_NE(absl::Poisson<int>(gen, 1.0), 500);
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillByDefault(Return(500));
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 500);
+
+  EXPECT_NE(absl::Bernoulli(gen, 0.000001), true);
+  ON_CALL(absl::MockBernoulli(), Call(gen, 0.000001))
+      .WillByDefault(Return(true));
+  EXPECT_EQ(absl::Bernoulli(gen, 0.000001), true);
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+
+  EXPECT_NE(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+  ON_CALL(absl::MockGaussian<double>(), Call(gen, 0.0, 1.0))
+      .WillByDefault(Return(0.001));
+  EXPECT_EQ(absl::Gaussian<double>(gen, 0.0, 1.0), 0.001);
+
+  EXPECT_NE(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+  ON_CALL(absl::MockLogUniform<int>(), Call(gen, 0, 1000000, 2))
+      .WillByDefault(Return(2040));
+  EXPECT_EQ(absl::LogUniform<int>(gen, 0, 1000000, 2), 2040);
+}
+
+TEST(BasicMocking, GMockMatchers) {
+  absl::MockingBitGen gen;
+
+  EXPECT_NE(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+  ON_CALL(absl::MockZipf<int>(), Call(gen, 1000000, 2.0, 1.0))
+      .WillByDefault(Return(1221));
+  EXPECT_EQ(absl::Zipf<int>(gen, 1000000, 2.0, 1.0), 1221);
+}
+
+TEST(BasicMocking, OverridesWithMultipleGMockExpectations) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 10000))
+      .WillOnce(Return(20))
+      .WillOnce(Return(40))
+      .WillOnce(Return(60));
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 20);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 40);
+  EXPECT_EQ(absl::Uniform(gen, 1, 10000), 60);
+}
+
+TEST(BasicMocking, DefaultArgument) {
+  absl::MockingBitGen gen;
+
+  ON_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+      .WillByDefault(Return(200));
+
+  EXPECT_EQ(absl::Exponential<double>(gen), 200);
+  EXPECT_EQ(absl::Exponential<double>(gen, 1.0), 200);
+}
+
+TEST(BasicMocking, MultipleGenerators) {
+  auto get_value = [](absl::BitGenRef gen_ref) {
+    return absl::Uniform(gen_ref, 1, 1000000);
+  };
+  absl::MockingBitGen unmocked_generator;
+  absl::MockingBitGen mocked_with_3;
+  absl::MockingBitGen mocked_with_11;
+
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_3, 1, 1000000))
+      .WillOnce(Return(3))
+      .WillRepeatedly(Return(17));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(mocked_with_11, 1, 1000000))
+      .WillOnce(Return(11))
+      .WillRepeatedly(Return(17));
+
+  // Ensure that unmocked generator generates neither value.
+  int unmocked_value = get_value(unmocked_generator);
+  EXPECT_NE(unmocked_value, 3);
+  EXPECT_NE(unmocked_value, 11);
+  // Mocked generators should generate their mocked values.
+  EXPECT_EQ(get_value(mocked_with_3), 3);
+  EXPECT_EQ(get_value(mocked_with_11), 11);
+  // Ensure that the mocks have expired.
+  EXPECT_NE(get_value(mocked_with_3), 3);
+  EXPECT_NE(get_value(mocked_with_11), 11);
+}
+
+TEST(BasicMocking, MocksNotTrigeredForIncorrectTypes) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen)).WillOnce(Return(42));
+
+  EXPECT_NE(absl::Uniform<uint16_t>(gen), 42);  // Not mocked
+  EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);  // Mock triggered
+}
+
+TEST(BasicMocking, FailsOnUnsatisfiedMocks) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockExponential<double>(), Call(gen, 1.0))
+            .WillOnce(Return(3.0));
+        // Does not call absl::Exponential().
+      }(),
+      "unsatisfied and active");
+}
+
+TEST(OnUniform, RespectsUniformIntervalSemantics) {
+  absl::MockingBitGen gen;
+
+  EXPECT_CALL(absl::MockUniform<int>(),
+              Call(absl::IntervalClosed, gen, 1, 1000000))
+      .WillOnce(Return(301));
+  EXPECT_NE(absl::Uniform(gen, 1, 1000000), 301);  // Not mocked
+  EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 1, 1000000), 301);
+}
+
+TEST(OnUniform, RespectsNoArgUnsignedShorthand) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<uint32_t>(), Call(gen)).WillOnce(Return(42));
+  EXPECT_EQ(absl::Uniform<uint32_t>(gen), 42);
+}
+
+TEST(RepeatedlyModifier, ForceSnakeEyesForManyDice) {
+  auto roll_some_dice = [](absl::BitGenRef gen_ref) {
+    std::vector<int> results(16);
+    for (auto& r : results) {
+      r = absl::Uniform(absl::IntervalClosed, gen_ref, 1, 6);
+    }
+    return results;
+  };
+  std::vector<int> results;
+  absl::MockingBitGen gen;
+
+  // Without any mocked calls, not all dice roll a "6".
+  results = roll_some_dice(gen);
+  EXPECT_LT(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+
+  // Verify that we can force all "6"-rolls, with mocking.
+  ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, 1, 6))
+      .WillByDefault(Return(6));
+  results = roll_some_dice(gen);
+  EXPECT_EQ(std::accumulate(std::begin(results), std::end(results), 0),
+            results.size() * 6);
+}
+
+TEST(WillOnce, DistinctCounters) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+      .Times(3)
+      .WillRepeatedly(Return(0));
+  EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1000001, 2000000))
+      .Times(3)
+      .WillRepeatedly(Return(1));
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+  EXPECT_EQ(absl::Uniform(gen, 1000001, 2000000), 1);
+  EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 0);
+}
+
+TEST(TimesModifier, ModifierSaturatesAndExpires) {
+  EXPECT_NONFATAL_FAILURE(
+      []() {
+        absl::MockingBitGen gen;
+        EXPECT_CALL(absl::MockUniform<int>(), Call(gen, 1, 1000000))
+            .Times(3)
+            .WillRepeatedly(Return(15))
+            .RetiresOnSaturation();
+
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 15);
+        // Times(3) has expired - Should get a different value now.
+
+        EXPECT_NE(absl::Uniform(gen, 1, 1000000), 15);
+      }(),
+      "");
+}
+
+TEST(TimesModifier, Times0) {
+  absl::MockingBitGen gen;
+  EXPECT_CALL(absl::MockBernoulli(), Call(gen, 0.0)).Times(0);
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).Times(0);
+}
+
+TEST(AnythingMatcher, MatchesAnyArgument) {
+  using testing::_;
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(absl::IntervalClosed, gen, _, 1000))
+        .WillByDefault(Return(11));
+    ON_CALL(absl::MockUniform<int>(),
+            Call(absl::IntervalClosed, gen, _, Ne(1000)))
+        .WillByDefault(Return(99));
+
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(absl::IntervalClosed, gen, 10, 1000), 11);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, 1, _))
+        .WillByDefault(Return(25));
+    ON_CALL(absl::MockUniform<int>(), Call(gen, Ne(1), _))
+        .WillByDefault(Return(99));
+    EXPECT_EQ(absl::Uniform(gen, 3, 1000000), 99);
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000000), 25);
+  }
+
+  {
+    absl::MockingBitGen gen;
+    ON_CALL(absl::MockUniform<int>(), Call(gen, _, _))
+        .WillByDefault(Return(145));
+    EXPECT_EQ(absl::Uniform(gen, 1, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 10, 1000), 145);
+    EXPECT_EQ(absl::Uniform(gen, 100, 1000), 145);
+  }
+}
+
+TEST(AnythingMatcher, WithWillByDefault) {
+  using testing::_;
+  absl::MockingBitGen gen;
+  std::vector<int> values = {11, 22, 33, 44, 55, 66, 77, 88, 99, 1010};
+
+  ON_CALL(absl::MockUniform<size_t>(), Call(gen, 0, _))
+      .WillByDefault(Return(0));
+  for (int i = 0; i < 100; i++) {
+    auto& elem = values[absl::Uniform(gen, 0u, values.size())];
+    EXPECT_EQ(elem, 11);
+  }
+}
+
+TEST(BasicMocking, WillByDefaultWithArgs) {
+  using testing::_;
+
+  absl::MockingBitGen gen;
+  ON_CALL(absl::MockPoisson<int>(), Call(gen, _))
+      .WillByDefault(
+          [](double lambda) { return static_cast<int>(lambda * 10); });
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.7), 17);
+  EXPECT_EQ(absl::Poisson<int>(gen, 0.03), 0);
+}
+
+TEST(MockingBitGen, InSequenceSucceedsInOrder) {
+  absl::MockingBitGen gen;
+
+  testing::InSequence seq;
+
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 1.0)).WillOnce(Return(3));
+  EXPECT_CALL(absl::MockPoisson<int>(), Call(gen, 2.0)).WillOnce(Return(4));
+
+  EXPECT_EQ(absl::Poisson<int>(gen, 1.0), 3);
+  EXPECT_EQ(absl::Poisson<int>(gen, 2.0), 4);
+}
+
+}  // namespace
diff --git a/absl/random/poisson_distribution.h b/absl/random/poisson_distribution.h
index 66c7509..cb5f5d5 100644
--- a/absl/random/poisson_distribution.h
+++ b/absl/random/poisson_distribution.h
@@ -22,13 +22,13 @@
 #include <ostream>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::poisson_distribution:
 // Generates discrete variates conforming to a Poisson distribution.
@@ -165,9 +165,9 @@
 poisson_distribution<IntType>::operator()(
     URBG& g,  // NOLINT(runtime/references)
     const param_type& p) {
-  using random_internal::PositiveValueT;
-  using random_internal::RandU64ToDouble;
-  using random_internal::SignedValueT;
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using random_internal::GenerateSignedTag;
 
   if (p.split_ != 0) {
     // Use Knuth's algorithm with range splitting to avoid floating-point
@@ -187,7 +187,8 @@
     for (int split = p.split_; split > 0; --split) {
       double r = 1.0;
       do {
-        r *= RandU64ToDouble<PositiveValueT, true>(fast_u64_(g));
+        r *= GenerateRealFromBits<double, GeneratePositiveTag, true>(
+            fast_u64_(g));  // U(-1, 0)
         ++n;
       } while (r > p.emu_);
       --n;
@@ -206,10 +207,11 @@
   // and k = max(f).
   const double a = p.mean_ + 0.5;
   for (;;) {
-    const double u =
-        RandU64ToDouble<PositiveValueT, false>(fast_u64_(g));  // (0, 1)
-    const double v =
-        RandU64ToDouble<SignedValueT, false>(fast_u64_(g));  // (-1, 1)
+    const double u = GenerateRealFromBits<double, GeneratePositiveTag, false>(
+        fast_u64_(g));  // U(0, 1)
+    const double v = GenerateRealFromBits<double, GenerateSignedTag, false>(
+        fast_u64_(g));  // U(-1, 1)
+
     const double x = std::floor(p.s_ * v / u + a);
     if (x < 0) continue;  // f(negative) = 0
     const double rhs = x * p.lmu_;
@@ -250,7 +252,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_POISSON_DISTRIBUTION_H_
diff --git a/absl/random/poisson_distribution_test.cc b/absl/random/poisson_distribution_test.cc
index 6d68999..9d215fb 100644
--- a/absl/random/poisson_distribution_test.cc
+++ b/absl/random/poisson_distribution_test.cc
@@ -339,7 +339,7 @@
   return absl::StrReplaceAll(name, {{"+", "_"}, {"-", "_"}, {".", "_"}});
 }
 
-INSTANTIATE_TEST_SUITE_P(, PoissonDistributionZTest,
+INSTANTIATE_TEST_SUITE_P(All, PoissonDistributionZTest,
                          ::testing::ValuesIn(GetZParams()), ZParamName);
 
 // The PoissonDistributionChiSquaredTest class provides a basic test framework
@@ -468,7 +468,7 @@
   EXPECT_LE(failures, 4);
 }
 
-INSTANTIATE_TEST_SUITE_P(, PoissonDistributionChiSquaredTest,
+INSTANTIATE_TEST_SUITE_P(All, PoissonDistributionChiSquaredTest,
                          ::testing::Values(0.5, 1.0, 2.0, 10.0, 50.0, 51.0,
                                            200.0));
 
diff --git a/absl/random/random.h b/absl/random/random.h
index 72a4cf5..c8f326e 100644
--- a/absl/random/random.h
+++ b/absl/random/random.h
@@ -41,7 +41,7 @@
 #include "absl/random/seed_sequences.h"  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // absl::BitGen
@@ -183,7 +183,7 @@
 // discards the intermediate results.
 // ---------------------------------------------------------------------------
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_RANDOM_H_
diff --git a/absl/random/seed_gen_exception.cc b/absl/random/seed_gen_exception.cc
index 5f01a30..fdcb54a 100644
--- a/absl/random/seed_gen_exception.cc
+++ b/absl/random/seed_gen_exception.cc
@@ -19,7 +19,7 @@
 #include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 static constexpr const char kExceptionMessage[] =
     "Failed generating seed-material for URBG.";
@@ -42,5 +42,5 @@
 }
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/seed_gen_exception.h b/absl/random/seed_gen_exception.h
index 52afe6c..5353900 100644
--- a/absl/random/seed_gen_exception.h
+++ b/absl/random/seed_gen_exception.h
@@ -28,8 +28,10 @@
 
 #include <exception>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // SeedGenException
@@ -47,7 +49,7 @@
 [[noreturn]] void ThrowSeedGenException();
 
 }  // namespace random_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_SEED_GEN_EXCEPTION_H_
diff --git a/absl/random/seed_sequences.cc b/absl/random/seed_sequences.cc
index fb7eb8d..426eafd 100644
--- a/absl/random/seed_sequences.cc
+++ b/absl/random/seed_sequences.cc
@@ -17,7 +17,7 @@
 #include "absl/random/internal/pool_urbg.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 SeedSeq MakeSeedSeq() {
   SeedSeq::result_type seed_material[8];
@@ -25,5 +25,5 @@
   return SeedSeq(std::begin(seed_material), std::end(seed_material));
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/random/seed_sequences.h b/absl/random/seed_sequences.h
index 73d075c..ff1340c 100644
--- a/absl/random/seed_sequences.h
+++ b/absl/random/seed_sequences.h
@@ -34,7 +34,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // absl::SeedSeq
@@ -104,7 +104,7 @@
 //
 SeedSeq MakeSeedSeq();
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_SEED_SEQUENCES_H_
diff --git a/absl/random/uniform_int_distribution.h b/absl/random/uniform_int_distribution.h
index 95eb04a..da66564 100644
--- a/absl/random/uniform_int_distribution.h
+++ b/absl/random/uniform_int_distribution.h
@@ -34,13 +34,13 @@
 #include <type_traits>
 
 #include "absl/base/optimization.h"
-#include "absl/random/internal/distribution_impl.h"
 #include "absl/random/internal/fast_uniform_bits.h"
 #include "absl/random/internal/iostream_state_saver.h"
 #include "absl/random/internal/traits.h"
+#include "absl/random/internal/wide_multiply.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::uniform_int_distribution<T>
 //
@@ -269,7 +269,7 @@
   return helper::hi(product);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_UNIFORM_INT_DISTRIBUTION_H_
diff --git a/absl/random/uniform_real_distribution.h b/absl/random/uniform_real_distribution.h
index 0ea3163..5ba17b2 100644
--- a/absl/random/uniform_real_distribution.h
+++ b/absl/random/uniform_real_distribution.h
@@ -39,12 +39,13 @@
 #include <limits>
 #include <type_traits>
 
-#include "absl/random/internal/distribution_impl.h"
+#include "absl/meta/type_traits.h"
 #include "absl/random/internal/fast_uniform_bits.h"
+#include "absl/random/internal/generate_real.h"
 #include "absl/random/internal/iostream_state_saver.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::uniform_real_distribution<T>
 //
@@ -57,7 +58,7 @@
 //
 //   // Use the distribution to produce a value between 0.0 (inclusive)
 //   // and 1.0 (exclusive).
-//   int value = absl::uniform_real_distribution<double>(0, 1)(gen);
+//   double value = absl::uniform_real_distribution<double>(0, 1)(gen);
 //
 template <typename RealType = double>
 class uniform_real_distribution {
@@ -77,6 +78,7 @@
       // is not possible, so value generation cannot use the full range of the
       // real type.
       assert(range_ <= (std::numeric_limits<result_type>::max)());
+      assert(std::isfinite(range_));
     }
 
     result_type a() const { return lo_; }
@@ -152,10 +154,15 @@
 typename uniform_real_distribution<RealType>::result_type
 uniform_real_distribution<RealType>::operator()(
     URBG& gen, const param_type& p) {  // NOLINT(runtime/references)
-  using random_internal::PositiveValueT;
+  using random_internal::GeneratePositiveTag;
+  using random_internal::GenerateRealFromBits;
+  using real_type =
+      absl::conditional_t<std::is_same<RealType, float>::value, float, double>;
+
   while (true) {
-    const result_type sample = random_internal::RandU64ToReal<
-        result_type>::template Value<PositiveValueT, true>(fast_u64_(gen));
+    const result_type sample =
+        GenerateRealFromBits<real_type, GeneratePositiveTag, true>(
+            fast_u64_(gen));
     const result_type res = p.a() + (sample * p.range_);
     if (res < p.b() || p.range_ <= 0 || !std::isfinite(p.range_)) {
       return res;
@@ -189,7 +196,7 @@
   }
   return is;
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_UNIFORM_REAL_DISTRIBUTION_H_
diff --git a/absl/random/uniform_real_distribution_test.cc b/absl/random/uniform_real_distribution_test.cc
index 597f0ee..a56374a 100644
--- a/absl/random/uniform_real_distribution_test.cc
+++ b/absl/random/uniform_real_distribution_test.cc
@@ -54,7 +54,12 @@
 template <typename RealType>
 class UniformRealDistributionTest : public ::testing::Test {};
 
+#if defined(__EMSCRIPTEN__)
+using RealTypes = ::testing::Types<float, double>;
+#else
 using RealTypes = ::testing::Types<float, double, long double>;
+#endif  // defined(__EMSCRIPTEN__)
+
 TYPED_TEST_SUITE(UniformRealDistributionTest, RealTypes);
 
 TYPED_TEST(UniformRealDistributionTest, ParamSerializeTest) {
@@ -156,6 +161,10 @@
   }
 }
 
+#ifdef _MSC_VER
+#pragma warning(push)
+#pragma warning(disable:4756)  // Constant arithmetic overflow.
+#endif
 TYPED_TEST(UniformRealDistributionTest, ViolatesPreconditionsDeathTest) {
 #if GTEST_HAS_DEATH_TEST
   // Hi < Lo
@@ -190,6 +199,9 @@
   }
 #endif  // NDEBUG
 }
+#ifdef _MSC_VER
+#pragma warning(pop)  // warning(disable:4756)
+#endif
 
 TYPED_TEST(UniformRealDistributionTest, TestMoments) {
   constexpr int kSize = 1000000;
diff --git a/absl/random/zipf_distribution.h b/absl/random/zipf_distribution.h
index bba98e8..22ebc75 100644
--- a/absl/random/zipf_distribution.h
+++ b/absl/random/zipf_distribution.h
@@ -26,7 +26,7 @@
 #include "absl/random/uniform_real_distribution.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // absl::zipf_distribution produces random integer-values in the range [0, k],
 // distributed according to the discrete probability function:
@@ -265,7 +265,7 @@
   return is;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_RANDOM_ZIPF_DISTRIBUTION_H_
diff --git a/absl/status/BUILD.bazel b/absl/status/BUILD.bazel
new file mode 100644
index 0000000..2b83077
--- /dev/null
+++ b/absl/status/BUILD.bazel
@@ -0,0 +1,65 @@
+#
+# Copyright 2017 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# This package contains `absl::Status`.
+# It will expand later to have utilities around `Status` like `StatusOr`,
+# `StatusBuilder` and macros.
+
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load(
+    "//absl:copts/configure_copts.bzl",
+    "ABSL_DEFAULT_COPTS",
+    "ABSL_TEST_COPTS",
+)
+
+package(default_visibility = ["//visibility:public"])
+
+licenses(["notice"])  # Apache 2.0
+
+cc_library(
+    name = "status",
+    srcs = [
+        "status.cc",
+        "status_payload_printer.cc",
+    ],
+    hdrs = [
+        "status.h",
+        "status_payload_printer.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        "//absl/base:config",
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:inlined_vector",
+        "//absl/debugging:stacktrace",
+        "//absl/debugging:symbolize",
+        "//absl/strings",
+        "//absl/strings:cord",
+        "//absl/strings:str_format",
+        "//absl/types:optional",
+    ],
+)
+
+cc_test(
+    name = "status_test",
+    srcs = ["status_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    deps = [
+        ":status",
+        "//absl/strings",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
diff --git a/absl/status/CMakeLists.txt b/absl/status/CMakeLists.txt
new file mode 100644
index 0000000..f05cee5
--- /dev/null
+++ b/absl/status/CMakeLists.txt
@@ -0,0 +1,52 @@
+#
+# Copyright 2020 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+absl_cc_library(
+  NAME
+    status
+  HDRS
+    "status.h"
+  SRCS
+    "status.cc"
+    "status_payload_printer.h"
+    "status_payload_printer.cc"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::config
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::inlined_vector
+    absl::stacktrace
+    absl::symbolize
+    absl::strings
+    absl::cord
+    absl::str_format
+    absl::optional
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    status_test
+  SRCS
+   "status_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::status
+    absl::strings
+    gmock_main
+)
diff --git a/absl/status/status.cc b/absl/status/status.cc
new file mode 100644
index 0000000..df3b740
--- /dev/null
+++ b/absl/status/status.cc
@@ -0,0 +1,447 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/status/status.h"
+
+#include <cassert>
+
+#include "absl/base/internal/raw_logging.h"
+#include "absl/debugging/stacktrace.h"
+#include "absl/debugging/symbolize.h"
+#include "absl/status/status_payload_printer.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_split.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// The implementation was intentionally kept same as util::error::Code_Name()
+// to ease the migration.
+std::string StatusCodeToString(StatusCode code) {
+  switch (code) {
+    case StatusCode::kOk:
+      return "OK";
+    case StatusCode::kCancelled:
+      return "CANCELLED";
+    case StatusCode::kUnknown:
+      return "UNKNOWN";
+    case StatusCode::kInvalidArgument:
+      return "INVALID_ARGUMENT";
+    case StatusCode::kDeadlineExceeded:
+      return "DEADLINE_EXCEEDED";
+    case StatusCode::kNotFound:
+      return "NOT_FOUND";
+    case StatusCode::kAlreadyExists:
+      return "ALREADY_EXISTS";
+    case StatusCode::kPermissionDenied:
+      return "PERMISSION_DENIED";
+    case StatusCode::kUnauthenticated:
+      return "UNAUTHENTICATED";
+    case StatusCode::kResourceExhausted:
+      return "RESOURCE_EXHAUSTED";
+    case StatusCode::kFailedPrecondition:
+      return "FAILED_PRECONDITION";
+    case StatusCode::kAborted:
+      return "ABORTED";
+    case StatusCode::kOutOfRange:
+      return "OUT_OF_RANGE";
+    case StatusCode::kUnimplemented:
+      return "UNIMPLEMENTED";
+    case StatusCode::kInternal:
+      return "INTERNAL";
+    case StatusCode::kUnavailable:
+      return "UNAVAILABLE";
+    case StatusCode::kDataLoss:
+      return "DATA_LOSS";
+    default:
+      return "";
+  }
+}
+
+std::ostream& operator<<(std::ostream& os, StatusCode code) {
+  return os << StatusCodeToString(code);
+}
+
+namespace status_internal {
+
+static int FindPayloadIndexByUrl(const Payloads* payloads,
+                                 absl::string_view type_url) {
+  if (payloads == nullptr) return -1;
+
+  for (int i = 0; i < payloads->size(); ++i) {
+    if ((*payloads)[i].type_url == type_url) return i;
+  }
+
+  return -1;
+}
+
+// Convert canonical code to a value known to this binary.
+absl::StatusCode MapToLocalCode(int value) {
+  absl::StatusCode code = static_cast<absl::StatusCode>(value);
+  switch (code) {
+    case absl::StatusCode::kOk:
+    case absl::StatusCode::kCancelled:
+    case absl::StatusCode::kUnknown:
+    case absl::StatusCode::kInvalidArgument:
+    case absl::StatusCode::kDeadlineExceeded:
+    case absl::StatusCode::kNotFound:
+    case absl::StatusCode::kAlreadyExists:
+    case absl::StatusCode::kPermissionDenied:
+    case absl::StatusCode::kResourceExhausted:
+    case absl::StatusCode::kFailedPrecondition:
+    case absl::StatusCode::kAborted:
+    case absl::StatusCode::kOutOfRange:
+    case absl::StatusCode::kUnimplemented:
+    case absl::StatusCode::kInternal:
+    case absl::StatusCode::kUnavailable:
+    case absl::StatusCode::kDataLoss:
+    case absl::StatusCode::kUnauthenticated:
+      return code;
+    default:
+      return absl::StatusCode::kUnknown;
+  }
+}
+}  // namespace status_internal
+
+absl::optional<absl::Cord> Status::GetPayload(
+    absl::string_view type_url) const {
+  const auto* payloads = GetPayloads();
+  int index = status_internal::FindPayloadIndexByUrl(payloads, type_url);
+  if (index != -1) return (*payloads)[index].payload;
+
+  return absl::nullopt;
+}
+
+void Status::SetPayload(absl::string_view type_url, absl::Cord payload) {
+  if (ok()) return;
+
+  PrepareToModify();
+
+  status_internal::StatusRep* rep = RepToPointer(rep_);
+  if (!rep->payloads) {
+    rep->payloads = absl::make_unique<status_internal::Payloads>();
+  }
+
+  int index =
+      status_internal::FindPayloadIndexByUrl(rep->payloads.get(), type_url);
+  if (index != -1) {
+    (*rep->payloads)[index].payload = std::move(payload);
+    return;
+  }
+
+  rep->payloads->push_back({std::string(type_url), std::move(payload)});
+}
+
+bool Status::ErasePayload(absl::string_view type_url) {
+  int index = status_internal::FindPayloadIndexByUrl(GetPayloads(), type_url);
+  if (index != -1) {
+    PrepareToModify();
+    GetPayloads()->erase(GetPayloads()->begin() + index);
+    if (GetPayloads()->empty() && message().empty()) {
+      // Special case: If this can be represented inlined, it MUST be
+      // inlined (EqualsSlow depends on this behavior).
+      StatusCode c = static_cast<StatusCode>(raw_code());
+      Unref(rep_);
+      rep_ = CodeToInlinedRep(c);
+    }
+    return true;
+  }
+
+  return false;
+}
+
+void Status::ForEachPayload(
+    const std::function<void(absl::string_view, const absl::Cord&)>& visitor)
+    const {
+  if (auto* payloads = GetPayloads()) {
+    bool in_reverse =
+        payloads->size() > 1 && reinterpret_cast<uintptr_t>(payloads) % 13 > 6;
+
+    for (int index = 0; index < payloads->size(); ++index) {
+      const auto& elem =
+          (*payloads)[in_reverse ? payloads->size() - 1 - index : index];
+
+#ifdef NDEBUG
+      visitor(elem.type_url, elem.payload);
+#else
+      // In debug mode invaldiate the type url to prevent users from relying on
+      // this std::string lifetime.
+
+      // NOLINTNEXTLINE intentional extra conversion to force temporary.
+      visitor(std::string(elem.type_url), elem.payload);
+#endif  // NDEBUG
+    }
+  }
+}
+
+const std::string* Status::EmptyString() {
+  static std::string* empty_string = new std::string();
+  return empty_string;
+}
+
+constexpr const char Status::kMovedFromString[];
+
+const std::string* Status::MovedFromString() {
+  static std::string* moved_from_string = new std::string(kMovedFromString);
+  return moved_from_string;
+}
+
+void Status::UnrefNonInlined(uintptr_t rep) {
+  status_internal::StatusRep* r = RepToPointer(rep);
+  // Fast path: if ref==1, there is no need for a RefCountDec (since
+  // this is the only reference and therefore no other thread is
+  // allowed to be mucking with r).
+  if (r->ref.load(std::memory_order_acquire) == 1 ||
+      r->ref.fetch_sub(1, std::memory_order_acq_rel) - 1 == 0) {
+    delete r;
+  }
+}
+
+uintptr_t Status::NewRep(absl::StatusCode code, absl::string_view msg,
+                         std::unique_ptr<status_internal::Payloads> payloads) {
+  status_internal::StatusRep* rep = new status_internal::StatusRep;
+  rep->ref.store(1, std::memory_order_relaxed);
+  rep->code = code;
+  rep->message.assign(msg.data(), msg.size());
+  rep->payloads = std::move(payloads);
+  return PointerToRep(rep);
+}
+
+Status::Status(absl::StatusCode code, absl::string_view msg)
+    : rep_(CodeToInlinedRep(code)) {
+  if (code != absl::StatusCode::kOk && !msg.empty()) {
+    rep_ = NewRep(code, msg, nullptr);
+  }
+}
+
+int Status::raw_code() const {
+  if (IsInlined(rep_)) {
+    return static_cast<int>(InlinedRepToCode(rep_));
+  }
+  status_internal::StatusRep* rep = RepToPointer(rep_);
+  return static_cast<int>(rep->code);
+}
+
+absl::StatusCode Status::code() const {
+  return status_internal::MapToLocalCode(raw_code());
+}
+
+void Status::PrepareToModify() {
+  ABSL_RAW_CHECK(!ok(), "PrepareToModify shouldn't be called on OK status.");
+  if (IsInlined(rep_)) {
+    rep_ = NewRep(static_cast<absl::StatusCode>(raw_code()),
+                  absl::string_view(), nullptr);
+    return;
+  }
+
+  uintptr_t rep_i = rep_;
+  status_internal::StatusRep* rep = RepToPointer(rep_);
+  if (rep->ref.load(std::memory_order_acquire) != 1) {
+    std::unique_ptr<status_internal::Payloads> payloads;
+    if (rep->payloads) {
+      payloads = absl::make_unique<status_internal::Payloads>(*rep->payloads);
+    }
+    rep_ = NewRep(rep->code, message(), std::move(payloads));
+    UnrefNonInlined(rep_i);
+  }
+}
+
+bool Status::EqualsSlow(const absl::Status& a, const absl::Status& b) {
+  if (IsInlined(a.rep_) != IsInlined(b.rep_)) return false;
+  if (a.message() != b.message()) return false;
+  if (a.raw_code() != b.raw_code()) return false;
+  if (a.GetPayloads() == b.GetPayloads()) return true;
+
+  const status_internal::Payloads no_payloads;
+  const status_internal::Payloads* larger_payloads =
+      a.GetPayloads() ? a.GetPayloads() : &no_payloads;
+  const status_internal::Payloads* smaller_payloads =
+      b.GetPayloads() ? b.GetPayloads() : &no_payloads;
+  if (larger_payloads->size() < smaller_payloads->size()) {
+    std::swap(larger_payloads, smaller_payloads);
+  }
+  if ((larger_payloads->size() - smaller_payloads->size()) > 1) return false;
+  // Payloads can be ordered differently, so we can't just compare payload
+  // vectors.
+  for (const auto& payload : *larger_payloads) {
+
+    bool found = false;
+    for (const auto& other_payload : *smaller_payloads) {
+      if (payload.type_url == other_payload.type_url) {
+        if (payload.payload != other_payload.payload) {
+          return false;
+        }
+        found = true;
+        break;
+      }
+    }
+    if (!found) return false;
+  }
+  return true;
+}
+
+std::string Status::ToStringSlow() const {
+  std::string text;
+  absl::StrAppend(&text, absl::StatusCodeToString(code()), ": ", message());
+  status_internal::StatusPayloadPrinter printer =
+      status_internal::GetStatusPayloadPrinter();
+  this->ForEachPayload([&](absl::string_view type_url,
+                           const absl::Cord& payload) {
+    absl::optional<std::string> result;
+    if (printer) result = printer(type_url, payload);
+    absl::StrAppend(
+        &text, " [", type_url, "='",
+        result.has_value() ? *result : absl::CHexEscape(std::string(payload)),
+        "']");
+  });
+
+  return text;
+}
+
+std::ostream& operator<<(std::ostream& os, const Status& x) {
+  os << x.ToString();
+  return os;
+}
+
+Status AbortedError(absl::string_view message) {
+  return Status(absl::StatusCode::kAborted, message);
+}
+
+Status AlreadyExistsError(absl::string_view message) {
+  return Status(absl::StatusCode::kAlreadyExists, message);
+}
+
+Status CancelledError(absl::string_view message) {
+  return Status(absl::StatusCode::kCancelled, message);
+}
+
+Status DataLossError(absl::string_view message) {
+  return Status(absl::StatusCode::kDataLoss, message);
+}
+
+Status DeadlineExceededError(absl::string_view message) {
+  return Status(absl::StatusCode::kDeadlineExceeded, message);
+}
+
+Status FailedPreconditionError(absl::string_view message) {
+  return Status(absl::StatusCode::kFailedPrecondition, message);
+}
+
+Status InternalError(absl::string_view message) {
+  return Status(absl::StatusCode::kInternal, message);
+}
+
+Status InvalidArgumentError(absl::string_view message) {
+  return Status(absl::StatusCode::kInvalidArgument, message);
+}
+
+Status NotFoundError(absl::string_view message) {
+  return Status(absl::StatusCode::kNotFound, message);
+}
+
+Status OutOfRangeError(absl::string_view message) {
+  return Status(absl::StatusCode::kOutOfRange, message);
+}
+
+Status PermissionDeniedError(absl::string_view message) {
+  return Status(absl::StatusCode::kPermissionDenied, message);
+}
+
+Status ResourceExhaustedError(absl::string_view message) {
+  return Status(absl::StatusCode::kResourceExhausted, message);
+}
+
+Status UnauthenticatedError(absl::string_view message) {
+  return Status(absl::StatusCode::kUnauthenticated, message);
+}
+
+Status UnavailableError(absl::string_view message) {
+  return Status(absl::StatusCode::kUnavailable, message);
+}
+
+Status UnimplementedError(absl::string_view message) {
+  return Status(absl::StatusCode::kUnimplemented, message);
+}
+
+Status UnknownError(absl::string_view message) {
+  return Status(absl::StatusCode::kUnknown, message);
+}
+
+bool IsAborted(const Status& status) {
+  return status.code() == absl::StatusCode::kAborted;
+}
+
+bool IsAlreadyExists(const Status& status) {
+  return status.code() == absl::StatusCode::kAlreadyExists;
+}
+
+bool IsCancelled(const Status& status) {
+  return status.code() == absl::StatusCode::kCancelled;
+}
+
+bool IsDataLoss(const Status& status) {
+  return status.code() == absl::StatusCode::kDataLoss;
+}
+
+bool IsDeadlineExceeded(const Status& status) {
+  return status.code() == absl::StatusCode::kDeadlineExceeded;
+}
+
+bool IsFailedPrecondition(const Status& status) {
+  return status.code() == absl::StatusCode::kFailedPrecondition;
+}
+
+bool IsInternal(const Status& status) {
+  return status.code() == absl::StatusCode::kInternal;
+}
+
+bool IsInvalidArgument(const Status& status) {
+  return status.code() == absl::StatusCode::kInvalidArgument;
+}
+
+bool IsNotFound(const Status& status) {
+  return status.code() == absl::StatusCode::kNotFound;
+}
+
+bool IsOutOfRange(const Status& status) {
+  return status.code() == absl::StatusCode::kOutOfRange;
+}
+
+bool IsPermissionDenied(const Status& status) {
+  return status.code() == absl::StatusCode::kPermissionDenied;
+}
+
+bool IsResourceExhausted(const Status& status) {
+  return status.code() == absl::StatusCode::kResourceExhausted;
+}
+
+bool IsUnauthenticated(const Status& status) {
+  return status.code() == absl::StatusCode::kUnauthenticated;
+}
+
+bool IsUnavailable(const Status& status) {
+  return status.code() == absl::StatusCode::kUnavailable;
+}
+
+bool IsUnimplemented(const Status& status) {
+  return status.code() == absl::StatusCode::kUnimplemented;
+}
+
+bool IsUnknown(const Status& status) {
+  return status.code() == absl::StatusCode::kUnknown;
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status.h b/absl/status/status.h
new file mode 100644
index 0000000..9706d4b
--- /dev/null
+++ b/absl/status/status.h
@@ -0,0 +1,428 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_STATUS_STATUS_H_
+#define ABSL_STATUS_STATUS_H_
+
+#include <iostream>
+#include <string>
+
+#include "absl/container/inlined_vector.h"
+#include "absl/strings/cord.h"
+#include "absl/types/optional.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+enum class StatusCode : int {
+  kOk = 0,
+  kCancelled = 1,
+  kUnknown = 2,
+  kInvalidArgument = 3,
+  kDeadlineExceeded = 4,
+  kNotFound = 5,
+  kAlreadyExists = 6,
+  kPermissionDenied = 7,
+  kResourceExhausted = 8,
+  kFailedPrecondition = 9,
+  kAborted = 10,
+  kOutOfRange = 11,
+  kUnimplemented = 12,
+  kInternal = 13,
+  kUnavailable = 14,
+  kDataLoss = 15,
+  kUnauthenticated = 16,
+  kDoNotUseReservedForFutureExpansionUseDefaultInSwitchInstead_ = 20
+};
+
+// Returns the name for the status code, or "" if it is an unknown value.
+std::string StatusCodeToString(StatusCode code);
+
+// Streams StatusCodeToString(code) to `os`.
+std::ostream& operator<<(std::ostream& os, StatusCode code);
+
+namespace status_internal {
+
+// Container for status payloads.
+struct Payload {
+  std::string type_url;
+  absl::Cord payload;
+};
+
+using Payloads = absl::InlinedVector<Payload, 1>;
+
+// Reference-counted representation of Status data.
+struct StatusRep {
+  std::atomic<int32_t> ref;
+  absl::StatusCode code;
+  std::string message;
+  std::unique_ptr<status_internal::Payloads> payloads;
+};
+
+absl::StatusCode MapToLocalCode(int value);
+}  // namespace status_internal
+
+class ABSL_MUST_USE_RESULT Status final {
+ public:
+  // Creates an OK status with no message or payload.
+  Status();
+
+  // Create a status in the canonical error space with the specified code and
+  // error message.  If `code == util::error::OK`, `msg` is ignored and an
+  // object identical to an OK status is constructed.
+  //
+  // `msg` must be in UTF-8. The implementation may complain (e.g.,
+  // by printing a warning) if it is not.
+  Status(absl::StatusCode code, absl::string_view msg);
+
+  Status(const Status&);
+  Status& operator=(const Status& x);
+
+  // Move operations.
+  // The moved-from state is valid but unspecified.
+  Status(Status&&) noexcept;
+  Status& operator=(Status&&);
+
+  ~Status();
+
+  // If `this->ok()`, stores `new_status` into *this. If `!this->ok()`,
+  // preserves the current data. May, in the future, augment the current status
+  // with additional information about `new_status`.
+  //
+  // Convenient way of keeping track of the first error encountered.
+  // Instead of:
+  //   if (overall_status.ok()) overall_status = new_status
+  // Use:
+  //   overall_status.Update(new_status);
+  //
+  // Style guide exception for rvalue reference granted in CL 153567220.
+  void Update(const Status& new_status);
+  void Update(Status&& new_status);
+
+  // Returns true if the Status is OK.
+  ABSL_MUST_USE_RESULT bool ok() const;
+
+  // Returns the (canonical) error code.
+  absl::StatusCode code() const;
+
+  // Returns the raw (canonical) error code which could be out of the range of
+  // the local `absl::StatusCode` enum. NOTE: This should only be called when
+  // converting to wire format. Use `code` for error handling.
+  int raw_code() const;
+
+  // Returns the error message.  Note: prefer ToString() for debug logging.
+  // This message rarely describes the error code.  It is not unusual for the
+  // error message to be the empty std::string.
+  absl::string_view message() const;
+
+  friend bool operator==(const Status&, const Status&);
+  friend bool operator!=(const Status&, const Status&);
+
+  // Returns a combination of the error code name, the message and the payloads.
+  // You can expect the code name and the message to be substrings of the
+  // result, and the payloads to be printed by the registered printer extensions
+  // if they are recognized.
+  // WARNING: Do not depend on the exact format of the result of `ToString()`
+  // which is subject to change.
+  std::string ToString() const;
+
+  // Ignores any errors. This method does nothing except potentially suppress
+  // complaints from any tools that are checking that errors are not dropped on
+  // the floor.
+  void IgnoreError() const;
+
+  // Swap the contents of `a` with `b`
+  friend void swap(Status& a, Status& b);
+
+  // Payload management APIs
+
+  // Type URL should be unique and follow the naming convention below:
+  // The idea of type URL comes from `google.protobuf.Any`
+  // (https://developers.google.com/protocol-buffers/docs/proto3#any). The
+  // type URL should be globally unique and follow the format of URL
+  // (https://en.wikipedia.org/wiki/URL). The default type URL for a given
+  // protobuf message type is "type.googleapis.com/packagename.messagename". For
+  // other custom wire formats, users should define the format of type URL in a
+  // similar practice so as to minimize the chance of conflict between type
+  // URLs. Users should make sure that the type URL can be mapped to a concrete
+  // C++ type if they want to deserialize the payload and read it effectively.
+
+  // Gets the payload based for `type_url` key, if it is present.
+  absl::optional<absl::Cord> GetPayload(absl::string_view type_url) const;
+
+  // Sets the payload for `type_url` key for a non-ok status, overwriting any
+  // existing payload for `type_url`.
+  //
+  // NOTE: Does nothing if the Status is ok.
+  void SetPayload(absl::string_view type_url, absl::Cord payload);
+
+  // Erases the payload corresponding to the `type_url` key.  Returns true if
+  // the payload was present.
+  bool ErasePayload(absl::string_view type_url);
+
+  // Iterates over the stored payloads and calls `visitor(type_key, payload)`
+  // for each one.
+  //
+  // NOTE: The order of calls to `visitor` is not specified and may change at
+  // any time.
+  //
+  // NOTE: Any mutation on the same 'Status' object during visitation is
+  // forbidden and could result in undefined behavior.
+  void ForEachPayload(
+      const std::function<void(absl::string_view, const absl::Cord&)>& visitor)
+      const;
+
+ private:
+  friend Status CancelledError();
+
+  // Creates a status in the canonical error space with the specified
+  // code, and an empty error message.
+  explicit Status(absl::StatusCode code);
+
+  static void UnrefNonInlined(uintptr_t rep);
+  static void Ref(uintptr_t rep);
+  static void Unref(uintptr_t rep);
+
+  // REQUIRES: !ok()
+  // Ensures rep_ is not shared with any other Status.
+  void PrepareToModify();
+
+  const status_internal::Payloads* GetPayloads() const;
+  status_internal::Payloads* GetPayloads();
+
+  // Takes ownership of payload.
+  static uintptr_t NewRep(absl::StatusCode code, absl::string_view msg,
+                          std::unique_ptr<status_internal::Payloads> payload);
+  static bool EqualsSlow(const absl::Status& a, const absl::Status& b);
+
+  // MSVC 14.0 limitation requires the const.
+  static constexpr const char kMovedFromString[] =
+      "Status accessed after move.";
+
+  static const std::string* EmptyString();
+  static const std::string* MovedFromString();
+
+  // Returns whether rep contains an inlined representation.
+  // See rep_ for details.
+  static bool IsInlined(uintptr_t rep);
+
+  // Indicates whether this Status was the rhs of a move operation. See rep_
+  // for details.
+  static bool IsMovedFrom(uintptr_t rep);
+  static uintptr_t MovedFromRep();
+
+  // Convert between error::Code and the inlined uintptr_t representation used
+  // by rep_. See rep_ for details.
+  static uintptr_t CodeToInlinedRep(absl::StatusCode code);
+  static absl::StatusCode InlinedRepToCode(uintptr_t rep);
+
+  // Converts between StatusRep* and the external uintptr_t representation used
+  // by rep_. See rep_ for details.
+  static uintptr_t PointerToRep(status_internal::StatusRep* r);
+  static status_internal::StatusRep* RepToPointer(uintptr_t r);
+
+  // Returns std::string for non-ok Status.
+  std::string ToStringSlow() const;
+
+  // Status supports two different representations.
+  //  - When the low bit is off it is an inlined representation.
+  //    It uses the canonical error space, no message or payload.
+  //    The error code is (rep_ >> 2).
+  //    The (rep_ & 2) bit is the "moved from" indicator, used in IsMovedFrom().
+  //  - When the low bit is on it is an external representation.
+  //    In this case all the data comes from a heap allocated Rep object.
+  //    (rep_ - 1) is a status_internal::StatusRep* pointer to that structure.
+  uintptr_t rep_;
+};
+
+// Returns an OK status, equivalent to a default constructed instance.
+Status OkStatus();
+
+// Prints a human-readable representation of `x` to `os`.
+std::ostream& operator<<(std::ostream& os, const Status& x);
+
+// -----------------------------------------------------------------
+// Implementation details follow
+
+inline Status::Status() : rep_(CodeToInlinedRep(absl::StatusCode::kOk)) {}
+
+inline Status::Status(absl::StatusCode code) : rep_(CodeToInlinedRep(code)) {}
+
+inline Status::Status(const Status& x) : rep_(x.rep_) { Ref(rep_); }
+
+inline Status& Status::operator=(const Status& x) {
+  uintptr_t old_rep = rep_;
+  if (x.rep_ != old_rep) {
+    Ref(x.rep_);
+    rep_ = x.rep_;
+    Unref(old_rep);
+  }
+  return *this;
+}
+
+inline Status::Status(Status&& x) noexcept : rep_(x.rep_) {
+  x.rep_ = MovedFromRep();
+}
+
+inline Status& Status::operator=(Status&& x) {
+  uintptr_t old_rep = rep_;
+  rep_ = x.rep_;
+  x.rep_ = MovedFromRep();
+  Unref(old_rep);
+  return *this;
+}
+
+inline void Status::Update(const Status& new_status) {
+  if (ok()) {
+    *this = new_status;
+  }
+}
+
+inline void Status::Update(Status&& new_status) {
+  if (ok()) {
+    *this = std::move(new_status);
+  }
+}
+
+inline Status::~Status() { Unref(rep_); }
+
+inline bool Status::ok() const {
+  return rep_ == CodeToInlinedRep(absl::StatusCode::kOk);
+}
+
+inline absl::string_view Status::message() const {
+  return !IsInlined(rep_)
+             ? RepToPointer(rep_)->message
+             : (IsMovedFrom(rep_) ? absl::string_view(kMovedFromString)
+                                  : absl::string_view());
+}
+
+inline bool operator==(const Status& lhs, const Status& rhs) {
+  return lhs.rep_ == rhs.rep_ || Status::EqualsSlow(lhs, rhs);
+}
+
+inline bool operator!=(const Status& lhs, const Status& rhs) {
+  return !(lhs == rhs);
+}
+
+inline std::string Status::ToString() const {
+  return ok() ? "OK" : ToStringSlow();
+}
+
+inline void Status::IgnoreError() const {
+  // no-op
+}
+
+inline void swap(absl::Status& a, absl::Status& b) {
+  using std::swap;
+  swap(a.rep_, b.rep_);
+}
+
+inline const status_internal::Payloads* Status::GetPayloads() const {
+  return IsInlined(rep_) ? nullptr : RepToPointer(rep_)->payloads.get();
+}
+
+inline status_internal::Payloads* Status::GetPayloads() {
+  return IsInlined(rep_) ? nullptr : RepToPointer(rep_)->payloads.get();
+}
+
+inline bool Status::IsInlined(uintptr_t rep) { return (rep & 1) == 0; }
+
+inline bool Status::IsMovedFrom(uintptr_t rep) {
+  return IsInlined(rep) && (rep & 2) != 0;
+}
+
+inline uintptr_t Status::MovedFromRep() {
+  return CodeToInlinedRep(absl::StatusCode::kInternal) | 2;
+}
+
+inline uintptr_t Status::CodeToInlinedRep(absl::StatusCode code) {
+  return static_cast<uintptr_t>(code) << 2;
+}
+
+inline absl::StatusCode Status::InlinedRepToCode(uintptr_t rep) {
+  assert(IsInlined(rep));
+  return static_cast<absl::StatusCode>(rep >> 2);
+}
+
+inline status_internal::StatusRep* Status::RepToPointer(uintptr_t rep) {
+  assert(!IsInlined(rep));
+  return reinterpret_cast<status_internal::StatusRep*>(rep - 1);
+}
+
+inline uintptr_t Status::PointerToRep(status_internal::StatusRep* rep) {
+  return reinterpret_cast<uintptr_t>(rep) + 1;
+}
+
+inline void Status::Ref(uintptr_t rep) {
+  if (!IsInlined(rep)) {
+    RepToPointer(rep)->ref.fetch_add(1, std::memory_order_relaxed);
+  }
+}
+
+inline void Status::Unref(uintptr_t rep) {
+  if (!IsInlined(rep)) {
+    UnrefNonInlined(rep);
+  }
+}
+
+inline Status OkStatus() { return Status(); }
+
+// Each of the functions below creates a Status object with a particular error
+// code and the given message. The error code of the returned status object
+// matches the name of the function.
+Status AbortedError(absl::string_view message);
+Status AlreadyExistsError(absl::string_view message);
+Status CancelledError(absl::string_view message);
+Status DataLossError(absl::string_view message);
+Status DeadlineExceededError(absl::string_view message);
+Status FailedPreconditionError(absl::string_view message);
+Status InternalError(absl::string_view message);
+Status InvalidArgumentError(absl::string_view message);
+Status NotFoundError(absl::string_view message);
+Status OutOfRangeError(absl::string_view message);
+Status PermissionDeniedError(absl::string_view message);
+Status ResourceExhaustedError(absl::string_view message);
+Status UnauthenticatedError(absl::string_view message);
+Status UnavailableError(absl::string_view message);
+Status UnimplementedError(absl::string_view message);
+Status UnknownError(absl::string_view message);
+
+// Creates a `Status` object with the `absl::StatusCode::kCancelled` error code
+// and an empty message. It is provided only for efficiency, given that
+// message-less kCancelled errors are common in the infrastructure.
+inline Status CancelledError() { return Status(absl::StatusCode::kCancelled); }
+
+// Each of the functions below returns true if the given status matches the
+// error code implied by the function's name.
+ABSL_MUST_USE_RESULT bool IsAborted(const Status& status);
+ABSL_MUST_USE_RESULT bool IsAlreadyExists(const Status& status);
+ABSL_MUST_USE_RESULT bool IsCancelled(const Status& status);
+ABSL_MUST_USE_RESULT bool IsDataLoss(const Status& status);
+ABSL_MUST_USE_RESULT bool IsDeadlineExceeded(const Status& status);
+ABSL_MUST_USE_RESULT bool IsFailedPrecondition(const Status& status);
+ABSL_MUST_USE_RESULT bool IsInternal(const Status& status);
+ABSL_MUST_USE_RESULT bool IsInvalidArgument(const Status& status);
+ABSL_MUST_USE_RESULT bool IsNotFound(const Status& status);
+ABSL_MUST_USE_RESULT bool IsOutOfRange(const Status& status);
+ABSL_MUST_USE_RESULT bool IsPermissionDenied(const Status& status);
+ABSL_MUST_USE_RESULT bool IsResourceExhausted(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnauthenticated(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnavailable(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnimplemented(const Status& status);
+ABSL_MUST_USE_RESULT bool IsUnknown(const Status& status);
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_H_
diff --git a/absl/status/status_payload_printer.cc b/absl/status/status_payload_printer.cc
new file mode 100644
index 0000000..ad96d76
--- /dev/null
+++ b/absl/status/status_payload_printer.cc
@@ -0,0 +1,43 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#include "absl/status/status_payload_printer.h"
+
+#include <atomic>
+
+#include "absl/base/attributes.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+namespace {
+// Tried constant initialized global variable but it doesn't work with Lexan
+// (MSVC's `std::atomic` has trouble constant initializing).
+std::atomic<StatusPayloadPrinter>& GetStatusPayloadPrinterStorage() {
+  ABSL_CONST_INIT static std::atomic<StatusPayloadPrinter> instance{nullptr};
+  return instance;
+}
+}  // namespace
+
+void SetStatusPayloadPrinter(StatusPayloadPrinter printer) {
+  GetStatusPayloadPrinterStorage().store(printer, std::memory_order_relaxed);
+}
+
+StatusPayloadPrinter GetStatusPayloadPrinter() {
+  return GetStatusPayloadPrinterStorage().load(std::memory_order_relaxed);
+}
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/status/status_payload_printer.h b/absl/status/status_payload_printer.h
new file mode 100644
index 0000000..5e0937f
--- /dev/null
+++ b/absl/status/status_payload_printer.h
@@ -0,0 +1,51 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+#ifndef ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
+#define ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
+
+#include <string>
+
+#include "absl/strings/cord.h"
+#include "absl/strings/string_view.h"
+#include "absl/types/optional.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace status_internal {
+
+// By default, `Status::ToString` and `operator<<(Status)` print a payload by
+// dumping the type URL and the raw bytes. To help debugging, we provide an
+// extension point, which is a global printer function that can be set by users
+// to specify how to print payloads. The function takes the type URL and the
+// payload as input, and should return a valid human-readable string on success
+// or `absl::nullopt` on failure (in which case it falls back to the default
+// approach of printing the raw bytes).
+// NOTE: This is an internal API and the design is subject to change in the
+// future in a non-backward-compatible way. Since it's only meant for debugging
+// purpose, you should not rely on it in any critical logic.
+using StatusPayloadPrinter = absl::optional<std::string> (*)(absl::string_view,
+                                                             const absl::Cord&);
+
+// Sets the global payload printer. Only one printer should be set per process.
+// If multiple printers are set, it's undefined which one will be used.
+void SetStatusPayloadPrinter(StatusPayloadPrinter);
+
+// Returns the global payload printer if previously set, otherwise `nullptr`.
+StatusPayloadPrinter GetStatusPayloadPrinter();
+
+}  // namespace status_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STATUS_STATUS_PAYLOAD_PRINTER_H_
diff --git a/absl/status/status_test.cc b/absl/status/status_test.cc
new file mode 100644
index 0000000..ca9488a
--- /dev/null
+++ b/absl/status/status_test.cc
@@ -0,0 +1,458 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/status/status.h"
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/strings/str_cat.h"
+
+namespace {
+
+using ::testing::Eq;
+using ::testing::HasSubstr;
+using ::testing::Optional;
+using ::testing::UnorderedElementsAreArray;
+
+TEST(StatusCode, InsertionOperator) {
+  const absl::StatusCode code = absl::StatusCode::kUnknown;
+  std::ostringstream oss;
+  oss << code;
+  EXPECT_EQ(oss.str(), absl::StatusCodeToString(code));
+}
+
+// This structure holds the details for testing a single error code,
+// its creator, and its classifier.
+struct ErrorTest {
+  absl::StatusCode code;
+  using Creator = absl::Status (*)(absl::string_view);
+  using Classifier = bool (*)(const absl::Status&);
+  Creator creator;
+  Classifier classifier;
+};
+
+constexpr ErrorTest kErrorTests[]{
+    {absl::StatusCode::kCancelled, absl::CancelledError, absl::IsCancelled},
+    {absl::StatusCode::kUnknown, absl::UnknownError, absl::IsUnknown},
+    {absl::StatusCode::kInvalidArgument, absl::InvalidArgumentError,
+     absl::IsInvalidArgument},
+    {absl::StatusCode::kDeadlineExceeded, absl::DeadlineExceededError,
+     absl::IsDeadlineExceeded},
+    {absl::StatusCode::kNotFound, absl::NotFoundError, absl::IsNotFound},
+    {absl::StatusCode::kAlreadyExists, absl::AlreadyExistsError,
+     absl::IsAlreadyExists},
+    {absl::StatusCode::kPermissionDenied, absl::PermissionDeniedError,
+     absl::IsPermissionDenied},
+    {absl::StatusCode::kResourceExhausted, absl::ResourceExhaustedError,
+     absl::IsResourceExhausted},
+    {absl::StatusCode::kFailedPrecondition, absl::FailedPreconditionError,
+     absl::IsFailedPrecondition},
+    {absl::StatusCode::kAborted, absl::AbortedError, absl::IsAborted},
+    {absl::StatusCode::kOutOfRange, absl::OutOfRangeError, absl::IsOutOfRange},
+    {absl::StatusCode::kUnimplemented, absl::UnimplementedError,
+     absl::IsUnimplemented},
+    {absl::StatusCode::kInternal, absl::InternalError, absl::IsInternal},
+    {absl::StatusCode::kUnavailable, absl::UnavailableError,
+     absl::IsUnavailable},
+    {absl::StatusCode::kDataLoss, absl::DataLossError, absl::IsDataLoss},
+    {absl::StatusCode::kUnauthenticated, absl::UnauthenticatedError,
+     absl::IsUnauthenticated},
+};
+
+TEST(Status, CreateAndClassify) {
+  for (const auto& test : kErrorTests) {
+    SCOPED_TRACE(absl::StatusCodeToString(test.code));
+
+    // Ensure that the creator does, in fact, create status objects with the
+    // expected error code and message.
+    std::string message =
+        absl::StrCat("error code ", test.code, " test message");
+    absl::Status status = test.creator(message);
+    EXPECT_EQ(test.code, status.code());
+    EXPECT_EQ(message, status.message());
+
+    // Ensure that the classifier returns true for a status produced by the
+    // creator.
+    EXPECT_TRUE(test.classifier(status));
+
+    // Ensure that the classifier returns false for status with a different
+    // code.
+    for (const auto& other : kErrorTests) {
+      if (other.code != test.code) {
+        EXPECT_FALSE(test.classifier(absl::Status(other.code, "")))
+            << " other.code = " << other.code;
+      }
+    }
+  }
+}
+
+TEST(Status, DefaultConstructor) {
+  absl::Status status;
+  EXPECT_TRUE(status.ok());
+  EXPECT_EQ(absl::StatusCode::kOk, status.code());
+  EXPECT_EQ("", status.message());
+}
+
+TEST(Status, OkStatus) {
+  absl::Status status = absl::OkStatus();
+  EXPECT_TRUE(status.ok());
+  EXPECT_EQ(absl::StatusCode::kOk, status.code());
+  EXPECT_EQ("", status.message());
+}
+
+TEST(Status, ConstructorWithCodeMessage) {
+  {
+    absl::Status status(absl::StatusCode::kCancelled, "");
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kCancelled, status.code());
+    EXPECT_EQ("", status.message());
+  }
+  {
+    absl::Status status(absl::StatusCode::kInternal, "message");
+    EXPECT_FALSE(status.ok());
+    EXPECT_EQ(absl::StatusCode::kInternal, status.code());
+    EXPECT_EQ("message", status.message());
+  }
+}
+
+TEST(Status, ConstructOutOfRangeCode) {
+  const int kRawCode = 9999;
+  absl::Status status(static_cast<absl::StatusCode>(kRawCode), "");
+  EXPECT_EQ(absl::StatusCode::kUnknown, status.code());
+  EXPECT_EQ(kRawCode, status.raw_code());
+}
+
+constexpr char kUrl1[] = "url.payload.1";
+constexpr char kUrl2[] = "url.payload.2";
+constexpr char kUrl3[] = "url.payload.3";
+constexpr char kUrl4[] = "url.payload.xx";
+
+constexpr char kPayload1[] = "aaaaa";
+constexpr char kPayload2[] = "bbbbb";
+constexpr char kPayload3[] = "ccccc";
+
+using PayloadsVec = std::vector<std::pair<std::string, absl::Cord>>;
+
+TEST(Status, TestGetSetPayload) {
+  absl::Status ok_status = absl::OkStatus();
+  ok_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  ok_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  EXPECT_FALSE(ok_status.GetPayload(kUrl1));
+  EXPECT_FALSE(ok_status.GetPayload(kUrl2));
+
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  EXPECT_THAT(bad_status.GetPayload(kUrl1), Optional(Eq(kPayload1)));
+  EXPECT_THAT(bad_status.GetPayload(kUrl2), Optional(Eq(kPayload2)));
+
+  EXPECT_FALSE(bad_status.GetPayload(kUrl3));
+
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload3));
+  EXPECT_THAT(bad_status.GetPayload(kUrl1), Optional(Eq(kPayload3)));
+
+  // Testing dynamically generated type_url
+  bad_status.SetPayload(absl::StrCat(kUrl1, ".1"), absl::Cord(kPayload1));
+  EXPECT_THAT(bad_status.GetPayload(absl::StrCat(kUrl1, ".1")),
+              Optional(Eq(kPayload1)));
+}
+
+TEST(Status, TestErasePayload) {
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  EXPECT_FALSE(bad_status.ErasePayload(kUrl4));
+
+  EXPECT_TRUE(bad_status.GetPayload(kUrl2));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl2));
+  EXPECT_FALSE(bad_status.GetPayload(kUrl2));
+  EXPECT_FALSE(bad_status.ErasePayload(kUrl2));
+
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl1));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl3));
+
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  EXPECT_TRUE(bad_status.ErasePayload(kUrl1));
+}
+
+TEST(Status, TestComparePayloads) {
+  absl::Status bad_status1(absl::StatusCode::kInternal, "fail");
+  bad_status1.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status1.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status1.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  absl::Status bad_status2(absl::StatusCode::kInternal, "fail");
+  bad_status2.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status2.SetPayload(kUrl3, absl::Cord(kPayload3));
+  bad_status2.SetPayload(kUrl1, absl::Cord(kPayload1));
+
+  EXPECT_EQ(bad_status1, bad_status2);
+}
+
+TEST(Status, TestComparePayloadsAfterErase) {
+  absl::Status payload_status(absl::StatusCode::kInternal, "");
+  payload_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  payload_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+
+  absl::Status empty_status(absl::StatusCode::kInternal, "");
+
+  // Different payloads, not equal
+  EXPECT_NE(payload_status, empty_status);
+  EXPECT_TRUE(payload_status.ErasePayload(kUrl1));
+
+  // Still Different payloads, still not equal.
+  EXPECT_NE(payload_status, empty_status);
+  EXPECT_TRUE(payload_status.ErasePayload(kUrl2));
+
+  // Both empty payloads, should be equal
+  EXPECT_EQ(payload_status, empty_status);
+}
+
+PayloadsVec AllVisitedPayloads(const absl::Status& s) {
+  PayloadsVec result;
+
+  s.ForEachPayload([&](absl::string_view type_url, const absl::Cord& payload) {
+    result.push_back(std::make_pair(std::string(type_url), payload));
+  });
+
+  return result;
+}
+
+TEST(Status, TestForEachPayload) {
+  absl::Status bad_status(absl::StatusCode::kInternal, "fail");
+  bad_status.SetPayload(kUrl1, absl::Cord(kPayload1));
+  bad_status.SetPayload(kUrl2, absl::Cord(kPayload2));
+  bad_status.SetPayload(kUrl3, absl::Cord(kPayload3));
+
+  int count = 0;
+
+  bad_status.ForEachPayload(
+      [&count](absl::string_view, const absl::Cord&) { ++count; });
+
+  EXPECT_EQ(count, 3);
+
+  PayloadsVec expected_payloads = {{kUrl1, absl::Cord(kPayload1)},
+                                   {kUrl2, absl::Cord(kPayload2)},
+                                   {kUrl3, absl::Cord(kPayload3)}};
+
+  // Test that we visit all the payloads in the status.
+  PayloadsVec visited_payloads = AllVisitedPayloads(bad_status);
+  EXPECT_THAT(visited_payloads, UnorderedElementsAreArray(expected_payloads));
+
+  // Test that visitation order is not consistent between run.
+  std::vector<absl::Status> scratch;
+  while (true) {
+    scratch.emplace_back(absl::StatusCode::kInternal, "fail");
+
+    scratch.back().SetPayload(kUrl1, absl::Cord(kPayload1));
+    scratch.back().SetPayload(kUrl2, absl::Cord(kPayload2));
+    scratch.back().SetPayload(kUrl3, absl::Cord(kPayload3));
+
+    if (AllVisitedPayloads(scratch.back()) != visited_payloads) {
+      break;
+    }
+  }
+}
+
+TEST(Status, ToString) {
+  absl::Status s(absl::StatusCode::kInternal, "fail");
+  EXPECT_EQ("INTERNAL: fail", s.ToString());
+  s.SetPayload("foo", absl::Cord("bar"));
+  EXPECT_EQ("INTERNAL: fail [foo='bar']", s.ToString());
+  s.SetPayload("bar", absl::Cord("\377"));
+  EXPECT_THAT(s.ToString(),
+              AllOf(HasSubstr("INTERNAL: fail"), HasSubstr("[foo='bar']"),
+                    HasSubstr("[bar='\\xff']")));
+}
+
+absl::Status EraseAndReturn(const absl::Status& base) {
+  absl::Status copy = base;
+  EXPECT_TRUE(copy.ErasePayload(kUrl1));
+  return copy;
+}
+
+TEST(Status, CopyOnWriteForErasePayload) {
+  {
+    absl::Status base(absl::StatusCode::kInvalidArgument, "fail");
+    base.SetPayload(kUrl1, absl::Cord(kPayload1));
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    absl::Status copy = EraseAndReturn(base);
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    EXPECT_FALSE(copy.GetPayload(kUrl1).has_value());
+  }
+  {
+    absl::Status base(absl::StatusCode::kInvalidArgument, "fail");
+    base.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy = base;
+
+    EXPECT_TRUE(base.GetPayload(kUrl1).has_value());
+    EXPECT_TRUE(copy.GetPayload(kUrl1).has_value());
+
+    EXPECT_TRUE(base.ErasePayload(kUrl1));
+
+    EXPECT_FALSE(base.GetPayload(kUrl1).has_value());
+    EXPECT_TRUE(copy.GetPayload(kUrl1).has_value());
+  }
+}
+
+TEST(Status, CopyConstructor) {
+  {
+    absl::Status status;
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy(status);
+    EXPECT_EQ(copy, status);
+  }
+}
+
+TEST(Status, CopyAssignment) {
+  absl::Status assignee;
+  {
+    absl::Status status;
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    assignee = status;
+    EXPECT_EQ(assignee, status);
+  }
+}
+
+TEST(Status, CopyAssignmentIsNotRef) {
+  const absl::Status status_orig(absl::StatusCode::kInvalidArgument, "message");
+  absl::Status status_copy = status_orig;
+  EXPECT_EQ(status_orig, status_copy);
+  status_copy.SetPayload(kUrl1, absl::Cord(kPayload1));
+  EXPECT_NE(status_orig, status_copy);
+}
+
+TEST(Status, MoveConstructor) {
+  {
+    absl::Status status;
+    absl::Status copy(absl::Status{});
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    absl::Status copy(
+        absl::Status(absl::StatusCode::kInvalidArgument, "message"));
+    EXPECT_EQ(copy, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy1(status);
+    absl::Status copy2(std::move(status));
+    EXPECT_EQ(copy1, copy2);
+  }
+}
+
+TEST(Status, MoveAssignment) {
+  absl::Status assignee;
+  {
+    absl::Status status;
+    assignee = absl::Status();
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    assignee = absl::Status(absl::StatusCode::kInvalidArgument, "message");
+    EXPECT_EQ(assignee, status);
+  }
+  {
+    absl::Status status(absl::StatusCode::kInvalidArgument, "message");
+    status.SetPayload(kUrl1, absl::Cord(kPayload1));
+    absl::Status copy(status);
+    assignee = std::move(status);
+    EXPECT_EQ(assignee, copy);
+  }
+}
+
+TEST(Status, Update) {
+  absl::Status s;
+  s.Update(absl::OkStatus());
+  EXPECT_TRUE(s.ok());
+  const absl::Status a(absl::StatusCode::kCancelled, "message");
+  s.Update(a);
+  EXPECT_EQ(s, a);
+  const absl::Status b(absl::StatusCode::kInternal, "other message");
+  s.Update(b);
+  EXPECT_EQ(s, a);
+  s.Update(absl::OkStatus());
+  EXPECT_EQ(s, a);
+  EXPECT_FALSE(s.ok());
+}
+
+TEST(Status, Equality) {
+  absl::Status ok;
+  absl::Status no_payload = absl::CancelledError("no payload");
+  absl::Status one_payload = absl::InvalidArgumentError("one payload");
+  one_payload.SetPayload(kUrl1, absl::Cord(kPayload1));
+  absl::Status two_payloads = one_payload;
+  two_payloads.SetPayload(kUrl2, absl::Cord(kPayload2));
+  const std::array<absl::Status, 4> status_arr = {ok, no_payload, one_payload,
+                                                  two_payloads};
+  for (int i = 0; i < status_arr.size(); i++) {
+    for (int j = 0; j < status_arr.size(); j++) {
+      if (i == j) {
+        EXPECT_TRUE(status_arr[i] == status_arr[j]);
+        EXPECT_FALSE(status_arr[i] != status_arr[j]);
+      } else {
+        EXPECT_TRUE(status_arr[i] != status_arr[j]);
+        EXPECT_FALSE(status_arr[i] == status_arr[j]);
+      }
+    }
+  }
+}
+
+TEST(Status, Swap) {
+  auto test_swap = [](const absl::Status& s1, const absl::Status& s2) {
+    absl::Status copy1 = s1, copy2 = s2;
+    swap(copy1, copy2);
+    EXPECT_EQ(copy1, s2);
+    EXPECT_EQ(copy2, s1);
+  };
+  const absl::Status ok;
+  const absl::Status no_payload(absl::StatusCode::kAlreadyExists, "no payload");
+  absl::Status with_payload(absl::StatusCode::kInternal, "with payload");
+  with_payload.SetPayload(kUrl1, absl::Cord(kPayload1));
+  test_swap(ok, no_payload);
+  test_swap(no_payload, ok);
+  test_swap(ok, with_payload);
+  test_swap(with_payload, ok);
+  test_swap(no_payload, with_payload);
+  test_swap(with_payload, no_payload);
+}
+
+}  // namespace
diff --git a/absl/strings/BUILD.bazel b/absl/strings/BUILD.bazel
index 20511a3..b950ec7 100644
--- a/absl/strings/BUILD.bazel
+++ b/absl/strings/BUILD.bazel
@@ -13,11 +13,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
@@ -26,7 +25,7 @@
     features = ["parse_headers"],
 )
 
-licenses(["notice"])  # Apache 2.0
+licenses(["notice"])
 
 cc_library(
     name = "strings",
@@ -73,6 +72,7 @@
         "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
         "//absl/base:throw_delegate",
         "//absl/memory",
         "//absl/meta:type_traits",
@@ -83,19 +83,23 @@
 cc_library(
     name = "internal",
     srcs = [
+        "internal/escaping.cc",
         "internal/ostringstream.cc",
         "internal/utf8.cc",
     ],
     hdrs = [
         "internal/char_map.h",
+        "internal/escaping.h",
         "internal/ostringstream.h",
         "internal/resize_uninitialized.h",
         "internal/utf8.h",
     ],
     copts = ABSL_DEFAULT_COPTS,
     deps = [
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
         "//absl/meta:type_traits",
     ],
 )
@@ -122,6 +126,7 @@
     copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
+        ":cord",
         ":strings",
         "//absl/base:core_headers",
         "//absl/container:fixed_array",
@@ -140,7 +145,7 @@
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -225,8 +230,8 @@
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -235,8 +240,7 @@
     name = "string_view_test",
     size = "small",
     srcs = ["string_view_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
@@ -247,6 +251,74 @@
     ],
 )
 
+cc_library(
+    name = "cord_internal",
+    hdrs = ["internal/cord_internal.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":strings",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_library(
+    name = "cord",
+    srcs = [
+        "cord.cc",
+    ],
+    hdrs = [
+        "cord.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":cord_internal",
+        ":internal",
+        ":str_format",
+        ":strings",
+        "//absl/base",
+        "//absl/base:base_internal",
+        "//absl/base:core_headers",
+        "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:fixed_array",
+        "//absl/container:inlined_vector",
+        "//absl/functional:function_ref",
+        "//absl/meta:type_traits",
+    ],
+)
+
+cc_library(
+    name = "cord_test_helpers",
+    testonly = 1,
+    hdrs = [
+        "cord_test_helpers.h",
+    ],
+    copts = ABSL_DEFAULT_COPTS,
+    deps = [
+        ":cord",
+    ],
+)
+
+cc_test(
+    name = "cord_test",
+    size = "medium",
+    srcs = ["cord_test.cc"],
+    copts = ABSL_TEST_COPTS,
+    visibility = ["//visibility:private"],
+    deps = [
+        ":cord",
+        ":cord_test_helpers",
+        ":strings",
+        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:endian",
+        "//absl/base:raw_logging_internal",
+        "//absl/container:fixed_array",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 cc_test(
     name = "substitute_test",
     size = "small",
@@ -268,7 +340,7 @@
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -306,7 +378,7 @@
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -414,8 +486,10 @@
     deps = [
         ":pow10_helper",
         ":strings",
-        "//absl/base",
-        "//absl/base:core_headers",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
+        "//absl/random",
+        "//absl/random:distributions",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -428,7 +502,9 @@
     visibility = ["//visibility:private"],
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
+        "//absl/random",
+        "//absl/random:distributions",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -474,7 +550,6 @@
         ":pow10_helper",
         ":str_format",
         ":strings",
-        "//absl/base",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -488,7 +563,8 @@
     copts = ABSL_TEST_COPTS,
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:config",
+        "//absl/base:raw_logging_internal",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -503,7 +579,7 @@
     copts = ABSL_TEST_COPTS,
     deps = [
         ":strings",
-        "//absl/base",
+        "//absl/base:config",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -518,7 +594,6 @@
     ],
     deps = [
         ":strings",
-        "//absl/base",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -634,6 +709,7 @@
     visibility = ["//visibility:private"],
     deps = [
         ":str_format_internal",
+        "//absl/base:raw_logging_internal",
         "//absl/numeric:int128",
         "@com_google_googletest//:gtest_main",
     ],
@@ -668,6 +744,7 @@
     srcs = ["internal/pow10_helper.cc"],
     hdrs = ["internal/pow10_helper.h"],
     visibility = ["//visibility:private"],
+    deps = ["//absl/base:config"],
 )
 
 cc_test(
diff --git a/absl/strings/CMakeLists.txt b/absl/strings/CMakeLists.txt
index e63eec1..fbf0f5a 100644
--- a/absl/strings/CMakeLists.txt
+++ b/absl/strings/CMakeLists.txt
@@ -59,10 +59,11 @@
     absl::config
     absl::core_headers
     absl::endian
-    absl::throw_delegate
-    absl::memory
-    absl::type_traits
     absl::int128
+    absl::memory
+    absl::raw_logging_internal
+    absl::throw_delegate
+    absl::type_traits
   PUBLIC
 )
 
@@ -71,6 +72,8 @@
     strings_internal
   HDRS
     "internal/char_map.h"
+    "internal/escaping.cc"
+    "internal/escaping.h"
     "internal/ostringstream.h"
     "internal/resize_uninitialized.h"
     "internal/utf8.h"
@@ -80,8 +83,10 @@
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
+    absl::config
     absl::core_headers
     absl::endian
+    absl::raw_logging_internal
     absl::type_traits
 )
 
@@ -160,9 +165,6 @@
     "string_view_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::strings
     absl::config
@@ -276,9 +278,12 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    absl::base
     absl::core_headers
     absl::pow10_helper
+    absl::config
+    absl::raw_logging_internal
+    absl::random_random
+    absl::random_distributions
     gmock_main
 )
 
@@ -317,7 +322,6 @@
   DEPS
     absl::strings
     absl::str_format
-    absl::base
     absl::pow10_helper
     gmock_main
 )
@@ -332,7 +336,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    absl::base
+    absl::config
+    absl::raw_logging_internal
     gmock_main
 )
 
@@ -347,7 +352,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::strings
-    absl::base
+    absl::config
     gmock_main
 )
 
@@ -465,6 +470,7 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::str_format_internal
+    absl::raw_logging_internal
     absl::int128
     gmock_main
 )
@@ -503,6 +509,8 @@
     "internal/pow10_helper.cc"
   COPTS
     ${ABSL_TEST_COPTS}
+  DEPS
+    absl::config
   TESTONLY
 )
 
@@ -518,3 +526,58 @@
     absl::str_format
     gmock_main
 )
+
+absl_cc_library(
+  NAME
+    cord
+  HDRS
+    "cord.h"
+  SRCS
+    "cord.cc"
+    "internal/cord_internal.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::strings
+    absl::strings_internal
+    absl::base
+    absl::base_internal
+    absl::core_headers
+    absl::endian
+    absl::fixed_array
+    absl::function_ref
+    absl::inlined_vector
+    absl::raw_logging_internal
+    absl::type_traits
+  PUBLIC
+)
+
+absl_cc_library(
+  NAME
+    cord_test_helpers
+  HDRS
+    "cord_test_helpers.h"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::cord
+  TESTONLY
+)
+
+absl_cc_test(
+  NAME
+    cord_test
+  SRCS
+    "cord_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::cord
+    absl::strings
+    absl::base
+    absl::config
+    absl::endian
+    absl::raw_logging_internal
+    absl::fixed_array
+    gmock_main
+)
diff --git a/absl/strings/ascii.cc b/absl/strings/ascii.cc
index 045a5e2..93bb03e 100644
--- a/absl/strings/ascii.cc
+++ b/absl/strings/ascii.cc
@@ -15,7 +15,7 @@
 #include "absl/strings/ascii.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace ascii_internal {
 
 // # Table generated by this Python code (bit 0x02 is currently unused):
@@ -57,7 +57,7 @@
 // of these bits is tightly coupled to this implementation, the individual bits
 // are not named. Note that bitfields for all characters above ASCII 127 are
 // zero-initialized.
-const unsigned char kPropertyBits[256] = {
+ABSL_DLL const unsigned char kPropertyBits[256] = {
     0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x00
     0x40, 0x68, 0x48, 0x48, 0x48, 0x48, 0x40, 0x40,
     0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40, 0x40,  // 0x10
@@ -79,7 +79,7 @@
 // Array of characters for the ascii_tolower() function. For values 'A'
 // through 'Z', return the lower-case character; otherwise, return the
 // identity of the passed character.
-const char kToLower[256] = {
+ABSL_DLL const char kToLower[256] = {
   '\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
   '\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
   '\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
@@ -117,7 +117,7 @@
 // Array of characters for the ascii_toupper() function. For values 'a'
 // through 'z', return the upper-case character; otherwise, return the
 // identity of the passed character.
-const char kToUpper[256] = {
+ABSL_DLL const char kToUpper[256] = {
   '\x00', '\x01', '\x02', '\x03', '\x04', '\x05', '\x06', '\x07',
   '\x08', '\x09', '\x0a', '\x0b', '\x0c', '\x0d', '\x0e', '\x0f',
   '\x10', '\x11', '\x12', '\x13', '\x14', '\x15', '\x16', '\x17',
@@ -196,5 +196,5 @@
   str->erase(output_it - &(*str)[0]);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/ascii.h b/absl/strings/ascii.h
index ebcbb11..b46bc71 100644
--- a/absl/strings/ascii.h
+++ b/absl/strings/ascii.h
@@ -56,20 +56,21 @@
 #include <string>
 
 #include "absl/base/attributes.h"
+#include "absl/base/config.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace ascii_internal {
 
 // Declaration for an array of bitfields holding character information.
-extern const unsigned char kPropertyBits[256];
+ABSL_DLL extern const unsigned char kPropertyBits[256];
 
 // Declaration for the array of characters to upper-case characters.
-extern const char kToUpper[256];
+ABSL_DLL extern const char kToUpper[256];
 
 // Declaration for the array of characters to lower-case characters.
-extern const char kToLower[256];
+ABSL_DLL extern const char kToLower[256];
 
 }  // namespace ascii_internal
 
@@ -235,7 +236,7 @@
 // Removes leading, trailing, and consecutive internal whitespace.
 void RemoveExtraAsciiWhitespace(std::string*);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_ASCII_H_
diff --git a/absl/strings/charconv.cc b/absl/strings/charconv.cc
index 866a163..bdba768 100644
--- a/absl/strings/charconv.cc
+++ b/absl/strings/charconv.cc
@@ -57,7 +57,7 @@
 // narrower mantissas.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 template <typename FloatType>
@@ -673,7 +673,6 @@
     EncodeResult(calculated, negative, &result, &value);
     return result;
   }
-  return result;
 }
 }  // namespace
 
@@ -981,5 +980,5 @@
 };
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/charconv.h b/absl/strings/charconv.h
index 0b84cca..e04be32 100644
--- a/absl/strings/charconv.h
+++ b/absl/strings/charconv.h
@@ -17,8 +17,10 @@
 
 #include <system_error>  // NOLINT(build/c++11)
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Workalike compatibilty version of std::chars_format from C++17.
 //
@@ -111,7 +113,7 @@
   return lhs;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_CHARCONV_H_
diff --git a/absl/strings/charconv_test.cc b/absl/strings/charconv_test.cc
index b58fad2..9090e9c 100644
--- a/absl/strings/charconv_test.cc
+++ b/absl/strings/charconv_test.cc
@@ -511,6 +511,13 @@
   EXPECT_EQ(f, std::numeric_limits<float>::max());
 }
 
+TEST(FromChars, RegressionTestsFromFuzzer) {
+  absl::string_view src = "0x21900000p00000000099";
+  float f;
+  auto result = absl::from_chars(src.data(), src.data() + src.size(), f);
+  EXPECT_EQ(result.ec, std::errc::result_out_of_range);
+}
+
 TEST(FromChars, ReturnValuePtr) {
   // Check that `ptr` points one past the number scanned, even if that number
   // is not representable.
diff --git a/absl/strings/cord.cc b/absl/strings/cord.cc
new file mode 100644
index 0000000..d9503ae
--- /dev/null
+++ b/absl/strings/cord.cc
@@ -0,0 +1,2019 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/cord.h"
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdio>
+#include <cstdlib>
+#include <iomanip>
+#include <limits>
+#include <ostream>
+#include <sstream>
+#include <type_traits>
+#include <unordered_set>
+#include <vector>
+
+#include "absl/base/casts.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/base/port.h"
+#include "absl/container/fixed_array.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/strings/escaping.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/str_format.h"
+#include "absl/strings/str_join.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+using ::absl::cord_internal::CordRep;
+using ::absl::cord_internal::CordRepConcat;
+using ::absl::cord_internal::CordRepExternal;
+using ::absl::cord_internal::CordRepSubstring;
+
+// Various representations that we allow
+enum CordRepKind {
+  CONCAT        = 0,
+  EXTERNAL      = 1,
+  SUBSTRING     = 2,
+
+  // We have different tags for different sized flat arrays,
+  // starting with FLAT
+  FLAT          = 3,
+};
+
+namespace {
+
+// Type used with std::allocator for allocating and deallocating
+// `CordRepExternal`. std::allocator is used because it opaquely handles the
+// different new / delete overloads available on a given platform.
+struct alignas(absl::cord_internal::ExternalRepAlignment()) ExternalAllocType {
+  unsigned char value[absl::cord_internal::ExternalRepAlignment()];
+};
+
+// Returns the number of objects to pass in to std::allocator<ExternalAllocType>
+// allocate() and deallocate() to create enough room for `CordRepExternal` with
+// `releaser_size` bytes on the end.
+constexpr size_t GetExternalAllocNumObjects(size_t releaser_size) {
+  // Be sure to round up since `releaser_size` could be smaller than
+  // `sizeof(ExternalAllocType)`.
+  return (sizeof(CordRepExternal) + releaser_size + sizeof(ExternalAllocType) -
+          1) /
+         sizeof(ExternalAllocType);
+}
+
+// Allocates enough memory for `CordRepExternal` and a releaser with size
+// `releaser_size` bytes.
+void* AllocateExternal(size_t releaser_size) {
+  return std::allocator<ExternalAllocType>().allocate(
+      GetExternalAllocNumObjects(releaser_size));
+}
+
+// Deallocates the memory for a `CordRepExternal` assuming it was allocated with
+// a releaser of given size and alignment.
+void DeallocateExternal(CordRepExternal* p, size_t releaser_size) {
+  std::allocator<ExternalAllocType>().deallocate(
+      reinterpret_cast<ExternalAllocType*>(p),
+      GetExternalAllocNumObjects(releaser_size));
+}
+
+// Returns a pointer to the type erased releaser for the given CordRepExternal.
+void* GetExternalReleaser(CordRepExternal* rep) {
+  return rep + 1;
+}
+
+}  // namespace
+
+namespace cord_internal {
+
+inline CordRepConcat* CordRep::concat() {
+  assert(tag == CONCAT);
+  return static_cast<CordRepConcat*>(this);
+}
+
+inline const CordRepConcat* CordRep::concat() const {
+  assert(tag == CONCAT);
+  return static_cast<const CordRepConcat*>(this);
+}
+
+inline CordRepSubstring* CordRep::substring() {
+  assert(tag == SUBSTRING);
+  return static_cast<CordRepSubstring*>(this);
+}
+
+inline const CordRepSubstring* CordRep::substring() const {
+  assert(tag == SUBSTRING);
+  return static_cast<const CordRepSubstring*>(this);
+}
+
+inline CordRepExternal* CordRep::external() {
+  assert(tag == EXTERNAL);
+  return static_cast<CordRepExternal*>(this);
+}
+
+inline const CordRepExternal* CordRep::external() const {
+  assert(tag == EXTERNAL);
+  return static_cast<const CordRepExternal*>(this);
+}
+
+}  // namespace cord_internal
+
+static const size_t kFlatOverhead = offsetof(CordRep, data);
+
+static_assert(kFlatOverhead == 13, "Unittests assume kFlatOverhead == 13");
+
+// Largest and smallest flat node lengths we are willing to allocate
+// Flat allocation size is stored in tag, which currently can encode sizes up
+// to 4K, encoded as multiple of either 8 or 32 bytes.
+// If we allow for larger sizes, we need to change this to 8/64, 16/128, etc.
+static constexpr size_t kMaxFlatSize = 4096;
+static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead;
+static constexpr size_t kMinFlatLength = 32 - kFlatOverhead;
+
+// Prefer copying blocks of at most this size, otherwise reference count.
+static const size_t kMaxBytesToCopy = 511;
+
+// Helper functions for rounded div, and rounding to exact sizes.
+static size_t DivUp(size_t n, size_t m) { return (n + m - 1) / m; }
+static size_t RoundUp(size_t n, size_t m) { return DivUp(n, m) * m; }
+
+// Returns the size to the nearest equal or larger value that can be
+// expressed exactly as a tag value.
+static size_t RoundUpForTag(size_t size) {
+  return RoundUp(size, (size <= 1024) ? 8 : 32);
+}
+
+// Converts the allocated size to a tag, rounding down if the size
+// does not exactly match a 'tag expressible' size value. The result is
+// undefined if the size exceeds the maximum size that can be encoded in
+// a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
+static uint8_t AllocatedSizeToTag(size_t size) {
+  const size_t tag = (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32;
+  assert(tag <= std::numeric_limits<uint8_t>::max());
+  return tag;
+}
+
+// Converts the provided tag to the corresponding allocated size
+static constexpr size_t TagToAllocatedSize(uint8_t tag) {
+  return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32);
+}
+
+// Converts the provided tag to the corresponding available data length
+static constexpr size_t TagToLength(uint8_t tag) {
+  return TagToAllocatedSize(tag) - kFlatOverhead;
+}
+
+// Enforce that kMaxFlatSize maps to a well-known exact tag value.
+static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic");
+
+constexpr uint64_t Fibonacci(unsigned char n, uint64_t a = 0, uint64_t b = 1) {
+  return n == 0 ? a : Fibonacci(n - 1, b, a + b);
+}
+
+static_assert(Fibonacci(63) == 6557470319842,
+              "Fibonacci values computed incorrectly");
+
+// Minimum length required for a given depth tree -- a tree is considered
+// balanced if
+//      length(t) >= min_length[depth(t)]
+// The root node depth is allowed to become twice as large to reduce rebalancing
+// for larger strings (see IsRootBalanced).
+static constexpr uint64_t min_length[] = {
+    Fibonacci(2),
+    Fibonacci(3),
+    Fibonacci(4),
+    Fibonacci(5),
+    Fibonacci(6),
+    Fibonacci(7),
+    Fibonacci(8),
+    Fibonacci(9),
+    Fibonacci(10),
+    Fibonacci(11),
+    Fibonacci(12),
+    Fibonacci(13),
+    Fibonacci(14),
+    Fibonacci(15),
+    Fibonacci(16),
+    Fibonacci(17),
+    Fibonacci(18),
+    Fibonacci(19),
+    Fibonacci(20),
+    Fibonacci(21),
+    Fibonacci(22),
+    Fibonacci(23),
+    Fibonacci(24),
+    Fibonacci(25),
+    Fibonacci(26),
+    Fibonacci(27),
+    Fibonacci(28),
+    Fibonacci(29),
+    Fibonacci(30),
+    Fibonacci(31),
+    Fibonacci(32),
+    Fibonacci(33),
+    Fibonacci(34),
+    Fibonacci(35),
+    Fibonacci(36),
+    Fibonacci(37),
+    Fibonacci(38),
+    Fibonacci(39),
+    Fibonacci(40),
+    Fibonacci(41),
+    Fibonacci(42),
+    Fibonacci(43),
+    Fibonacci(44),
+    Fibonacci(45),
+    Fibonacci(46),
+    Fibonacci(47),
+    0xffffffffffffffffull,  // Avoid overflow
+};
+
+static const int kMinLengthSize = ABSL_ARRAYSIZE(min_length);
+
+// The inlined size to use with absl::InlinedVector.
+//
+// Note: The InlinedVectors in this file (and in cord.h) do not need to use
+// the same value for their inlined size. The fact that they do is historical.
+// It may be desirable for each to use a different inlined size optimized for
+// that InlinedVector's usage.
+//
+// TODO(jgm): Benchmark to see if there's a more optimal value than 47 for
+// the inlined vector size (47 exists for backward compatibility).
+static const int kInlinedVectorSize = 47;
+
+static inline bool IsRootBalanced(CordRep* node) {
+  if (node->tag != CONCAT) {
+    return true;
+  } else if (node->concat()->depth() <= 15) {
+    return true;
+  } else if (node->concat()->depth() > kMinLengthSize) {
+    return false;
+  } else {
+    // Allow depth to become twice as large as implied by fibonacci rule to
+    // reduce rebalancing for larger strings.
+    return (node->length >= min_length[node->concat()->depth() / 2]);
+  }
+}
+
+static CordRep* Rebalance(CordRep* node);
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os);
+static bool VerifyNode(CordRep* root, CordRep* start_node,
+                       bool full_validation);
+
+static inline CordRep* VerifyTree(CordRep* node) {
+  // Verification is expensive, so only do it in debug mode.
+  // Even in debug mode we normally do only light validation.
+  // If you are debugging Cord itself, you should define the
+  // macro EXTRA_CORD_VALIDATION, e.g. by adding
+  // --copt=-DEXTRA_CORD_VALIDATION to the blaze line.
+#ifdef EXTRA_CORD_VALIDATION
+  assert(node == nullptr || VerifyNode(node, node, /*full_validation=*/true));
+#else   // EXTRA_CORD_VALIDATION
+  assert(node == nullptr || VerifyNode(node, node, /*full_validation=*/false));
+#endif  // EXTRA_CORD_VALIDATION
+  static_cast<void>(&VerifyNode);
+
+  return node;
+}
+
+// --------------------------------------------------------------------
+// Memory management
+
+inline CordRep* Ref(CordRep* rep) {
+  if (rep != nullptr) {
+    rep->refcount.Increment();
+  }
+  return rep;
+}
+
+// This internal routine is called from the cold path of Unref below. Keeping it
+// in a separate routine allows good inlining of Unref into many profitable call
+// sites. However, the call to this function can be highly disruptive to the
+// register pressure in those callers. To minimize the cost to callers, we use
+// a special LLVM calling convention that preserves most registers. This allows
+// the call to this routine in cold paths to not disrupt the caller's register
+// pressure. This calling convention is not available on all platforms; we
+// intentionally allow LLVM to ignore the attribute rather than attempting to
+// hardcode the list of supported platforms.
+#if defined(__clang__) && !defined(__i386__)
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wattributes"
+__attribute__((preserve_most))
+#pragma clang diagnostic pop
+#endif
+static void UnrefInternal(CordRep* rep) {
+  assert(rep != nullptr);
+
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> pending;
+  while (true) {
+    if (rep->tag == CONCAT) {
+      CordRepConcat* rep_concat = rep->concat();
+      CordRep* right = rep_concat->right;
+      if (!right->refcount.Decrement()) {
+        pending.push_back(right);
+      }
+      CordRep* left = rep_concat->left;
+      delete rep_concat;
+      rep = nullptr;
+      if (!left->refcount.Decrement()) {
+        rep = left;
+        continue;
+      }
+    } else if (rep->tag == EXTERNAL) {
+      CordRepExternal* rep_external = rep->external();
+      absl::string_view data(rep_external->base, rep->length);
+      void* releaser = GetExternalReleaser(rep_external);
+      size_t releaser_size = rep_external->releaser_invoker(releaser, data);
+      rep_external->~CordRepExternal();
+      DeallocateExternal(rep_external, releaser_size);
+      rep = nullptr;
+    } else if (rep->tag == SUBSTRING) {
+      CordRepSubstring* rep_substring = rep->substring();
+      CordRep* child = rep_substring->child;
+      delete rep_substring;
+      rep = nullptr;
+      if (!child->refcount.Decrement()) {
+        rep = child;
+        continue;
+      }
+    } else {
+      // Flat CordReps are allocated and constructed with raw ::operator new
+      // and placement new, and must be destructed and deallocated
+      // accordingly.
+#if defined(__cpp_sized_deallocation)
+      size_t size = TagToAllocatedSize(rep->tag);
+      rep->~CordRep();
+      ::operator delete(rep, size);
+#else
+      rep->~CordRep();
+      ::operator delete(rep);
+#endif
+      rep = nullptr;
+    }
+
+    if (!pending.empty()) {
+      rep = pending.back();
+      pending.pop_back();
+    } else {
+      break;
+    }
+  }
+}
+
+inline void Unref(CordRep* rep) {
+  // Fast-path for two common, hot cases: a null rep and a shared root.
+  if (ABSL_PREDICT_TRUE(rep == nullptr ||
+                        rep->refcount.DecrementExpectHighRefcount())) {
+    return;
+  }
+
+  UnrefInternal(rep);
+}
+
+// Return the depth of a node
+static int Depth(const CordRep* rep) {
+  if (rep->tag == CONCAT) {
+    return rep->concat()->depth();
+  } else {
+    return 0;
+  }
+}
+
+static void SetConcatChildren(CordRepConcat* concat, CordRep* left,
+                              CordRep* right) {
+  concat->left = left;
+  concat->right = right;
+
+  concat->length = left->length + right->length;
+  concat->set_depth(1 + std::max(Depth(left), Depth(right)));
+}
+
+// Create a concatenation of the specified nodes.
+// Does not change the refcounts of "left" and "right".
+// The returned node has a refcount of 1.
+static CordRep* RawConcat(CordRep* left, CordRep* right) {
+  // Avoid making degenerate concat nodes (one child is empty)
+  if (left == nullptr || left->length == 0) {
+    Unref(left);
+    return right;
+  }
+  if (right == nullptr || right->length == 0) {
+    Unref(right);
+    return left;
+  }
+
+  CordRepConcat* rep = new CordRepConcat();
+  rep->tag = CONCAT;
+  SetConcatChildren(rep, left, right);
+
+  return rep;
+}
+
+static CordRep* Concat(CordRep* left, CordRep* right) {
+  CordRep* rep = RawConcat(left, right);
+  if (rep != nullptr && !IsRootBalanced(rep)) {
+    rep = Rebalance(rep);
+  }
+  return VerifyTree(rep);
+}
+
+// Make a balanced tree out of an array of leaf nodes.
+static CordRep* MakeBalancedTree(CordRep** reps, size_t n) {
+  // Make repeated passes over the array, merging adjacent pairs
+  // until we are left with just a single node.
+  while (n > 1) {
+    size_t dst = 0;
+    for (size_t src = 0; src < n; src += 2) {
+      if (src + 1 < n) {
+        reps[dst] = Concat(reps[src], reps[src + 1]);
+      } else {
+        reps[dst] = reps[src];
+      }
+      dst++;
+    }
+    n = dst;
+  }
+
+  return reps[0];
+}
+
+// Create a new flat node.
+static CordRep* NewFlat(size_t length_hint) {
+  if (length_hint <= kMinFlatLength) {
+    length_hint = kMinFlatLength;
+  } else if (length_hint > kMaxFlatLength) {
+    length_hint = kMaxFlatLength;
+  }
+
+  // Round size up so it matches a size we can exactly express in a tag.
+  const size_t size = RoundUpForTag(length_hint + kFlatOverhead);
+  void* const raw_rep = ::operator new(size);
+  CordRep* rep = new (raw_rep) CordRep();
+  rep->tag = AllocatedSizeToTag(size);
+  return VerifyTree(rep);
+}
+
+// Create a new tree out of the specified array.
+// The returned node has a refcount of 1.
+static CordRep* NewTree(const char* data,
+                        size_t length,
+                        size_t alloc_hint) {
+  if (length == 0) return nullptr;
+  absl::FixedArray<CordRep*> reps((length - 1) / kMaxFlatLength + 1);
+  size_t n = 0;
+  do {
+    const size_t len = std::min(length, kMaxFlatLength);
+    CordRep* rep = NewFlat(len + alloc_hint);
+    rep->length = len;
+    memcpy(rep->data, data, len);
+    reps[n++] = VerifyTree(rep);
+    data += len;
+    length -= len;
+  } while (length != 0);
+  return MakeBalancedTree(reps.data(), n);
+}
+
+namespace cord_internal {
+
+ExternalRepReleaserPair NewExternalWithUninitializedReleaser(
+    absl::string_view data, ExternalReleaserInvoker invoker,
+    size_t releaser_size) {
+  assert(!data.empty());
+
+  void* raw_rep = AllocateExternal(releaser_size);
+  auto* rep = new (raw_rep) CordRepExternal();
+  rep->length = data.size();
+  rep->tag = EXTERNAL;
+  rep->base = data.data();
+  rep->releaser_invoker = invoker;
+  return {VerifyTree(rep), GetExternalReleaser(rep)};
+}
+
+}  // namespace cord_internal
+
+static CordRep* NewSubstring(CordRep* child, size_t offset, size_t length) {
+  // Never create empty substring nodes
+  if (length == 0) {
+    Unref(child);
+    return nullptr;
+  } else {
+    CordRepSubstring* rep = new CordRepSubstring();
+    assert((offset + length) <= child->length);
+    rep->length = length;
+    rep->tag = SUBSTRING;
+    rep->start = offset;
+    rep->child = child;
+    return VerifyTree(rep);
+  }
+}
+
+// --------------------------------------------------------------------
+// Cord::InlineRep functions
+
+// This will trigger LNK2005 in MSVC.
+#ifndef COMPILER_MSVC
+const unsigned char Cord::InlineRep::kMaxInline;
+#endif  // COMPILER_MSVC
+
+inline void Cord::InlineRep::set_data(const char* data, size_t n,
+                                      bool nullify_tail) {
+  static_assert(kMaxInline == 15, "set_data is hard-coded for a length of 15");
+
+  cord_internal::SmallMemmove(data_, data, n, nullify_tail);
+  data_[kMaxInline] = static_cast<char>(n);
+}
+
+inline char* Cord::InlineRep::set_data(size_t n) {
+  assert(n <= kMaxInline);
+  memset(data_, 0, sizeof(data_));
+  data_[kMaxInline] = static_cast<char>(n);
+  return data_;
+}
+
+inline CordRep* Cord::InlineRep::force_tree(size_t extra_hint) {
+  size_t len = data_[kMaxInline];
+  CordRep* result;
+  if (len > kMaxInline) {
+    memcpy(&result, data_, sizeof(result));
+  } else {
+    result = NewFlat(len + extra_hint);
+    result->length = len;
+    memcpy(result->data, data_, len);
+    set_tree(result);
+  }
+  return result;
+}
+
+inline void Cord::InlineRep::reduce_size(size_t n) {
+  size_t tag = data_[kMaxInline];
+  assert(tag <= kMaxInline);
+  assert(tag >= n);
+  tag -= n;
+  memset(data_ + tag, 0, n);
+  data_[kMaxInline] = static_cast<char>(tag);
+}
+
+inline void Cord::InlineRep::remove_prefix(size_t n) {
+  cord_internal::SmallMemmove(data_, data_ + n, data_[kMaxInline] - n);
+  reduce_size(n);
+}
+
+void Cord::InlineRep::AppendTree(CordRep* tree) {
+  if (tree == nullptr) return;
+  size_t len = data_[kMaxInline];
+  if (len == 0) {
+    set_tree(tree);
+  } else {
+    set_tree(Concat(force_tree(0), tree));
+  }
+}
+
+void Cord::InlineRep::PrependTree(CordRep* tree) {
+  if (tree == nullptr) return;
+  size_t len = data_[kMaxInline];
+  if (len == 0) {
+    set_tree(tree);
+  } else {
+    set_tree(Concat(tree, force_tree(0)));
+  }
+}
+
+// Searches for a non-full flat node at the rightmost leaf of the tree. If a
+// suitable leaf is found, the function will update the length field for all
+// nodes to account for the size increase. The append region address will be
+// written to region and the actual size increase will be written to size.
+static inline bool PrepareAppendRegion(CordRep* root, char** region,
+                                       size_t* size, size_t max_length) {
+  // Search down the right-hand path for a non-full FLAT node.
+  CordRep* dst = root;
+  while (dst->tag == CONCAT && dst->refcount.IsOne()) {
+    dst = dst->concat()->right;
+  }
+
+  if (dst->tag < FLAT || !dst->refcount.IsOne()) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  const size_t in_use = dst->length;
+  const size_t capacity = TagToLength(dst->tag);
+  if (in_use == capacity) {
+    *region = nullptr;
+    *size = 0;
+    return false;
+  }
+
+  size_t size_increase = std::min(capacity - in_use, max_length);
+
+  // We need to update the length fields for all nodes, including the leaf node.
+  for (CordRep* rep = root; rep != dst; rep = rep->concat()->right) {
+    rep->length += size_increase;
+  }
+  dst->length += size_increase;
+
+  *region = dst->data + in_use;
+  *size = size_increase;
+  return true;
+}
+
+void Cord::InlineRep::GetAppendRegion(char** region, size_t* size,
+                                      size_t max_length) {
+  if (max_length == 0) {
+    *region = nullptr;
+    *size = 0;
+    return;
+  }
+
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline && max_length <= kMaxInline - inline_length) {
+    *region = data_ + inline_length;
+    *size = max_length;
+    data_[kMaxInline] = static_cast<char>(inline_length + max_length);
+    return;
+  }
+
+  CordRep* root = force_tree(max_length);
+
+  if (PrepareAppendRegion(root, region, size, max_length)) {
+    return;
+  }
+
+  // Allocate new node.
+  CordRep* new_node =
+      NewFlat(std::max(static_cast<size_t>(root->length), max_length));
+  new_node->length =
+      std::min(static_cast<size_t>(TagToLength(new_node->tag)), max_length);
+  *region = new_node->data;
+  *size = new_node->length;
+  replace_tree(Concat(root, new_node));
+}
+
+void Cord::InlineRep::GetAppendRegion(char** region, size_t* size) {
+  const size_t max_length = std::numeric_limits<size_t>::max();
+
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline) {
+    *region = data_ + inline_length;
+    *size = kMaxInline - inline_length;
+    data_[kMaxInline] = kMaxInline;
+    return;
+  }
+
+  CordRep* root = force_tree(max_length);
+
+  if (PrepareAppendRegion(root, region, size, max_length)) {
+    return;
+  }
+
+  // Allocate new node.
+  CordRep* new_node = NewFlat(root->length);
+  new_node->length = TagToLength(new_node->tag);
+  *region = new_node->data;
+  *size = new_node->length;
+  replace_tree(Concat(root, new_node));
+}
+
+// If the rep is a leaf, this will increment the value at total_mem_usage and
+// will return true.
+static bool RepMemoryUsageLeaf(const CordRep* rep, size_t* total_mem_usage) {
+  if (rep->tag >= FLAT) {
+    *total_mem_usage += TagToAllocatedSize(rep->tag);
+    return true;
+  }
+  if (rep->tag == EXTERNAL) {
+    *total_mem_usage += sizeof(CordRepConcat) + rep->length;
+    return true;
+  }
+  return false;
+}
+
+void Cord::InlineRep::AssignSlow(const Cord::InlineRep& src) {
+  ClearSlow();
+
+  memcpy(data_, src.data_, sizeof(data_));
+  if (is_tree()) {
+    Ref(tree());
+  }
+}
+
+void Cord::InlineRep::ClearSlow() {
+  if (is_tree()) {
+    Unref(tree());
+  }
+  memset(data_, 0, sizeof(data_));
+}
+
+// --------------------------------------------------------------------
+// Constructors and destructors
+
+Cord::Cord(const Cord& src) : contents_(src.contents_) {
+  Ref(contents_.tree());  // Does nothing if contents_ has embedded data
+}
+
+Cord::Cord(absl::string_view src) {
+  const size_t n = src.size();
+  if (n <= InlineRep::kMaxInline) {
+    contents_.set_data(src.data(), n, false);
+  } else {
+    contents_.set_tree(NewTree(src.data(), n, 0));
+  }
+}
+
+// The destruction code is separate so that the compiler can determine
+// that it does not need to call the destructor on a moved-from Cord.
+void Cord::DestroyCordSlow() {
+  Unref(VerifyTree(contents_.tree()));
+}
+
+// --------------------------------------------------------------------
+// Mutators
+
+void Cord::Clear() {
+  Unref(contents_.clear());
+}
+
+Cord& Cord::operator=(absl::string_view src) {
+
+  const char* data = src.data();
+  size_t length = src.size();
+  CordRep* tree = contents_.tree();
+  if (length <= InlineRep::kMaxInline) {
+    // Embed into this->contents_
+    contents_.set_data(data, length, true);
+    Unref(tree);
+    return *this;
+  }
+  if (tree != nullptr && tree->tag >= FLAT &&
+      TagToLength(tree->tag) >= length && tree->refcount.IsOne()) {
+    // Copy in place if the existing FLAT node is reusable.
+    memmove(tree->data, data, length);
+    tree->length = length;
+    VerifyTree(tree);
+    return *this;
+  }
+  contents_.set_tree(NewTree(data, length, 0));
+  Unref(tree);
+  return *this;
+}
+
+// TODO(sanjay): Move to Cord::InlineRep section of file.  For now,
+// we keep it here to make diffs easier.
+void Cord::InlineRep::AppendArray(const char* src_data, size_t src_size) {
+  if (src_size == 0) return;  // memcpy(_, nullptr, 0) is undefined.
+  // Try to fit in the inline buffer if possible.
+  size_t inline_length = data_[kMaxInline];
+  if (inline_length < kMaxInline && src_size <= kMaxInline - inline_length) {
+    // Append new data to embedded array
+    data_[kMaxInline] = static_cast<char>(inline_length + src_size);
+    memcpy(data_ + inline_length, src_data, src_size);
+    return;
+  }
+
+  CordRep* root = tree();
+
+  size_t appended = 0;
+  if (root) {
+    char* region;
+    if (PrepareAppendRegion(root, &region, &appended, src_size)) {
+      memcpy(region, src_data, appended);
+    }
+  } else {
+    // It is possible that src_data == data_, but when we transition from an
+    // InlineRep to a tree we need to assign data_ = root via set_tree. To
+    // avoid corrupting the source data before we copy it, delay calling
+    // set_tree until after we've copied data.
+    // We are going from an inline size to beyond inline size. Make the new size
+    // either double the inlined size, or the added size + 10%.
+    const size_t size1 = inline_length * 2 + src_size;
+    const size_t size2 = inline_length + src_size / 10;
+    root = NewFlat(std::max<size_t>(size1, size2));
+    appended = std::min(src_size, TagToLength(root->tag) - inline_length);
+    memcpy(root->data, data_, inline_length);
+    memcpy(root->data + inline_length, src_data, appended);
+    root->length = inline_length + appended;
+    set_tree(root);
+  }
+
+  src_data += appended;
+  src_size -= appended;
+  if (src_size == 0) {
+    return;
+  }
+
+  // Use new block(s) for any remaining bytes that were not handled above.
+  // Alloc extra memory only if the right child of the root of the new tree is
+  // going to be a FLAT node, which will permit further inplace appends.
+  size_t length = src_size;
+  if (src_size < kMaxFlatLength) {
+    // The new length is either
+    // - old size + 10%
+    // - old_size + src_size
+    // This will cause a reasonable conservative step-up in size that is still
+    // large enough to avoid excessive amounts of small fragments being added.
+    length = std::max<size_t>(root->length / 10, src_size);
+  }
+  set_tree(Concat(root, NewTree(src_data, src_size, length - src_size)));
+}
+
+inline CordRep* Cord::TakeRep() const& {
+  return Ref(contents_.tree());
+}
+
+inline CordRep* Cord::TakeRep() && {
+  CordRep* rep = contents_.tree();
+  contents_.clear();
+  return rep;
+}
+
+template <typename C>
+inline void Cord::AppendImpl(C&& src) {
+  if (empty()) {
+    // In case of an empty destination avoid allocating a new node, do not copy
+    // data.
+    *this = std::forward<C>(src);
+    return;
+  }
+
+  // For short cords, it is faster to copy data if there is room in dst.
+  const size_t src_size = src.contents_.size();
+  if (src_size <= kMaxBytesToCopy) {
+    CordRep* src_tree = src.contents_.tree();
+    if (src_tree == nullptr) {
+      // src has embedded data.
+      contents_.AppendArray(src.contents_.data(), src_size);
+      return;
+    }
+    if (src_tree->tag >= FLAT) {
+      // src tree just has one flat node.
+      contents_.AppendArray(src_tree->data, src_size);
+      return;
+    }
+    if (&src == this) {
+      // ChunkIterator below assumes that src is not modified during traversal.
+      Append(Cord(src));
+      return;
+    }
+    // TODO(mec): Should we only do this if "dst" has space?
+    for (absl::string_view chunk : src.Chunks()) {
+      Append(chunk);
+    }
+    return;
+  }
+
+  contents_.AppendTree(std::forward<C>(src).TakeRep());
+}
+
+void Cord::Append(const Cord& src) { AppendImpl(src); }
+
+void Cord::Append(Cord&& src) { AppendImpl(std::move(src)); }
+
+void Cord::Prepend(const Cord& src) {
+  CordRep* src_tree = src.contents_.tree();
+  if (src_tree != nullptr) {
+    Ref(src_tree);
+    contents_.PrependTree(src_tree);
+    return;
+  }
+
+  // `src` cord is inlined.
+  absl::string_view src_contents(src.contents_.data(), src.contents_.size());
+  return Prepend(src_contents);
+}
+
+void Cord::Prepend(absl::string_view src) {
+  if (src.empty()) return;  // memcpy(_, nullptr, 0) is undefined.
+  size_t cur_size = contents_.size();
+  if (!contents_.is_tree() && cur_size + src.size() <= InlineRep::kMaxInline) {
+    // Use embedded storage.
+    char data[InlineRep::kMaxInline + 1] = {0};
+    data[InlineRep::kMaxInline] = cur_size + src.size();  // set size
+    memcpy(data, src.data(), src.size());
+    memcpy(data + src.size(), contents_.data(), cur_size);
+    memcpy(reinterpret_cast<void*>(&contents_), data,
+           InlineRep::kMaxInline + 1);
+  } else {
+    contents_.PrependTree(NewTree(src.data(), src.size(), 0));
+  }
+}
+
+static CordRep* RemovePrefixFrom(CordRep* node, size_t n) {
+  if (n >= node->length) return nullptr;
+  if (n == 0) return Ref(node);
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> rhs_stack;
+
+  while (node->tag == CONCAT) {
+    assert(n <= node->length);
+    if (n < node->concat()->left->length) {
+      // Push right to stack, descend left.
+      rhs_stack.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Drop left, descend right.
+      n -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+  assert(n <= node->length);
+
+  if (n == 0) {
+    Ref(node);
+  } else {
+    size_t start = n;
+    size_t len = node->length - n;
+    if (node->tag == SUBSTRING) {
+      // Consider in-place update of node, similar to in RemoveSuffixFrom().
+      start += node->substring()->start;
+      node = node->substring()->child;
+    }
+    node = NewSubstring(Ref(node), start, len);
+  }
+  while (!rhs_stack.empty()) {
+    node = Concat(node, Ref(rhs_stack.back()));
+    rhs_stack.pop_back();
+  }
+  return node;
+}
+
+// RemoveSuffixFrom() is very similar to RemovePrefixFrom(), with the
+// exception that removing a suffix has an optimization where a node may be
+// edited in place iff that node and all its ancestors have a refcount of 1.
+static CordRep* RemoveSuffixFrom(CordRep* node, size_t n) {
+  if (n >= node->length) return nullptr;
+  if (n == 0) return Ref(node);
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> lhs_stack;
+  bool inplace_ok = node->refcount.IsOne();
+
+  while (node->tag == CONCAT) {
+    assert(n <= node->length);
+    if (n < node->concat()->right->length) {
+      // Push left to stack, descend right.
+      lhs_stack.push_back(node->concat()->left);
+      node = node->concat()->right;
+    } else {
+      // Drop right, descend left.
+      n -= node->concat()->right->length;
+      node = node->concat()->left;
+    }
+    inplace_ok = inplace_ok && node->refcount.IsOne();
+  }
+  assert(n <= node->length);
+
+  if (n == 0) {
+    Ref(node);
+  } else if (inplace_ok && node->tag != EXTERNAL) {
+    // Consider making a new buffer if the current node capacity is much
+    // larger than the new length.
+    Ref(node);
+    node->length -= n;
+  } else {
+    size_t start = 0;
+    size_t len = node->length - n;
+    if (node->tag == SUBSTRING) {
+      start = node->substring()->start;
+      node = node->substring()->child;
+    }
+    node = NewSubstring(Ref(node), start, len);
+  }
+  while (!lhs_stack.empty()) {
+    node = Concat(Ref(lhs_stack.back()), node);
+    lhs_stack.pop_back();
+  }
+  return node;
+}
+
+void Cord::RemovePrefix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested prefix size ", n,
+                                   " exceeds Cord's size ", size()));
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.remove_prefix(n);
+  } else {
+    CordRep* newrep = RemovePrefixFrom(tree, n);
+    Unref(tree);
+    contents_.replace_tree(VerifyTree(newrep));
+  }
+}
+
+void Cord::RemoveSuffix(size_t n) {
+  ABSL_INTERNAL_CHECK(n <= size(),
+                      absl::StrCat("Requested suffix size ", n,
+                                   " exceeds Cord's size ", size()));
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    contents_.reduce_size(n);
+  } else {
+    CordRep* newrep = RemoveSuffixFrom(tree, n);
+    Unref(tree);
+    contents_.replace_tree(VerifyTree(newrep));
+  }
+}
+
+// Work item for NewSubRange().
+struct SubRange {
+  SubRange(CordRep* a_node, size_t a_pos, size_t a_n)
+      : node(a_node), pos(a_pos), n(a_n) {}
+  CordRep* node;  // nullptr means concat last 2 results.
+  size_t pos;
+  size_t n;
+};
+
+static CordRep* NewSubRange(CordRep* node, size_t pos, size_t n) {
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> results;
+  absl::InlinedVector<SubRange, kInlinedVectorSize> todo;
+  todo.push_back(SubRange(node, pos, n));
+  do {
+    const SubRange& sr = todo.back();
+    node = sr.node;
+    pos = sr.pos;
+    n = sr.n;
+    todo.pop_back();
+
+    if (node == nullptr) {
+      assert(results.size() >= 2);
+      CordRep* right = results.back();
+      results.pop_back();
+      CordRep* left = results.back();
+      results.pop_back();
+      results.push_back(Concat(left, right));
+    } else if (pos == 0 && n == node->length) {
+      results.push_back(Ref(node));
+    } else if (node->tag != CONCAT) {
+      if (node->tag == SUBSTRING) {
+        pos += node->substring()->start;
+        node = node->substring()->child;
+      }
+      results.push_back(NewSubstring(Ref(node), pos, n));
+    } else if (pos + n <= node->concat()->left->length) {
+      todo.push_back(SubRange(node->concat()->left, pos, n));
+    } else if (pos >= node->concat()->left->length) {
+      pos -= node->concat()->left->length;
+      todo.push_back(SubRange(node->concat()->right, pos, n));
+    } else {
+      size_t left_n = node->concat()->left->length - pos;
+      todo.push_back(SubRange(nullptr, 0, 0));  // Concat()
+      todo.push_back(SubRange(node->concat()->right, 0, n - left_n));
+      todo.push_back(SubRange(node->concat()->left, pos, left_n));
+    }
+  } while (!todo.empty());
+  assert(results.size() == 1);
+  return results[0];
+}
+
+Cord Cord::Subcord(size_t pos, size_t new_size) const {
+  Cord sub_cord;
+  size_t length = size();
+  if (pos > length) pos = length;
+  if (new_size > length - pos) new_size = length - pos;
+  CordRep* tree = contents_.tree();
+  if (tree == nullptr) {
+    // sub_cord is newly constructed, no need to re-zero-out the tail of
+    // contents_ memory.
+    sub_cord.contents_.set_data(contents_.data() + pos, new_size, false);
+  } else if (new_size == 0) {
+    // We want to return empty subcord, so nothing to do.
+  } else if (new_size <= InlineRep::kMaxInline) {
+    Cord::ChunkIterator it = chunk_begin();
+    it.AdvanceBytes(pos);
+    char* dest = sub_cord.contents_.data_;
+    size_t remaining_size = new_size;
+    while (remaining_size > it->size()) {
+      cord_internal::SmallMemmove(dest, it->data(), it->size());
+      remaining_size -= it->size();
+      dest += it->size();
+      ++it;
+    }
+    cord_internal::SmallMemmove(dest, it->data(), remaining_size);
+    sub_cord.contents_.data_[InlineRep::kMaxInline] = new_size;
+  } else {
+    sub_cord.contents_.set_tree(NewSubRange(tree, pos, new_size));
+  }
+  return sub_cord;
+}
+
+// --------------------------------------------------------------------
+// Balancing
+
+class CordForest {
+ public:
+  explicit CordForest(size_t length)
+      : root_length_(length), trees_(kMinLengthSize, nullptr) {}
+
+  void Build(CordRep* cord_root) {
+    std::vector<CordRep*> pending = {cord_root};
+
+    while (!pending.empty()) {
+      CordRep* node = pending.back();
+      pending.pop_back();
+      CheckNode(node);
+      if (ABSL_PREDICT_FALSE(node->tag != CONCAT)) {
+        AddNode(node);
+        continue;
+      }
+
+      CordRepConcat* concat_node = node->concat();
+      if (concat_node->depth() >= kMinLengthSize ||
+          concat_node->length < min_length[concat_node->depth()]) {
+        pending.push_back(concat_node->right);
+        pending.push_back(concat_node->left);
+
+        if (concat_node->refcount.IsOne()) {
+          concat_node->left = concat_freelist_;
+          concat_freelist_ = concat_node;
+        } else {
+          Ref(concat_node->right);
+          Ref(concat_node->left);
+          Unref(concat_node);
+        }
+      } else {
+        AddNode(node);
+      }
+    }
+  }
+
+  CordRep* ConcatNodes() {
+    CordRep* sum = nullptr;
+    for (auto* node : trees_) {
+      if (node == nullptr) continue;
+
+      sum = PrependNode(node, sum);
+      root_length_ -= node->length;
+      if (root_length_ == 0) break;
+    }
+    ABSL_INTERNAL_CHECK(sum != nullptr, "Failed to locate sum node");
+    return VerifyTree(sum);
+  }
+
+ private:
+  CordRep* AppendNode(CordRep* node, CordRep* sum) {
+    return (sum == nullptr) ? node : MakeConcat(sum, node);
+  }
+
+  CordRep* PrependNode(CordRep* node, CordRep* sum) {
+    return (sum == nullptr) ? node : MakeConcat(node, sum);
+  }
+
+  void AddNode(CordRep* node) {
+    CordRep* sum = nullptr;
+
+    // Collect together everything with which we will merge node
+    int i = 0;
+    for (; node->length > min_length[i + 1]; ++i) {
+      auto& tree_at_i = trees_[i];
+
+      if (tree_at_i == nullptr) continue;
+      sum = PrependNode(tree_at_i, sum);
+      tree_at_i = nullptr;
+    }
+
+    sum = AppendNode(node, sum);
+
+    // Insert sum into appropriate place in the forest
+    for (; sum->length >= min_length[i]; ++i) {
+      auto& tree_at_i = trees_[i];
+      if (tree_at_i == nullptr) continue;
+
+      sum = MakeConcat(tree_at_i, sum);
+      tree_at_i = nullptr;
+    }
+
+    // min_length[0] == 1, which means sum->length >= min_length[0]
+    assert(i > 0);
+    trees_[i - 1] = sum;
+  }
+
+  // Make concat node trying to resue existing CordRepConcat nodes we
+  // already collected in the concat_freelist_.
+  CordRep* MakeConcat(CordRep* left, CordRep* right) {
+    if (concat_freelist_ == nullptr) return RawConcat(left, right);
+
+    CordRepConcat* rep = concat_freelist_;
+    if (concat_freelist_->left == nullptr) {
+      concat_freelist_ = nullptr;
+    } else {
+      concat_freelist_ = concat_freelist_->left->concat();
+    }
+    SetConcatChildren(rep, left, right);
+
+    return rep;
+  }
+
+  static void CheckNode(CordRep* node) {
+    ABSL_INTERNAL_CHECK(node->length != 0u, "");
+    if (node->tag == CONCAT) {
+      ABSL_INTERNAL_CHECK(node->concat()->left != nullptr, "");
+      ABSL_INTERNAL_CHECK(node->concat()->right != nullptr, "");
+      ABSL_INTERNAL_CHECK(node->length == (node->concat()->left->length +
+                                           node->concat()->right->length),
+                          "");
+    }
+  }
+
+  size_t root_length_;
+
+  // use an inlined vector instead of a flat array to get bounds checking
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> trees_;
+
+  // List of concat nodes we can re-use for Cord balancing.
+  CordRepConcat* concat_freelist_ = nullptr;
+};
+
+static CordRep* Rebalance(CordRep* node) {
+  VerifyTree(node);
+  assert(node->tag == CONCAT);
+
+  if (node->length == 0) {
+    return nullptr;
+  }
+
+  CordForest forest(node->length);
+  forest.Build(node);
+  return forest.ConcatNodes();
+}
+
+// --------------------------------------------------------------------
+// Comparators
+
+namespace {
+
+int ClampResult(int memcmp_res) {
+  return static_cast<int>(memcmp_res > 0) - static_cast<int>(memcmp_res < 0);
+}
+
+int CompareChunks(absl::string_view* lhs, absl::string_view* rhs,
+                  size_t* size_to_compare) {
+  size_t compared_size = std::min(lhs->size(), rhs->size());
+  assert(*size_to_compare >= compared_size);
+  *size_to_compare -= compared_size;
+
+  int memcmp_res = ::memcmp(lhs->data(), rhs->data(), compared_size);
+  if (memcmp_res != 0) return memcmp_res;
+
+  lhs->remove_prefix(compared_size);
+  rhs->remove_prefix(compared_size);
+
+  return 0;
+}
+
+// This overload set computes comparison results from memcmp result. This
+// interface is used inside GenericCompare below. Differet implementations
+// are specialized for int and bool. For int we clamp result to {-1, 0, 1}
+// set. For bool we just interested in "value == 0".
+template <typename ResultType>
+ResultType ComputeCompareResult(int memcmp_res) {
+  return ClampResult(memcmp_res);
+}
+template <>
+bool ComputeCompareResult<bool>(int memcmp_res) {
+  return memcmp_res == 0;
+}
+
+}  // namespace
+
+// Helper routine. Locates the first flat chunk of the Cord without
+// initializing the iterator.
+inline absl::string_view Cord::InlineRep::FindFlatStartPiece() const {
+  size_t n = data_[kMaxInline];
+  if (n <= kMaxInline) {
+    return absl::string_view(data_, n);
+  }
+
+  CordRep* node = tree();
+  if (node->tag >= FLAT) {
+    return absl::string_view(node->data, node->length);
+  }
+
+  if (node->tag == EXTERNAL) {
+    return absl::string_view(node->external()->base, node->length);
+  }
+
+  // Walk down the left branches until we hit a non-CONCAT node.
+  while (node->tag == CONCAT) {
+    node = node->concat()->left;
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  assert(length != 0);
+
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  if (node->tag >= FLAT) {
+    return absl::string_view(node->data + offset, length);
+  }
+
+  assert((node->tag == EXTERNAL) && "Expect FLAT or EXTERNAL node here");
+
+  return absl::string_view(node->external()->base + offset, length);
+}
+
+inline int Cord::CompareSlowPath(absl::string_view rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+
+  // compared_size is inside first chunk.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && !rhs.empty()) {
+    int comparison_result = CompareChunks(&lhs_chunk, &rhs, &size_to_compare);
+    if (comparison_result != 0) return comparison_result;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs.empty()) - static_cast<int>(lhs_chunk.empty());
+}
+
+inline int Cord::CompareSlowPath(const Cord& rhs, size_t compared_size,
+                                 size_t size_to_compare) const {
+  auto advance = [](Cord::ChunkIterator* it, absl::string_view* chunk) {
+    if (!chunk->empty()) return true;
+    ++*it;
+    if (it->bytes_remaining_ == 0) return false;
+    *chunk = **it;
+    return true;
+  };
+
+  Cord::ChunkIterator lhs_it = chunk_begin();
+  Cord::ChunkIterator rhs_it = rhs.chunk_begin();
+
+  // compared_size is inside both first chunks.
+  absl::string_view lhs_chunk =
+      (lhs_it.bytes_remaining_ != 0) ? *lhs_it : absl::string_view();
+  absl::string_view rhs_chunk =
+      (rhs_it.bytes_remaining_ != 0) ? *rhs_it : absl::string_view();
+  assert(compared_size <= lhs_chunk.size());
+  assert(compared_size <= rhs_chunk.size());
+  lhs_chunk.remove_prefix(compared_size);
+  rhs_chunk.remove_prefix(compared_size);
+  size_to_compare -= compared_size;  // skip already compared size.
+
+  while (advance(&lhs_it, &lhs_chunk) && advance(&rhs_it, &rhs_chunk)) {
+    int memcmp_res = CompareChunks(&lhs_chunk, &rhs_chunk, &size_to_compare);
+    if (memcmp_res != 0) return memcmp_res;
+    if (size_to_compare == 0) return 0;
+  }
+
+  return static_cast<int>(rhs_chunk.empty()) -
+         static_cast<int>(lhs_chunk.empty());
+}
+
+inline absl::string_view Cord::GetFirstChunk(const Cord& c) {
+  return c.contents_.FindFlatStartPiece();
+}
+inline absl::string_view Cord::GetFirstChunk(absl::string_view sv) {
+  return sv;
+}
+
+// Compares up to 'size_to_compare' bytes of 'lhs' with 'rhs'. It is assumed
+// that 'size_to_compare' is greater that size of smallest of first chunks.
+template <typename ResultType, typename RHS>
+ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
+                          size_t size_to_compare) {
+  absl::string_view lhs_chunk = Cord::GetFirstChunk(lhs);
+  absl::string_view rhs_chunk = Cord::GetFirstChunk(rhs);
+
+  size_t compared_size = std::min(lhs_chunk.size(), rhs_chunk.size());
+  assert(size_to_compare >= compared_size);
+  int memcmp_res = ::memcmp(lhs_chunk.data(), rhs_chunk.data(), compared_size);
+  if (compared_size == size_to_compare || memcmp_res != 0) {
+    return ComputeCompareResult<ResultType>(memcmp_res);
+  }
+
+  return ComputeCompareResult<ResultType>(
+      lhs.CompareSlowPath(rhs, compared_size, size_to_compare));
+}
+
+bool Cord::EqualsImpl(absl::string_view rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+bool Cord::EqualsImpl(const Cord& rhs, size_t size_to_compare) const {
+  return GenericCompare<bool>(*this, rhs, size_to_compare);
+}
+
+template <typename RHS>
+inline int SharedCompareImpl(const Cord& lhs, const RHS& rhs) {
+  size_t lhs_size = lhs.size();
+  size_t rhs_size = rhs.size();
+  if (lhs_size == rhs_size) {
+    return GenericCompare<int>(lhs, rhs, lhs_size);
+  }
+  if (lhs_size < rhs_size) {
+    auto data_comp_res = GenericCompare<int>(lhs, rhs, lhs_size);
+    return data_comp_res == 0 ? -1 : data_comp_res;
+  }
+
+  auto data_comp_res = GenericCompare<int>(lhs, rhs, rhs_size);
+  return data_comp_res == 0 ? +1 : data_comp_res;
+}
+
+int Cord::Compare(absl::string_view rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+int Cord::CompareImpl(const Cord& rhs) const {
+  return SharedCompareImpl(*this, rhs);
+}
+
+bool Cord::EndsWith(absl::string_view rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+bool Cord::EndsWith(const Cord& rhs) const {
+  size_t my_size = size();
+  size_t rhs_size = rhs.size();
+
+  if (my_size < rhs_size) return false;
+
+  Cord tmp(*this);
+  tmp.RemovePrefix(my_size - rhs_size);
+  return tmp.EqualsImpl(rhs, rhs_size);
+}
+
+// --------------------------------------------------------------------
+// Misc.
+
+Cord::operator std::string() const {
+  std::string s;
+  absl::CopyCordToString(*this, &s);
+  return s;
+}
+
+void CopyCordToString(const Cord& src, std::string* dst) {
+  if (!src.contents_.is_tree()) {
+    src.contents_.CopyTo(dst);
+  } else {
+    absl::strings_internal::STLStringResizeUninitialized(dst, src.size());
+    src.CopyToArraySlowPath(&(*dst)[0]);
+  }
+}
+
+void Cord::CopyToArraySlowPath(char* dst) const {
+  assert(contents_.is_tree());
+  absl::string_view fragment;
+  if (GetFlatAux(contents_.tree(), &fragment)) {
+    memcpy(dst, fragment.data(), fragment.size());
+    return;
+  }
+  for (absl::string_view chunk : Chunks()) {
+    memcpy(dst, chunk.data(), chunk.size());
+    dst += chunk.size();
+  }
+}
+
+Cord::ChunkIterator& Cord::ChunkIterator::operator++() {
+  assert(bytes_remaining_ > 0 && "Attempted to iterate past `end()`");
+  assert(bytes_remaining_ >= current_chunk_.size());
+  bytes_remaining_ -= current_chunk_.size();
+
+  if (stack_of_right_children_.empty()) {
+    assert(!current_chunk_.empty());  // Called on invalid iterator.
+    // We have reached the end of the Cord.
+    return *this;
+  }
+
+  // Process the next node on the stack.
+  CordRep* node = stack_of_right_children_.back();
+  stack_of_right_children_.pop_back();
+
+  // Walk down the left branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    stack_of_right_children_.push_back(node->concat()->right);
+    node = node->concat()->left;
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length != 0);
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset, length);
+  current_leaf_ = node;
+  return *this;
+}
+
+Cord Cord::ChunkIterator::AdvanceAndReadBytes(size_t n) {
+  assert(bytes_remaining_ >= n && "Attempted to iterate past `end()`");
+  Cord subcord;
+
+  if (n <= InlineRep::kMaxInline) {
+    // Range to read fits in inline data. Flatten it.
+    char* data = subcord.contents_.set_data(n);
+    while (n > current_chunk_.size()) {
+      memcpy(data, current_chunk_.data(), current_chunk_.size());
+      data += current_chunk_.size();
+      n -= current_chunk_.size();
+      ++*this;
+    }
+    memcpy(data, current_chunk_.data(), n);
+    if (n < current_chunk_.size()) {
+      RemoveChunkPrefix(n);
+    } else if (n > 0) {
+      ++*this;
+    }
+    return subcord;
+  }
+  if (n < current_chunk_.size()) {
+    // Range to read is a proper subrange of the current chunk.
+    assert(current_leaf_ != nullptr);
+    CordRep* subnode = Ref(current_leaf_);
+    const char* data =
+        subnode->tag == EXTERNAL ? subnode->external()->base : subnode->data;
+    subnode = NewSubstring(subnode, current_chunk_.data() - data, n);
+    subcord.contents_.set_tree(VerifyTree(subnode));
+    RemoveChunkPrefix(n);
+    return subcord;
+  }
+
+  // Range to read begins with a proper subrange of the current chunk.
+  assert(!current_chunk_.empty());
+  assert(current_leaf_ != nullptr);
+  CordRep* subnode = Ref(current_leaf_);
+  if (current_chunk_.size() < subnode->length) {
+    const char* data =
+        subnode->tag == EXTERNAL ? subnode->external()->base : subnode->data;
+    subnode = NewSubstring(subnode, current_chunk_.data() - data,
+                           current_chunk_.size());
+  }
+  n -= current_chunk_.size();
+  bytes_remaining_ -= current_chunk_.size();
+
+  // Process the next node(s) on the stack, reading whole subtrees depending on
+  // their length and how many bytes we are advancing.
+  CordRep* node = nullptr;
+  while (!stack_of_right_children_.empty()) {
+    node = stack_of_right_children_.back();
+    stack_of_right_children_.pop_back();
+    if (node->length > n) break;
+    // TODO(qrczak): This might unnecessarily recreate existing concat nodes.
+    // Avoiding that would need pretty complicated logic (instead of
+    // current_leaf_, keep current_subtree_ which points to the highest node
+    // such that the current leaf can be found on the path of left children
+    // starting from current_subtree_; delay creating subnode while node is
+    // below current_subtree_; find the proper node along the path of left
+    // children starting from current_subtree_ if this loop exits while staying
+    // below current_subtree_; etc.; alternatively, push parents instead of
+    // right children on the stack).
+    subnode = Concat(subnode, Ref(node));
+    n -= node->length;
+    bytes_remaining_ -= node->length;
+    node = nullptr;
+  }
+
+  if (node == nullptr) {
+    // We have reached the end of the Cord.
+    assert(bytes_remaining_ == 0);
+    subcord.contents_.set_tree(VerifyTree(subnode));
+    return subcord;
+  }
+
+  // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    if (node->concat()->left->length > n) {
+      // Push right, descend left.
+      stack_of_right_children_.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Read left, descend right.
+      subnode = Concat(subnode, Ref(node->concat()->left));
+      n -= node->concat()->left->length;
+      bytes_remaining_ -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  // Range to read ends with a proper (possibly empty) subrange of the current
+  // chunk.
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length > n);
+  if (n > 0) subnode = Concat(subnode, NewSubstring(Ref(node), offset, n));
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset + n, length - n);
+  current_leaf_ = node;
+  bytes_remaining_ -= n;
+  subcord.contents_.set_tree(VerifyTree(subnode));
+  return subcord;
+}
+
+void Cord::ChunkIterator::AdvanceBytesSlowPath(size_t n) {
+  assert(bytes_remaining_ >= n && "Attempted to iterate past `end()`");
+  assert(n >= current_chunk_.size());  // This should only be called when
+                                       // iterating to a new node.
+
+  n -= current_chunk_.size();
+  bytes_remaining_ -= current_chunk_.size();
+
+  // Process the next node(s) on the stack, skipping whole subtrees depending on
+  // their length and how many bytes we are advancing.
+  CordRep* node = nullptr;
+  while (!stack_of_right_children_.empty()) {
+    node = stack_of_right_children_.back();
+    stack_of_right_children_.pop_back();
+    if (node->length > n) break;
+    n -= node->length;
+    bytes_remaining_ -= node->length;
+    node = nullptr;
+  }
+
+  if (node == nullptr) {
+    // We have reached the end of the Cord.
+    assert(bytes_remaining_ == 0);
+    return;
+  }
+
+  // Walk down the appropriate branches until we hit a non-CONCAT node. Save the
+  // right children to the stack for subsequent traversal.
+  while (node->tag == CONCAT) {
+    if (node->concat()->left->length > n) {
+      // Push right, descend left.
+      stack_of_right_children_.push_back(node->concat()->right);
+      node = node->concat()->left;
+    } else {
+      // Skip left, descend right.
+      n -= node->concat()->left->length;
+      bytes_remaining_ -= node->concat()->left->length;
+      node = node->concat()->right;
+    }
+  }
+
+  // Get the child node if we encounter a SUBSTRING.
+  size_t offset = 0;
+  size_t length = node->length;
+  if (node->tag == SUBSTRING) {
+    offset = node->substring()->start;
+    node = node->substring()->child;
+  }
+
+  assert(node->tag == EXTERNAL || node->tag >= FLAT);
+  assert(length > n);
+  const char* data =
+      node->tag == EXTERNAL ? node->external()->base : node->data;
+  current_chunk_ = absl::string_view(data + offset + n, length - n);
+  current_leaf_ = node;
+  bytes_remaining_ -= n;
+}
+
+char Cord::operator[](size_t i) const {
+  assert(i < size());
+  size_t offset = i;
+  const CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return contents_.data()[i];
+  }
+  while (true) {
+    assert(rep != nullptr);
+    assert(offset < rep->length);
+    if (rep->tag >= FLAT) {
+      // Get the "i"th character directly from the flat array.
+      return rep->data[offset];
+    } else if (rep->tag == EXTERNAL) {
+      // Get the "i"th character from the external array.
+      return rep->external()->base[offset];
+    } else if (rep->tag == CONCAT) {
+      // Recursively branch to the side of the concatenation that the "i"th
+      // character is on.
+      size_t left_length = rep->concat()->left->length;
+      if (offset < left_length) {
+        rep = rep->concat()->left;
+      } else {
+        offset -= left_length;
+        rep = rep->concat()->right;
+      }
+    } else {
+      // This must be a substring a node, so bypass it to get to the child.
+      assert(rep->tag == SUBSTRING);
+      offset += rep->substring()->start;
+      rep = rep->substring()->child;
+    }
+  }
+}
+
+absl::string_view Cord::FlattenSlowPath() {
+  size_t total_size = size();
+  CordRep* new_rep;
+  char* new_buffer;
+
+  // Try to put the contents into a new flat rep. If they won't fit in the
+  // biggest possible flat node, use an external rep instead.
+  if (total_size <= kMaxFlatLength) {
+    new_rep = NewFlat(total_size);
+    new_rep->length = total_size;
+    new_buffer = new_rep->data;
+    CopyToArraySlowPath(new_buffer);
+  } else {
+    new_buffer = std::allocator<char>().allocate(total_size);
+    CopyToArraySlowPath(new_buffer);
+    new_rep = absl::cord_internal::NewExternalRep(
+        absl::string_view(new_buffer, total_size), [](absl::string_view s) {
+          std::allocator<char>().deallocate(const_cast<char*>(s.data()),
+                                            s.size());
+        });
+  }
+  Unref(contents_.tree());
+  contents_.set_tree(new_rep);
+  return absl::string_view(new_buffer, total_size);
+}
+
+/* static */ bool Cord::GetFlatAux(CordRep* rep, absl::string_view* fragment) {
+  assert(rep != nullptr);
+  if (rep->tag >= FLAT) {
+    *fragment = absl::string_view(rep->data, rep->length);
+    return true;
+  } else if (rep->tag == EXTERNAL) {
+    *fragment = absl::string_view(rep->external()->base, rep->length);
+    return true;
+  } else if (rep->tag == SUBSTRING) {
+    CordRep* child = rep->substring()->child;
+    if (child->tag >= FLAT) {
+      *fragment =
+          absl::string_view(child->data + rep->substring()->start, rep->length);
+      return true;
+    } else if (child->tag == EXTERNAL) {
+      *fragment = absl::string_view(
+          child->external()->base + rep->substring()->start, rep->length);
+      return true;
+    }
+  }
+  return false;
+}
+
+/* static */ void Cord::ForEachChunkAux(
+    absl::cord_internal::CordRep* rep,
+    absl::FunctionRef<void(absl::string_view)> callback) {
+  assert(rep != nullptr);
+  int stack_pos = 0;
+  constexpr int stack_max = 128;
+  // Stack of right branches for tree traversal
+  absl::cord_internal::CordRep* stack[stack_max];
+  absl::cord_internal::CordRep* current_node = rep;
+  while (true) {
+    if (current_node->tag == CONCAT) {
+      if (stack_pos == stack_max) {
+        // There's no more room on our stack array to add another right branch,
+        // and the idea is to avoid allocations, so call this function
+        // recursively to navigate this subtree further.  (This is not something
+        // we expect to happen in practice).
+        ForEachChunkAux(current_node, callback);
+
+        // Pop the next right branch and iterate.
+        current_node = stack[--stack_pos];
+        continue;
+      } else {
+        // Save the right branch for later traversal and continue down the left
+        // branch.
+        stack[stack_pos++] = current_node->concat()->right;
+        current_node = current_node->concat()->left;
+        continue;
+      }
+    }
+    // This is a leaf node, so invoke our callback.
+    absl::string_view chunk;
+    bool success = GetFlatAux(current_node, &chunk);
+    assert(success);
+    if (success) {
+      callback(chunk);
+    }
+    if (stack_pos == 0) {
+      // end of traversal
+      return;
+    }
+    current_node = stack[--stack_pos];
+  }
+}
+
+static void DumpNode(CordRep* rep, bool include_data, std::ostream* os) {
+  const int kIndentStep = 1;
+  int indent = 0;
+  absl::InlinedVector<CordRep*, kInlinedVectorSize> stack;
+  absl::InlinedVector<int, kInlinedVectorSize> indents;
+  for (;;) {
+    *os << std::setw(3) << rep->refcount.Get();
+    *os << " " << std::setw(7) << rep->length;
+    *os << " [";
+    if (include_data) *os << static_cast<void*>(rep);
+    *os << "]";
+    *os << " " << (IsRootBalanced(rep) ? 'b' : 'u');
+    *os << " " << std::setw(indent) << "";
+    if (rep->tag == CONCAT) {
+      *os << "CONCAT depth=" << Depth(rep) << "\n";
+      indent += kIndentStep;
+      indents.push_back(indent);
+      stack.push_back(rep->concat()->right);
+      rep = rep->concat()->left;
+    } else if (rep->tag == SUBSTRING) {
+      *os << "SUBSTRING @ " << rep->substring()->start << "\n";
+      indent += kIndentStep;
+      rep = rep->substring()->child;
+    } else {  // Leaf
+      if (rep->tag == EXTERNAL) {
+        *os << "EXTERNAL [";
+        if (include_data)
+          *os << absl::CEscape(std::string(rep->external()->base, rep->length));
+        *os << "]\n";
+      } else {
+        *os << "FLAT cap=" << TagToLength(rep->tag) << " [";
+        if (include_data)
+          *os << absl::CEscape(std::string(rep->data, rep->length));
+        *os << "]\n";
+      }
+      if (stack.empty()) break;
+      rep = stack.back();
+      stack.pop_back();
+      indent = indents.back();
+      indents.pop_back();
+    }
+  }
+  ABSL_INTERNAL_CHECK(indents.empty(), "");
+}
+
+static std::string ReportError(CordRep* root, CordRep* node) {
+  std::ostringstream buf;
+  buf << "Error at node " << node << " in:";
+  DumpNode(root, true, &buf);
+  return buf.str();
+}
+
+static bool VerifyNode(CordRep* root, CordRep* start_node,
+                       bool full_validation) {
+  absl::InlinedVector<CordRep*, 2> worklist;
+  worklist.push_back(start_node);
+  do {
+    CordRep* node = worklist.back();
+    worklist.pop_back();
+
+    ABSL_INTERNAL_CHECK(node != nullptr, ReportError(root, node));
+    if (node != root) {
+      ABSL_INTERNAL_CHECK(node->length != 0, ReportError(root, node));
+    }
+
+    if (node->tag == CONCAT) {
+      ABSL_INTERNAL_CHECK(node->concat()->left != nullptr,
+                          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->concat()->right != nullptr,
+                          ReportError(root, node));
+      ABSL_INTERNAL_CHECK((node->length == node->concat()->left->length +
+                                               node->concat()->right->length),
+                          ReportError(root, node));
+      if (full_validation) {
+        worklist.push_back(node->concat()->right);
+        worklist.push_back(node->concat()->left);
+      }
+    } else if (node->tag >= FLAT) {
+      ABSL_INTERNAL_CHECK(node->length <= TagToLength(node->tag),
+                          ReportError(root, node));
+    } else if (node->tag == EXTERNAL) {
+      ABSL_INTERNAL_CHECK(node->external()->base != nullptr,
+                          ReportError(root, node));
+    } else if (node->tag == SUBSTRING) {
+      ABSL_INTERNAL_CHECK(
+          node->substring()->start < node->substring()->child->length,
+          ReportError(root, node));
+      ABSL_INTERNAL_CHECK(node->substring()->start + node->length <=
+                              node->substring()->child->length,
+                          ReportError(root, node));
+    }
+  } while (!worklist.empty());
+  return true;
+}
+
+// Traverses the tree and computes the total memory allocated.
+/* static */ size_t Cord::MemoryUsageAux(const CordRep* rep) {
+  size_t total_mem_usage = 0;
+
+  // Allow a quick exit for the common case that the root is a leaf.
+  if (RepMemoryUsageLeaf(rep, &total_mem_usage)) {
+    return total_mem_usage;
+  }
+
+  // Iterate over the tree. cur_node is never a leaf node and leaf nodes will
+  // never be appended to tree_stack. This reduces overhead from manipulating
+  // tree_stack.
+  absl::InlinedVector<const CordRep*, kInlinedVectorSize> tree_stack;
+  const CordRep* cur_node = rep;
+  while (true) {
+    const CordRep* next_node = nullptr;
+
+    if (cur_node->tag == CONCAT) {
+      total_mem_usage += sizeof(CordRepConcat);
+      const CordRep* left = cur_node->concat()->left;
+      if (!RepMemoryUsageLeaf(left, &total_mem_usage)) {
+        next_node = left;
+      }
+
+      const CordRep* right = cur_node->concat()->right;
+      if (!RepMemoryUsageLeaf(right, &total_mem_usage)) {
+        if (next_node) {
+          tree_stack.push_back(next_node);
+        }
+        next_node = right;
+      }
+    } else {
+      // Since cur_node is not a leaf or a concat node it must be a substring.
+      assert(cur_node->tag == SUBSTRING);
+      total_mem_usage += sizeof(CordRepSubstring);
+      next_node = cur_node->substring()->child;
+      if (RepMemoryUsageLeaf(next_node, &total_mem_usage)) {
+        next_node = nullptr;
+      }
+    }
+
+    if (!next_node) {
+      if (tree_stack.empty()) {
+        return total_mem_usage;
+      }
+      next_node = tree_stack.back();
+      tree_stack.pop_back();
+    }
+    cur_node = next_node;
+  }
+}
+
+std::ostream& operator<<(std::ostream& out, const Cord& cord) {
+  for (absl::string_view chunk : cord.Chunks()) {
+    out.write(chunk.data(), chunk.size());
+  }
+  return out;
+}
+
+namespace strings_internal {
+size_t CordTestAccess::FlatOverhead() { return kFlatOverhead; }
+size_t CordTestAccess::MaxFlatLength() { return kMaxFlatLength; }
+size_t CordTestAccess::FlatTagToLength(uint8_t tag) {
+  return TagToLength(tag);
+}
+uint8_t CordTestAccess::LengthToTag(size_t s) {
+  ABSL_INTERNAL_CHECK(s <= kMaxFlatLength, absl::StrCat("Invalid length ", s));
+  return AllocatedSizeToTag(s + kFlatOverhead);
+}
+size_t CordTestAccess::SizeofCordRepConcat() { return sizeof(CordRepConcat); }
+size_t CordTestAccess::SizeofCordRepExternal() {
+  return sizeof(CordRepExternal);
+}
+size_t CordTestAccess::SizeofCordRepSubstring() {
+  return sizeof(CordRepSubstring);
+}
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/cord.h b/absl/strings/cord.h
new file mode 100644
index 0000000..40566cb
--- /dev/null
+++ b/absl/strings/cord.h
@@ -0,0 +1,1121 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// A Cord is a sequence of characters with some unusual access propreties.
+// A Cord supports efficient insertions and deletions at the start and end of
+// the byte sequence, but random access reads are slower, and random access
+// modifications are not supported by the API.  Cord also provides cheap copies
+// (using a copy-on-write strategy) and cheap substring operations.
+//
+// Thread safety
+// -------------
+// Cord has the same thread-safety properties as many other types like
+// std::string, std::vector<>, int, etc -- it is thread-compatible. In
+// particular, if no thread may call a non-const method, then it is safe to
+// concurrently call const methods. Copying a Cord produces a new instance that
+// can be used concurrently with the original in arbitrary ways.
+//
+// Implementation is similar to the "Ropes" described in:
+//    Ropes: An alternative to strings
+//    Hans J. Boehm, Russ Atkinson, Michael Plass
+//    Software Practice and Experience, December 1995
+
+#ifndef ABSL_STRINGS_CORD_H_
+#define ABSL_STRINGS_CORD_H_
+
+#include <algorithm>
+#include <cstddef>
+#include <cstdint>
+#include <cstring>
+#include <iostream>
+#include <iterator>
+#include <string>
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/invoke.h"
+#include "absl/base/internal/per_thread_tls.h"
+#include "absl/base/macros.h"
+#include "absl/base/port.h"
+#include "absl/container/inlined_vector.h"
+#include "absl/functional/function_ref.h"
+#include "absl/meta/type_traits.h"
+#include "absl/strings/internal/cord_internal.h"
+#include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+class Cord;
+class CordTestPeer;
+template <typename Releaser>
+Cord MakeCordFromExternal(absl::string_view, Releaser&&);
+void CopyCordToString(const Cord& src, std::string* dst);
+namespace hash_internal {
+template <typename H>
+H HashFragmentedCord(H, const Cord&);
+}
+
+// A Cord is a sequence of characters.
+class Cord {
+ private:
+  template <typename T>
+  using EnableIfString =
+      absl::enable_if_t<std::is_same<T, std::string>::value, int>;
+
+ public:
+  // --------------------------------------------------------------------
+  // Constructors, destructors and helper factories
+
+  // Create an empty cord
+  constexpr Cord() noexcept;
+
+  // Cord is copyable and efficiently movable.
+  // The moved-from state is valid but unspecified.
+  Cord(const Cord& src);
+  Cord(Cord&& src) noexcept;
+  Cord& operator=(const Cord& x);
+  Cord& operator=(Cord&& x) noexcept;
+
+  // Create a cord out of "src". This constructor is explicit on
+  // purpose so that people do not get automatic type conversions.
+  explicit Cord(absl::string_view src);
+  Cord& operator=(absl::string_view src);
+
+  // These are templated to avoid ambiguities for types that are convertible to
+  // both `absl::string_view` and `std::string`, such as `const char*`.
+  //
+  // Note that these functions reserve the right to reuse the `string&&`'s
+  // memory and that they will do so in the future.
+  template <typename T, EnableIfString<T> = 0>
+  explicit Cord(T&& src) : Cord(absl::string_view(src)) {}
+  template <typename T, EnableIfString<T> = 0>
+  Cord& operator=(T&& src);
+
+  // Destroy the cord
+  ~Cord() {
+    if (contents_.is_tree()) DestroyCordSlow();
+  }
+
+  // Creates a Cord that takes ownership of external memory. The contents of
+  // `data` are not copied.
+  //
+  // This function takes a callable that is invoked when all Cords are
+  // finished with `data`. The data must remain live and unchanging until the
+  // releaser is called. The requirements for the releaser are that it:
+  //   * is move constructible,
+  //   * supports `void operator()(absl::string_view) const`,
+  //   * does not have alignment requirement greater than what is guaranteed by
+  //     ::operator new. This is dictated by alignof(std::max_align_t) before
+  //     C++17 and __STDCPP_DEFAULT_NEW_ALIGNMENT__ if compiling with C++17 or
+  //     it is supported by the implementation.
+  //
+  // Example:
+  //
+  // Cord MakeCord(BlockPool* pool) {
+  //   Block* block = pool->NewBlock();
+  //   FillBlock(block);
+  //   return absl::MakeCordFromExternal(
+  //       block->ToStringView(),
+  //       [pool, block](absl::string_view /*ignored*/) {
+  //         pool->FreeBlock(block);
+  //       });
+  // }
+  //
+  // WARNING: It's likely a bug if your releaser doesn't do anything.
+  // For example, consider the following:
+  //
+  // void Foo(const char* buffer, int len) {
+  //   auto c = absl::MakeCordFromExternal(absl::string_view(buffer, len),
+  //                                       [](absl::string_view) {});
+  //
+  //   // BUG: If Bar() copies its cord for any reason, including keeping a
+  //   // substring of it, the lifetime of buffer might be extended beyond
+  //   // when Foo() returns.
+  //   Bar(c);
+  // }
+  template <typename Releaser>
+  friend Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser);
+
+  // --------------------------------------------------------------------
+  // Mutations
+
+  void Clear();
+
+  void Append(const Cord& src);
+  void Append(Cord&& src);
+  void Append(absl::string_view src);
+  template <typename T, EnableIfString<T> = 0>
+  void Append(T&& src);
+
+  void Prepend(const Cord& src);
+  void Prepend(absl::string_view src);
+  template <typename T, EnableIfString<T> = 0>
+  void Prepend(T&& src);
+
+  void RemovePrefix(size_t n);
+  void RemoveSuffix(size_t n);
+
+  // Returns a new cord representing the subrange [pos, pos + new_size) of
+  // *this. If pos >= size(), the result is empty(). If
+  // (pos + new_size) >= size(), the result is the subrange [pos, size()).
+  Cord Subcord(size_t pos, size_t new_size) const;
+
+  friend void swap(Cord& x, Cord& y) noexcept;
+
+  // --------------------------------------------------------------------
+  // Accessors
+
+  size_t size() const;
+  bool empty() const;
+
+  // Returns the approximate number of bytes pinned by this Cord.  Note that
+  // Cords that share memory could each be "charged" independently for the same
+  // shared memory.
+  size_t EstimatedMemoryUsage() const;
+
+  // --------------------------------------------------------------------
+  // Comparators
+
+  // Compares 'this' Cord with rhs. This function and its relatives
+  // treat Cords as sequences of unsigned bytes. The comparison is a
+  // straightforward lexicographic comparison. Return value:
+  //   -1  'this' Cord is smaller
+  //    0  two Cords are equal
+  //    1  'this' Cord is larger
+  int Compare(absl::string_view rhs) const;
+  int Compare(const Cord& rhs) const;
+
+  // Does 'this' cord start/end with rhs
+  bool StartsWith(const Cord& rhs) const;
+  bool StartsWith(absl::string_view rhs) const;
+  bool EndsWith(absl::string_view rhs) const;
+  bool EndsWith(const Cord& rhs) const;
+
+  // --------------------------------------------------------------------
+  // Conversion to other types
+
+  explicit operator std::string() const;
+
+  // Copies the contents from `src` to `*dst`.
+  //
+  // This function optimizes the case of reusing the destination std::string since it
+  // can reuse previously allocated capacity. However, this function does not
+  // guarantee that pointers previously returned by `dst->data()` remain valid
+  // even if `*dst` had enough capacity to hold `src`. If `*dst` is a new
+  // object, prefer to simply use the conversion operator to `std::string`.
+  friend void CopyCordToString(const Cord& src, std::string* dst);
+
+  // --------------------------------------------------------------------
+  // Iteration
+
+  class CharIterator;
+
+  // Type for iterating over the chunks of a `Cord`. See comments for
+  // `Cord::chunk_begin()`, `Cord::chunk_end()` and `Cord::Chunks()` below for
+  // preferred usage.
+  //
+  // Additional notes:
+  //   * The `string_view` returned by dereferencing a valid, non-`end()`
+  //     iterator is guaranteed to be non-empty.
+  //   * A `ChunkIterator` object is invalidated after any non-const
+  //     operation on the `Cord` object over which it iterates.
+  //   * Two `ChunkIterator` objects can be equality compared if and only if
+  //     they remain valid and iterate over the same `Cord`.
+  //   * This is a proxy iterator. This means the `string_view` returned by the
+  //     iterator does not live inside the Cord, and its lifetime is limited to
+  //     the lifetime of the iterator itself. To help prevent issues,
+  //     `ChunkIterator::reference` is not a true reference type and is
+  //     equivalent to `value_type`.
+  //   * The iterator keeps state that can grow for `Cord`s that contain many
+  //     nodes and are imbalanced due to sharing. Prefer to pass this type by
+  //     const reference instead of by value.
+  class ChunkIterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = absl::string_view;
+    using difference_type = ptrdiff_t;
+    using pointer = const value_type*;
+    using reference = value_type;
+
+    ChunkIterator() = default;
+
+    ChunkIterator& operator++();
+    ChunkIterator operator++(int);
+    bool operator==(const ChunkIterator& other) const;
+    bool operator!=(const ChunkIterator& other) const;
+    reference operator*() const;
+    pointer operator->() const;
+
+    friend class Cord;
+    friend class CharIterator;
+
+   private:
+    // Constructs a `begin()` iterator from `cord`.
+    explicit ChunkIterator(const Cord* cord);
+
+    // Removes `n` bytes from `current_chunk_`. Expects `n` to be smaller than
+    // `current_chunk_.size()`.
+    void RemoveChunkPrefix(size_t n);
+    Cord AdvanceAndReadBytes(size_t n);
+    void AdvanceBytes(size_t n);
+    // Iterates `n` bytes, where `n` is expected to be greater than or equal to
+    // `current_chunk_.size()`.
+    void AdvanceBytesSlowPath(size_t n);
+
+    // A view into bytes of the current `CordRep`. It may only be a view to a
+    // suffix of bytes if this is being used by `CharIterator`.
+    absl::string_view current_chunk_;
+    // The current leaf, or `nullptr` if the iterator points to short data.
+    // If the current chunk is a substring node, current_leaf_ points to the
+    // underlying flat or external node.
+    absl::cord_internal::CordRep* current_leaf_ = nullptr;
+    // The number of bytes left in the `Cord` over which we are iterating.
+    size_t bytes_remaining_ = 0;
+    absl::InlinedVector<absl::cord_internal::CordRep*, 4>
+        stack_of_right_children_;
+  };
+
+  // Returns an iterator to the first chunk of the `Cord`.
+  //
+  // This is useful for getting a `ChunkIterator` outside the context of a
+  // range-based for-loop (in which case see `Cord::Chunks()` below).
+  //
+  // Example:
+  //
+  //   absl::Cord::ChunkIterator FindAsChunk(const absl::Cord& c,
+  //                                         absl::string_view s) {
+  //     return std::find(c.chunk_begin(), c.chunk_end(), s);
+  //   }
+  ChunkIterator chunk_begin() const;
+  // Returns an iterator one increment past the last chunk of the `Cord`.
+  ChunkIterator chunk_end() const;
+
+  // Convenience wrapper over `Cord::chunk_begin()` and `Cord::chunk_end()` to
+  // enable range-based for-loop iteration over `Cord` chunks.
+  //
+  // Prefer to use `Cord::Chunks()` below instead of constructing this directly.
+  class ChunkRange {
+   public:
+    explicit ChunkRange(const Cord* cord) : cord_(cord) {}
+
+    ChunkIterator begin() const;
+    ChunkIterator end() const;
+
+   private:
+    const Cord* cord_;
+  };
+
+  // Returns a range for iterating over the chunks of a `Cord` with a
+  // range-based for-loop.
+  //
+  // Example:
+  //
+  //   void ProcessChunks(const Cord& cord) {
+  //     for (absl::string_view chunk : cord.Chunks()) { ... }
+  //   }
+  //
+  // Note that the ordinary caveats of temporary lifetime extension apply:
+  //
+  //   void Process() {
+  //     for (absl::string_view chunk : CordFactory().Chunks()) {
+  //       // The temporary Cord returned by CordFactory has been destroyed!
+  //     }
+  //   }
+  ChunkRange Chunks() const;
+
+  // Type for iterating over the characters of a `Cord`. See comments for
+  // `Cord::char_begin()`, `Cord::char_end()` and `Cord::Chars()` below for
+  // preferred usage.
+  //
+  // Additional notes:
+  //   * A `CharIterator` object is invalidated after any non-const
+  //     operation on the `Cord` object over which it iterates.
+  //   * Two `CharIterator` objects can be equality compared if and only if
+  //     they remain valid and iterate over the same `Cord`.
+  //   * The iterator keeps state that can grow for `Cord`s that contain many
+  //     nodes and are imbalanced due to sharing. Prefer to pass this type by
+  //     const reference instead of by value.
+  //   * This type cannot be a forward iterator because a `Cord` can reuse
+  //     sections of memory. This violates the requirement that if dereferencing
+  //     two iterators returns the same object, the iterators must compare
+  //     equal.
+  class CharIterator {
+   public:
+    using iterator_category = std::input_iterator_tag;
+    using value_type = char;
+    using difference_type = ptrdiff_t;
+    using pointer = const char*;
+    using reference = const char&;
+
+    CharIterator() = default;
+
+    CharIterator& operator++();
+    CharIterator operator++(int);
+    bool operator==(const CharIterator& other) const;
+    bool operator!=(const CharIterator& other) const;
+    reference operator*() const;
+    pointer operator->() const;
+
+    friend Cord;
+
+   private:
+    explicit CharIterator(const Cord* cord) : chunk_iterator_(cord) {}
+
+    ChunkIterator chunk_iterator_;
+  };
+
+  // Advances `*it` by `n_bytes` and returns the bytes passed as a `Cord`.
+  //
+  // `n_bytes` must be less than or equal to the number of bytes remaining for
+  // iteration. Otherwise the behavior is undefined. It is valid to pass
+  // `char_end()` and 0.
+  static Cord AdvanceAndRead(CharIterator* it, size_t n_bytes);
+
+  // Advances `*it` by `n_bytes`.
+  //
+  // `n_bytes` must be less than or equal to the number of bytes remaining for
+  // iteration. Otherwise the behavior is undefined. It is valid to pass
+  // `char_end()` and 0.
+  static void Advance(CharIterator* it, size_t n_bytes);
+
+  // Returns the longest contiguous view starting at the iterator's position.
+  //
+  // `it` must be dereferenceable.
+  static absl::string_view ChunkRemaining(const CharIterator& it);
+
+  // Returns an iterator to the first character of the `Cord`.
+  CharIterator char_begin() const;
+  // Returns an iterator to one past the last character of the `Cord`.
+  CharIterator char_end() const;
+
+  // Convenience wrapper over `Cord::char_begin()` and `Cord::char_end()` to
+  // enable range-based for-loop iterator over the characters of a `Cord`.
+  //
+  // Prefer to use `Cord::Chars()` below instead of constructing this directly.
+  class CharRange {
+   public:
+    explicit CharRange(const Cord* cord) : cord_(cord) {}
+
+    CharIterator begin() const;
+    CharIterator end() const;
+
+   private:
+    const Cord* cord_;
+  };
+
+  // Returns a range for iterating over the characters of a `Cord` with a
+  // range-based for-loop.
+  //
+  // Example:
+  //
+  //   void ProcessCord(const Cord& cord) {
+  //     for (char c : cord.Chars()) { ... }
+  //   }
+  //
+  // Note that the ordinary caveats of temporary lifetime extension apply:
+  //
+  //   void Process() {
+  //     for (char c : CordFactory().Chars()) {
+  //       // The temporary Cord returned by CordFactory has been destroyed!
+  //     }
+  //   }
+  CharRange Chars() const;
+
+  // --------------------------------------------------------------------
+  // Miscellaneous
+
+  // Get the "i"th character of 'this' and return it.
+  // NOTE: This routine is reasonably efficient.  It is roughly
+  // logarithmic in the number of nodes that make up the cord.  Still,
+  // if you need to iterate over the contents of a cord, you should
+  // use a CharIterator/CordIterator rather than call operator[] or Get()
+  //  repeatedly in a loop.
+  //
+  // REQUIRES: 0 <= i < size()
+  char operator[](size_t i) const;
+
+  // Flattens the cord into a single array and returns a view of the data.
+  //
+  // If the cord was already flat, the contents are not modified.
+  absl::string_view Flatten();
+
+ private:
+  friend class CordTestPeer;
+  template <typename H>
+  friend H absl::hash_internal::HashFragmentedCord(H, const Cord&);
+  friend bool operator==(const Cord& lhs, const Cord& rhs);
+  friend bool operator==(const Cord& lhs, absl::string_view rhs);
+
+  // Call the provided function once for each cord chunk, in order.  Unlike
+  // Chunks(), this API will not allocate memory.
+  void ForEachChunk(absl::FunctionRef<void(absl::string_view)>) const;
+
+  // Allocates new contiguous storage for the contents of the cord. This is
+  // called by Flatten() when the cord was not already flat.
+  absl::string_view FlattenSlowPath();
+
+  // Actual cord contents are hidden inside the following simple
+  // class so that we can isolate the bulk of cord.cc from changes
+  // to the representation.
+  //
+  // InlineRep holds either either a tree pointer, or an array of kMaxInline
+  // bytes.
+  class InlineRep {
+   public:
+    static const unsigned char kMaxInline = 15;
+    static_assert(kMaxInline >= sizeof(absl::cord_internal::CordRep*), "");
+    // Tag byte & kMaxInline means we are storing a pointer.
+    static const unsigned char kTreeFlag = 1 << 4;
+    // Tag byte & kProfiledFlag means we are profiling the Cord.
+    static const unsigned char kProfiledFlag = 1 << 5;
+
+    constexpr InlineRep() : data_{} {}
+    InlineRep(const InlineRep& src);
+    InlineRep(InlineRep&& src);
+    InlineRep& operator=(const InlineRep& src);
+    InlineRep& operator=(InlineRep&& src) noexcept;
+
+    void Swap(InlineRep* rhs);
+    bool empty() const;
+    size_t size() const;
+    const char* data() const;  // Returns nullptr if holding pointer
+    void set_data(const char* data, size_t n,
+                  bool nullify_tail);  // Discards pointer, if any
+    char* set_data(size_t n);  // Write data to the result
+    // Returns nullptr if holding bytes
+    absl::cord_internal::CordRep* tree() const;
+    // Discards old pointer, if any
+    void set_tree(absl::cord_internal::CordRep* rep);
+    // Replaces a tree with a new root. This is faster than set_tree, but it
+    // should only be used when it's clear that the old rep was a tree.
+    void replace_tree(absl::cord_internal::CordRep* rep);
+    // Returns non-null iff was holding a pointer
+    absl::cord_internal::CordRep* clear();
+    // Convert to pointer if necessary
+    absl::cord_internal::CordRep* force_tree(size_t extra_hint);
+    void reduce_size(size_t n);  // REQUIRES: holding data
+    void remove_prefix(size_t n);  // REQUIRES: holding data
+    void AppendArray(const char* src_data, size_t src_size);
+    absl::string_view FindFlatStartPiece() const;
+    void AppendTree(absl::cord_internal::CordRep* tree);
+    void PrependTree(absl::cord_internal::CordRep* tree);
+    void GetAppendRegion(char** region, size_t* size, size_t max_length);
+    void GetAppendRegion(char** region, size_t* size);
+    bool IsSame(const InlineRep& other) const {
+      return memcmp(data_, other.data_, sizeof(data_)) == 0;
+    }
+    int BitwiseCompare(const InlineRep& other) const {
+      uint64_t x, y;
+      // Use memcpy to avoid anti-aliasing issues.
+      memcpy(&x, data_, sizeof(x));
+      memcpy(&y, other.data_, sizeof(y));
+      if (x == y) {
+        memcpy(&x, data_ + 8, sizeof(x));
+        memcpy(&y, other.data_ + 8, sizeof(y));
+        if (x == y) return 0;
+      }
+      return absl::big_endian::FromHost64(x) < absl::big_endian::FromHost64(y)
+                 ? -1
+                 : 1;
+    }
+    void CopyTo(std::string* dst) const {
+      // memcpy is much faster when operating on a known size. On most supported
+      // platforms, the small std::string optimization is large enough that resizing
+      // to 15 bytes does not cause a memory allocation.
+      absl::strings_internal::STLStringResizeUninitialized(dst,
+                                                           sizeof(data_) - 1);
+      memcpy(&(*dst)[0], data_, sizeof(data_) - 1);
+      // erase is faster than resize because the logic for memory allocation is
+      // not needed.
+      dst->erase(data_[kMaxInline]);
+    }
+
+    // Copies the inline contents into `dst`. Assumes the cord is not empty.
+    void CopyToArray(char* dst) const;
+
+    bool is_tree() const { return data_[kMaxInline] > kMaxInline; }
+
+   private:
+    friend class Cord;
+
+    void AssignSlow(const InlineRep& src);
+    // Unrefs the tree, stops profiling, and zeroes the contents
+    void ClearSlow();
+
+    // If the data has length <= kMaxInline, we store it in data_[0..len-1],
+    // and store the length in data_[kMaxInline].  Else we store it in a tree
+    // and store a pointer to that tree in data_[0..sizeof(CordRep*)-1].
+    alignas(absl::cord_internal::CordRep*) char data_[kMaxInline + 1];
+  };
+  InlineRep contents_;
+
+  // Helper for MemoryUsage()
+  static size_t MemoryUsageAux(const absl::cord_internal::CordRep* rep);
+
+  // Helper for GetFlat()
+  static bool GetFlatAux(absl::cord_internal::CordRep* rep,
+                         absl::string_view* fragment);
+
+  // Helper for ForEachChunk()
+  static void ForEachChunkAux(
+      absl::cord_internal::CordRep* rep,
+      absl::FunctionRef<void(absl::string_view)> callback);
+
+  // The destructor for non-empty Cords.
+  void DestroyCordSlow();
+
+  // Out-of-line implementation of slower parts of logic.
+  void CopyToArraySlowPath(char* dst) const;
+  int CompareSlowPath(absl::string_view rhs, size_t compared_size,
+                      size_t size_to_compare) const;
+  int CompareSlowPath(const Cord& rhs, size_t compared_size,
+                      size_t size_to_compare) const;
+  bool EqualsImpl(absl::string_view rhs, size_t size_to_compare) const;
+  bool EqualsImpl(const Cord& rhs, size_t size_to_compare) const;
+  int CompareImpl(const Cord& rhs) const;
+
+  template <typename ResultType, typename RHS>
+  friend ResultType GenericCompare(const Cord& lhs, const RHS& rhs,
+                                   size_t size_to_compare);
+  static absl::string_view GetFirstChunk(const Cord& c);
+  static absl::string_view GetFirstChunk(absl::string_view sv);
+
+  // Returns a new reference to contents_.tree(), or steals an existing
+  // reference if called on an rvalue.
+  absl::cord_internal::CordRep* TakeRep() const&;
+  absl::cord_internal::CordRep* TakeRep() &&;
+
+  // Helper for Append()
+  template <typename C>
+  void AppendImpl(C&& src);
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// allow a Cord to be logged
+extern std::ostream& operator<<(std::ostream& out, const Cord& cord);
+
+// ------------------------------------------------------------------
+// Internal details follow.  Clients should ignore.
+
+namespace cord_internal {
+
+// Fast implementation of memmove for up to 15 bytes. This implementation is
+// safe for overlapping regions. If nullify_tail is true, the destination is
+// padded with '\0' up to 16 bytes.
+inline void SmallMemmove(char* dst, const char* src, size_t n,
+                         bool nullify_tail = false) {
+  if (n >= 8) {
+    assert(n <= 16);
+    uint64_t buf1;
+    uint64_t buf2;
+    memcpy(&buf1, src, 8);
+    memcpy(&buf2, src + n - 8, 8);
+    if (nullify_tail) {
+      memset(dst + 8, 0, 8);
+    }
+    memcpy(dst, &buf1, 8);
+    memcpy(dst + n - 8, &buf2, 8);
+  } else if (n >= 4) {
+    uint32_t buf1;
+    uint32_t buf2;
+    memcpy(&buf1, src, 4);
+    memcpy(&buf2, src + n - 4, 4);
+    if (nullify_tail) {
+      memset(dst + 4, 0, 4);
+      memset(dst + 8, 0, 8);
+    }
+    memcpy(dst, &buf1, 4);
+    memcpy(dst + n - 4, &buf2, 4);
+  } else {
+    if (n != 0) {
+      dst[0] = src[0];
+      dst[n / 2] = src[n / 2];
+      dst[n - 1] = src[n - 1];
+    }
+    if (nullify_tail) {
+      memset(dst + 8, 0, 8);
+      memset(dst + n, 0, 8);
+    }
+  }
+}
+
+struct ExternalRepReleaserPair {
+  CordRep* rep;
+  void* releaser_address;
+};
+
+// Allocates a new external `CordRep` and returns a pointer to it and a pointer
+// to `releaser_size` bytes where the desired releaser can be constructed.
+// Expects `data` to be non-empty.
+ExternalRepReleaserPair NewExternalWithUninitializedReleaser(
+    absl::string_view data, ExternalReleaserInvoker invoker,
+    size_t releaser_size);
+
+// Creates a new `CordRep` that owns `data` and `releaser` and returns a pointer
+// to it, or `nullptr` if `data` was empty.
+template <typename Releaser>
+// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
+CordRep* NewExternalRep(absl::string_view data, Releaser&& releaser) {
+  static_assert(
+#if defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
+      alignof(Releaser) <= __STDCPP_DEFAULT_NEW_ALIGNMENT__,
+#else
+      alignof(Releaser) <= alignof(max_align_t),
+#endif
+      "Releasers with alignment requirement greater than what is returned by "
+      "default `::operator new()` are not supported.");
+
+  using ReleaserType = absl::decay_t<Releaser>;
+  if (data.empty()) {
+    // Never create empty external nodes.
+    ::absl::base_internal::Invoke(
+        ReleaserType(std::forward<Releaser>(releaser)), data);
+    return nullptr;
+  }
+
+  auto releaser_invoker = [](void* type_erased_releaser, absl::string_view d) {
+    auto* my_releaser = static_cast<ReleaserType*>(type_erased_releaser);
+    ::absl::base_internal::Invoke(std::move(*my_releaser), d);
+    my_releaser->~ReleaserType();
+    return sizeof(Releaser);
+  };
+
+  ExternalRepReleaserPair external = NewExternalWithUninitializedReleaser(
+      data, releaser_invoker, sizeof(releaser));
+  ::new (external.releaser_address)
+      ReleaserType(std::forward<Releaser>(releaser));
+  return external.rep;
+}
+
+// Overload for function reference types that dispatches using a function
+// pointer because there are no `alignof()` or `sizeof()` a function reference.
+// NOLINTNEXTLINE - suppress clang-tidy raw pointer return.
+inline CordRep* NewExternalRep(absl::string_view data,
+                               void (&releaser)(absl::string_view)) {
+  return NewExternalRep(data, &releaser);
+}
+
+}  // namespace cord_internal
+
+template <typename Releaser>
+Cord MakeCordFromExternal(absl::string_view data, Releaser&& releaser) {
+  Cord cord;
+  cord.contents_.set_tree(::absl::cord_internal::NewExternalRep(
+      data, std::forward<Releaser>(releaser)));
+  return cord;
+}
+
+inline Cord::InlineRep::InlineRep(const Cord::InlineRep& src) {
+  cord_internal::SmallMemmove(data_, src.data_, sizeof(data_));
+}
+
+inline Cord::InlineRep::InlineRep(Cord::InlineRep&& src) {
+  memcpy(data_, src.data_, sizeof(data_));
+  memset(src.data_, 0, sizeof(data_));
+}
+
+inline Cord::InlineRep& Cord::InlineRep::operator=(const Cord::InlineRep& src) {
+  if (this == &src) {
+    return *this;
+  }
+  if (!is_tree() && !src.is_tree()) {
+    cord_internal::SmallMemmove(data_, src.data_, sizeof(data_));
+    return *this;
+  }
+  AssignSlow(src);
+  return *this;
+}
+
+inline Cord::InlineRep& Cord::InlineRep::operator=(
+    Cord::InlineRep&& src) noexcept {
+  if (is_tree()) {
+    ClearSlow();
+  }
+  memcpy(data_, src.data_, sizeof(data_));
+  memset(src.data_, 0, sizeof(data_));
+  return *this;
+}
+
+inline void Cord::InlineRep::Swap(Cord::InlineRep* rhs) {
+  if (rhs == this) {
+    return;
+  }
+
+  Cord::InlineRep tmp;
+  cord_internal::SmallMemmove(tmp.data_, data_, sizeof(data_));
+  cord_internal::SmallMemmove(data_, rhs->data_, sizeof(data_));
+  cord_internal::SmallMemmove(rhs->data_, tmp.data_, sizeof(data_));
+}
+
+inline const char* Cord::InlineRep::data() const {
+  return is_tree() ? nullptr : data_;
+}
+
+inline absl::cord_internal::CordRep* Cord::InlineRep::tree() const {
+  if (is_tree()) {
+    absl::cord_internal::CordRep* rep;
+    memcpy(&rep, data_, sizeof(rep));
+    return rep;
+  } else {
+    return nullptr;
+  }
+}
+
+inline bool Cord::InlineRep::empty() const { return data_[kMaxInline] == 0; }
+
+inline size_t Cord::InlineRep::size() const {
+  const char tag = data_[kMaxInline];
+  if (tag <= kMaxInline) return tag;
+  return static_cast<size_t>(tree()->length);
+}
+
+inline void Cord::InlineRep::set_tree(absl::cord_internal::CordRep* rep) {
+  if (rep == nullptr) {
+    memset(data_, 0, sizeof(data_));
+  } else {
+    bool was_tree = is_tree();
+    memcpy(data_, &rep, sizeof(rep));
+    memset(data_ + sizeof(rep), 0, sizeof(data_) - sizeof(rep) - 1);
+    if (!was_tree) {
+      data_[kMaxInline] = kTreeFlag;
+    }
+  }
+}
+
+inline void Cord::InlineRep::replace_tree(absl::cord_internal::CordRep* rep) {
+  ABSL_ASSERT(is_tree());
+  if (ABSL_PREDICT_FALSE(rep == nullptr)) {
+    set_tree(rep);
+    return;
+  }
+  memcpy(data_, &rep, sizeof(rep));
+  memset(data_ + sizeof(rep), 0, sizeof(data_) - sizeof(rep) - 1);
+}
+
+inline absl::cord_internal::CordRep* Cord::InlineRep::clear() {
+  const char tag = data_[kMaxInline];
+  absl::cord_internal::CordRep* result = nullptr;
+  if (tag > kMaxInline) {
+    memcpy(&result, data_, sizeof(result));
+  }
+  memset(data_, 0, sizeof(data_));  // Clear the cord
+  return result;
+}
+
+inline void Cord::InlineRep::CopyToArray(char* dst) const {
+  assert(!is_tree());
+  size_t n = data_[kMaxInline];
+  assert(n != 0);
+  cord_internal::SmallMemmove(dst, data_, n);
+}
+
+constexpr inline Cord::Cord() noexcept {}
+
+inline Cord& Cord::operator=(const Cord& x) {
+  contents_ = x.contents_;
+  return *this;
+}
+
+inline Cord::Cord(Cord&& src) noexcept : contents_(std::move(src.contents_)) {}
+
+inline Cord& Cord::operator=(Cord&& x) noexcept {
+  contents_ = std::move(x.contents_);
+  return *this;
+}
+
+template <typename T, Cord::EnableIfString<T>>
+inline Cord& Cord::operator=(T&& src) {
+  *this = absl::string_view(src);
+  return *this;
+}
+
+inline size_t Cord::size() const {
+  // Length is 1st field in str.rep_
+  return contents_.size();
+}
+
+inline bool Cord::empty() const { return contents_.empty(); }
+
+inline size_t Cord::EstimatedMemoryUsage() const {
+  size_t result = sizeof(Cord);
+  if (const absl::cord_internal::CordRep* rep = contents_.tree()) {
+    result += MemoryUsageAux(rep);
+  }
+  return result;
+}
+
+inline absl::string_view Cord::Flatten() {
+  absl::cord_internal::CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    return absl::string_view(contents_.data(), contents_.size());
+  } else {
+    absl::string_view already_flat_contents;
+    if (GetFlatAux(rep, &already_flat_contents)) {
+      return already_flat_contents;
+    }
+  }
+  return FlattenSlowPath();
+}
+
+inline void Cord::Append(absl::string_view src) {
+  contents_.AppendArray(src.data(), src.size());
+}
+
+template <typename T, Cord::EnableIfString<T>>
+inline void Cord::Append(T&& src) {
+  // Note that this function reserves the right to reuse the `string&&`'s
+  // memory and that it will do so in the future.
+  Append(absl::string_view(src));
+}
+
+template <typename T, Cord::EnableIfString<T>>
+inline void Cord::Prepend(T&& src) {
+  // Note that this function reserves the right to reuse the `string&&`'s
+  // memory and that it will do so in the future.
+  Prepend(absl::string_view(src));
+}
+
+inline int Cord::Compare(const Cord& rhs) const {
+  if (!contents_.is_tree() && !rhs.contents_.is_tree()) {
+    return contents_.BitwiseCompare(rhs.contents_);
+  }
+
+  return CompareImpl(rhs);
+}
+
+// Does 'this' cord start/end with rhs
+inline bool Cord::StartsWith(const Cord& rhs) const {
+  if (contents_.IsSame(rhs.contents_)) return true;
+  size_t rhs_size = rhs.size();
+  if (size() < rhs_size) return false;
+  return EqualsImpl(rhs, rhs_size);
+}
+
+inline bool Cord::StartsWith(absl::string_view rhs) const {
+  size_t rhs_size = rhs.size();
+  if (size() < rhs_size) return false;
+  return EqualsImpl(rhs, rhs_size);
+}
+
+inline Cord::ChunkIterator::ChunkIterator(const Cord* cord)
+    : bytes_remaining_(cord->size()) {
+  if (cord->empty()) return;
+  if (cord->contents_.is_tree()) {
+    stack_of_right_children_.push_back(cord->contents_.tree());
+    operator++();
+  } else {
+    current_chunk_ = absl::string_view(cord->contents_.data(), cord->size());
+  }
+}
+
+inline Cord::ChunkIterator Cord::ChunkIterator::operator++(int) {
+  ChunkIterator tmp(*this);
+  operator++();
+  return tmp;
+}
+
+inline bool Cord::ChunkIterator::operator==(const ChunkIterator& other) const {
+  return bytes_remaining_ == other.bytes_remaining_;
+}
+
+inline bool Cord::ChunkIterator::operator!=(const ChunkIterator& other) const {
+  return !(*this == other);
+}
+
+inline Cord::ChunkIterator::reference Cord::ChunkIterator::operator*() const {
+  assert(bytes_remaining_ != 0);
+  return current_chunk_;
+}
+
+inline Cord::ChunkIterator::pointer Cord::ChunkIterator::operator->() const {
+  assert(bytes_remaining_ != 0);
+  return &current_chunk_;
+}
+
+inline void Cord::ChunkIterator::RemoveChunkPrefix(size_t n) {
+  assert(n < current_chunk_.size());
+  current_chunk_.remove_prefix(n);
+  bytes_remaining_ -= n;
+}
+
+inline void Cord::ChunkIterator::AdvanceBytes(size_t n) {
+  if (ABSL_PREDICT_TRUE(n < current_chunk_.size())) {
+    RemoveChunkPrefix(n);
+  } else if (n != 0) {
+    AdvanceBytesSlowPath(n);
+  }
+}
+
+inline Cord::ChunkIterator Cord::chunk_begin() const {
+  return ChunkIterator(this);
+}
+
+inline Cord::ChunkIterator Cord::chunk_end() const { return ChunkIterator(); }
+
+inline Cord::ChunkIterator Cord::ChunkRange::begin() const {
+  return cord_->chunk_begin();
+}
+
+inline Cord::ChunkIterator Cord::ChunkRange::end() const {
+  return cord_->chunk_end();
+}
+
+inline Cord::ChunkRange Cord::Chunks() const { return ChunkRange(this); }
+
+inline Cord::CharIterator& Cord::CharIterator::operator++() {
+  if (ABSL_PREDICT_TRUE(chunk_iterator_->size() > 1)) {
+    chunk_iterator_.RemoveChunkPrefix(1);
+  } else {
+    ++chunk_iterator_;
+  }
+  return *this;
+}
+
+inline Cord::CharIterator Cord::CharIterator::operator++(int) {
+  CharIterator tmp(*this);
+  operator++();
+  return tmp;
+}
+
+inline bool Cord::CharIterator::operator==(const CharIterator& other) const {
+  return chunk_iterator_ == other.chunk_iterator_;
+}
+
+inline bool Cord::CharIterator::operator!=(const CharIterator& other) const {
+  return !(*this == other);
+}
+
+inline Cord::CharIterator::reference Cord::CharIterator::operator*() const {
+  return *chunk_iterator_->data();
+}
+
+inline Cord::CharIterator::pointer Cord::CharIterator::operator->() const {
+  return chunk_iterator_->data();
+}
+
+inline Cord Cord::AdvanceAndRead(CharIterator* it, size_t n_bytes) {
+  assert(it != nullptr);
+  return it->chunk_iterator_.AdvanceAndReadBytes(n_bytes);
+}
+
+inline void Cord::Advance(CharIterator* it, size_t n_bytes) {
+  assert(it != nullptr);
+  it->chunk_iterator_.AdvanceBytes(n_bytes);
+}
+
+inline absl::string_view Cord::ChunkRemaining(const CharIterator& it) {
+  return *it.chunk_iterator_;
+}
+
+inline Cord::CharIterator Cord::char_begin() const {
+  return CharIterator(this);
+}
+
+inline Cord::CharIterator Cord::char_end() const { return CharIterator(); }
+
+inline Cord::CharIterator Cord::CharRange::begin() const {
+  return cord_->char_begin();
+}
+
+inline Cord::CharIterator Cord::CharRange::end() const {
+  return cord_->char_end();
+}
+
+inline Cord::CharRange Cord::Chars() const { return CharRange(this); }
+
+inline void Cord::ForEachChunk(
+    absl::FunctionRef<void(absl::string_view)> callback) const {
+  absl::cord_internal::CordRep* rep = contents_.tree();
+  if (rep == nullptr) {
+    callback(absl::string_view(contents_.data(), contents_.size()));
+  } else {
+    return ForEachChunkAux(rep, callback);
+  }
+}
+
+// Nonmember Cord-to-Cord relational operarators.
+inline bool operator==(const Cord& lhs, const Cord& rhs) {
+  if (lhs.contents_.IsSame(rhs.contents_)) return true;
+  size_t rhs_size = rhs.size();
+  if (lhs.size() != rhs_size) return false;
+  return lhs.EqualsImpl(rhs, rhs_size);
+}
+
+inline bool operator!=(const Cord& x, const Cord& y) { return !(x == y); }
+inline bool operator<(const Cord& x, const Cord& y) {
+  return x.Compare(y) < 0;
+}
+inline bool operator>(const Cord& x, const Cord& y) {
+  return x.Compare(y) > 0;
+}
+inline bool operator<=(const Cord& x, const Cord& y) {
+  return x.Compare(y) <= 0;
+}
+inline bool operator>=(const Cord& x, const Cord& y) {
+  return x.Compare(y) >= 0;
+}
+
+// Nonmember Cord-to-absl::string_view relational operators.
+//
+// Due to implicit conversions, these also enable comparisons of Cord with
+// with std::string, ::string, and const char*.
+inline bool operator==(const Cord& lhs, absl::string_view rhs) {
+  size_t lhs_size = lhs.size();
+  size_t rhs_size = rhs.size();
+  if (lhs_size != rhs_size) return false;
+  return lhs.EqualsImpl(rhs, rhs_size);
+}
+
+inline bool operator==(absl::string_view x, const Cord& y) { return y == x; }
+inline bool operator!=(const Cord& x, absl::string_view y) { return !(x == y); }
+inline bool operator!=(absl::string_view x, const Cord& y) { return !(x == y); }
+inline bool operator<(const Cord& x, absl::string_view y) {
+  return x.Compare(y) < 0;
+}
+inline bool operator<(absl::string_view x, const Cord& y) {
+  return y.Compare(x) > 0;
+}
+inline bool operator>(const Cord& x, absl::string_view y) { return y < x; }
+inline bool operator>(absl::string_view x, const Cord& y) { return y < x; }
+inline bool operator<=(const Cord& x, absl::string_view y) { return !(y < x); }
+inline bool operator<=(absl::string_view x, const Cord& y) { return !(y < x); }
+inline bool operator>=(const Cord& x, absl::string_view y) { return !(x < y); }
+inline bool operator>=(absl::string_view x, const Cord& y) { return !(x < y); }
+
+// Overload of swap for Cord. The use of non-const references is
+// required. :(
+inline void swap(Cord& x, Cord& y) noexcept { y.contents_.Swap(&x.contents_); }
+
+// Some internals exposed to test code.
+namespace strings_internal {
+class CordTestAccess {
+ public:
+  static size_t FlatOverhead();
+  static size_t MaxFlatLength();
+  static size_t SizeofCordRepConcat();
+  static size_t SizeofCordRepExternal();
+  static size_t SizeofCordRepSubstring();
+  static size_t FlatTagToLength(uint8_t tag);
+  static uint8_t LengthToTag(size_t s);
+};
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORD_H_
diff --git a/absl/strings/cord_test.cc b/absl/strings/cord_test.cc
new file mode 100644
index 0000000..434f3a2
--- /dev/null
+++ b/absl/strings/cord_test.cc
@@ -0,0 +1,1526 @@
+#include "absl/strings/cord.h"
+
+#include <algorithm>
+#include <climits>
+#include <cstdio>
+#include <iterator>
+#include <map>
+#include <numeric>
+#include <random>
+#include <sstream>
+#include <type_traits>
+#include <utility>
+#include <vector>
+
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+#include "absl/base/casts.h"
+#include "absl/base/config.h"
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+#include "absl/container/fixed_array.h"
+#include "absl/strings/cord_test_helpers.h"
+#include "absl/strings/str_cat.h"
+#include "absl/strings/string_view.h"
+
+typedef std::mt19937_64 RandomEngine;
+
+static std::string RandomLowercaseString(RandomEngine* rng);
+static std::string RandomLowercaseString(RandomEngine* rng, size_t length);
+
+static int GetUniformRandomUpTo(RandomEngine* rng, int upper_bound) {
+  if (upper_bound > 0) {
+    std::uniform_int_distribution<int> uniform(0, upper_bound - 1);
+    return uniform(*rng);
+  } else {
+    return 0;
+  }
+}
+
+static size_t GetUniformRandomUpTo(RandomEngine* rng, size_t upper_bound) {
+  if (upper_bound > 0) {
+    std::uniform_int_distribution<size_t> uniform(0, upper_bound - 1);
+    return uniform(*rng);
+  } else {
+    return 0;
+  }
+}
+
+static int32_t GenerateSkewedRandom(RandomEngine* rng, int max_log) {
+  const uint32_t base = (*rng)() % (max_log + 1);
+  const uint32_t mask = ((base < 32) ? (1u << base) : 0u) - 1u;
+  return (*rng)() & mask;
+}
+
+static std::string RandomLowercaseString(RandomEngine* rng) {
+  int length;
+  std::bernoulli_distribution one_in_1k(0.001);
+  std::bernoulli_distribution one_in_10k(0.0001);
+  // With low probability, make a large fragment
+  if (one_in_10k(*rng)) {
+    length = GetUniformRandomUpTo(rng, 1048576);
+  } else if (one_in_1k(*rng)) {
+    length = GetUniformRandomUpTo(rng, 10000);
+  } else {
+    length = GenerateSkewedRandom(rng, 10);
+  }
+  return RandomLowercaseString(rng, length);
+}
+
+static std::string RandomLowercaseString(RandomEngine* rng, size_t length) {
+  std::string result(length, '\0');
+  std::uniform_int_distribution<int> chars('a', 'z');
+  std::generate(result.begin(), result.end(), [&]() {
+    return static_cast<char>(chars(*rng));
+  });
+  return result;
+}
+
+static void DoNothing(absl::string_view /* data */, void* /* arg */) {}
+
+static void DeleteExternalString(absl::string_view data, void* arg) {
+  std::string* s = reinterpret_cast<std::string*>(arg);
+  EXPECT_EQ(data, *s);
+  delete s;
+}
+
+// Add "s" to *dst via `MakeCordFromExternal`
+static void AddExternalMemory(absl::string_view s, absl::Cord* dst) {
+  std::string* str = new std::string(s.data(), s.size());
+  dst->Append(absl::MakeCordFromExternal(*str, [str](absl::string_view data) {
+    DeleteExternalString(data, str);
+  }));
+}
+
+static void DumpGrowth() {
+  absl::Cord str;
+  for (int i = 0; i < 1000; i++) {
+    char c = 'a' + i % 26;
+    str.Append(absl::string_view(&c, 1));
+  }
+}
+
+// Make a Cord with some number of fragments.  Return the size (in bytes)
+// of the smallest fragment.
+static size_t AppendWithFragments(const std::string& s, RandomEngine* rng,
+                                  absl::Cord* cord) {
+  size_t j = 0;
+  const size_t max_size = s.size() / 5;  // Make approx. 10 fragments
+  size_t min_size = max_size;            // size of smallest fragment
+  while (j < s.size()) {
+    size_t N = 1 + GetUniformRandomUpTo(rng, max_size);
+    if (N > (s.size() - j)) {
+      N = s.size() - j;
+    }
+    if (N < min_size) {
+      min_size = N;
+    }
+
+    std::bernoulli_distribution coin_flip(0.5);
+    if (coin_flip(*rng)) {
+      // Grow by adding an external-memory.
+      AddExternalMemory(absl::string_view(s.data() + j, N), cord);
+    } else {
+      cord->Append(absl::string_view(s.data() + j, N));
+    }
+    j += N;
+  }
+  return min_size;
+}
+
+// Add an external memory that contains the specified std::string to cord
+static void AddNewStringBlock(const std::string& str, absl::Cord* dst) {
+  char* data = new char[str.size()];
+  memcpy(data, str.data(), str.size());
+  dst->Append(absl::MakeCordFromExternal(
+      absl::string_view(data, str.size()),
+      [](absl::string_view s) { delete[] s.data(); }));
+}
+
+// Make a Cord out of many different types of nodes.
+static absl::Cord MakeComposite() {
+  absl::Cord cord;
+  cord.Append("the");
+  AddExternalMemory(" quick brown", &cord);
+  AddExternalMemory(" fox jumped", &cord);
+
+  absl::Cord full(" over");
+  AddExternalMemory(" the lazy", &full);
+  AddNewStringBlock(" dog slept the whole day away", &full);
+  absl::Cord substring = full.Subcord(0, 18);
+
+  // Make substring long enough to defeat the copying fast path in Append.
+  substring.Append(std::string(1000, '.'));
+  cord.Append(substring);
+  cord = cord.Subcord(0, cord.size() - 998);  // Remove most of extra junk
+
+  return cord;
+}
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+class CordTestPeer {
+ public:
+  static void ForEachChunk(
+      const Cord& c, absl::FunctionRef<void(absl::string_view)> callback) {
+    c.ForEachChunk(callback);
+  }
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+TEST(Cord, AllFlatSizes) {
+  using absl::strings_internal::CordTestAccess;
+
+  for (size_t s = 0; s < CordTestAccess::MaxFlatLength(); s++) {
+    // Make a std::string of length s.
+    std::string src;
+    while (src.size() < s) {
+      src.push_back('a' + (src.size() % 26));
+    }
+
+    absl::Cord dst(src);
+    EXPECT_EQ(std::string(dst), src) << s;
+  }
+}
+
+// We create a Cord at least 128GB in size using the fact that Cords can
+// internally reference-count; thus the Cord is enormous without actually
+// consuming very much memory.
+TEST(GigabyteCord, FromExternal) {
+  const size_t one_gig = 1024U * 1024U * 1024U;
+  size_t max_size = 2 * one_gig;
+  if (sizeof(max_size) > 4) max_size = 128 * one_gig;
+
+  size_t length = 128 * 1024;
+  char* data = new char[length];
+  absl::Cord from = absl::MakeCordFromExternal(
+      absl::string_view(data, length),
+      [](absl::string_view sv) { delete[] sv.data(); });
+
+  // This loop may seem odd due to its combination of exponential doubling of
+  // size and incremental size increases.  We do it incrementally to be sure the
+  // Cord will need rebalancing and will exercise code that, in the past, has
+  // caused crashes in production.  We grow exponentially so that the code will
+  // execute in a reasonable amount of time.
+  absl::Cord c;
+  ABSL_RAW_LOG(INFO, "Made a Cord with %zu bytes!", c.size());
+  c.Append(from);
+  while (c.size() < max_size) {
+    c.Append(c);
+    c.Append(from);
+    c.Append(from);
+    c.Append(from);
+    c.Append(from);
+  }
+
+  for (int i = 0; i < 1024; ++i) {
+    c.Append(from);
+  }
+  ABSL_RAW_LOG(INFO, "Made a Cord with %zu bytes!", c.size());
+  // Note: on a 32-bit build, this comes out to   2,818,048,000 bytes.
+  // Note: on a 64-bit build, this comes out to 171,932,385,280 bytes.
+}
+
+static absl::Cord MakeExternalCord(int size) {
+  char* buffer = new char[size];
+  memset(buffer, 'x', size);
+  absl::Cord cord;
+  cord.Append(absl::MakeCordFromExternal(
+      absl::string_view(buffer, size),
+      [](absl::string_view s) { delete[] s.data(); }));
+  return cord;
+}
+
+// Extern to fool clang that this is not constant. Needed to suppress
+// a warning of unsafe code we want to test.
+extern bool my_unique_true_boolean;
+bool my_unique_true_boolean = true;
+
+TEST(Cord, Assignment) {
+  absl::Cord x(absl::string_view("hi there"));
+  absl::Cord y(x);
+  ASSERT_EQ(std::string(x), "hi there");
+  ASSERT_EQ(std::string(y), "hi there");
+  ASSERT_TRUE(x == y);
+  ASSERT_TRUE(x <= y);
+  ASSERT_TRUE(y <= x);
+
+  x = absl::string_view("foo");
+  ASSERT_EQ(std::string(x), "foo");
+  ASSERT_EQ(std::string(y), "hi there");
+  ASSERT_TRUE(x < y);
+  ASSERT_TRUE(y > x);
+  ASSERT_TRUE(x != y);
+  ASSERT_TRUE(x <= y);
+  ASSERT_TRUE(y >= x);
+
+  x = "foo";
+  ASSERT_EQ(x, "foo");
+
+  // Test that going from inline rep to tree we don't leak memory.
+  std::vector<std::pair<absl::string_view, absl::string_view>>
+      test_string_pairs = {{"hi there", "foo"},
+                           {"loooooong coooooord", "short cord"},
+                           {"short cord", "loooooong coooooord"},
+                           {"loooooong coooooord1", "loooooong coooooord2"}};
+  for (std::pair<absl::string_view, absl::string_view> test_strings :
+       test_string_pairs) {
+    absl::Cord tmp(test_strings.first);
+    absl::Cord z(std::move(tmp));
+    ASSERT_EQ(std::string(z), test_strings.first);
+    tmp = test_strings.second;
+    z = std::move(tmp);
+    ASSERT_EQ(std::string(z), test_strings.second);
+  }
+  {
+    // Test that self-move assignment doesn't crash/leak.
+    // Do not write such code!
+    absl::Cord my_small_cord("foo");
+    absl::Cord my_big_cord("loooooong coooooord");
+    // Bypass clang's warning on self move-assignment.
+    absl::Cord* my_small_alias =
+        my_unique_true_boolean ? &my_small_cord : &my_big_cord;
+    absl::Cord* my_big_alias =
+        !my_unique_true_boolean ? &my_small_cord : &my_big_cord;
+
+    *my_small_alias = std::move(my_small_cord);
+    *my_big_alias = std::move(my_big_cord);
+    // my_small_cord and my_big_cord are in an unspecified but valid
+    // state, and will be correctly destroyed here.
+  }
+}
+
+TEST(Cord, StartsEndsWith) {
+  absl::Cord x(absl::string_view("abcde"));
+  absl::Cord empty("");
+
+  ASSERT_TRUE(x.StartsWith(absl::Cord("abcde")));
+  ASSERT_TRUE(x.StartsWith(absl::Cord("abc")));
+  ASSERT_TRUE(x.StartsWith(absl::Cord("")));
+  ASSERT_TRUE(empty.StartsWith(absl::Cord("")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("abcde")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("cde")));
+  ASSERT_TRUE(x.EndsWith(absl::Cord("")));
+  ASSERT_TRUE(empty.EndsWith(absl::Cord("")));
+
+  ASSERT_TRUE(!x.StartsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!empty.StartsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!x.EndsWith(absl::Cord("xyz")));
+  ASSERT_TRUE(!empty.EndsWith(absl::Cord("xyz")));
+
+  ASSERT_TRUE(x.StartsWith("abcde"));
+  ASSERT_TRUE(x.StartsWith("abc"));
+  ASSERT_TRUE(x.StartsWith(""));
+  ASSERT_TRUE(empty.StartsWith(""));
+  ASSERT_TRUE(x.EndsWith("abcde"));
+  ASSERT_TRUE(x.EndsWith("cde"));
+  ASSERT_TRUE(x.EndsWith(""));
+  ASSERT_TRUE(empty.EndsWith(""));
+
+  ASSERT_TRUE(!x.StartsWith("xyz"));
+  ASSERT_TRUE(!empty.StartsWith("xyz"));
+  ASSERT_TRUE(!x.EndsWith("xyz"));
+  ASSERT_TRUE(!empty.EndsWith("xyz"));
+}
+
+TEST(Cord, Subcord) {
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  const std::string s = RandomLowercaseString(&rng, 1024);
+
+  absl::Cord a;
+  AppendWithFragments(s, &rng, &a);
+  ASSERT_EQ(s.size(), a.size());
+
+  // Check subcords of a, from a variety of interesting points.
+  std::set<size_t> positions;
+  for (int i = 0; i <= 32; ++i) {
+    positions.insert(i);
+    positions.insert(i * 32 - 1);
+    positions.insert(i * 32);
+    positions.insert(i * 32 + 1);
+    positions.insert(a.size() - i);
+  }
+  positions.insert(237);
+  positions.insert(732);
+  for (size_t pos : positions) {
+    if (pos > a.size()) continue;
+    for (size_t end_pos : positions) {
+      if (end_pos < pos || end_pos > a.size()) continue;
+      absl::Cord sa = a.Subcord(pos, end_pos - pos);
+      EXPECT_EQ(absl::string_view(s).substr(pos, end_pos - pos),
+                std::string(sa))
+          << a;
+    }
+  }
+
+  // Do the same thing for an inline cord.
+  const std::string sh = "short";
+  absl::Cord c(sh);
+  for (size_t pos = 0; pos <= sh.size(); ++pos) {
+    for (size_t n = 0; n <= sh.size() - pos; ++n) {
+      absl::Cord sc = c.Subcord(pos, n);
+      EXPECT_EQ(sh.substr(pos, n), std::string(sc)) << c;
+    }
+  }
+
+  // Check subcords of subcords.
+  absl::Cord sa = a.Subcord(0, a.size());
+  std::string ss = s.substr(0, s.size());
+  while (sa.size() > 1) {
+    sa = sa.Subcord(1, sa.size() - 2);
+    ss = ss.substr(1, ss.size() - 2);
+    EXPECT_EQ(ss, std::string(sa)) << a;
+    if (HasFailure()) break;  // halt cascade
+  }
+
+  // It is OK to ask for too much.
+  sa = a.Subcord(0, a.size() + 1);
+  EXPECT_EQ(s, std::string(sa));
+
+  // It is OK to ask for something beyond the end.
+  sa = a.Subcord(a.size() + 1, 0);
+  EXPECT_TRUE(sa.empty());
+  sa = a.Subcord(a.size() + 1, 1);
+  EXPECT_TRUE(sa.empty());
+}
+
+TEST(Cord, Swap) {
+  absl::string_view a("Dexter");
+  absl::string_view b("Mandark");
+  absl::Cord x(a);
+  absl::Cord y(b);
+  swap(x, y);
+  ASSERT_EQ(x, absl::Cord(b));
+  ASSERT_EQ(y, absl::Cord(a));
+}
+
+static void VerifyCopyToString(const absl::Cord& cord) {
+  std::string initially_empty;
+  absl::CopyCordToString(cord, &initially_empty);
+  EXPECT_EQ(initially_empty, cord);
+
+  constexpr size_t kInitialLength = 1024;
+  std::string has_initial_contents(kInitialLength, 'x');
+  const char* address_before_copy = has_initial_contents.data();
+  absl::CopyCordToString(cord, &has_initial_contents);
+  EXPECT_EQ(has_initial_contents, cord);
+
+  if (cord.size() <= kInitialLength) {
+    EXPECT_EQ(has_initial_contents.data(), address_before_copy)
+        << "CopyCordToString allocated new std::string storage; "
+           "has_initial_contents = \""
+        << has_initial_contents << "\"";
+  }
+}
+
+TEST(Cord, CopyToString) {
+  VerifyCopyToString(absl::Cord());
+  VerifyCopyToString(absl::Cord("small cord"));
+  VerifyCopyToString(
+      absl::MakeFragmentedCord({"fragmented ", "cord ", "to ", "test ",
+                                "copying ", "to ", "a ", "string."}));
+}
+
+static bool IsFlat(const absl::Cord& c) {
+  return c.chunk_begin() == c.chunk_end() || ++c.chunk_begin() == c.chunk_end();
+}
+
+static void VerifyFlatten(absl::Cord c) {
+  std::string old_contents(c);
+  absl::string_view old_flat;
+  bool already_flat_and_non_empty = IsFlat(c) && !c.empty();
+  if (already_flat_and_non_empty) {
+    old_flat = *c.chunk_begin();
+  }
+  absl::string_view new_flat = c.Flatten();
+
+  // Verify that the contents of the flattened Cord are correct.
+  EXPECT_EQ(new_flat, old_contents);
+  EXPECT_EQ(std::string(c), old_contents);
+
+  // If the Cord contained data and was already flat, verify that the data
+  // wasn't copied.
+  if (already_flat_and_non_empty) {
+    EXPECT_EQ(old_flat.data(), new_flat.data())
+        << "Allocated new memory even though the Cord was already flat.";
+  }
+
+  // Verify that the flattened Cord is in fact flat.
+  EXPECT_TRUE(IsFlat(c));
+}
+
+TEST(Cord, Flatten) {
+  VerifyFlatten(absl::Cord());
+  VerifyFlatten(absl::Cord("small cord"));
+  VerifyFlatten(absl::Cord("larger than small buffer optimization"));
+  VerifyFlatten(absl::MakeFragmentedCord({"small ", "fragmented ", "cord"}));
+
+  // Test with a cord that is longer than the largest flat buffer
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  VerifyFlatten(absl::Cord(RandomLowercaseString(&rng, 8192)));
+}
+
+// Test data
+namespace {
+class TestData {
+ private:
+  std::vector<std::string> data_;
+
+  // Return a std::string of the specified length.
+  static std::string MakeString(int length) {
+    std::string result;
+    char buf[30];
+    snprintf(buf, sizeof(buf), "(%d)", length);
+    while (result.size() < length) {
+      result += buf;
+    }
+    result.resize(length);
+    return result;
+  }
+
+ public:
+  TestData() {
+    // short strings increasing in length by one
+    for (int i = 0; i < 30; i++) {
+      data_.push_back(MakeString(i));
+    }
+
+    // strings around half kMaxFlatLength
+    static const int kMaxFlatLength = 4096 - 9;
+    static const int kHalf = kMaxFlatLength / 2;
+
+    for (int i = -10; i <= +10; i++) {
+      data_.push_back(MakeString(kHalf + i));
+    }
+
+    for (int i = -10; i <= +10; i++) {
+      data_.push_back(MakeString(kMaxFlatLength + i));
+    }
+  }
+
+  size_t size() const { return data_.size(); }
+  const std::string& data(size_t i) const { return data_[i]; }
+};
+}  // namespace
+
+TEST(Cord, MultipleLengths) {
+  TestData d;
+  for (size_t i = 0; i < d.size(); i++) {
+    std::string a = d.data(i);
+
+    { // Construct from Cord
+      absl::Cord tmp(a);
+      absl::Cord x(tmp);
+      EXPECT_EQ(a, std::string(x)) << "'" << a << "'";
+    }
+
+    { // Construct from absl::string_view
+      absl::Cord x(a);
+      EXPECT_EQ(a, std::string(x)) << "'" << a << "'";
+    }
+
+    { // Append cord to self
+      absl::Cord self(a);
+      self.Append(self);
+      EXPECT_EQ(a + a, std::string(self)) << "'" << a << "' + '" << a << "'";
+    }
+
+    { // Prepend cord to self
+      absl::Cord self(a);
+      self.Prepend(self);
+      EXPECT_EQ(a + a, std::string(self)) << "'" << a << "' + '" << a << "'";
+    }
+
+    // Try to append/prepend others
+    for (size_t j = 0; j < d.size(); j++) {
+      std::string b = d.data(j);
+
+      { // CopyFrom Cord
+        absl::Cord x(a);
+        absl::Cord y(b);
+        x = y;
+        EXPECT_EQ(b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      { // CopyFrom absl::string_view
+        absl::Cord x(a);
+        x = b;
+        EXPECT_EQ(b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      { // Cord::Append(Cord)
+        absl::Cord x(a);
+        absl::Cord y(b);
+        x.Append(y);
+        EXPECT_EQ(a + b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      { // Cord::Append(absl::string_view)
+        absl::Cord x(a);
+        x.Append(b);
+        EXPECT_EQ(a + b, std::string(x)) << "'" << a << "' + '" << b << "'";
+      }
+
+      { // Cord::Prepend(Cord)
+        absl::Cord x(a);
+        absl::Cord y(b);
+        x.Prepend(y);
+        EXPECT_EQ(b + a, std::string(x)) << "'" << b << "' + '" << a << "'";
+      }
+
+      { // Cord::Prepend(absl::string_view)
+        absl::Cord x(a);
+        x.Prepend(b);
+        EXPECT_EQ(b + a, std::string(x)) << "'" << b << "' + '" << a << "'";
+      }
+    }
+  }
+}
+
+namespace {
+
+TEST(Cord, RemoveSuffixWithExternalOrSubstring) {
+  absl::Cord cord = absl::MakeCordFromExternal(
+      "foo bar baz", [](absl::string_view s) { DoNothing(s, nullptr); });
+
+  EXPECT_EQ("foo bar baz", std::string(cord));
+
+  // This RemoveSuffix() will wrap the EXTERNAL node in a SUBSTRING node.
+  cord.RemoveSuffix(4);
+  EXPECT_EQ("foo bar", std::string(cord));
+
+  // This RemoveSuffix() will adjust the SUBSTRING node in-place.
+  cord.RemoveSuffix(4);
+  EXPECT_EQ("foo", std::string(cord));
+}
+
+TEST(Cord, RemoveSuffixMakesZeroLengthNode) {
+  absl::Cord c;
+  c.Append(absl::Cord(std::string(100, 'x')));
+  absl::Cord other_ref = c;  // Prevent inplace appends
+  c.Append(absl::Cord(std::string(200, 'y')));
+  c.RemoveSuffix(200);
+  EXPECT_EQ(std::string(100, 'x'), std::string(c));
+}
+
+}  // namespace
+
+// CordSpliceTest contributed by hendrie.
+namespace {
+
+// Create a cord with an external memory block filled with 'z'
+absl::Cord CordWithZedBlock(size_t size) {
+  char* data = new char[size];
+  if (size > 0) {
+    memset(data, 'z', size);
+  }
+  absl::Cord cord = absl::MakeCordFromExternal(
+      absl::string_view(data, size),
+      [](absl::string_view s) { delete[] s.data(); });
+  return cord;
+}
+
+// Establish that ZedBlock does what we think it does.
+TEST(CordSpliceTest, ZedBlock) {
+  absl::Cord blob = CordWithZedBlock(10);
+  EXPECT_EQ(10, blob.size());
+  std::string s;
+  absl::CopyCordToString(blob, &s);
+  EXPECT_EQ("zzzzzzzzzz", s);
+}
+
+TEST(CordSpliceTest, ZedBlock0) {
+  absl::Cord blob = CordWithZedBlock(0);
+  EXPECT_EQ(0, blob.size());
+  std::string s;
+  absl::CopyCordToString(blob, &s);
+  EXPECT_EQ("", s);
+}
+
+TEST(CordSpliceTest, ZedBlockSuffix1) {
+  absl::Cord blob = CordWithZedBlock(10);
+  EXPECT_EQ(10, blob.size());
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(9);
+  EXPECT_EQ(1, suffix.size());
+  std::string s;
+  absl::CopyCordToString(suffix, &s);
+  EXPECT_EQ("z", s);
+}
+
+// Remove all of a prefix block
+TEST(CordSpliceTest, ZedBlockSuffix0) {
+  absl::Cord blob = CordWithZedBlock(10);
+  EXPECT_EQ(10, blob.size());
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(10);
+  EXPECT_EQ(0, suffix.size());
+  std::string s;
+  absl::CopyCordToString(suffix, &s);
+  EXPECT_EQ("", s);
+}
+
+absl::Cord BigCord(size_t len, char v) {
+  std::string s(len, v);
+  return absl::Cord(s);
+}
+
+// Splice block into cord.
+absl::Cord SpliceCord(const absl::Cord& blob, int64_t offset,
+                      const absl::Cord& block) {
+  ABSL_RAW_CHECK(offset >= 0, "");
+  ABSL_RAW_CHECK(offset + block.size() <= blob.size(), "");
+  absl::Cord result(blob);
+  result.RemoveSuffix(blob.size() - offset);
+  result.Append(block);
+  absl::Cord suffix(blob);
+  suffix.RemovePrefix(offset + block.size());
+  result.Append(suffix);
+  ABSL_RAW_CHECK(blob.size() == result.size(), "");
+  return result;
+}
+
+// Taking an empty suffix of a block breaks appending.
+TEST(CordSpliceTest, RemoveEntireBlock1) {
+  absl::Cord zero = CordWithZedBlock(10);
+  absl::Cord suffix(zero);
+  suffix.RemovePrefix(10);
+  absl::Cord result;
+  result.Append(suffix);
+}
+
+TEST(CordSpliceTest, RemoveEntireBlock2) {
+  absl::Cord zero = CordWithZedBlock(10);
+  absl::Cord prefix(zero);
+  prefix.RemoveSuffix(10);
+  absl::Cord suffix(zero);
+  suffix.RemovePrefix(10);
+  absl::Cord result(prefix);
+  result.Append(suffix);
+}
+
+TEST(CordSpliceTest, RemoveEntireBlock3) {
+  absl::Cord blob = CordWithZedBlock(10);
+  absl::Cord block = BigCord(10, 'b');
+  blob = SpliceCord(blob, 0, block);
+}
+
+struct CordCompareTestCase {
+  template <typename LHS, typename RHS>
+  CordCompareTestCase(const LHS& lhs, const RHS& rhs)
+      : lhs_cord(lhs), rhs_cord(rhs) {}
+
+  absl::Cord lhs_cord;
+  absl::Cord rhs_cord;
+};
+
+const auto sign = [](int x) { return x == 0 ? 0 : (x > 0 ? 1 : -1); };
+
+void VerifyComparison(const CordCompareTestCase& test_case) {
+  std::string lhs_string(test_case.lhs_cord);
+  std::string rhs_string(test_case.rhs_cord);
+  int expected = sign(lhs_string.compare(rhs_string));
+  EXPECT_EQ(expected, test_case.lhs_cord.Compare(test_case.rhs_cord))
+      << "LHS=" << lhs_string << "; RHS=" << rhs_string;
+  EXPECT_EQ(expected, test_case.lhs_cord.Compare(rhs_string))
+      << "LHS=" << lhs_string << "; RHS=" << rhs_string;
+  EXPECT_EQ(-expected, test_case.rhs_cord.Compare(test_case.lhs_cord))
+      << "LHS=" << rhs_string << "; RHS=" << lhs_string;
+  EXPECT_EQ(-expected, test_case.rhs_cord.Compare(lhs_string))
+      << "LHS=" << rhs_string << "; RHS=" << lhs_string;
+}
+
+TEST(Cord, Compare) {
+  absl::Cord subcord("aaaaaBBBBBcccccDDDDD");
+  subcord = subcord.Subcord(3, 10);
+
+  absl::Cord tmp("aaaaaaaaaaaaaaaa");
+  tmp.Append("BBBBBBBBBBBBBBBB");
+  absl::Cord concat = absl::Cord("cccccccccccccccc");
+  concat.Append("DDDDDDDDDDDDDDDD");
+  concat.Prepend(tmp);
+
+  absl::Cord concat2("aaaaaaaaaaaaa");
+  concat2.Append("aaaBBBBBBBBBBBBBBBBccccc");
+  concat2.Append("cccccccccccDDDDDDDDDDDDDD");
+  concat2.Append("DD");
+
+  std::vector<CordCompareTestCase> test_cases = {{
+      // Inline cords
+      {"abcdef", "abcdef"},
+      {"abcdef", "abcdee"},
+      {"abcdef", "abcdeg"},
+      {"bbcdef", "abcdef"},
+      {"bbcdef", "abcdeg"},
+      {"abcdefa", "abcdef"},
+      {"abcdef", "abcdefa"},
+
+      // Small flat cords
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBcccccDDDDD"},
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBxccccDDDDD"},
+      {"aaaaaBBBBBcxcccDDDDD", "aaaaaBBBBBcccccDDDDD"},
+      {"aaaaaBBBBBxccccDDDDD", "aaaaaBBBBBcccccDDDDX"},
+      {"aaaaaBBBBBcccccDDDDDa", "aaaaaBBBBBcccccDDDDD"},
+      {"aaaaaBBBBBcccccDDDDD", "aaaaaBBBBBcccccDDDDDa"},
+
+      // Subcords
+      {subcord, subcord},
+      {subcord, "aaBBBBBccc"},
+      {subcord, "aaBBBBBccd"},
+      {subcord, "aaBBBBBccb"},
+      {subcord, "aaBBBBBxcb"},
+      {subcord, "aaBBBBBccca"},
+      {subcord, "aaBBBBBcc"},
+
+      // Concats
+      {concat, concat},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDDD"},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBcccccccccccccccxDDDDDDDDDDDDDDDD"},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBacccccccccccccccDDDDDDDDDDDDDDDD"},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDD"},
+      {concat,
+       "aaaaaaaaaaaaaaaaBBBBBBBBBBBBBBBBccccccccccccccccDDDDDDDDDDDDDDDDe"},
+
+      {concat, concat2},
+  }};
+
+  for (const auto& tc : test_cases) {
+    VerifyComparison(tc);
+  }
+}
+
+TEST(Cord, CompareAfterAssign) {
+  absl::Cord a("aaaaaa1111111");
+  absl::Cord b("aaaaaa2222222");
+  a = "cccccc";
+  b = "cccccc";
+  EXPECT_EQ(a, b);
+  EXPECT_FALSE(a < b);
+
+  a = "aaaa";
+  b = "bbbbb";
+  a = "";
+  b = "";
+  EXPECT_EQ(a, b);
+  EXPECT_FALSE(a < b);
+}
+
+// Test CompareTo() and ComparePrefix() against string and substring
+// comparison methods from std::basic_string.
+static void TestCompare(const absl::Cord& c, const absl::Cord& d,
+                        RandomEngine* rng) {
+  typedef std::basic_string<uint8_t> ustring;
+  ustring cs(reinterpret_cast<const uint8_t*>(std::string(c).data()), c.size());
+  ustring ds(reinterpret_cast<const uint8_t*>(std::string(d).data()), d.size());
+  // ustring comparison is ideal because we expect Cord comparisons to be
+  // based on unsigned byte comparisons regardless of whether char is signed.
+  int expected = sign(cs.compare(ds));
+  EXPECT_EQ(expected, sign(c.Compare(d))) << c << ", " << d;
+}
+
+TEST(Compare, ComparisonIsUnsigned) {
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  std::uniform_int_distribution<uint32_t> uniform_uint8(0, 255);
+  char x = static_cast<char>(uniform_uint8(rng));
+  TestCompare(
+      absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100), x)),
+      absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100), x ^ 0x80)), &rng);
+}
+
+TEST(Compare, RandomComparisons) {
+  const int kIters = 5000;
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+
+  int n = GetUniformRandomUpTo(&rng, 5000);
+  absl::Cord a[] = {MakeExternalCord(n),
+                    absl::Cord("ant"),
+                    absl::Cord("elephant"),
+                    absl::Cord("giraffe"),
+                    absl::Cord(std::string(GetUniformRandomUpTo(&rng, 100),
+                                           GetUniformRandomUpTo(&rng, 100))),
+                    absl::Cord(""),
+                    absl::Cord("x"),
+                    absl::Cord("A"),
+                    absl::Cord("B"),
+                    absl::Cord("C")};
+  for (int i = 0; i < kIters; i++) {
+    absl::Cord c, d;
+    for (int j = 0; j < (i % 7) + 1; j++) {
+      c.Append(a[GetUniformRandomUpTo(&rng, ABSL_ARRAYSIZE(a))]);
+      d.Append(a[GetUniformRandomUpTo(&rng, ABSL_ARRAYSIZE(a))]);
+    }
+    std::bernoulli_distribution coin_flip(0.5);
+    TestCompare(coin_flip(rng) ? c : absl::Cord(std::string(c)),
+                coin_flip(rng) ? d : absl::Cord(std::string(d)), &rng);
+  }
+}
+
+template <typename T1, typename T2>
+void CompareOperators() {
+  const T1 a("a");
+  const T2 b("b");
+
+  EXPECT_TRUE(a == a);
+  // For pointer type (i.e. `const char*`), operator== compares the address
+  // instead of the std::string, so `a == const char*("a")` isn't necessarily true.
+  EXPECT_TRUE(std::is_pointer<T1>::value || a == T1("a"));
+  EXPECT_TRUE(std::is_pointer<T2>::value || a == T2("a"));
+  EXPECT_FALSE(a == b);
+
+  EXPECT_TRUE(a != b);
+  EXPECT_FALSE(a != a);
+
+  EXPECT_TRUE(a < b);
+  EXPECT_FALSE(b < a);
+
+  EXPECT_TRUE(b > a);
+  EXPECT_FALSE(a > b);
+
+  EXPECT_TRUE(a >= a);
+  EXPECT_TRUE(b >= a);
+  EXPECT_FALSE(a >= b);
+
+  EXPECT_TRUE(a <= a);
+  EXPECT_TRUE(a <= b);
+  EXPECT_FALSE(b <= a);
+}
+
+TEST(ComparisonOperators, Cord_Cord) {
+  CompareOperators<absl::Cord, absl::Cord>();
+}
+
+TEST(ComparisonOperators, Cord_StringPiece) {
+  CompareOperators<absl::Cord, absl::string_view>();
+}
+
+TEST(ComparisonOperators, StringPiece_Cord) {
+  CompareOperators<absl::string_view, absl::Cord>();
+}
+
+TEST(ComparisonOperators, Cord_string) {
+  CompareOperators<absl::Cord, std::string>();
+}
+
+TEST(ComparisonOperators, string_Cord) {
+  CompareOperators<std::string, absl::Cord>();
+}
+
+TEST(ComparisonOperators, stdstring_Cord) {
+  CompareOperators<std::string, absl::Cord>();
+}
+
+TEST(ComparisonOperators, Cord_stdstring) {
+  CompareOperators<absl::Cord, std::string>();
+}
+
+TEST(ComparisonOperators, charstar_Cord) {
+  CompareOperators<const char*, absl::Cord>();
+}
+
+TEST(ComparisonOperators, Cord_charstar) {
+  CompareOperators<absl::Cord, const char*>();
+}
+
+TEST(ConstructFromExternal, ReleaserInvoked) {
+  // Empty external memory means the releaser should be called immediately.
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      auto c = absl::MakeCordFromExternal("", releaser);
+      EXPECT_TRUE(invoked);
+    }
+  }
+
+  // If the size of the data is small enough, a future constructor
+  // implementation may copy the bytes and immediately invoke the releaser
+  // instead of creating an external node. We make a large dummy std::string to
+  // make this test independent of such an optimization.
+  std::string large_dummy(2048, 'c');
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      auto c = absl::MakeCordFromExternal(large_dummy, releaser);
+      EXPECT_FALSE(invoked);
+    }
+    EXPECT_TRUE(invoked);
+  }
+
+  {
+    bool invoked = false;
+    auto releaser = [&invoked](absl::string_view) { invoked = true; };
+    {
+      absl::Cord copy;
+      {
+        auto c = absl::MakeCordFromExternal(large_dummy, releaser);
+        copy = c;
+        EXPECT_FALSE(invoked);
+      }
+      EXPECT_FALSE(invoked);
+    }
+    EXPECT_TRUE(invoked);
+  }
+}
+
+TEST(ConstructFromExternal, CompareContents) {
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+
+  for (int length = 1; length <= 2048; length *= 2) {
+    std::string data = RandomLowercaseString(&rng, length);
+    auto* external = new std::string(data);
+    auto cord =
+        absl::MakeCordFromExternal(*external, [external](absl::string_view sv) {
+          EXPECT_EQ(external->data(), sv.data());
+          EXPECT_EQ(external->size(), sv.size());
+          delete external;
+        });
+    EXPECT_EQ(data, cord);
+  }
+}
+
+TEST(ConstructFromExternal, LargeReleaser) {
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  constexpr size_t kLength = 256;
+  std::string data = RandomLowercaseString(&rng, kLength);
+  std::array<char, kLength> data_array;
+  for (size_t i = 0; i < kLength; ++i) data_array[i] = data[i];
+  bool invoked = false;
+  auto releaser = [data_array, &invoked](absl::string_view data) {
+    EXPECT_EQ(data, absl::string_view(data_array.data(), data_array.size()));
+    invoked = true;
+  };
+  (void)absl::MakeCordFromExternal(data, releaser);
+  EXPECT_TRUE(invoked);
+}
+
+TEST(ConstructFromExternal, FunctionPointerReleaser) {
+  static absl::string_view data("hello world");
+  static bool invoked;
+  auto* releaser =
+      static_cast<void (*)(absl::string_view)>([](absl::string_view sv) {
+        EXPECT_EQ(data, sv);
+        invoked = true;
+      });
+  invoked = false;
+  (void)absl::MakeCordFromExternal(data, releaser);
+  EXPECT_TRUE(invoked);
+
+  invoked = false;
+  (void)absl::MakeCordFromExternal(data, *releaser);
+  EXPECT_TRUE(invoked);
+}
+
+TEST(ConstructFromExternal, MoveOnlyReleaser) {
+  struct Releaser {
+    explicit Releaser(bool* invoked) : invoked(invoked) {}
+    Releaser(Releaser&& other) noexcept : invoked(other.invoked) {}
+    void operator()(absl::string_view) const { *invoked = true; }
+
+    bool* invoked;
+  };
+
+  bool invoked = false;
+  (void)absl::MakeCordFromExternal("dummy", Releaser(&invoked));
+  EXPECT_TRUE(invoked);
+}
+
+TEST(ConstructFromExternal, NonTrivialReleaserDestructor) {
+  struct Releaser {
+    explicit Releaser(bool* destroyed) : destroyed(destroyed) {}
+    ~Releaser() { *destroyed = true; }
+    void operator()(absl::string_view) const {}
+
+    bool* destroyed;
+  };
+
+  bool destroyed = false;
+  Releaser releaser(&destroyed);
+  (void)absl::MakeCordFromExternal("dummy", releaser);
+  EXPECT_TRUE(destroyed);
+}
+
+TEST(ConstructFromExternal, ReferenceQualifierOverloads) {
+  struct Releaser {
+    void operator()(absl::string_view) & { *lvalue_invoked = true; }
+    void operator()(absl::string_view) && { *rvalue_invoked = true; }
+
+    bool* lvalue_invoked;
+    bool* rvalue_invoked;
+  };
+
+  bool lvalue_invoked = false;
+  bool rvalue_invoked = false;
+  Releaser releaser = {&lvalue_invoked, &rvalue_invoked};
+  (void)absl::MakeCordFromExternal("", releaser);
+  EXPECT_FALSE(lvalue_invoked);
+  EXPECT_TRUE(rvalue_invoked);
+  rvalue_invoked = false;
+
+  (void)absl::MakeCordFromExternal("dummy", releaser);
+  EXPECT_FALSE(lvalue_invoked);
+  EXPECT_TRUE(rvalue_invoked);
+  rvalue_invoked = false;
+
+  // NOLINTNEXTLINE: suppress clang-tidy std::move on trivially copyable type.
+  (void)absl::MakeCordFromExternal("dummy", std::move(releaser));
+  EXPECT_FALSE(lvalue_invoked);
+  EXPECT_TRUE(rvalue_invoked);
+}
+
+TEST(ExternalMemory, BasicUsage) {
+  static const char* strings[] = { "", "hello", "there" };
+  for (const char* str : strings) {
+    absl::Cord dst("(prefix)");
+    AddExternalMemory(str, &dst);
+    dst.Append("(suffix)");
+    EXPECT_EQ((std::string("(prefix)") + str + std::string("(suffix)")),
+              std::string(dst));
+  }
+}
+
+TEST(ExternalMemory, RemovePrefixSuffix) {
+  // Exhaustively try all sub-strings.
+  absl::Cord cord = MakeComposite();
+  std::string s = std::string(cord);
+  for (int offset = 0; offset <= s.size(); offset++) {
+    for (int length = 0; length <= s.size() - offset; length++) {
+      absl::Cord result(cord);
+      result.RemovePrefix(offset);
+      result.RemoveSuffix(result.size() - length);
+      EXPECT_EQ(s.substr(offset, length), std::string(result))
+          << offset << " " << length;
+    }
+  }
+}
+
+TEST(ExternalMemory, Get) {
+  absl::Cord cord("hello");
+  AddExternalMemory(" world!", &cord);
+  AddExternalMemory(" how are ", &cord);
+  cord.Append(" you?");
+  std::string s = std::string(cord);
+  for (int i = 0; i < s.size(); i++) {
+    EXPECT_EQ(s[i], cord[i]);
+  }
+}
+
+// CordMemoryUsage tests verify the correctness of the EstimatedMemoryUsage()
+// These tests take into account that the reported memory usage is approximate
+// and non-deterministic. For all tests, We verify that the reported memory
+// usage is larger than `size()`, and less than `size() * 1.5` as a cord should
+// never reserve more 'extra' capacity than half of its size as it grows.
+// Additionally we have some whiteboxed expectations based on our knowledge of
+// the layout and size of empty and inlined cords, and flat nodes.
+
+TEST(CordMemoryUsage, Empty) {
+  EXPECT_EQ(sizeof(absl::Cord), absl::Cord().EstimatedMemoryUsage());
+}
+
+TEST(CordMemoryUsage, Embedded) {
+  absl::Cord a("hello");
+  EXPECT_EQ(a.EstimatedMemoryUsage(), sizeof(absl::Cord));
+}
+
+TEST(CordMemoryUsage, EmbeddedAppend) {
+  absl::Cord a("a");
+  absl::Cord b("bcd");
+  EXPECT_EQ(b.EstimatedMemoryUsage(), sizeof(absl::Cord));
+  a.Append(b);
+  EXPECT_EQ(a.EstimatedMemoryUsage(), sizeof(absl::Cord));
+}
+
+TEST(CordMemoryUsage, ExternalMemory) {
+  static const int kLength = 1000;
+  absl::Cord cord;
+  AddExternalMemory(std::string(kLength, 'x'), &cord);
+  EXPECT_GT(cord.EstimatedMemoryUsage(), kLength);
+  EXPECT_LE(cord.EstimatedMemoryUsage(), kLength * 1.5);
+}
+
+TEST(CordMemoryUsage, Flat) {
+  static const int kLength = 125;
+  absl::Cord a(std::string(kLength, 'a'));
+  EXPECT_GT(a.EstimatedMemoryUsage(), kLength);
+  EXPECT_LE(a.EstimatedMemoryUsage(), kLength * 1.5);
+}
+
+TEST(CordMemoryUsage, AppendFlat) {
+  using absl::strings_internal::CordTestAccess;
+  absl::Cord a(std::string(CordTestAccess::MaxFlatLength(), 'a'));
+  size_t length = a.EstimatedMemoryUsage();
+  a.Append(std::string(CordTestAccess::MaxFlatLength(), 'b'));
+  size_t delta = a.EstimatedMemoryUsage() - length;
+  EXPECT_GT(delta, CordTestAccess::MaxFlatLength());
+  EXPECT_LE(delta, CordTestAccess::MaxFlatLength() * 1.5);
+}
+
+// Regtest for a change that had to be rolled back because it expanded out
+// of the InlineRep too soon, which was observable through MemoryUsage().
+TEST(CordMemoryUsage, InlineRep) {
+  constexpr size_t kMaxInline = 15;  // Cord::InlineRep::N
+  const std::string small_string(kMaxInline, 'x');
+  absl::Cord c1(small_string);
+
+  absl::Cord c2;
+  c2.Append(small_string);
+  EXPECT_EQ(c1, c2);
+  EXPECT_EQ(c1.EstimatedMemoryUsage(), c2.EstimatedMemoryUsage());
+}
+
+}  // namespace
+
+// Regtest for 7510292 (fix a bug introduced by 7465150)
+TEST(Cord, Concat_Append) {
+  // Create a rep of type CONCAT
+  absl::Cord s1("foobarbarbarbarbar");
+  s1.Append("abcdefgabcdefgabcdefgabcdefgabcdefgabcdefgabcdefg");
+  size_t size = s1.size();
+
+  // Create a copy of s1 and append to it.
+  absl::Cord s2 = s1;
+  s2.Append("x");
+
+  // 7465150 modifies s1 when it shouldn't.
+  EXPECT_EQ(s1.size(), size);
+  EXPECT_EQ(s2.size(), size + 1);
+}
+
+TEST(MakeFragmentedCord, MakeFragmentedCordFromInitializerList) {
+  absl::Cord fragmented =
+      absl::MakeFragmentedCord({"A ", "fragmented ", "Cord"});
+
+  EXPECT_EQ("A fragmented Cord", fragmented);
+
+  auto chunk_it = fragmented.chunk_begin();
+
+  ASSERT_TRUE(chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("A ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("fragmented ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("Cord", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
+}
+
+TEST(MakeFragmentedCord, MakeFragmentedCordFromVector) {
+  std::vector<absl::string_view> chunks = {"A ", "fragmented ", "Cord"};
+  absl::Cord fragmented = absl::MakeFragmentedCord(chunks);
+
+  EXPECT_EQ("A fragmented Cord", fragmented);
+
+  auto chunk_it = fragmented.chunk_begin();
+
+  ASSERT_TRUE(chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("A ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("fragmented ", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it != fragmented.chunk_end());
+  EXPECT_EQ("Cord", *chunk_it);
+
+  ASSERT_TRUE(++chunk_it == fragmented.chunk_end());
+}
+
+TEST(CordChunkIterator, Traits) {
+  static_assert(std::is_copy_constructible<absl::Cord::ChunkIterator>::value,
+                "");
+  static_assert(std::is_copy_assignable<absl::Cord::ChunkIterator>::value, "");
+
+  // Move semantics to satisfy swappable via std::swap
+  static_assert(std::is_move_constructible<absl::Cord::ChunkIterator>::value,
+                "");
+  static_assert(std::is_move_assignable<absl::Cord::ChunkIterator>::value, "");
+
+  static_assert(
+      std::is_same<
+          std::iterator_traits<absl::Cord::ChunkIterator>::iterator_category,
+          std::input_iterator_tag>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::ChunkIterator>::value_type,
+                   absl::string_view>::value,
+      "");
+  static_assert(
+      std::is_same<
+          std::iterator_traits<absl::Cord::ChunkIterator>::difference_type,
+          ptrdiff_t>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::ChunkIterator>::pointer,
+                   const absl::string_view*>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::ChunkIterator>::reference,
+                   absl::string_view>::value,
+      "");
+}
+
+static void VerifyChunkIterator(const absl::Cord& cord,
+                                size_t expected_chunks) {
+  EXPECT_EQ(cord.chunk_begin() == cord.chunk_end(), cord.empty()) << cord;
+  EXPECT_EQ(cord.chunk_begin() != cord.chunk_end(), !cord.empty());
+
+  absl::Cord::ChunkRange range = cord.Chunks();
+  EXPECT_EQ(range.begin() == range.end(), cord.empty());
+  EXPECT_EQ(range.begin() != range.end(), !cord.empty());
+
+  std::string content(cord);
+  size_t pos = 0;
+  auto pre_iter = cord.chunk_begin(), post_iter = cord.chunk_begin();
+  size_t n_chunks = 0;
+  while (pre_iter != cord.chunk_end() && post_iter != cord.chunk_end()) {
+    EXPECT_FALSE(pre_iter == cord.chunk_end());   // NOLINT: explicitly test ==
+    EXPECT_FALSE(post_iter == cord.chunk_end());  // NOLINT
+
+    EXPECT_EQ(pre_iter, post_iter);
+    EXPECT_EQ(*pre_iter, *post_iter);
+
+    EXPECT_EQ(pre_iter->data(), (*pre_iter).data());
+    EXPECT_EQ(pre_iter->size(), (*pre_iter).size());
+
+    absl::string_view chunk = *pre_iter;
+    EXPECT_FALSE(chunk.empty());
+    EXPECT_LE(pos + chunk.size(), content.size());
+    EXPECT_EQ(absl::string_view(content.c_str() + pos, chunk.size()), chunk);
+
+    int n_equal_iterators = 0;
+    for (absl::Cord::ChunkIterator it = range.begin(); it != range.end();
+         ++it) {
+      n_equal_iterators += static_cast<int>(it == pre_iter);
+    }
+    EXPECT_EQ(n_equal_iterators, 1);
+
+    ++pre_iter;
+    EXPECT_EQ(*post_iter++, chunk);
+
+    pos += chunk.size();
+    ++n_chunks;
+  }
+  EXPECT_EQ(expected_chunks, n_chunks);
+  EXPECT_EQ(pos, content.size());
+  EXPECT_TRUE(pre_iter == cord.chunk_end());   // NOLINT: explicitly test ==
+  EXPECT_TRUE(post_iter == cord.chunk_end());  // NOLINT
+}
+
+TEST(CordChunkIterator, Operations) {
+  absl::Cord empty_cord;
+  VerifyChunkIterator(empty_cord, 0);
+
+  absl::Cord small_buffer_cord("small cord");
+  VerifyChunkIterator(small_buffer_cord, 1);
+
+  absl::Cord flat_node_cord("larger than small buffer optimization");
+  VerifyChunkIterator(flat_node_cord, 1);
+
+  VerifyChunkIterator(
+      absl::MakeFragmentedCord({"a ", "small ", "fragmented ", "cord ", "for ",
+                                "testing ", "chunk ", "iterations."}),
+      8);
+
+  absl::Cord reused_nodes_cord(std::string(40, 'c'));
+  reused_nodes_cord.Prepend(absl::Cord(std::string(40, 'b')));
+  reused_nodes_cord.Prepend(absl::Cord(std::string(40, 'a')));
+  size_t expected_chunks = 3;
+  for (int i = 0; i < 8; ++i) {
+    reused_nodes_cord.Prepend(reused_nodes_cord);
+    expected_chunks *= 2;
+    VerifyChunkIterator(reused_nodes_cord, expected_chunks);
+  }
+
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  absl::Cord flat_cord(RandomLowercaseString(&rng, 256));
+  absl::Cord subcords;
+  for (int i = 0; i < 128; ++i) subcords.Prepend(flat_cord.Subcord(i, 128));
+  VerifyChunkIterator(subcords, 128);
+}
+
+TEST(CordCharIterator, Traits) {
+  static_assert(std::is_copy_constructible<absl::Cord::CharIterator>::value,
+                "");
+  static_assert(std::is_copy_assignable<absl::Cord::CharIterator>::value, "");
+
+  // Move semantics to satisfy swappable via std::swap
+  static_assert(std::is_move_constructible<absl::Cord::CharIterator>::value,
+                "");
+  static_assert(std::is_move_assignable<absl::Cord::CharIterator>::value, "");
+
+  static_assert(
+      std::is_same<
+          std::iterator_traits<absl::Cord::CharIterator>::iterator_category,
+          std::input_iterator_tag>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::CharIterator>::value_type,
+                   char>::value,
+      "");
+  static_assert(
+      std::is_same<
+          std::iterator_traits<absl::Cord::CharIterator>::difference_type,
+          ptrdiff_t>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::CharIterator>::pointer,
+                   const char*>::value,
+      "");
+  static_assert(
+      std::is_same<std::iterator_traits<absl::Cord::CharIterator>::reference,
+                   const char&>::value,
+      "");
+}
+
+static void VerifyCharIterator(const absl::Cord& cord) {
+  EXPECT_EQ(cord.char_begin() == cord.char_end(), cord.empty());
+  EXPECT_EQ(cord.char_begin() != cord.char_end(), !cord.empty());
+
+  absl::Cord::CharRange range = cord.Chars();
+  EXPECT_EQ(range.begin() == range.end(), cord.empty());
+  EXPECT_EQ(range.begin() != range.end(), !cord.empty());
+
+  size_t i = 0;
+  absl::Cord::CharIterator pre_iter = cord.char_begin();
+  absl::Cord::CharIterator post_iter = cord.char_begin();
+  std::string content(cord);
+  while (pre_iter != cord.char_end() && post_iter != cord.char_end()) {
+    EXPECT_FALSE(pre_iter == cord.char_end());   // NOLINT: explicitly test ==
+    EXPECT_FALSE(post_iter == cord.char_end());  // NOLINT
+
+    EXPECT_LT(i, cord.size());
+    EXPECT_EQ(content[i], *pre_iter);
+
+    EXPECT_EQ(pre_iter, post_iter);
+    EXPECT_EQ(*pre_iter, *post_iter);
+    EXPECT_EQ(&*pre_iter, &*post_iter);
+
+    EXPECT_EQ(&*pre_iter, pre_iter.operator->());
+
+    const char* character_address = &*pre_iter;
+    absl::Cord::CharIterator copy = pre_iter;
+    ++copy;
+    EXPECT_EQ(character_address, &*pre_iter);
+
+    int n_equal_iterators = 0;
+    for (absl::Cord::CharIterator it = range.begin(); it != range.end(); ++it) {
+      n_equal_iterators += static_cast<int>(it == pre_iter);
+    }
+    EXPECT_EQ(n_equal_iterators, 1);
+
+    absl::Cord::CharIterator advance_iter = range.begin();
+    absl::Cord::Advance(&advance_iter, i);
+    EXPECT_EQ(pre_iter, advance_iter);
+
+    advance_iter = range.begin();
+    EXPECT_EQ(absl::Cord::AdvanceAndRead(&advance_iter, i), cord.Subcord(0, i));
+    EXPECT_EQ(pre_iter, advance_iter);
+
+    advance_iter = pre_iter;
+    absl::Cord::Advance(&advance_iter, cord.size() - i);
+    EXPECT_EQ(range.end(), advance_iter);
+
+    advance_iter = pre_iter;
+    EXPECT_EQ(absl::Cord::AdvanceAndRead(&advance_iter, cord.size() - i),
+              cord.Subcord(i, cord.size() - i));
+    EXPECT_EQ(range.end(), advance_iter);
+
+    ++i;
+    ++pre_iter;
+    post_iter++;
+  }
+  EXPECT_EQ(i, cord.size());
+  EXPECT_TRUE(pre_iter == cord.char_end());   // NOLINT: explicitly test ==
+  EXPECT_TRUE(post_iter == cord.char_end());  // NOLINT
+
+  absl::Cord::CharIterator zero_advanced_end = cord.char_end();
+  absl::Cord::Advance(&zero_advanced_end, 0);
+  EXPECT_EQ(zero_advanced_end, cord.char_end());
+
+  absl::Cord::CharIterator it = cord.char_begin();
+  for (absl::string_view chunk : cord.Chunks()) {
+    while (!chunk.empty()) {
+      EXPECT_EQ(absl::Cord::ChunkRemaining(it), chunk);
+      chunk.remove_prefix(1);
+      ++it;
+    }
+  }
+}
+
+TEST(CordCharIterator, Operations) {
+  absl::Cord empty_cord;
+  VerifyCharIterator(empty_cord);
+
+  absl::Cord small_buffer_cord("small cord");
+  VerifyCharIterator(small_buffer_cord);
+
+  absl::Cord flat_node_cord("larger than small buffer optimization");
+  VerifyCharIterator(flat_node_cord);
+
+  VerifyCharIterator(
+      absl::MakeFragmentedCord({"a ", "small ", "fragmented ", "cord ", "for ",
+                                "testing ", "character ", "iteration."}));
+
+  absl::Cord reused_nodes_cord("ghi");
+  reused_nodes_cord.Prepend(absl::Cord("def"));
+  reused_nodes_cord.Prepend(absl::Cord("abc"));
+  for (int i = 0; i < 4; ++i) {
+    reused_nodes_cord.Prepend(reused_nodes_cord);
+    VerifyCharIterator(reused_nodes_cord);
+  }
+
+  RandomEngine rng(testing::GTEST_FLAG(random_seed));
+  absl::Cord flat_cord(RandomLowercaseString(&rng, 256));
+  absl::Cord subcords;
+  for (int i = 0; i < 4; ++i) subcords.Prepend(flat_cord.Subcord(16 * i, 128));
+  VerifyCharIterator(subcords);
+}
+
+TEST(Cord, StreamingOutput) {
+  absl::Cord c =
+      absl::MakeFragmentedCord({"A ", "small ", "fragmented ", "Cord", "."});
+  std::stringstream output;
+  output << c;
+  EXPECT_EQ("A small fragmented Cord.", output.str());
+}
+
+TEST(Cord, ForEachChunk) {
+  for (int num_elements : {1, 10, 200}) {
+    SCOPED_TRACE(num_elements);
+    std::vector<std::string> cord_chunks;
+    for (int i = 0; i < num_elements; ++i) {
+      cord_chunks.push_back(absl::StrCat("[", i, "]"));
+    }
+    absl::Cord c = absl::MakeFragmentedCord(cord_chunks);
+
+    std::vector<std::string> iterated_chunks;
+    absl::CordTestPeer::ForEachChunk(c,
+                                     [&iterated_chunks](absl::string_view sv) {
+                                       iterated_chunks.emplace_back(sv);
+                                     });
+    EXPECT_EQ(iterated_chunks, cord_chunks);
+  }
+}
+
+TEST(Cord, SmallBufferAssignFromOwnData) {
+  constexpr size_t kMaxInline = 15;
+  std::string contents = "small buff cord";
+  EXPECT_EQ(contents.size(), kMaxInline);
+  for (size_t pos = 0; pos < contents.size(); ++pos) {
+    for (size_t count = contents.size() - pos; count > 0; --count) {
+      absl::Cord c(contents);
+      absl::string_view flat = c.Flatten();
+      c = flat.substr(pos, count);
+      EXPECT_EQ(c, contents.substr(pos, count))
+          << "pos = " << pos << "; count = " << count;
+    }
+  }
+}
diff --git a/absl/strings/cord_test_helpers.h b/absl/strings/cord_test_helpers.h
new file mode 100644
index 0000000..f1036e3
--- /dev/null
+++ b/absl/strings/cord_test_helpers.h
@@ -0,0 +1,60 @@
+//
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+#ifndef ABSL_STRINGS_CORD_TEST_HELPERS_H_
+#define ABSL_STRINGS_CORD_TEST_HELPERS_H_
+
+#include "absl/strings/cord.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// Creates a multi-segment Cord from an iterable container of strings.  The
+// resulting Cord is guaranteed to have one segment for every string in the
+// container.  This allows code to be unit tested with multi-segment Cord
+// inputs.
+//
+// Example:
+//
+//   absl::Cord c = absl::MakeFragmentedCord({"A ", "fragmented ", "Cord"});
+//   EXPECT_FALSE(c.GetFlat(&unused));
+//
+// The mechanism by which this Cord is created is an implementation detail.  Any
+// implementation that produces a multi-segment Cord may produce a flat Cord in
+// the future as new optimizations are added to the Cord class.
+// MakeFragmentedCord will, however, always be updated to return a multi-segment
+// Cord.
+template <typename Container>
+Cord MakeFragmentedCord(const Container& c) {
+  Cord result;
+  for (const auto& s : c) {
+    auto* external = new std::string(s);
+    Cord tmp = absl::MakeCordFromExternal(
+        *external, [external](absl::string_view) { delete external; });
+    tmp.Prepend(result);
+    result = tmp;
+  }
+  return result;
+}
+
+inline Cord MakeFragmentedCord(std::initializer_list<absl::string_view> list) {
+  return MakeFragmentedCord<std::initializer_list<absl::string_view>>(list);
+}
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_CORD_TEST_HELPERS_H_
diff --git a/absl/strings/escaping.cc b/absl/strings/escaping.cc
index eb0974d..7adc1b6 100644
--- a/absl/strings/escaping.cc
+++ b/absl/strings/escaping.cc
@@ -26,6 +26,7 @@
 #include "absl/base/internal/raw_logging.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/strings/internal/char_map.h"
+#include "absl/strings/internal/escaping.h"
 #include "absl/strings/internal/resize_uninitialized.h"
 #include "absl/strings/internal/utf8.h"
 #include "absl/strings/str_cat.h"
@@ -33,30 +34,9 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
-// Digit conversion.
-constexpr char kHexChar[] = "0123456789abcdef";
-
-constexpr char kHexTable[513] =
-    "000102030405060708090a0b0c0d0e0f"
-    "101112131415161718191a1b1c1d1e1f"
-    "202122232425262728292a2b2c2d2e2f"
-    "303132333435363738393a3b3c3d3e3f"
-    "404142434445464748494a4b4c4d4e4f"
-    "505152535455565758595a5b5c5d5e5f"
-    "606162636465666768696a6b6c6d6e6f"
-    "707172737475767778797a7b7c7d7e7f"
-    "808182838485868788898a8b8c8d8e8f"
-    "909192939495969798999a9b9c9d9e9f"
-    "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
-    "b0b1b2b3b4b5b6b7b8b9babbbcbdbebf"
-    "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf"
-    "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf"
-    "e0e1e2e3e4e5e6e7e8e9eaebecedeeef"
-    "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff";
-
 // These are used for the leave_nulls_escaped argument to CUnescapeInternal().
 constexpr bool kUnescapeNulls = false;
 
@@ -349,14 +329,14 @@
              (last_hex_escape && absl::ascii_isxdigit(c)))) {
           if (use_hex) {
             dest.append("\\" "x");
-            dest.push_back(kHexChar[c / 16]);
-            dest.push_back(kHexChar[c % 16]);
+            dest.push_back(numbers_internal::kHexChar[c / 16]);
+            dest.push_back(numbers_internal::kHexChar[c % 16]);
             is_hex_escape = true;
           } else {
             dest.append("\\");
-            dest.push_back(kHexChar[c / 64]);
-            dest.push_back(kHexChar[(c % 64) / 8]);
-            dest.push_back(kHexChar[c % 8]);
+            dest.push_back(numbers_internal::kHexChar[c / 64]);
+            dest.push_back(numbers_internal::kHexChar[(c % 64) / 8]);
+            dest.push_back(numbers_internal::kHexChar[c % 8]);
           }
         } else {
           dest.push_back(c);
@@ -785,177 +765,10 @@
 };
 /* clang-format on */
 
-size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
-  // Base64 encodes three bytes of input at a time. If the input is not
-  // divisible by three, we pad as appropriate.
-  //
-  // (from https://tools.ietf.org/html/rfc3548)
-  // Special processing is performed if fewer than 24 bits are available
-  // at the end of the data being encoded.  A full encoding quantum is
-  // always completed at the end of a quantity.  When fewer than 24 input
-  // bits are available in an input group, zero bits are added (on the
-  // right) to form an integral number of 6-bit groups.  Padding at the
-  // end of the data is performed using the '=' character.  Since all base
-  // 64 input is an integral number of octets, only the following cases
-  // can arise:
-
-  // Base64 encodes each three bytes of input into four bytes of output.
-  size_t len = (input_len / 3) * 4;
-
-  if (input_len % 3 == 0) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (1) the final quantum of encoding input is an integral multiple of 24
-    // bits; here, the final unit of encoded output will be an integral
-    // multiple of 4 characters with no "=" padding,
-  } else if (input_len % 3 == 1) {
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (2) the final quantum of encoding input is exactly 8 bits; here, the
-    // final unit of encoded output will be two characters followed by two
-    // "=" padding characters, or
-    len += 2;
-    if (do_padding) {
-      len += 2;
-    }
-  } else {  // (input_len % 3 == 2)
-    // (from https://tools.ietf.org/html/rfc3548)
-    // (3) the final quantum of encoding input is exactly 16 bits; here, the
-    // final unit of encoded output will be three characters followed by one
-    // "=" padding character.
-    len += 3;
-    if (do_padding) {
-      len += 1;
-    }
-  }
-
-  assert(len >= input_len);  // make sure we didn't overflow
-  return len;
-}
-
-size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
-                            size_t szdest, const char* base64,
-                            bool do_padding) {
-  static const char kPad64 = '=';
-
-  if (szsrc * 4 > szdest * 3) return 0;
-
-  char* cur_dest = dest;
-  const unsigned char* cur_src = src;
-
-  char* const limit_dest = dest + szdest;
-  const unsigned char* const limit_src = src + szsrc;
-
-  // Three bytes of data encodes to four characters of cyphertext.
-  // So we can pump through three-byte chunks atomically.
-  if (szsrc >= 3) {  // "limit_src - 3" is UB if szsrc < 3.
-    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
-      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
-
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-
-      cur_dest += 4;
-      cur_src += 3;
-    }
-  }
-  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
-  szdest = limit_dest - cur_dest;
-  szsrc = limit_src - cur_src;
-
-  /* now deal with the tail (<=3 bytes) */
-  switch (szsrc) {
-    case 0:
-      // Nothing left; nothing more to do.
-      break;
-    case 1: {
-      // One byte left: this encodes to two characters, and (optionally)
-      // two pad characters to round out the four-character cypherblock.
-      if (szdest < 2) return 0;
-      uint32_t in = cur_src[0];
-      cur_dest[0] = base64[in >> 2];
-      in &= 0x3;
-      cur_dest[1] = base64[in << 4];
-      cur_dest += 2;
-      szdest -= 2;
-      if (do_padding) {
-        if (szdest < 2) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest[1] = kPad64;
-        cur_dest += 2;
-        szdest -= 2;
-      }
-      break;
-    }
-    case 2: {
-      // Two bytes left: this encodes to three characters, and (optionally)
-      // one pad character to round out the four-character cypherblock.
-      if (szdest < 3) return 0;
-      uint32_t in = absl::big_endian::Load16(cur_src);
-      cur_dest[0] = base64[in >> 10];
-      in &= 0x3FF;
-      cur_dest[1] = base64[in >> 4];
-      in &= 0x00F;
-      cur_dest[2] = base64[in << 2];
-      cur_dest += 3;
-      szdest -= 3;
-      if (do_padding) {
-        if (szdest < 1) return 0;
-        cur_dest[0] = kPad64;
-        cur_dest += 1;
-        szdest -= 1;
-      }
-      break;
-    }
-    case 3: {
-      // Three bytes left: same as in the big loop above.  We can't do this in
-      // the loop because the loop above always reads 4 bytes, and the fourth
-      // byte is past the end of the input.
-      if (szdest < 4) return 0;
-      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
-      cur_dest[0] = base64[in >> 18];
-      in &= 0x3FFFF;
-      cur_dest[1] = base64[in >> 12];
-      in &= 0xFFF;
-      cur_dest[2] = base64[in >> 6];
-      in &= 0x3F;
-      cur_dest[3] = base64[in];
-      cur_dest += 4;
-      szdest -= 4;
-      break;
-    }
-    default:
-      // Should not be reached: blocks of 4 bytes are handled
-      // in the while loop before this switch statement.
-      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
-      break;
-  }
-  return (cur_dest - dest);
-}
-
-constexpr char kBase64Chars[] =
-    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
-
 constexpr char kWebSafeBase64Chars[] =
     "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
 
 template <typename String>
-void Base64EscapeInternal(const unsigned char* src, size_t szsrc, String* dest,
-                          bool do_padding, const char* base64_chars) {
-  const size_t calc_escaped_size =
-      CalculateBase64EscapedLenInternal(szsrc, do_padding);
-  strings_internal::STLStringResizeUninitialized(dest, calc_escaped_size);
-
-  const size_t escaped_len = Base64EscapeInternal(
-      src, szsrc, &(*dest)[0], dest->size(), base64_chars, do_padding);
-  assert(calc_escaped_size == escaped_len);
-  dest->erase(escaped_len);
-}
-
-template <typename String>
 bool Base64UnescapeInternal(const char* src, size_t slen, String* dest,
                             const signed char* unbase64) {
   // Determine the size of the output std::string.  Base64 encodes every 3 bytes into
@@ -983,7 +796,7 @@
 }
 
 /* clang-format off */
-constexpr char kHexValue[256] = {
+constexpr char kHexValueLenient[256] = {
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
@@ -999,8 +812,9 @@
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
     0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
-    0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+    0,  0,  0,  0,  0,  0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 };
+
 /* clang-format on */
 
 // This is a templated function so that T can be either a char*
@@ -1009,8 +823,8 @@
 template <typename T>
 void HexStringToBytesInternal(const char* from, T to, ptrdiff_t num) {
   for (int i = 0; i < num; i++) {
-    to[i] = (kHexValue[from[i * 2] & 0xFF] << 4) +
-            (kHexValue[from[i * 2 + 1] & 0xFF]);
+    to[i] = (kHexValueLenient[from[i * 2] & 0xFF] << 4) +
+            (kHexValueLenient[from[i * 2 + 1] & 0xFF]);
   }
 }
 
@@ -1020,7 +834,7 @@
 void BytesToHexStringInternal(const unsigned char* src, T dest, ptrdiff_t num) {
   auto dest_ptr = &dest[0];
   for (auto src_ptr = src; src_ptr != (src + num); ++src_ptr, dest_ptr += 2) {
-    const char* hex_p = &kHexTable[*src_ptr * 2];
+    const char* hex_p = &numbers_internal::kHexTable[*src_ptr * 2];
     std::copy(hex_p, hex_p + 2, dest_ptr);
   }
 }
@@ -1088,26 +902,30 @@
 }
 
 void Base64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, true, kBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), dest,
+      true, strings_internal::kBase64Chars);
 }
 
 void WebSafeBase64Escape(absl::string_view src, std::string* dest) {
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), dest, false, kWebSafeBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), dest,
+      false, kWebSafeBase64Chars);
 }
 
 std::string Base64Escape(absl::string_view src) {
   std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, true, kBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), &dest,
+      true, strings_internal::kBase64Chars);
   return dest;
 }
 
 std::string WebSafeBase64Escape(absl::string_view src) {
   std::string dest;
-  Base64EscapeInternal(reinterpret_cast<const unsigned char*>(src.data()),
-                       src.size(), &dest, false, kWebSafeBase64Chars);
+  strings_internal::Base64EscapeInternal(
+      reinterpret_cast<const unsigned char*>(src.data()), src.size(), &dest,
+      false, kWebSafeBase64Chars);
   return dest;
 }
 
@@ -1127,5 +945,5 @@
   return result;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/escaping.h b/absl/strings/escaping.h
index 7f1c5d4..f5ca26c 100644
--- a/absl/strings/escaping.h
+++ b/absl/strings/escaping.h
@@ -33,7 +33,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // CUnescape()
 //
@@ -158,7 +158,7 @@
 // `2*from.size()`.
 std::string BytesToHexString(absl::string_view from);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_ESCAPING_H_
diff --git a/absl/strings/internal/char_map.h b/absl/strings/internal/char_map.h
index 772ae86..a76e603 100644
--- a/absl/strings/internal/char_map.h
+++ b/absl/strings/internal/char_map.h
@@ -28,7 +28,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 class Charmap {
@@ -150,7 +150,7 @@
 constexpr Charmap PunctCharmap() { return GraphCharmap() & ~AlnumCharmap(); }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CHAR_MAP_H_
diff --git a/absl/strings/internal/charconv_bigint.cc b/absl/strings/internal/charconv_bigint.cc
index 58c909f..66f33e7 100644
--- a/absl/strings/internal/charconv_bigint.cc
+++ b/absl/strings/internal/charconv_bigint.cc
@@ -19,7 +19,7 @@
 #include <string>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -158,12 +158,12 @@
 int LargePowerOfFiveSize(int i) { return 2 * i; }
 }  // namespace
 
-const uint32_t kFiveToNth[14] = {
+ABSL_DLL const uint32_t kFiveToNth[14] = {
     1,     5,      25,      125,     625,      3125,      15625,
     78125, 390625, 1953125, 9765625, 48828125, 244140625, 1220703125,
 };
 
-const uint32_t kTenToNth[10] = {
+ABSL_DLL const uint32_t kTenToNth[10] = {
     1, 10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000, 1000000000,
 };
 
@@ -355,5 +355,5 @@
 template class BigUnsigned<84>;
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_bigint.h b/absl/strings/internal/charconv_bigint.h
index 5aef416..999e9ae 100644
--- a/absl/strings/internal/charconv_bigint.h
+++ b/absl/strings/internal/charconv_bigint.h
@@ -20,12 +20,13 @@
 #include <iostream>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/internal/charconv_parse.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // The largest power that 5 that can be raised to, and still fit in a uint32_t.
@@ -33,8 +34,9 @@
 // The largest power that 10 that can be raised to, and still fit in a uint32_t.
 constexpr int kMaxSmallPowerOfTen = 9;
 
-extern const uint32_t kFiveToNth[kMaxSmallPowerOfFive + 1];
-extern const uint32_t kTenToNth[kMaxSmallPowerOfTen + 1];
+ABSL_DLL extern const uint32_t
+    kFiveToNth[kMaxSmallPowerOfFive + 1];
+ABSL_DLL extern const uint32_t kTenToNth[kMaxSmallPowerOfTen + 1];
 
 // Large, fixed-width unsigned integer.
 //
@@ -415,7 +417,7 @@
 extern template class BigUnsigned<84>;
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_CHARCONV_BIGINT_H_
diff --git a/absl/strings/internal/charconv_bigint_test.cc b/absl/strings/internal/charconv_bigint_test.cc
index 590511d..363bcb0 100644
--- a/absl/strings/internal/charconv_bigint_test.cc
+++ b/absl/strings/internal/charconv_bigint_test.cc
@@ -19,7 +19,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 TEST(BigUnsigned, ShiftLeft) {
@@ -201,5 +201,5 @@
 
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_parse.cc b/absl/strings/internal/charconv_parse.cc
index 4dd4ecb..d9a57a7 100644
--- a/absl/strings/internal/charconv_parse.cc
+++ b/absl/strings/internal/charconv_parse.cc
@@ -22,7 +22,7 @@
 #include "absl/strings/internal/memutil.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // ParseFloat<10> will read the first 19 significant digits of the mantissa.
@@ -254,6 +254,12 @@
     assert(max_digits * 4 <= std::numeric_limits<T>::digits);
   }
   const char* const original_begin = begin;
+
+  // Skip leading zeros, but only if *out is zero.
+  // They don't cause an overflow so we don't have to count them for
+  // `max_digits`.
+  while (!*out && end != begin && *begin == '0') ++begin;
+
   T accumulator = *out;
   const char* significant_digits_end =
       (end - begin > max_digits) ? begin + max_digits : end;
@@ -494,5 +500,5 @@
                                     chars_format format_flags);
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/charconv_parse.h b/absl/strings/internal/charconv_parse.h
index ddfc87f..505998b 100644
--- a/absl/strings/internal/charconv_parse.h
+++ b/absl/strings/internal/charconv_parse.h
@@ -17,10 +17,11 @@
 
 #include <cstdint>
 
+#include "absl/base/config.h"
 #include "absl/strings/charconv.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // Enum indicating whether a parsed float is a number or special value.
@@ -93,6 +94,6 @@
                                            absl::chars_format format_flags);
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_STRINGS_INTERNAL_CHARCONV_PARSE_H_
diff --git a/absl/strings/internal/cord_internal.h b/absl/strings/internal/cord_internal.h
new file mode 100644
index 0000000..5b5d108
--- /dev/null
+++ b/absl/strings/internal/cord_internal.h
@@ -0,0 +1,151 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
+#define ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
+
+#include <atomic>
+#include <cassert>
+#include <cstddef>
+#include <cstdint>
+#include <type_traits>
+
+#include "absl/meta/type_traits.h"
+#include "absl/strings/string_view.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace cord_internal {
+
+// Wraps std::atomic for reference counting.
+class Refcount {
+ public:
+  Refcount() : count_{1} {}
+  ~Refcount() {}
+
+  // Increments the reference count by 1. Imposes no memory ordering.
+  inline void Increment() { count_.fetch_add(1, std::memory_order_relaxed); }
+
+  // Asserts that the current refcount is greater than 0. If the refcount is
+  // greater than 1, decrements the reference count by 1.
+  //
+  // Returns false if there are no references outstanding; true otherwise.
+  // Inserts barriers to ensure that state written before this method returns
+  // false will be visible to a thread that just observed this method returning
+  // false.
+  inline bool Decrement() {
+    int32_t refcount = count_.load(std::memory_order_acquire);
+    assert(refcount > 0);
+    return refcount != 1 && count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
+  }
+
+  // Same as Decrement but expect that refcount is greater than 1.
+  inline bool DecrementExpectHighRefcount() {
+    int32_t refcount = count_.fetch_sub(1, std::memory_order_acq_rel);
+    assert(refcount > 0);
+    return refcount != 1;
+  }
+
+  // Returns the current reference count using acquire semantics.
+  inline int32_t Get() const { return count_.load(std::memory_order_acquire); }
+
+  // Returns whether the atomic integer is 1.
+  // If the reference count is used in the conventional way, a
+  // reference count of 1 implies that the current thread owns the
+  // reference and no other thread shares it.
+  // This call performs the test for a reference count of one, and
+  // performs the memory barrier needed for the owning thread
+  // to act on the object, knowing that it has exclusive access to the
+  // object.
+  inline bool IsOne() { return count_.load(std::memory_order_acquire) == 1; }
+
+ private:
+  std::atomic<int32_t> count_;
+};
+
+// The overhead of a vtable is too much for Cord, so we roll our own subclasses
+// using only a single byte to differentiate classes from each other - the "tag"
+// byte.  Define the subclasses first so we can provide downcasting helper
+// functions in the base class.
+
+struct CordRepConcat;
+struct CordRepSubstring;
+struct CordRepExternal;
+
+struct CordRep {
+  // The following three fields have to be less than 32 bytes since
+  // that is the smallest supported flat node size.
+  // We use uint64_t for the length even in 32-bit binaries.
+  uint64_t length;
+  Refcount refcount;
+  // If tag < FLAT, it represents CordRepKind and indicates the type of node.
+  // Otherwise, the node type is CordRepFlat and the tag is the encoded size.
+  uint8_t tag;
+  char data[1];  // Starting point for flat array: MUST BE LAST FIELD of CordRep
+
+  inline CordRepConcat* concat();
+  inline const CordRepConcat* concat() const;
+  inline CordRepSubstring* substring();
+  inline const CordRepSubstring* substring() const;
+  inline CordRepExternal* external();
+  inline const CordRepExternal* external() const;
+};
+
+struct CordRepConcat : public CordRep {
+  CordRep* left;
+  CordRep* right;
+
+  uint8_t depth() const { return static_cast<uint8_t>(data[0]); }
+  void set_depth(uint8_t depth) { data[0] = static_cast<char>(depth); }
+};
+
+struct CordRepSubstring : public CordRep {
+  size_t start;  // Starting offset of substring in child
+  CordRep* child;
+};
+
+// TODO(strel): replace the following logic (and related functions in cord.cc)
+// with container_internal::Layout.
+
+// Alignment requirement for CordRepExternal so that the type erased releaser
+// will be stored at a suitably aligned address.
+constexpr size_t ExternalRepAlignment() {
+#if defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
+  return __STDCPP_DEFAULT_NEW_ALIGNMENT__;
+#else
+  return alignof(max_align_t);
+#endif
+}
+
+// Type for function pointer that will invoke and destroy the type-erased
+// releaser function object. Accepts a pointer to the releaser and the
+// `string_view` that were passed in to `NewExternalRep` below. The return value
+// is the size of the `Releaser` type.
+using ExternalReleaserInvoker = size_t (*)(void*, absl::string_view);
+
+// External CordReps are allocated together with a type erased releaser. The
+// releaser is stored in the memory directly following the CordRepExternal.
+struct alignas(ExternalRepAlignment()) CordRepExternal : public CordRep {
+  const char* base;
+  // Pointer to function that knows how to call and destroy the releaser.
+  ExternalReleaserInvoker releaser_invoker;
+};
+
+// TODO(strel): look into removing, it doesn't seem like anything relies on this
+static_assert(sizeof(CordRepConcat) == sizeof(CordRepSubstring), "");
+
+}  // namespace cord_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+#endif  // ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
diff --git a/absl/strings/internal/escaping.cc b/absl/strings/internal/escaping.cc
new file mode 100644
index 0000000..c527128
--- /dev/null
+++ b/absl/strings/internal/escaping.cc
@@ -0,0 +1,180 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/strings/internal/escaping.h"
+
+#include "absl/base/internal/endian.h"
+#include "absl/base/internal/raw_logging.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+const char kBase64Chars[] =
+    "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
+  // Base64 encodes three bytes of input at a time. If the input is not
+  // divisible by three, we pad as appropriate.
+  //
+  // (from https://tools.ietf.org/html/rfc3548)
+  // Special processing is performed if fewer than 24 bits are available
+  // at the end of the data being encoded.  A full encoding quantum is
+  // always completed at the end of a quantity.  When fewer than 24 input
+  // bits are available in an input group, zero bits are added (on the
+  // right) to form an integral number of 6-bit groups.  Padding at the
+  // end of the data is performed using the '=' character.  Since all base
+  // 64 input is an integral number of octets, only the following cases
+  // can arise:
+
+  // Base64 encodes each three bytes of input into four bytes of output.
+  size_t len = (input_len / 3) * 4;
+
+  if (input_len % 3 == 0) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (1) the final quantum of encoding input is an integral multiple of 24
+    // bits; here, the final unit of encoded output will be an integral
+    // multiple of 4 characters with no "=" padding,
+  } else if (input_len % 3 == 1) {
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (2) the final quantum of encoding input is exactly 8 bits; here, the
+    // final unit of encoded output will be two characters followed by two
+    // "=" padding characters, or
+    len += 2;
+    if (do_padding) {
+      len += 2;
+    }
+  } else {  // (input_len % 3 == 2)
+    // (from https://tools.ietf.org/html/rfc3548)
+    // (3) the final quantum of encoding input is exactly 16 bits; here, the
+    // final unit of encoded output will be three characters followed by one
+    // "=" padding character.
+    len += 3;
+    if (do_padding) {
+      len += 1;
+    }
+  }
+
+  assert(len >= input_len);  // make sure we didn't overflow
+  return len;
+}
+
+size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
+                            size_t szdest, const char* base64,
+                            bool do_padding) {
+  static const char kPad64 = '=';
+
+  if (szsrc * 4 > szdest * 3) return 0;
+
+  char* cur_dest = dest;
+  const unsigned char* cur_src = src;
+
+  char* const limit_dest = dest + szdest;
+  const unsigned char* const limit_src = src + szsrc;
+
+  // Three bytes of data encodes to four characters of cyphertext.
+  // So we can pump through three-byte chunks atomically.
+  if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
+    while (cur_src < limit_src - 3) {  // While we have >= 32 bits.
+      uint32_t in = absl::big_endian::Load32(cur_src) >> 8;
+
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+
+      cur_dest += 4;
+      cur_src += 3;
+    }
+  }
+  // To save time, we didn't update szdest or szsrc in the loop.  So do it now.
+  szdest = limit_dest - cur_dest;
+  szsrc = limit_src - cur_src;
+
+  /* now deal with the tail (<=3 bytes) */
+  switch (szsrc) {
+    case 0:
+      // Nothing left; nothing more to do.
+      break;
+    case 1: {
+      // One byte left: this encodes to two characters, and (optionally)
+      // two pad characters to round out the four-character cypherblock.
+      if (szdest < 2) return 0;
+      uint32_t in = cur_src[0];
+      cur_dest[0] = base64[in >> 2];
+      in &= 0x3;
+      cur_dest[1] = base64[in << 4];
+      cur_dest += 2;
+      szdest -= 2;
+      if (do_padding) {
+        if (szdest < 2) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest[1] = kPad64;
+        cur_dest += 2;
+        szdest -= 2;
+      }
+      break;
+    }
+    case 2: {
+      // Two bytes left: this encodes to three characters, and (optionally)
+      // one pad character to round out the four-character cypherblock.
+      if (szdest < 3) return 0;
+      uint32_t in = absl::big_endian::Load16(cur_src);
+      cur_dest[0] = base64[in >> 10];
+      in &= 0x3FF;
+      cur_dest[1] = base64[in >> 4];
+      in &= 0x00F;
+      cur_dest[2] = base64[in << 2];
+      cur_dest += 3;
+      szdest -= 3;
+      if (do_padding) {
+        if (szdest < 1) return 0;
+        cur_dest[0] = kPad64;
+        cur_dest += 1;
+        szdest -= 1;
+      }
+      break;
+    }
+    case 3: {
+      // Three bytes left: same as in the big loop above.  We can't do this in
+      // the loop because the loop above always reads 4 bytes, and the fourth
+      // byte is past the end of the input.
+      if (szdest < 4) return 0;
+      uint32_t in = (cur_src[0] << 16) + absl::big_endian::Load16(cur_src + 1);
+      cur_dest[0] = base64[in >> 18];
+      in &= 0x3FFFF;
+      cur_dest[1] = base64[in >> 12];
+      in &= 0xFFF;
+      cur_dest[2] = base64[in >> 6];
+      in &= 0x3F;
+      cur_dest[3] = base64[in];
+      cur_dest += 4;
+      szdest -= 4;
+      break;
+    }
+    default:
+      // Should not be reached: blocks of 4 bytes are handled
+      // in the while loop before this switch statement.
+      ABSL_RAW_LOG(FATAL, "Logic problem? szsrc = %zu", szsrc);
+      break;
+  }
+  return (cur_dest - dest);
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
diff --git a/absl/strings/internal/escaping.h b/absl/strings/internal/escaping.h
new file mode 100644
index 0000000..6a9ce60
--- /dev/null
+++ b/absl/strings/internal/escaping.h
@@ -0,0 +1,58 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_STRINGS_INTERNAL_ESCAPING_H_
+#define ABSL_STRINGS_INTERNAL_ESCAPING_H_
+
+#include <cassert>
+
+#include "absl/strings/internal/resize_uninitialized.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace strings_internal {
+
+ABSL_CONST_INIT extern const char kBase64Chars[];
+
+// Calculates how long a string will be when it is base64 encoded given its
+// length and whether or not the result should be padded.
+size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding);
+
+// Base64-encodes `src` using the alphabet provided in `base64` and writes the
+// result to `dest`. If `do_padding` is true, `dest` is padded with '=' chars
+// until its length is a multiple of 3. Returns the length of `dest`.
+size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
+                            size_t szdest, const char* base64, bool do_padding);
+
+// Base64-encodes `src` using the alphabet provided in `base64` and writes the
+// result to `dest`. If `do_padding` is true, `dest` is padded with '=' chars
+// until its length is a multiple of 3.
+template <typename String>
+void Base64EscapeInternal(const unsigned char* src, size_t szsrc, String* dest,
+                          bool do_padding, const char* base64_chars) {
+  const size_t calc_escaped_size =
+      CalculateBase64EscapedLenInternal(szsrc, do_padding);
+  STLStringResizeUninitialized(dest, calc_escaped_size);
+
+  const size_t escaped_len = Base64EscapeInternal(
+      src, szsrc, &(*dest)[0], dest->size(), base64_chars, do_padding);
+  assert(calc_escaped_size == escaped_len);
+  dest->erase(escaped_len);
+}
+
+}  // namespace strings_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_STRINGS_INTERNAL_ESCAPING_H_
diff --git a/absl/strings/internal/escaping_test_common.h b/absl/strings/internal/escaping_test_common.h
index ecd3aa3..7b18017 100644
--- a/absl/strings/internal/escaping_test_common.h
+++ b/absl/strings/internal/escaping_test_common.h
@@ -22,7 +22,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 struct base64_testcase {
@@ -127,7 +127,7 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_ESCAPING_TEST_COMMON_H_
diff --git a/absl/strings/internal/memutil.cc b/absl/strings/internal/memutil.cc
index 0525137..2519c68 100644
--- a/absl/strings/internal/memutil.cc
+++ b/absl/strings/internal/memutil.cc
@@ -17,7 +17,7 @@
 #include <cstdlib>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 int memcasecmp(const char* s1, const char* s2, size_t len) {
@@ -108,5 +108,5 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/memutil.h b/absl/strings/internal/memutil.h
index 4efac98..9ad0535 100644
--- a/absl/strings/internal/memutil.h
+++ b/absl/strings/internal/memutil.h
@@ -69,7 +69,7 @@
 #include "absl/strings/ascii.h"  // for absl::ascii_tolower
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 inline char* memcat(char* dest, size_t destlen, const char* src,
@@ -142,7 +142,7 @@
                      size_t neelen);
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_MEMUTIL_H_
diff --git a/absl/strings/internal/numbers_test_common.h b/absl/strings/internal/numbers_test_common.h
index 3f6965f..1a1e50c 100644
--- a/absl/strings/internal/numbers_test_common.h
+++ b/absl/strings/internal/numbers_test_common.h
@@ -23,8 +23,10 @@
 #include <limits>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 template <typename IntType>
@@ -43,8 +45,9 @@
   while (value != 0) {
     const IntType next_value = value / base;
     // Can't use std::abs here because of problems when IntType is unsigned.
-    int remainder = value > next_value * base ? value - next_value * base
-                                              : next_value * base - value;
+    int remainder =
+        static_cast<int>(value > next_value * base ? value - next_value * base
+                                                   : next_value * base - value);
     char c = remainder < 10 ? '0' + remainder : 'A' + remainder - 10;
     destination->insert(0, 1, c);
     value = next_value;
@@ -175,7 +178,7 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_NUMBERS_TEST_COMMON_H_
diff --git a/absl/strings/internal/ostringstream.cc b/absl/strings/internal/ostringstream.cc
index ce2dd6c..05324c7 100644
--- a/absl/strings/internal/ostringstream.cc
+++ b/absl/strings/internal/ostringstream.cc
@@ -15,7 +15,7 @@
 #include "absl/strings/internal/ostringstream.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 OStringStream::Buf::int_type OStringStream::overflow(int c) {
@@ -32,5 +32,5 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/ostringstream.h b/absl/strings/internal/ostringstream.h
index 2cf6513..d25d604 100644
--- a/absl/strings/internal/ostringstream.h
+++ b/absl/strings/internal/ostringstream.h
@@ -23,7 +23,7 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // The same as std::ostringstream but appends to a user-specified std::string,
@@ -83,7 +83,7 @@
 };
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_OSTRINGSTREAM_H_
diff --git a/absl/strings/internal/pow10_helper.cc b/absl/strings/internal/pow10_helper.cc
index 5c02ab8..42e96c3 100644
--- a/absl/strings/internal/pow10_helper.cc
+++ b/absl/strings/internal/pow10_helper.cc
@@ -17,7 +17,7 @@
 #include <cmath>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -118,5 +118,5 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/pow10_helper.h b/absl/strings/internal/pow10_helper.h
index c9a1b27..c37c2c3 100644
--- a/absl/strings/internal/pow10_helper.h
+++ b/absl/strings/internal/pow10_helper.h
@@ -22,8 +22,10 @@
 
 #include <vector>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // Computes the precise value of 10^exp. (I.e. the nearest representable
@@ -32,7 +34,7 @@
 double Pow10(int exp);
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_POW10_HELPER_H_
diff --git a/absl/strings/internal/pow10_helper_test.cc b/absl/strings/internal/pow10_helper_test.cc
index 4a62a70..a4ff76d 100644
--- a/absl/strings/internal/pow10_helper_test.cc
+++ b/absl/strings/internal/pow10_helper_test.cc
@@ -20,7 +20,7 @@
 #include "absl/strings/str_format.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 namespace {
@@ -118,5 +118,5 @@
 
 }  // namespace
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/resize_uninitialized.h b/absl/strings/internal/resize_uninitialized.h
index ab1d868..e42628e 100644
--- a/absl/strings/internal/resize_uninitialized.h
+++ b/absl/strings/internal/resize_uninitialized.h
@@ -25,7 +25,7 @@
 #include "absl/meta/type_traits.h"  //  for void_t
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // Is a subclass of true_type or false_type, depending on whether or not
@@ -36,8 +36,7 @@
   static void Resize(string_type* s, size_t new_size) { s->resize(new_size); }
 };
 
-// __resize_default_init is provided by libc++ >= 8.0 and by Google's internal
-// ::string implementation.
+// __resize_default_init is provided by libc++ >= 8.0
 template <typename string_type>
 struct ResizeUninitializedTraits<
     string_type, absl::void_t<decltype(std::declval<string_type&>()
@@ -68,7 +67,7 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_RESIZE_UNINITIALIZED_H_
diff --git a/absl/strings/internal/resize_uninitialized_test.cc b/absl/strings/internal/resize_uninitialized_test.cc
index c5be0b1..0f8b3c2 100644
--- a/absl/strings/internal/resize_uninitialized_test.cc
+++ b/absl/strings/internal/resize_uninitialized_test.cc
@@ -20,13 +20,27 @@
 
 int resize_call_count = 0;
 
+// A mock string class whose only purpose is to track how many times its
+// resize() method has been called.
 struct resizable_string {
+  size_t size() const { return 0; }
+  char& operator[](size_t) {
+    static char c = '\0';
+    return c;
+  }
   void resize(size_t) { resize_call_count += 1; }
 };
 
 int resize_default_init_call_count = 0;
 
+// A mock string class whose only purpose is to track how many times its
+// resize() and __resize_default_init() methods have been called.
 struct resize_default_init_string {
+  size_t size() const { return 0; }
+  char& operator[](size_t) {
+    static char c = '\0';
+    return c;
+  }
   void resize(size_t) { resize_call_count += 1; }
   void __resize_default_init(size_t) { resize_default_init_call_count += 1; }
 };
diff --git a/absl/strings/internal/stl_type_traits.h b/absl/strings/internal/stl_type_traits.h
index af50be7..6035ca4 100644
--- a/absl/strings/internal/stl_type_traits.h
+++ b/absl/strings/internal/stl_type_traits.h
@@ -40,7 +40,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 template <typename C, template <typename...> class T>
@@ -243,6 +243,6 @@
                         IsConvertibleToSTLContainer<C>> {};
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_STRINGS_INTERNAL_STL_TYPE_TRAITS_H_
diff --git a/absl/strings/internal/str_format/arg.cc b/absl/strings/internal/str_format/arg.cc
index 667cc13..4d0604e 100644
--- a/absl/strings/internal/str_format/arg.cc
+++ b/absl/strings/internal/str_format/arg.cc
@@ -14,7 +14,7 @@
 #include "absl/strings/internal/str_format/float_conversion.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -33,6 +33,10 @@
 template <typename T>
 struct MakeUnsigned : std::make_unsigned<T> {};
 template <>
+struct MakeUnsigned<absl::int128> {
+  using type = absl::uint128;
+};
+template <>
 struct MakeUnsigned<absl::uint128> {
   using type = absl::uint128;
 };
@@ -40,6 +44,8 @@
 template <typename T>
 struct IsSigned : std::is_signed<T> {};
 template <>
+struct IsSigned<absl::int128> : std::true_type {};
+template <>
 struct IsSigned<absl::uint128> : std::false_type {};
 
 class ConvertedIntInfo {
@@ -82,7 +88,7 @@
   template <typename T>
   void UnsignedToStringRight(T u, ConversionChar conv) {
     char *p = end();
-    switch (conv.radix()) {
+    switch (FormatConversionCharRadix(conv)) {
       default:
       case 10:
         for (; u; u /= 10)
@@ -93,7 +99,7 @@
           *--p = static_cast<char>('0' + static_cast<size_t>(u % 8));
         break;
       case 16: {
-        const char *digits = kDigit[conv.upper() ? 1 : 0];
+        const char *digits = kDigit[FormatConversionCharIsUpper(conv) ? 1 : 0];
         for (; u; u /= 16) *--p = digits[static_cast<size_t>(u % 16)];
         break;
       }
@@ -115,21 +121,20 @@
 string_view BaseIndicator(const ConvertedIntInfo &info,
                           const ConversionSpec conv) {
   bool alt = conv.flags().alt;
-  int radix = conv.conv().radix();
-  if (conv.conv().id() == ConversionChar::p)
-    alt = true;  // always show 0x for %p.
+  int radix = FormatConversionCharRadix(conv.conv());
+  if (conv.conv() == ConversionChar::p) alt = true;  // always show 0x for %p.
   // From the POSIX description of '#' flag:
   //   "For x or X conversion specifiers, a non-zero result shall have
   //   0x (or 0X) prefixed to it."
   if (alt && radix == 16 && !info.digits().empty()) {
-    if (conv.conv().upper()) return "0X";
+    if (FormatConversionCharIsUpper(conv.conv())) return "0X";
     return "0x";
   }
   return {};
 }
 
 string_view SignColumn(bool neg, const ConversionSpec conv) {
-  if (conv.conv().is_signed()) {
+  if (FormatConversionCharIsSigned(conv.conv())) {
     if (neg) return "-";
     if (conv.flags().show_pos) return "+";
     if (conv.flags().sign_col) return " ";
@@ -169,7 +174,7 @@
   if (!precision_specified)
     precision = 1;
 
-  if (conv.flags().alt && conv.conv().id() == ConversionChar::o) {
+  if (conv.flags().alt && conv.conv() == ConversionChar::o) {
     // From POSIX description of the '#' (alt) flag:
     //   "For o conversion, it increases the precision (if necessary) to
     //   force the first digit of the result to be zero."
@@ -205,7 +210,7 @@
 template <typename T>
 bool ConvertIntImplInner(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
   ConvertedIntInfo info(v, conv.conv());
-  if (conv.flags().basic && conv.conv().id() != ConversionChar::p) {
+  if (conv.flags().basic && (conv.conv() != ConversionChar::p)) {
     if (info.is_neg()) sink->Append(1, '-');
     if (info.digits().empty()) {
       sink->Append(1, '0');
@@ -219,14 +224,13 @@
 
 template <typename T>
 bool ConvertIntArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  if (conv.conv().is_float()) {
+  if (FormatConversionCharIsFloat(conv.conv())) {
     return FormatConvertImpl(static_cast<double>(v), conv, sink).value;
   }
-  if (conv.conv().id() == ConversionChar::c)
+  if (conv.conv() == ConversionChar::c)
     return ConvertCharImpl(static_cast<unsigned char>(v), conv, sink);
-  if (!conv.conv().is_integral())
-    return false;
-  if (!conv.conv().is_signed() && IsSigned<T>::value) {
+  if (!FormatConversionCharIsIntegral(conv.conv())) return false;
+  if (!FormatConversionCharIsSigned(conv.conv()) && IsSigned<T>::value) {
     using U = typename MakeUnsigned<T>::type;
     return FormatConvertImpl(static_cast<U>(v), conv, sink).value;
   }
@@ -235,13 +239,13 @@
 
 template <typename T>
 bool ConvertFloatArg(T v, const ConversionSpec conv, FormatSinkImpl *sink) {
-  return conv.conv().is_float() && ConvertFloatImpl(v, conv, sink);
+  return FormatConversionCharIsFloat(conv.conv()) &&
+         ConvertFloatImpl(v, conv, sink);
 }
 
 inline bool ConvertStringArg(string_view v, const ConversionSpec conv,
                              FormatSinkImpl *sink) {
-  if (conv.conv().id() != ConversionChar::s)
-    return false;
+  if (conv.conv() != ConversionChar::s) return false;
   if (conv.flags().basic) {
     sink->Append(v);
     return true;
@@ -268,7 +272,7 @@
 ConvertResult<Conv::s | Conv::p> FormatConvertImpl(const char *v,
                                                    const ConversionSpec conv,
                                                    FormatSinkImpl *sink) {
-  if (conv.conv().id() == ConversionChar::p)
+  if (conv.conv() == ConversionChar::p)
     return {FormatConvertImpl(VoidPtr(v), conv, sink).value};
   size_t len;
   if (v == nullptr) {
@@ -276,7 +280,7 @@
   } else if (conv.precision() < 0) {
     len = std::strlen(v);
   } else {
-    // If precision is set, we look for the null terminator on the valid range.
+    // If precision is set, we look for the NUL-terminator on the valid range.
     len = std::find(v, v + conv.precision(), '\0') - v;
   }
   return {ConvertStringArg(string_view(v, len), conv, sink)};
@@ -285,8 +289,7 @@
 // ==================== Raw pointers ====================
 ConvertResult<Conv::p> FormatConvertImpl(VoidPtr v, const ConversionSpec conv,
                                          FormatSinkImpl *sink) {
-  if (conv.conv().id() != ConversionChar::p)
-    return {false};
+  if (conv.conv() != ConversionChar::p) return {false};
   if (!v.value) {
     sink->Append("(nil)");
     return {true};
@@ -364,6 +367,11 @@
                                         FormatSinkImpl *sink) {
   return {ConvertIntArg(v, conv, sink)};
 }
+IntegralConvertResult FormatConvertImpl(absl::int128 v,
+                                        const ConversionSpec conv,
+                                        FormatSinkImpl *sink) {
+  return {ConvertIntArg(v, conv, sink)};
+}
 IntegralConvertResult FormatConvertImpl(absl::uint128 v,
                                         const ConversionSpec conv,
                                         FormatSinkImpl *sink) {
@@ -373,7 +381,8 @@
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_();
 
 
+
 }  // namespace str_format_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index b6f708c..7a93756 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h
@@ -18,12 +18,10 @@
 #include "absl/strings/internal/str_format/extension.h"
 #include "absl/strings/string_view.h"
 
-class Cord;
-class CordReader;
-
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
+class Cord;
 class FormatCountCapture;
 class FormatSink;
 
@@ -68,12 +66,11 @@
                                                    FormatSinkImpl* sink);
 template <class AbslCord,
           typename std::enable_if<
-              std::is_same<AbslCord, ::Cord>::value>::type* = nullptr,
-          class AbslCordReader = ::CordReader>
+              std::is_same<AbslCord, absl::Cord>::value>::type* = nullptr>
 ConvertResult<Conv::s> FormatConvertImpl(const AbslCord& value,
                                          ConversionSpec conv,
                                          FormatSinkImpl* sink) {
-  if (conv.conv().id() != ConversionChar::s) return {false};
+  if (conv.conv() != ConversionChar::s) return {false};
 
   bool is_left = conv.flags().left;
   size_t space_remaining = 0;
@@ -91,11 +88,17 @@
 
   if (space_remaining > 0 && !is_left) sink->Append(space_remaining, ' ');
 
-  string_view piece;
-  for (AbslCordReader reader(value);
-       to_write > 0 && reader.ReadFragment(&piece); to_write -= piece.size()) {
-    if (piece.size() > to_write) piece.remove_suffix(piece.size() - to_write);
+  for (string_view piece : value.Chunks()) {
+    if (piece.size() > to_write) {
+      piece.remove_suffix(piece.size() - to_write);
+      to_write = 0;
+    } else {
+      to_write -= piece.size();
+    }
     sink->Append(piece);
+    if (to_write == 0) {
+      break;
+    }
   }
 
   if (space_remaining > 0 && is_left) sink->Append(space_remaining, ' ');
@@ -145,6 +148,8 @@
 IntegralConvertResult FormatConvertImpl(unsigned long long v,  // NOLINT
                                         ConversionSpec conv,
                                         FormatSinkImpl* sink);
+IntegralConvertResult FormatConvertImpl(int128 v, ConversionSpec conv,
+                                        FormatSinkImpl* sink);
 IntegralConvertResult FormatConvertImpl(uint128 v, ConversionSpec conv,
                                         FormatSinkImpl* sink);
 template <typename T, enable_if_t<std::is_same<T, bool>::value, int> = 0>
@@ -180,8 +185,7 @@
                                               FormatSinkImpl* sink) {
     const absl::enable_if_t<sizeof(T) != 0, FormatCountCapture>& v2 = v;
 
-    if (conv.conv().id() != str_format_internal::ConversionChar::n)
-      return {false};
+    if (conv.conv() != str_format_internal::ConversionChar::n) return {false};
     *v2.p_ = static_cast<int>(sink->size());
     return {true};
   }
@@ -373,7 +377,7 @@
   template <typename T>
   static bool Dispatch(Data arg, ConversionSpec spec, void* out) {
     // A `none` conv indicates that we want the `int` conversion.
-    if (ABSL_PREDICT_FALSE(spec.conv().id() == ConversionChar::none)) {
+    if (ABSL_PREDICT_FALSE(spec.conv() == ConversionChar::none)) {
       return ToInt<T>(arg, static_cast<int*>(out), std::is_integral<T>(),
                       std::is_enum<T>());
     }
@@ -409,6 +413,7 @@
                                              __VA_ARGS__);                     \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(unsigned long long, /* NOLINT */  \
                                              __VA_ARGS__);                     \
+  ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(int128, __VA_ARGS__);             \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(uint128, __VA_ARGS__);            \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(float, __VA_ARGS__);              \
   ABSL_INTERNAL_FORMAT_DISPATCH_INSTANTIATE_(double, __VA_ARGS__);             \
@@ -419,8 +424,9 @@
 
 ABSL_INTERNAL_FORMAT_DISPATCH_OVERLOADS_EXPAND_(extern);
 
+
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_ARG_H_
diff --git a/absl/strings/internal/str_format/arg_test.cc b/absl/strings/internal/str_format/arg_test.cc
index c9c5195..8d30d8b 100644
--- a/absl/strings/internal/str_format/arg_test.cc
+++ b/absl/strings/internal/str_format/arg_test.cc
@@ -14,7 +14,7 @@
 #include "absl/strings/str_format.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -96,10 +96,10 @@
   std::string s;
   FormatSinkImpl sink(&s);
   ConversionSpec conv;
-  conv.set_conv(ConversionChar::FromChar('s'));
-  conv.set_flags(Flags());
-  conv.set_width(-1);
-  conv.set_precision(-1);
+  FormatConversionSpecImplFriend::SetConversionChar(ConversionChar::s, &conv);
+  FormatConversionSpecImplFriend::SetFlags(Flags(), &conv);
+  FormatConversionSpecImplFriend::SetWidth(-1, &conv);
+  FormatConversionSpecImplFriend::SetPrecision(-1, &conv);
   EXPECT_TRUE(
       FormatArgImplFriend::Convert(FormatArgImpl(kMyArray), conv, &sink));
   sink.Flush();
@@ -109,5 +109,5 @@
 
 }  // namespace
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/bind.cc b/absl/strings/internal/str_format/bind.cc
index 48a306d..27522fd 100644
--- a/absl/strings/internal/str_format/bind.cc
+++ b/absl/strings/internal/str_format/bind.cc
@@ -6,7 +6,7 @@
 #include <string>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -66,19 +66,22 @@
         return false;
     }
 
-    bound->set_width(width);
-    bound->set_precision(precision);
-    bound->set_flags(unbound->flags);
-    if (force_left)
-      bound->set_left(true);
-  } else {
-    bound->set_flags(unbound->flags);
-    bound->set_width(-1);
-    bound->set_precision(-1);
-  }
+    FormatConversionSpecImplFriend::SetWidth(width, bound);
+    FormatConversionSpecImplFriend::SetPrecision(precision, bound);
 
-  bound->set_length_mod(unbound->length_mod);
-  bound->set_conv(unbound->conv);
+    if (force_left) {
+      Flags flags = unbound->flags;
+      flags.left = true;
+      FormatConversionSpecImplFriend::SetFlags(flags, bound);
+    } else {
+      FormatConversionSpecImplFriend::SetFlags(unbound->flags, bound);
+    }
+  } else {
+    FormatConversionSpecImplFriend::SetFlags(unbound->flags, bound);
+    FormatConversionSpecImplFriend::SetWidth(-1, bound);
+    FormatConversionSpecImplFriend::SetPrecision(-1, bound);
+  }
+  FormatConversionSpecImplFriend::SetConversionChar(unbound->conv, bound);
   bound->set_arg(arg);
   return true;
 }
@@ -140,10 +143,11 @@
     UntypedFormatSpecImpl spec("%d");
 
     std::ostringstream ss;
-    ss << "{" << Streamable(spec, {*bound.arg()}) << ":" << bound.flags();
+    ss << "{" << Streamable(spec, {*bound.arg()}) << ":"
+       << FormatConversionSpecImplFriend::FlagsToString(bound);
     if (bound.width() >= 0) ss << bound.width();
     if (bound.precision() >= 0) ss << "." << bound.precision();
-    ss << bound.length_mod() << bound.conv() << "}";
+    ss << bound.conv() << "}";
     Append(ss.str());
     return true;
   }
@@ -197,6 +201,15 @@
   return *out;
 }
 
+std::string FormatPack(const UntypedFormatSpecImpl format,
+                       absl::Span<const FormatArgImpl> args) {
+  std::string out;
+  if (ABSL_PREDICT_FALSE(!FormatUntyped(&out, format, args))) {
+    out.clear();
+  }
+  return out;
+}
+
 int FprintF(std::FILE* output, const UntypedFormatSpecImpl format,
             absl::Span<const FormatArgImpl> args) {
   FILERawSink sink(output);
@@ -228,5 +241,5 @@
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/bind.h b/absl/strings/internal/str_format/bind.h
index db79eb6..cf41b19 100644
--- a/absl/strings/internal/str_format/bind.h
+++ b/absl/strings/internal/str_format/bind.h
@@ -13,7 +13,7 @@
 #include "absl/types/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 class UntypedFormatSpec;
 
@@ -74,7 +74,7 @@
   using Base = typename MakeDependent<UntypedFormatSpec, Args...>::type;
 
  public:
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
   // Honeypot overload for when the std::string is not constexpr.
   // We use the 'unavailable' attribute to give a better compiler error than
@@ -122,8 +122,8 @@
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
   template <Conv... C, typename = typename std::enable_if<
-                           sizeof...(C) == sizeof...(Args) &&
-                           AllOf(Contains(ArgumentToConv<Args>(),
+                           AllOf(sizeof...(C) == sizeof...(Args),
+                             Contains(ArgumentToConv<Args>(),
                                           C)...)>::type>
   FormatSpecTemplate(const ExtendedParsedFormat<C...>& pc)  // NOLINT
       : Base(&pc) {}
@@ -179,12 +179,8 @@
 std::string& AppendPack(std::string* out, UntypedFormatSpecImpl format,
                         absl::Span<const FormatArgImpl> args);
 
-inline std::string FormatPack(const UntypedFormatSpecImpl format,
-                              absl::Span<const FormatArgImpl> args) {
-  std::string out;
-  AppendPack(&out, format, args);
-  return out;
-}
+std::string FormatPack(const UntypedFormatSpecImpl format,
+                       absl::Span<const FormatArgImpl> args);
 
 int FprintF(std::FILE* output, UntypedFormatSpecImpl format,
             absl::Span<const FormatArgImpl> args);
@@ -207,7 +203,7 @@
 };
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_BIND_H_
diff --git a/absl/strings/internal/str_format/bind_test.cc b/absl/strings/internal/str_format/bind_test.cc
index 8bccc92..64790a8 100644
--- a/absl/strings/internal/str_format/bind_test.cc
+++ b/absl/strings/internal/str_format/bind_test.cc
@@ -6,7 +6,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -113,18 +113,18 @@
                                 FormatArgImpl(ia[2]), FormatArgImpl(ia[3]),
                                 FormatArgImpl(ia[4])};
   const Expectation kExpect[] = {
-    {__LINE__, "a%4db%dc", "a{10:4d}b{20:d}c"},
-    {__LINE__, "a%.4db%dc", "a{10:.4d}b{20:d}c"},
-    {__LINE__, "a%4.5db%dc", "a{10:4.5d}b{20:d}c"},
-    {__LINE__, "a%db%4.5dc", "a{10:d}b{20:4.5d}c"},
-    {__LINE__, "a%db%*.*dc", "a{10:d}b{40:20.30d}c"},
-    {__LINE__, "a%.*fb", "a{20:.10f}b"},
-    {__LINE__, "a%1$db%2$*3$.*4$dc", "a{10:d}b{20:30.40d}c"},
-    {__LINE__, "a%4$db%3$*2$.*1$dc", "a{40:d}b{30:20.10d}c"},
-    {__LINE__, "a%04ldb", "a{10:04ld}b"},
-    {__LINE__, "a%-#04lldb", "a{10:-#04lld}b"},
-    {__LINE__, "a%1$*5$db", "a{10:-10d}b"},
-    {__LINE__, "a%1$.*5$db", "a{10:d}b"},
+      {__LINE__, "a%4db%dc", "a{10:4d}b{20:d}c"},
+      {__LINE__, "a%.4db%dc", "a{10:.4d}b{20:d}c"},
+      {__LINE__, "a%4.5db%dc", "a{10:4.5d}b{20:d}c"},
+      {__LINE__, "a%db%4.5dc", "a{10:d}b{20:4.5d}c"},
+      {__LINE__, "a%db%*.*dc", "a{10:d}b{40:20.30d}c"},
+      {__LINE__, "a%.*fb", "a{20:.10f}b"},
+      {__LINE__, "a%1$db%2$*3$.*4$dc", "a{10:d}b{20:30.40d}c"},
+      {__LINE__, "a%4$db%3$*2$.*1$dc", "a{40:d}b{30:20.10d}c"},
+      {__LINE__, "a%04ldb", "a{10:04d}b"},
+      {__LINE__, "a%-#04lldb", "a{10:-#04d}b"},
+      {__LINE__, "a%1$*5$db", "a{10:-10d}b"},
+      {__LINE__, "a%1$.*5$db", "a{10:d}b"},
   };
   for (const Expectation &e : kExpect) {
     absl::string_view fmt = e.fmt;
@@ -139,5 +139,5 @@
 
 }  // namespace
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/checker.h b/absl/strings/internal/str_format/checker.h
index 262887c..8993a79 100644
--- a/absl/strings/internal/str_format/checker.h
+++ b/absl/strings/internal/str_format/checker.h
@@ -1,21 +1,20 @@
 #ifndef ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
 
+#include "absl/base/attributes.h"
 #include "absl/strings/internal/str_format/arg.h"
 #include "absl/strings/internal/str_format/extension.h"
 
 // Compile time check support for entry points.
 
 #ifndef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
-#if defined(__clang__) && !defined(__native_client__)
-#if __has_attribute(enable_if)
+#if ABSL_HAVE_ATTRIBUTE(enable_if) && !defined(__native_client__)
 #define ABSL_INTERNAL_ENABLE_FORMAT_CHECKER 1
-#endif  // __has_attribute(enable_if)
-#endif  // defined(__clang__) && !defined(__native_client__)
+#endif  // ABSL_HAVE_ATTRIBUTE(enable_if) && !defined(__native_client__)
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 constexpr bool AllOf() { return true; }
@@ -32,7 +31,7 @@
       std::declval<FormatSinkImpl*>()))::kConv;
 }
 
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 constexpr bool ContainsChar(const char* chars, char c) {
   return *chars == c || (*chars && ContainsChar(chars + 1, c));
@@ -321,7 +320,7 @@
 #endif  // ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_CHECKER_H_
diff --git a/absl/strings/internal/str_format/checker_test.cc b/absl/strings/internal/str_format/checker_test.cc
index 2aa3b12..ea2a768 100644
--- a/absl/strings/internal/str_format/checker_test.cc
+++ b/absl/strings/internal/str_format/checker_test.cc
@@ -5,7 +5,7 @@
 #include "absl/strings/str_format.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -13,7 +13,7 @@
   std::string out;
 #define CONV_SET_CASE(c) \
   if (Contains(conv, Conv::c)) out += #c;
-  ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
 #undef CONV_SET_CASE
   if (Contains(conv, Conv::star)) out += "*";
   return out;
@@ -36,7 +36,7 @@
   EXPECT_EQ(ConvToString(conv), "p");
 }
 
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
 
 struct Case {
   bool result;
@@ -148,5 +148,5 @@
 
 }  // namespace
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/convert_test.cc b/absl/strings/internal/str_format/convert_test.cc
index 751bfc3..cbcd7ca 100644
--- a/absl/strings/internal/str_format/convert_test.cc
+++ b/absl/strings/internal/str_format/convert_test.cc
@@ -5,11 +5,13 @@
 #include <cmath>
 #include <string>
 
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/internal/raw_logging.h"
 #include "absl/strings/internal/str_format/bind.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 namespace {
 
@@ -152,18 +154,26 @@
   UntypedFormatSpecImpl format("%.1s");
   EXPECT_EQ("a", FormatPack(format, {FormatArgImpl(p)}));
 
-  // We cap at the nul terminator.
+  // We cap at the NUL-terminator.
   p = "ABC";
   UntypedFormatSpecImpl format2("%.10s");
   EXPECT_EQ("ABC", FormatPack(format2, {FormatArgImpl(p)}));
 }
 
+// Pointer formatting is implementation defined. This checks that the argument
+// can be matched to `ptr`.
+MATCHER_P(MatchesPointerString, ptr, "") {
+  if (ptr == nullptr && arg == "(nil)") {
+    return true;
+  }
+  void* parsed = nullptr;
+  if (sscanf(arg.c_str(), "%p", &parsed) != 1) {
+    ABSL_RAW_LOG(FATAL, "Could not parse %s", arg.c_str());
+  }
+  return ptr == parsed;
+}
+
 TEST_F(FormatConvertTest, Pointer) {
-#ifdef _MSC_VER
-  // MSVC's printf implementation prints pointers differently. We can't easily
-  // compare our implementation to theirs.
-  return;
-#endif
   static int x = 0;
   const int *xp = &x;
   char c = 'h';
@@ -174,48 +184,62 @@
   using VoidF = void (*)();
   VoidF fp = [] {}, fnil = nullptr;
   volatile char vc;
-  volatile char* vcp = &vc;
-  volatile char* vcnil = nullptr;
-  const FormatArgImpl args[] = {
+  volatile char *vcp = &vc;
+  volatile char *vcnil = nullptr;
+  const FormatArgImpl args_array[] = {
       FormatArgImpl(xp),   FormatArgImpl(cp),  FormatArgImpl(inil),
       FormatArgImpl(cnil), FormatArgImpl(mcp), FormatArgImpl(fp),
       FormatArgImpl(fnil), FormatArgImpl(vcp), FormatArgImpl(vcnil),
   };
-  struct Expectation {
-    std::string out;
-    const char *fmt;
-  };
-  const Expectation kExpect[] = {
-      {StrPrint("%p", &x), "%p"},
-      {StrPrint("%20p", &x), "%20p"},
-      {StrPrint("%.1p", &x), "%.1p"},
-      {StrPrint("%.20p", &x), "%.20p"},
-      {StrPrint("%30.20p", &x), "%30.20p"},
+  auto args = absl::MakeConstSpan(args_array);
 
-      {StrPrint("%-p", &x), "%-p"},
-      {StrPrint("%-20p", &x), "%-20p"},
-      {StrPrint("%-.1p", &x), "%-.1p"},
-      {StrPrint("%.20p", &x), "%.20p"},
-      {StrPrint("%-30.20p", &x), "%-30.20p"},
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.1p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%30.20p"), args),
+              MatchesPointerString(&x));
 
-      {StrPrint("%p", cp), "%2$p"},   // const char*
-      {"(nil)", "%3$p"},              // null const char *
-      {"(nil)", "%4$p"},              // null const int *
-      {StrPrint("%p", mcp), "%5$p"},  // nonconst char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-.1p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%.20p"), args),
+              MatchesPointerString(&x));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%-30.20p"), args),
+              MatchesPointerString(&x));
 
-      {StrPrint("%p", fp), "%6$p"},   // function pointer
-      {StrPrint("%p", vcp), "%8$p"},  // function pointer
+  // const char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%2$p"), args),
+              MatchesPointerString(cp));
+  // null const int*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%3$p"), args),
+              MatchesPointerString(nullptr));
+  // null const char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%4$p"), args),
+              MatchesPointerString(nullptr));
+  // nonconst char*
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%5$p"), args),
+              MatchesPointerString(mcp));
 
-#ifndef __APPLE__
-      // Apple's printf differs here (0x0 vs. nil)
-      {StrPrint("%p", fnil), "%7$p"},   // null function pointer
-      {StrPrint("%p", vcnil), "%9$p"},  // null function pointer
-#endif
-  };
-  for (const Expectation &e : kExpect) {
-    UntypedFormatSpecImpl format(e.fmt);
-    EXPECT_EQ(e.out, FormatPack(format, absl::MakeSpan(args))) << e.fmt;
-  }
+  // function pointers
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%6$p"), args),
+              MatchesPointerString(reinterpret_cast<const void*>(fp)));
+  EXPECT_THAT(
+      FormatPack(UntypedFormatSpecImpl("%8$p"), args),
+      MatchesPointerString(reinterpret_cast<volatile const void *>(vcp)));
+
+  // null function pointers
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%7$p"), args),
+              MatchesPointerString(nullptr));
+  EXPECT_THAT(FormatPack(UntypedFormatSpecImpl("%9$p"), args),
+              MatchesPointerString(nullptr));
 }
 
 struct Cardinal {
@@ -367,7 +391,6 @@
     AllIntTypes;
 INSTANTIATE_TYPED_TEST_CASE_P(TypedFormatConvertTestWithAllIntTypes,
                               TypedFormatConvertTest, AllIntTypes);
-
 TEST_F(FormatConvertTest, VectorBool) {
   // Make sure vector<bool>'s values behave as bools.
   std::vector<bool> v = {true, false};
@@ -379,6 +402,42 @@
                             FormatArgImpl(cv[0]), FormatArgImpl(cv[1])})));
 }
 
+
+TEST_F(FormatConvertTest, Int128) {
+  absl::int128 positive = static_cast<absl::int128>(0x1234567890abcdef) * 1979;
+  absl::int128 negative = -positive;
+  absl::int128 max = absl::Int128Max(), min = absl::Int128Min();
+  const FormatArgImpl args[] = {FormatArgImpl(positive),
+                                FormatArgImpl(negative), FormatArgImpl(max),
+                                FormatArgImpl(min)};
+
+  struct Case {
+    const char* format;
+    const char* expected;
+  } cases[] = {
+      {"%1$d", "2595989796776606496405"},
+      {"%1$30d", "        2595989796776606496405"},
+      {"%1$-30d", "2595989796776606496405        "},
+      {"%1$u", "2595989796776606496405"},
+      {"%1$x", "8cba9876066020f695"},
+      {"%2$d", "-2595989796776606496405"},
+      {"%2$30d", "       -2595989796776606496405"},
+      {"%2$-30d", "-2595989796776606496405       "},
+      {"%2$u", "340282366920938460867384810655161715051"},
+      {"%2$x", "ffffffffffffff73456789f99fdf096b"},
+      {"%3$d", "170141183460469231731687303715884105727"},
+      {"%3$u", "170141183460469231731687303715884105727"},
+      {"%3$x", "7fffffffffffffffffffffffffffffff"},
+      {"%4$d", "-170141183460469231731687303715884105728"},
+      {"%4$x", "80000000000000000000000000000000"},
+  };
+
+  for (auto c : cases) {
+    UntypedFormatSpecImpl format(c.format);
+    EXPECT_EQ(c.expected, FormatPack(format, absl::MakeSpan(args)));
+  }
+}
+
 TEST_F(FormatConvertTest, Uint128) {
   absl::uint128 v = static_cast<absl::uint128>(0x1234567890abcdef) * 1979;
   absl::uint128 max = absl::Uint128Max();
@@ -588,5 +647,5 @@
 
 }  // namespace
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/extension.cc b/absl/strings/internal/str_format/extension.cc
index 1a0c417..2e5bc2c 100644
--- a/absl/strings/internal/str_format/extension.cc
+++ b/absl/strings/internal/str_format/extension.cc
@@ -20,39 +20,8 @@
 #include <string>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
-namespace {
-// clang-format off
-#define ABSL_LENGTH_MODS_EXPAND_ \
-  X_VAL(h) X_SEP \
-  X_VAL(hh) X_SEP \
-  X_VAL(l) X_SEP \
-  X_VAL(ll) X_SEP \
-  X_VAL(L) X_SEP \
-  X_VAL(j) X_SEP \
-  X_VAL(z) X_SEP \
-  X_VAL(t) X_SEP \
-  X_VAL(q)
-// clang-format on
-}  // namespace
-
-const LengthMod::Spec LengthMod::kSpecs[] = {
-#define X_VAL(id) { LengthMod::id, #id, strlen(#id) }
-#define X_SEP ,
-    ABSL_LENGTH_MODS_EXPAND_, {LengthMod::none, "", 0}
-#undef X_VAL
-#undef X_SEP
-};
-
-const ConversionChar::Spec ConversionChar::kSpecs[] = {
-#define X_VAL(id) { ConversionChar::id, #id[0] }
-#define X_SEP ,
-    ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP),
-    {ConversionChar::none, '\0'},
-#undef X_VAL
-#undef X_SEP
-};
 
 std::string Flags::ToString() const {
   std::string s;
@@ -64,10 +33,6 @@
   return s;
 }
 
-const size_t LengthMod::kNumValues;
-
-const size_t ConversionChar::kNumValues;
-
 bool FormatSinkImpl::PutPaddedString(string_view v, int w, int p, bool l) {
   size_t space_remaining = 0;
   if (w >= 0) space_remaining = w;
@@ -82,5 +47,5 @@
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/extension.h b/absl/strings/internal/str_format/extension.h
index 4fbe4a0..d166575 100644
--- a/absl/strings/internal/str_format/extension.h
+++ b/absl/strings/internal/str_format/extension.h
@@ -17,19 +17,18 @@
 #define ABSL_STRINGS_INTERNAL_STR_FORMAT_EXTENSION_H_
 
 #include <limits.h>
+
 #include <cstddef>
 #include <cstring>
 #include <ostream>
 
+#include "absl/base/config.h"
 #include "absl/base/port.h"
 #include "absl/strings/internal/str_format/output.h"
 #include "absl/strings/string_view.h"
 
-class Cord;
-
 namespace absl {
-inline namespace lts_2019_08_08 {
-
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 class FormatRawSinkImpl {
@@ -137,56 +136,8 @@
   }
 };
 
-struct LengthMod {
- public:
-  enum Id : uint8_t {
-    h, hh, l, ll, L, j, z, t, q, none
-  };
-  static const size_t kNumValues = none + 1;
-
-  LengthMod() : id_(none) {}
-
-  // Index into the opaque array of LengthMod enums.
-  // Requires: i < kNumValues
-  static LengthMod FromIndex(size_t i) {
-    return LengthMod(kSpecs[i].value);
-  }
-
-  static LengthMod FromId(Id id) { return LengthMod(id); }
-
-  // The length modifier std::string associated with a specified LengthMod.
-  string_view name() const {
-    const Spec& spec = kSpecs[id_];
-    return {spec.name, spec.name_length};
-  }
-
-  Id id() const { return id_; }
-
-  friend bool operator==(const LengthMod& a, const LengthMod& b) {
-    return a.id() == b.id();
-  }
-  friend bool operator!=(const LengthMod& a, const LengthMod& b) {
-    return !(a == b);
-  }
-  friend std::ostream& operator<<(std::ostream& os, const LengthMod& v) {
-    return os << v.name();
-  }
-
- private:
-  struct Spec {
-    Id value;
-    const char *name;
-    size_t name_length;
-  };
-  static const Spec kSpecs[];
-
-  explicit LengthMod(Id id) : id_(id) {}
-
-  Id id_;
-};
-
 // clang-format off
-#define ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP) \
+#define ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(X_VAL, X_SEP) \
   /* text */ \
   X_VAL(c) X_SEP X_VAL(C) X_SEP X_VAL(s) X_SEP X_VAL(S) X_SEP \
   /* ints */ \
@@ -197,121 +148,135 @@
   X_VAL(g) X_SEP X_VAL(G) X_SEP X_VAL(a) X_SEP X_VAL(A) X_SEP \
   /* misc */ \
   X_VAL(n) X_SEP X_VAL(p)
-// clang-format on
 
-struct ConversionChar {
- public:
-  enum Id : uint8_t {
+enum class FormatConversionChar : uint8_t {
     c, C, s, S,              // text
     d, i, o, u, x, X,        // int
     f, F, e, E, g, G, a, A,  // float
     n, p,                    // misc
-    none
-  };
-  static const size_t kNumValues = none + 1;
-
-  ConversionChar() : id_(none) {}
-
- public:
-  // Index into the opaque array of ConversionChar enums.
-  // Requires: i < kNumValues
-  static ConversionChar FromIndex(size_t i) {
-    return ConversionChar(kSpecs[i].value);
-  }
-
-  static ConversionChar FromChar(char c) {
-    ConversionChar::Id out_id = ConversionChar::none;
-    switch (c) {
-#define X_VAL(id)                \
-  case #id[0]:                   \
-    out_id = ConversionChar::id; \
-    break;
-      ABSL_CONVERSION_CHARS_EXPAND_(X_VAL, )
-#undef X_VAL
-      default:
-        break;
-    }
-    return ConversionChar(out_id);
-  }
-
-  static ConversionChar FromId(Id id) { return ConversionChar(id); }
-  Id id() const { return id_; }
-
-  int radix() const {
-    switch (id()) {
-      case x: case X: case a: case A: case p: return 16;
-      case o: return 8;
-      default: return 10;
-    }
-  }
-
-  bool upper() const {
-    switch (id()) {
-      case X: case F: case E: case G: case A: return true;
-      default: return false;
-    }
-  }
-
-  bool is_signed() const {
-    switch (id()) {
-      case d: case i: return true;
-      default: return false;
-    }
-  }
-
-  bool is_integral() const {
-    switch (id()) {
-      case d: case i: case u: case o: case x: case X:
-        return true;
-      default: return false;
-    }
-  }
-
-  bool is_float() const {
-    switch (id()) {
-      case a: case e: case f: case g: case A: case E: case F: case G:
-        return true;
-      default: return false;
-    }
-  }
-
-  bool IsValid() const { return id() != none; }
-
-  // The associated char.
-  char Char() const { return kSpecs[id_].name; }
-
-  friend bool operator==(const ConversionChar& a, const ConversionChar& b) {
-    return a.id() == b.id();
-  }
-  friend bool operator!=(const ConversionChar& a, const ConversionChar& b) {
-    return !(a == b);
-  }
-  friend std::ostream& operator<<(std::ostream& os, const ConversionChar& v) {
-    char c = v.Char();
-    if (!c) c = '?';
-    return os << c;
-  }
-
- private:
-  struct Spec {
-    Id value;
-    char name;
-  };
-  static const Spec kSpecs[];
-
-  explicit ConversionChar(Id id) : id_(id) {}
-
-  Id id_;
+    kNone,
+    none = kNone
 };
+// clang-format on
 
-class ConversionSpec {
+inline FormatConversionChar FormatConversionCharFromChar(char c) {
+  switch (c) {
+#define ABSL_INTERNAL_X_VAL(id) \
+  case #id[0]:                  \
+    return FormatConversionChar::id;
+    ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL, )
+#undef ABSL_INTERNAL_X_VAL
+  }
+  return FormatConversionChar::kNone;
+}
+
+inline int FormatConversionCharRadix(FormatConversionChar c) {
+  switch (c) {
+    case FormatConversionChar::x:
+    case FormatConversionChar::X:
+    case FormatConversionChar::a:
+    case FormatConversionChar::A:
+    case FormatConversionChar::p:
+      return 16;
+    case FormatConversionChar::o:
+      return 8;
+    default:
+      return 10;
+  }
+}
+
+inline bool FormatConversionCharIsUpper(FormatConversionChar c) {
+  switch (c) {
+    case FormatConversionChar::X:
+    case FormatConversionChar::F:
+    case FormatConversionChar::E:
+    case FormatConversionChar::G:
+    case FormatConversionChar::A:
+      return true;
+    default:
+      return false;
+  }
+}
+
+inline bool FormatConversionCharIsSigned(FormatConversionChar c) {
+  switch (c) {
+    case FormatConversionChar::d:
+    case FormatConversionChar::i:
+      return true;
+    default:
+      return false;
+  }
+}
+
+inline bool FormatConversionCharIsIntegral(FormatConversionChar c) {
+  switch (c) {
+    case FormatConversionChar::d:
+    case FormatConversionChar::i:
+    case FormatConversionChar::u:
+    case FormatConversionChar::o:
+    case FormatConversionChar::x:
+    case FormatConversionChar::X:
+      return true;
+    default:
+      return false;
+  }
+}
+
+inline bool FormatConversionCharIsFloat(FormatConversionChar c) {
+  switch (c) {
+    case FormatConversionChar::a:
+    case FormatConversionChar::e:
+    case FormatConversionChar::f:
+    case FormatConversionChar::g:
+    case FormatConversionChar::A:
+    case FormatConversionChar::E:
+    case FormatConversionChar::F:
+    case FormatConversionChar::G:
+      return true;
+    default:
+      return false;
+  }
+}
+
+inline char FormatConversionCharToChar(FormatConversionChar c) {
+  switch (c) {
+#define ABSL_INTERNAL_X_VAL(e)  \
+  case FormatConversionChar::e: \
+    return #e[0];
+#define ABSL_INTERNAL_X_SEP
+    ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_X_VAL,
+                                           ABSL_INTERNAL_X_SEP)
+    case FormatConversionChar::kNone:
+      return '\0';
+#undef ABSL_INTERNAL_X_VAL
+#undef ABSL_INTERNAL_X_SEP
+  }
+  return '\0';
+}
+
+// The associated char.
+inline std::ostream& operator<<(std::ostream& os, FormatConversionChar v) {
+  char c = FormatConversionCharToChar(v);
+  if (!c) c = '?';
+  return os << c;
+}
+
+struct FormatConversionSpecImplFriend;
+
+class FormatConversionSpec {
  public:
-  Flags flags() const { return flags_; }
-  LengthMod length_mod() const { return length_mod_; }
-  ConversionChar conv() const {
+  // Width and precison are not specified, no flags are set.
+  bool is_basic() const { return flags_.basic; }
+  bool has_left_flag() const { return flags_.left; }
+  bool has_show_pos_flag() const { return flags_.show_pos; }
+  bool has_sign_col_flag() const { return flags_.sign_col; }
+  bool has_alt_flag() const { return flags_.alt; }
+  bool has_zero_flag() const { return flags_.zero; }
+
+  FormatConversionChar conversion_char() const {
     // Keep this field first in the struct . It generates better code when
     // accessing it when ConversionSpec is passed by value in registers.
-    static_assert(offsetof(ConversionSpec, conv_) == 0, "");
+    static_assert(offsetof(FormatConversionSpec, conv_) == 0, "");
     return conv_;
   }
 
@@ -322,46 +287,71 @@
   // negative value.
   int precision() const { return precision_; }
 
-  void set_flags(Flags f) { flags_ = f; }
-  void set_length_mod(LengthMod lm) { length_mod_ = lm; }
-  void set_conv(ConversionChar c) { conv_ = c; }
-  void set_width(int w) { width_ = w; }
-  void set_precision(int p) { precision_ = p; }
-  void set_left(bool b) { flags_.left = b; }
+  // Deprecated (use has_x_flag() instead).
+  Flags flags() const { return flags_; }
+  // Deprecated
+  FormatConversionChar conv() const { return conversion_char(); }
 
  private:
-  ConversionChar conv_;
+  friend struct str_format_internal::FormatConversionSpecImplFriend;
+  FormatConversionChar conv_ = FormatConversionChar::kNone;
   Flags flags_;
-  LengthMod length_mod_;
   int width_;
   int precision_;
 };
 
-constexpr uint64_t ConversionCharToConvValue(char conv) {
+struct FormatConversionSpecImplFriend final {
+  static void SetFlags(Flags f, FormatConversionSpec* conv) {
+    conv->flags_ = f;
+  }
+  static void SetConversionChar(FormatConversionChar c,
+                                FormatConversionSpec* conv) {
+    conv->conv_ = c;
+  }
+  static void SetWidth(int w, FormatConversionSpec* conv) { conv->width_ = w; }
+  static void SetPrecision(int p, FormatConversionSpec* conv) {
+    conv->precision_ = p;
+  }
+  static std::string FlagsToString(const FormatConversionSpec& spec) {
+    return spec.flags_.ToString();
+  }
+};
+
+constexpr uint64_t FormatConversionCharToConvValue(char conv) {
   return
-#define CONV_SET_CASE(c) \
-  conv == #c[0] ? (uint64_t{1} << (1 + ConversionChar::Id::c)):
-      ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
-#undef CONV_SET_CASE
+#define ABSL_INTERNAL_CHAR_SET_CASE(c)                                       \
+  conv == #c[0]                                                              \
+      ? (uint64_t{1} << (1 + static_cast<uint8_t>(FormatConversionChar::c))) \
+      :
+      ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
+#undef ABSL_INTERNAL_CHAR_SET_CASE
                   conv == '*'
           ? 1
           : 0;
 }
 
-enum class Conv : uint64_t {
-#define CONV_SET_CASE(c) c = ConversionCharToConvValue(#c[0]),
-  ABSL_CONVERSION_CHARS_EXPAND_(CONV_SET_CASE, )
-#undef CONV_SET_CASE
+enum class FormatConversionCharSet : uint64_t {
+#define ABSL_INTERNAL_CHAR_SET_CASE(c) \
+  c = FormatConversionCharToConvValue(#c[0]),
+  ABSL_INTERNAL_CONVERSION_CHARS_EXPAND_(ABSL_INTERNAL_CHAR_SET_CASE, )
+#undef ABSL_INTERNAL_CHAR_SET_CASE
 
   // Used for width/precision '*' specification.
-  star = ConversionCharToConvValue('*'),
-
+  kStar = FormatConversionCharToConvValue('*'),
   // Some predefined values:
-  integral = d | i | u | o | x | X,
-  floating = a | e | f | g | A | E | F | G,
-  numeric = integral | floating,
-  string = s,
-  pointer = p
+  kIntegral = d | i | u | o | x | X,
+  kFloating = a | e | f | g | A | E | F | G,
+  kNumeric = kIntegral | kFloating,
+  kString = s,
+  kPointer = p,
+
+  // The following are deprecated
+  star = kStar,
+  integral = kIntegral,
+  floating = kFloating,
+  numeric = kNumeric,
+  string = kString,
+  pointer = kPointer
 };
 
 // Type safe OR operator.
@@ -369,45 +359,57 @@
 //  1. operator| on enums makes them decay to integers and the result is an
 //     integer. We need the result to stay as an enum.
 //  2. We use "enum class" which would not work even if we accepted the decay.
-constexpr Conv operator|(Conv a, Conv b) {
-  return Conv(static_cast<uint64_t>(a) | static_cast<uint64_t>(b));
+constexpr FormatConversionCharSet operator|(FormatConversionCharSet a,
+                                            FormatConversionCharSet b) {
+  return FormatConversionCharSet(static_cast<uint64_t>(a) |
+                                 static_cast<uint64_t>(b));
 }
 
 // Get a conversion with a single character in it.
-constexpr Conv ConversionCharToConv(char c) {
-  return Conv(ConversionCharToConvValue(c));
+constexpr FormatConversionCharSet ConversionCharToConv(char c) {
+  return FormatConversionCharSet(FormatConversionCharToConvValue(c));
 }
 
 // Checks whether `c` exists in `set`.
-constexpr bool Contains(Conv set, char c) {
-  return (static_cast<uint64_t>(set) & ConversionCharToConvValue(c)) != 0;
+constexpr bool Contains(FormatConversionCharSet set, char c) {
+  return (static_cast<uint64_t>(set) & FormatConversionCharToConvValue(c)) != 0;
 }
 
 // Checks whether all the characters in `c` are contained in `set`
-constexpr bool Contains(Conv set, Conv c) {
+constexpr bool Contains(FormatConversionCharSet set,
+                        FormatConversionCharSet c) {
   return (static_cast<uint64_t>(set) & static_cast<uint64_t>(c)) ==
          static_cast<uint64_t>(c);
 }
 
 // Return type of the AbslFormatConvert() functions.
-// The Conv template parameter is used to inform the framework of what
-// conversion characters are supported by that AbslFormatConvert routine.
-template <Conv C>
-struct ConvertResult {
-  static constexpr Conv kConv = C;
+// The FormatConversionCharSet template parameter is used to inform the
+// framework of what conversion characters are supported by that
+// AbslFormatConvert routine.
+template <FormatConversionCharSet C>
+struct FormatConvertResult {
+  static constexpr FormatConversionCharSet kConv = C;
   bool value;
 };
-template <Conv C>
-constexpr Conv ConvertResult<C>::kConv;
+
+template <FormatConversionCharSet C>
+constexpr FormatConversionCharSet FormatConvertResult<C>::kConv;
 
 // Return capacity - used, clipped to a minimum of 0.
 inline size_t Excess(size_t used, size_t capacity) {
   return used < capacity ? capacity - used : 0;
 }
 
+// Type alias for use during migration.
+using ConversionChar = FormatConversionChar;
+using ConversionSpec = FormatConversionSpec;
+using Conv = FormatConversionCharSet;
+template <FormatConversionCharSet C>
+using ConvertResult = FormatConvertResult<C>;
+
 }  // namespace str_format_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_EXTENSION_H_
diff --git a/absl/strings/internal/str_format/float_conversion.cc b/absl/strings/internal/str_format/float_conversion.cc
index a0484fe..d4c647c 100644
--- a/absl/strings/internal/str_format/float_conversion.cc
+++ b/absl/strings/internal/str_format/float_conversion.cc
@@ -9,7 +9,7 @@
 #include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -28,12 +28,12 @@
   {
     char *fp = fmt;
     *fp++ = '%';
-    fp = CopyStringTo(conv.flags().ToString(), fp);
+    fp = CopyStringTo(FormatConversionSpecImplFriend::FlagsToString(conv), fp);
     fp = CopyStringTo("*.*", fp);
     if (std::is_same<long double, Float>()) {
       *fp++ = 'L';
     }
-    *fp++ = conv.conv().Char();
+    *fp++ = FormatConversionCharToChar(conv.conv());
     *fp = 0;
     assert(fp < fmt + sizeof(fmt));
   }
@@ -100,9 +100,11 @@
   char text[4], *ptr = text;
   if (sign_char) *ptr++ = sign_char;
   if (std::isnan(v)) {
-    ptr = std::copy_n(conv.conv().upper() ? "NAN" : "nan", 3, ptr);
+    ptr = std::copy_n(FormatConversionCharIsUpper(conv.conv()) ? "NAN" : "nan",
+                      3, ptr);
   } else if (std::isinf(v)) {
-    ptr = std::copy_n(conv.conv().upper() ? "INF" : "inf", 3, ptr);
+    ptr = std::copy_n(FormatConversionCharIsUpper(conv.conv()) ? "INF" : "inf",
+                      3, ptr);
   } else {
     return false;
   }
@@ -399,7 +401,7 @@
 
   Buffer buffer;
 
-  switch (conv.conv().id()) {
+  switch (conv.conv()) {
     case ConversionChar::f:
     case ConversionChar::F:
       if (!FloatToBuffer<FormatStyle::Fixed>(decomposed, precision, &buffer,
@@ -416,7 +418,8 @@
         return FallbackToSnprintf(v, conv, sink);
       }
       if (!conv.flags().alt && buffer.back() == '.') buffer.pop_back();
-      PrintExponent(exp, conv.conv().upper() ? 'E' : 'e', &buffer);
+      PrintExponent(exp, FormatConversionCharIsUpper(conv.conv()) ? 'E' : 'e',
+                    &buffer);
       break;
 
     case ConversionChar::g:
@@ -447,7 +450,10 @@
         while (buffer.back() == '0') buffer.pop_back();
         if (buffer.back() == '.') buffer.pop_back();
       }
-      if (exp) PrintExponent(exp, conv.conv().upper() ? 'E' : 'e', &buffer);
+      if (exp) {
+        PrintExponent(exp, FormatConversionCharIsUpper(conv.conv()) ? 'E' : 'e',
+                      &buffer);
+      }
       break;
 
     case ConversionChar::a:
@@ -483,5 +489,5 @@
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/float_conversion.h b/absl/strings/internal/str_format/float_conversion.h
index 5d76ce2..49a6a63 100644
--- a/absl/strings/internal/str_format/float_conversion.h
+++ b/absl/strings/internal/str_format/float_conversion.h
@@ -4,7 +4,7 @@
 #include "absl/strings/internal/str_format/extension.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 bool ConvertFloatImpl(float v, const ConversionSpec &conv,
@@ -17,7 +17,7 @@
                       FormatSinkImpl *sink);
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_FLOAT_CONVERSION_H_
diff --git a/absl/strings/internal/str_format/output.cc b/absl/strings/internal/str_format/output.cc
index d5ead8d..c4b2470 100644
--- a/absl/strings/internal/str_format/output.cc
+++ b/absl/strings/internal/str_format/output.cc
@@ -18,7 +18,7 @@
 #include <cstring>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -68,5 +68,5 @@
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/output.h b/absl/strings/internal/str_format/output.h
index ba84747..28b288b 100644
--- a/absl/strings/internal/str_format/output.h
+++ b/absl/strings/internal/str_format/output.h
@@ -28,10 +28,11 @@
 #include "absl/base/port.h"
 #include "absl/strings/string_view.h"
 
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
 class Cord;
 
-namespace absl {
-inline namespace lts_2019_08_08 {
 namespace str_format_internal {
 
 // RawSink implementation that writes into a char* buffer.
@@ -77,7 +78,7 @@
 }
 
 template <class AbslCord, typename = typename std::enable_if<
-                              std::is_same<AbslCord, ::Cord>::value>::type>
+                              std::is_same<AbslCord, absl::Cord>::value>::type>
 inline void AbslFormatFlush(AbslCord* out, string_view s) {
   out->Append(s);
 }
@@ -97,7 +98,7 @@
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_OUTPUT_H_
diff --git a/absl/strings/internal/str_format/output_test.cc b/absl/strings/internal/str_format/output_test.cc
index 5c3c4af..e54e6f7 100644
--- a/absl/strings/internal/str_format/output_test.cc
+++ b/absl/strings/internal/str_format/output_test.cc
@@ -21,7 +21,7 @@
 #include "gtest/gtest.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 TEST(InvokeFlush, String) {
@@ -68,6 +68,6 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
diff --git a/absl/strings/internal/str_format/parser.cc b/absl/strings/internal/str_format/parser.cc
index 4b8ca4d..aab68db 100644
--- a/absl/strings/internal/str_format/parser.cc
+++ b/absl/strings/internal/str_format/parser.cc
@@ -14,11 +14,12 @@
 #include <unordered_set>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
-using CC = ConversionChar::Id;
-using LM = LengthMod::Id;
+using CC = ConversionChar;
+using LM = LengthMod;
+
 ABSL_CONST_INIT const ConvTag kTags[256] = {
     {},    {},    {},    {},    {},    {},    {},    {},     // 00-07
     {},    {},    {},    {},    {},    {},    {},    {},     // 08-0f
@@ -205,11 +206,11 @@
     using str_format_internal::LengthMod;
     LengthMod length_mod = tag.as_length();
     ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    if (c == 'h' && length_mod.id() == LengthMod::h) {
-      conv->length_mod = LengthMod::FromId(LengthMod::hh);
+    if (c == 'h' && length_mod == LengthMod::h) {
+      conv->length_mod = LengthMod::hh;
       ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
-    } else if (c == 'l' && length_mod.id() == LengthMod::l) {
-      conv->length_mod = LengthMod::FromId(LengthMod::ll);
+    } else if (c == 'l' && length_mod == LengthMod::l) {
+      conv->length_mod = LengthMod::ll;
       ABSL_FORMAT_PARSER_INTERNAL_GET_CHAR();
     } else {
       conv->length_mod = length_mod;
@@ -228,6 +229,32 @@
 
 }  // namespace
 
+std::string LengthModToString(LengthMod v) {
+  switch (v) {
+    case LengthMod::h:
+      return "h";
+    case LengthMod::hh:
+      return "hh";
+    case LengthMod::l:
+      return "l";
+    case LengthMod::ll:
+      return "ll";
+    case LengthMod::L:
+      return "L";
+    case LengthMod::j:
+      return "j";
+    case LengthMod::z:
+      return "z";
+    case LengthMod::t:
+      return "t";
+    case LengthMod::q:
+      return "q";
+    case LengthMod::none:
+      return "";
+  }
+  return "";
+}
+
 const char *ConsumeUnboundConversion(const char *p, const char *end,
                                      UnboundConversion *conv, int *next_arg) {
   if (*next_arg < 0) return ConsumeConversion<true>(p, end, conv, next_arg);
@@ -295,11 +322,13 @@
     if (conv.width.is_from_arg() &&
         !add_if_valid_conv(conv.width.get_from_arg(), '*'))
       return false;
-    if (!add_if_valid_conv(conv.arg_position, conv.conv.Char())) return false;
+    if (!add_if_valid_conv(conv.arg_position,
+                           FormatConversionCharToChar(conv.conv)))
+      return false;
   }
   return used.size() == convs.size() || allow_ignored;
 }
 
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_format/parser.h b/absl/strings/internal/str_format/parser.h
index d3357fd..45c90d1 100644
--- a/absl/strings/internal/str_format/parser.h
+++ b/absl/strings/internal/str_format/parser.h
@@ -6,19 +6,25 @@
 #include <stdlib.h>
 
 #include <cassert>
+#include <cstdint>
 #include <initializer_list>
 #include <iosfwd>
 #include <iterator>
 #include <memory>
+#include <string>
 #include <vector>
 
 #include "absl/strings/internal/str_format/checker.h"
 #include "absl/strings/internal/str_format/extension.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
+enum class LengthMod : std::uint8_t { h, hh, l, ll, L, j, z, t, q, none };
+
+std::string LengthModToString(LengthMod v);
+
 // The analyzed properties of a single specified conversion.
 struct UnboundConversion {
   UnboundConversion()
@@ -60,8 +66,8 @@
   InputValue precision;
 
   Flags flags;
-  LengthMod length_mod;
-  ConversionChar conv;
+  LengthMod length_mod = LengthMod::none;
+  ConversionChar conv = FormatConversionChar::kNone;
 };
 
 // Consume conversion spec prefix (not including '%') of [p, end) if valid.
@@ -73,13 +79,16 @@
                                      UnboundConversion* conv, int* next_arg);
 
 // Helper tag class for the table below.
-// It allows fast `char -> ConversionChar/LengthMod` checking and conversions.
+// It allows fast `char -> ConversionChar/LengthMod` checking and
+// conversions.
 class ConvTag {
  public:
-  constexpr ConvTag(ConversionChar::Id id) : tag_(id) {}  // NOLINT
+  constexpr ConvTag(ConversionChar conversion_char)  // NOLINT
+      : tag_(static_cast<int8_t>(conversion_char)) {}
   // We invert the length modifiers to make them negative so that we can easily
   // test for them.
-  constexpr ConvTag(LengthMod::Id id) : tag_(~id) {}  // NOLINT
+  constexpr ConvTag(LengthMod length_mod)  // NOLINT
+      : tag_(~static_cast<std::int8_t>(length_mod)) {}
   // Everything else is -128, which is negative to make is_conv() simpler.
   constexpr ConvTag() : tag_(-128) {}
 
@@ -87,11 +96,11 @@
   bool is_length() const { return tag_ < 0 && tag_ != -128; }
   ConversionChar as_conv() const {
     assert(is_conv());
-    return ConversionChar::FromId(static_cast<ConversionChar::Id>(tag_));
+    return static_cast<ConversionChar>(tag_);
   }
   LengthMod as_length() const {
     assert(is_length());
-    return LengthMod::FromId(static_cast<LengthMod::Id>(~tag_));
+    return static_cast<LengthMod>(~tag_);
   }
 
  private:
@@ -275,7 +284,7 @@
 class ExtendedParsedFormat : public str_format_internal::ParsedFormatBase {
  public:
   explicit ExtendedParsedFormat(string_view format)
-#if ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
+#ifdef ABSL_INTERNAL_ENABLE_FORMAT_CHECKER
       __attribute__((
           enable_if(str_format_internal::EnsureConstexpr(format),
                     "Format std::string is not constexpr."),
@@ -318,7 +327,7 @@
       : ParsedFormatBase(s, allow_ignored, {C...}) {}
 };
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_FORMAT_PARSER_H_
diff --git a/absl/strings/internal/str_format/parser_test.cc b/absl/strings/internal/str_format/parser_test.cc
index d77a8ea..1b1ee03 100644
--- a/absl/strings/internal/str_format/parser_test.cc
+++ b/absl/strings/internal/str_format/parser_test.cc
@@ -7,7 +7,7 @@
 #include "absl/base/macros.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace str_format_internal {
 
 namespace {
@@ -17,7 +17,7 @@
 TEST(LengthModTest, Names) {
   struct Expectation {
     int line;
-    LengthMod::Id id;
+    LengthMod mod;
     const char *name;
   };
   const Expectation kExpect[] = {
@@ -32,18 +32,16 @@
     {__LINE__, LengthMod::t,    "t" },
     {__LINE__, LengthMod::q,    "q" },
   };
-  EXPECT_EQ(ABSL_ARRAYSIZE(kExpect), LengthMod::kNumValues);
+  EXPECT_EQ(ABSL_ARRAYSIZE(kExpect), 10);
   for (auto e : kExpect) {
     SCOPED_TRACE(e.line);
-    LengthMod mod = LengthMod::FromId(e.id);
-    EXPECT_EQ(e.id, mod.id());
-    EXPECT_EQ(e.name, mod.name());
+    EXPECT_EQ(e.name, LengthModToString(e.mod));
   }
 }
 
 TEST(ConversionCharTest, Names) {
   struct Expectation {
-    ConversionChar::Id id;
+    ConversionChar id;
     char name;
   };
   // clang-format off
@@ -57,12 +55,10 @@
     {ConversionChar::none, '\0'},
   };
   // clang-format on
-  EXPECT_EQ(ABSL_ARRAYSIZE(kExpect), ConversionChar::kNumValues);
   for (auto e : kExpect) {
     SCOPED_TRACE(e.name);
-    ConversionChar v = ConversionChar::FromId(e.id);
-    EXPECT_EQ(e.id, v.id());
-    EXPECT_EQ(e.name, v.Char());
+    ConversionChar v = e.id;
+    EXPECT_EQ(e.name, FormatConversionCharToChar(v));
   }
 }
 
@@ -121,13 +117,12 @@
   EXPECT_FALSE(Run("dd"));  // no excess allowed
 
   EXPECT_TRUE(Run("d"));
-  EXPECT_EQ('d', o.conv.Char());
+  EXPECT_EQ('d', FormatConversionCharToChar(o.conv));
   EXPECT_FALSE(o.width.is_from_arg());
   EXPECT_LT(o.width.value(), 0);
   EXPECT_FALSE(o.precision.is_from_arg());
   EXPECT_LT(o.precision.value(), 0);
   EXPECT_EQ(1, o.arg_position);
-  EXPECT_EQ(LengthMod::none, o.length_mod.id());
 }
 
 TEST_F(ConsumeUnboundConversionTest, ArgPosition) {
@@ -163,7 +158,7 @@
 
 TEST_F(ConsumeUnboundConversionTest, WidthAndPrecision) {
   EXPECT_TRUE(Run("14d"));
-  EXPECT_EQ('d', o.conv.Char());
+  EXPECT_EQ('d', FormatConversionCharToChar(o.conv));
   EXPECT_FALSE(o.width.is_from_arg());
   EXPECT_EQ(14, o.width.value());
   EXPECT_FALSE(o.precision.is_from_arg());
@@ -290,6 +285,29 @@
   }
 }
 
+TEST_F(ConsumeUnboundConversionTest, LengthMod) {
+  EXPECT_TRUE(Run("d"));
+  EXPECT_EQ(LengthMod::none, o.length_mod);
+  EXPECT_TRUE(Run("hd"));
+  EXPECT_EQ(LengthMod::h, o.length_mod);
+  EXPECT_TRUE(Run("hhd"));
+  EXPECT_EQ(LengthMod::hh, o.length_mod);
+  EXPECT_TRUE(Run("ld"));
+  EXPECT_EQ(LengthMod::l, o.length_mod);
+  EXPECT_TRUE(Run("lld"));
+  EXPECT_EQ(LengthMod::ll, o.length_mod);
+  EXPECT_TRUE(Run("Lf"));
+  EXPECT_EQ(LengthMod::L, o.length_mod);
+  EXPECT_TRUE(Run("qf"));
+  EXPECT_EQ(LengthMod::q, o.length_mod);
+  EXPECT_TRUE(Run("jd"));
+  EXPECT_EQ(LengthMod::j, o.length_mod);
+  EXPECT_TRUE(Run("zd"));
+  EXPECT_EQ(LengthMod::z, o.length_mod);
+  EXPECT_TRUE(Run("td"));
+  EXPECT_EQ(LengthMod::t, o.length_mod);
+}
+
 struct SummarizeConsumer {
   std::string* out;
   explicit SummarizeConsumer(std::string* out) : out(out) {}
@@ -310,7 +328,7 @@
     if (conv.precision.is_from_arg()) {
       *out += "." + std::to_string(conv.precision.get_from_arg()) + "$*";
     }
-    *out += conv.conv.Char();
+    *out += FormatConversionCharToChar(conv.conv);
     *out += "}";
     return true;
   }
@@ -390,5 +408,5 @@
 
 }  // namespace
 }  // namespace str_format_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/str_join_internal.h b/absl/strings/internal/str_join_internal.h
index 02787dd..31dbf67 100644
--- a/absl/strings/internal/str_join_internal.h
+++ b/absl/strings/internal/str_join_internal.h
@@ -43,7 +43,7 @@
 #include "absl/strings/str_cat.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 //
@@ -308,7 +308,7 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_JOIN_INTERNAL_H_
diff --git a/absl/strings/internal/str_split_internal.h b/absl/strings/internal/str_split_internal.h
index 92a678e..b54f6eb 100644
--- a/absl/strings/internal/str_split_internal.h
+++ b/absl/strings/internal/str_split_internal.h
@@ -47,7 +47,7 @@
 #endif  // _GLIBCXX_DEBUG
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // This class is implicitly constructible from everything that absl::string_view
@@ -449,7 +449,7 @@
 };
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_STR_SPLIT_INTERNAL_H_
diff --git a/absl/strings/internal/utf8.cc b/absl/strings/internal/utf8.cc
index fe4a9be..8fd8edc 100644
--- a/absl/strings/internal/utf8.cc
+++ b/absl/strings/internal/utf8.cc
@@ -17,7 +17,7 @@
 #include "absl/strings/internal/utf8.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 size_t EncodeUTF8Char(char *buffer, char32_t utf8_char) {
@@ -49,5 +49,5 @@
 }
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/internal/utf8.h b/absl/strings/internal/utf8.h
index b1bb954..32fb109 100644
--- a/absl/strings/internal/utf8.h
+++ b/absl/strings/internal/utf8.h
@@ -20,8 +20,10 @@
 #include <cstddef>
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 // For Unicode code points 0 through 0x10FFFF, EncodeUTF8Char writes
@@ -42,7 +44,7 @@
 size_t EncodeUTF8Char(char *buffer, char32_t utf8_char);
 
 }  // namespace strings_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_INTERNAL_UTF8_H_
diff --git a/absl/strings/match.cc b/absl/strings/match.cc
index 01ed1f1..8127cb0 100644
--- a/absl/strings/match.cc
+++ b/absl/strings/match.cc
@@ -17,7 +17,7 @@
 #include "absl/strings/internal/memutil.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 bool EqualsIgnoreCase(absl::string_view piece1, absl::string_view piece2) {
   return (piece1.size() == piece2.size() &&
@@ -36,5 +36,5 @@
          EqualsIgnoreCase(text.substr(text.size() - suffix.size()), suffix);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/match.h b/absl/strings/match.h
index 15fdc0c..90fca98 100644
--- a/absl/strings/match.h
+++ b/absl/strings/match.h
@@ -20,7 +20,7 @@
 // This file contains simple utilities for performing string matching checks.
 // All of these function parameters are specified as `absl::string_view`,
 // meaning that these functions can accept `std::string`, `absl::string_view` or
-// nul-terminated C-style strings.
+// NUL-terminated C-style strings.
 //
 // Examples:
 //   std::string s = "foo";
@@ -38,7 +38,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // StrContains()
 //
@@ -84,7 +84,7 @@
 // case in the comparison.
 bool EndsWithIgnoreCase(absl::string_view text, absl::string_view suffix);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_MATCH_H_
diff --git a/absl/strings/numbers.cc b/absl/strings/numbers.cc
index 2bac4ad..68c26dd 100644
--- a/absl/strings/numbers.cc
+++ b/absl/strings/numbers.cc
@@ -19,8 +19,8 @@
 
 #include <algorithm>
 #include <cassert>
-#include <cfloat>          // for DBL_DIG and FLT_DIG
-#include <cmath>           // for HUGE_VAL
+#include <cfloat>  // for DBL_DIG and FLT_DIG
+#include <cmath>   // for HUGE_VAL
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
@@ -30,16 +30,18 @@
 #include <memory>
 #include <utility>
 
+#include "absl/base/attributes.h"
 #include "absl/base/internal/bits.h"
 #include "absl/base/internal/raw_logging.h"
 #include "absl/strings/ascii.h"
 #include "absl/strings/charconv.h"
+#include "absl/strings/escaping.h"
 #include "absl/strings/internal/memutil.h"
 #include "absl/strings/match.h"
 #include "absl/strings/str_cat.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 bool SimpleAtof(absl::string_view str, float* out) {
   *out = 0.0;
@@ -93,43 +95,6 @@
   return true;
 }
 
-namespace {
-
-// Writes a two-character representation of 'i' to 'buf'. 'i' must be in the
-// range 0 <= i < 100, and buf must have space for two characters. Example:
-//   char buf[2];
-//   PutTwoDigits(42, buf);
-//   // buf[0] == '4'
-//   // buf[1] == '2'
-inline void PutTwoDigits(size_t i, char* buf) {
-  static const char two_ASCII_digits[100][2] = {
-    {'0', '0'}, {'0', '1'}, {'0', '2'}, {'0', '3'}, {'0', '4'},
-    {'0', '5'}, {'0', '6'}, {'0', '7'}, {'0', '8'}, {'0', '9'},
-    {'1', '0'}, {'1', '1'}, {'1', '2'}, {'1', '3'}, {'1', '4'},
-    {'1', '5'}, {'1', '6'}, {'1', '7'}, {'1', '8'}, {'1', '9'},
-    {'2', '0'}, {'2', '1'}, {'2', '2'}, {'2', '3'}, {'2', '4'},
-    {'2', '5'}, {'2', '6'}, {'2', '7'}, {'2', '8'}, {'2', '9'},
-    {'3', '0'}, {'3', '1'}, {'3', '2'}, {'3', '3'}, {'3', '4'},
-    {'3', '5'}, {'3', '6'}, {'3', '7'}, {'3', '8'}, {'3', '9'},
-    {'4', '0'}, {'4', '1'}, {'4', '2'}, {'4', '3'}, {'4', '4'},
-    {'4', '5'}, {'4', '6'}, {'4', '7'}, {'4', '8'}, {'4', '9'},
-    {'5', '0'}, {'5', '1'}, {'5', '2'}, {'5', '3'}, {'5', '4'},
-    {'5', '5'}, {'5', '6'}, {'5', '7'}, {'5', '8'}, {'5', '9'},
-    {'6', '0'}, {'6', '1'}, {'6', '2'}, {'6', '3'}, {'6', '4'},
-    {'6', '5'}, {'6', '6'}, {'6', '7'}, {'6', '8'}, {'6', '9'},
-    {'7', '0'}, {'7', '1'}, {'7', '2'}, {'7', '3'}, {'7', '4'},
-    {'7', '5'}, {'7', '6'}, {'7', '7'}, {'7', '8'}, {'7', '9'},
-    {'8', '0'}, {'8', '1'}, {'8', '2'}, {'8', '3'}, {'8', '4'},
-    {'8', '5'}, {'8', '6'}, {'8', '7'}, {'8', '8'}, {'8', '9'},
-    {'9', '0'}, {'9', '1'}, {'9', '2'}, {'9', '3'}, {'9', '4'},
-    {'9', '5'}, {'9', '6'}, {'9', '7'}, {'9', '8'}, {'9', '9'}
-  };
-  assert(i < 100);
-  memcpy(buf, two_ASCII_digits[i], 2);
-}
-
-}  // namespace
-
 bool SimpleAtob(absl::string_view str, bool* out) {
   ABSL_RAW_CHECK(out != nullptr, "Output pointer must not be nullptr.");
   if (EqualsIgnoreCase(str, "true") || EqualsIgnoreCase(str, "t") ||
@@ -496,13 +461,13 @@
 
   int two_digits = dddddd / 10000;
   dddddd -= two_digits * 10000;
-  PutTwoDigits(two_digits, &exp_dig.digits[0]);
+  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[0]);
 
   two_digits = dddddd / 100;
   dddddd -= two_digits * 100;
-  PutTwoDigits(two_digits, &exp_dig.digits[2]);
+  numbers_internal::PutTwoDigits(two_digits, &exp_dig.digits[2]);
 
-  PutTwoDigits(dddddd, &exp_dig.digits[4]);
+  numbers_internal::PutTwoDigits(dddddd, &exp_dig.digits[4]);
   return exp_dig;
 }
 
@@ -755,8 +720,8 @@
 // commonly used bases.
 template <typename IntType>
 struct LookupTables {
-  static const IntType kVmaxOverBase[];
-  static const IntType kVminOverBase[];
+  ABSL_CONST_INIT static const IntType kVmaxOverBase[];
+  ABSL_CONST_INIT static const IntType kVminOverBase[];
 };
 
 // An array initializer macro for X/base where base in [0, 36].
@@ -771,6 +736,49 @@
         X / 35, X / 36,                                                   \
   }
 
+// uint128& operator/=(uint128) is not constexpr, so hardcode the resulting
+// array to avoid a static initializer.
+template <>
+const uint128 LookupTables<uint128>::kVmaxOverBase[] = {
+    0,
+    0,
+    MakeUint128(9223372036854775807u, 18446744073709551615u),
+    MakeUint128(6148914691236517205u, 6148914691236517205u),
+    MakeUint128(4611686018427387903u, 18446744073709551615u),
+    MakeUint128(3689348814741910323u, 3689348814741910323u),
+    MakeUint128(3074457345618258602u, 12297829382473034410u),
+    MakeUint128(2635249153387078802u, 5270498306774157604u),
+    MakeUint128(2305843009213693951u, 18446744073709551615u),
+    MakeUint128(2049638230412172401u, 14347467612885206812u),
+    MakeUint128(1844674407370955161u, 11068046444225730969u),
+    MakeUint128(1676976733973595601u, 8384883669867978007u),
+    MakeUint128(1537228672809129301u, 6148914691236517205u),
+    MakeUint128(1418980313362273201u, 4256940940086819603u),
+    MakeUint128(1317624576693539401u, 2635249153387078802u),
+    MakeUint128(1229782938247303441u, 1229782938247303441u),
+    MakeUint128(1152921504606846975u, 18446744073709551615u),
+    MakeUint128(1085102592571150095u, 1085102592571150095u),
+    MakeUint128(1024819115206086200u, 16397105843297379214u),
+    MakeUint128(970881267037344821u, 16504981539634861972u),
+    MakeUint128(922337203685477580u, 14757395258967641292u),
+    MakeUint128(878416384462359600u, 14054662151397753612u),
+    MakeUint128(838488366986797800u, 13415813871788764811u),
+    MakeUint128(802032351030850070u, 4812194106185100421u),
+    MakeUint128(768614336404564650u, 12297829382473034410u),
+    MakeUint128(737869762948382064u, 11805916207174113034u),
+    MakeUint128(709490156681136600u, 11351842506898185609u),
+    MakeUint128(683212743470724133u, 17080318586768103348u),
+    MakeUint128(658812288346769700u, 10540996613548315209u),
+    MakeUint128(636094623231363848u, 15266270957552732371u),
+    MakeUint128(614891469123651720u, 9838263505978427528u),
+    MakeUint128(595056260442243600u, 9520900167075897608u),
+    MakeUint128(576460752303423487u, 18446744073709551615u),
+    MakeUint128(558992244657865200u, 8943875914525843207u),
+    MakeUint128(542551296285575047u, 9765923333140350855u),
+    MakeUint128(527049830677415760u, 8432797290838652167u),
+    MakeUint128(512409557603043100u, 8198552921648689607u),
+};
+
 template <typename IntType>
 const IntType LookupTables<IntType>::kVmaxOverBase[] =
     X_OVER_BASE_INITIALIZER(std::numeric_limits<IntType>::max());
@@ -790,6 +798,8 @@
   assert(base >= 0);
   assert(vmax >= static_cast<IntType>(base));
   const IntType vmax_over_base = LookupTables<IntType>::kVmaxOverBase[base];
+  assert(base < 2 ||
+         std::numeric_limits<IntType>::max() / base == vmax_over_base);
   const char* start = text.data();
   const char* end = start + text.size();
   // loop over digits
@@ -823,6 +833,8 @@
   assert(vmin < 0);
   assert(vmin <= 0 - base);
   IntType vmin_over_base = LookupTables<IntType>::kVminOverBase[base];
+  assert(base < 2 ||
+         std::numeric_limits<IntType>::min() / base == vmin_over_base);
   // 2003 c++ standard [expr.mul]
   // "... the sign of the remainder is implementation-defined."
   // Although (vmin/base)*base + vmin%base is always vmin.
@@ -886,6 +898,48 @@
 }  // anonymous namespace
 
 namespace numbers_internal {
+
+// Digit conversion.
+ABSL_CONST_INIT ABSL_DLL const char kHexChar[] =
+    "0123456789abcdef";
+
+ABSL_CONST_INIT ABSL_DLL const char kHexTable[513] =
+    "000102030405060708090a0b0c0d0e0f"
+    "101112131415161718191a1b1c1d1e1f"
+    "202122232425262728292a2b2c2d2e2f"
+    "303132333435363738393a3b3c3d3e3f"
+    "404142434445464748494a4b4c4d4e4f"
+    "505152535455565758595a5b5c5d5e5f"
+    "606162636465666768696a6b6c6d6e6f"
+    "707172737475767778797a7b7c7d7e7f"
+    "808182838485868788898a8b8c8d8e8f"
+    "909192939495969798999a9b9c9d9e9f"
+    "a0a1a2a3a4a5a6a7a8a9aaabacadaeaf"
+    "b0b1b2b3b4b5b6b7b8b9babbbcbdbebf"
+    "c0c1c2c3c4c5c6c7c8c9cacbcccdcecf"
+    "d0d1d2d3d4d5d6d7d8d9dadbdcdddedf"
+    "e0e1e2e3e4e5e6e7e8e9eaebecedeeef"
+    "f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff";
+
+ABSL_CONST_INIT ABSL_DLL const char two_ASCII_digits[100][2] = {
+    {'0', '0'}, {'0', '1'}, {'0', '2'}, {'0', '3'}, {'0', '4'}, {'0', '5'},
+    {'0', '6'}, {'0', '7'}, {'0', '8'}, {'0', '9'}, {'1', '0'}, {'1', '1'},
+    {'1', '2'}, {'1', '3'}, {'1', '4'}, {'1', '5'}, {'1', '6'}, {'1', '7'},
+    {'1', '8'}, {'1', '9'}, {'2', '0'}, {'2', '1'}, {'2', '2'}, {'2', '3'},
+    {'2', '4'}, {'2', '5'}, {'2', '6'}, {'2', '7'}, {'2', '8'}, {'2', '9'},
+    {'3', '0'}, {'3', '1'}, {'3', '2'}, {'3', '3'}, {'3', '4'}, {'3', '5'},
+    {'3', '6'}, {'3', '7'}, {'3', '8'}, {'3', '9'}, {'4', '0'}, {'4', '1'},
+    {'4', '2'}, {'4', '3'}, {'4', '4'}, {'4', '5'}, {'4', '6'}, {'4', '7'},
+    {'4', '8'}, {'4', '9'}, {'5', '0'}, {'5', '1'}, {'5', '2'}, {'5', '3'},
+    {'5', '4'}, {'5', '5'}, {'5', '6'}, {'5', '7'}, {'5', '8'}, {'5', '9'},
+    {'6', '0'}, {'6', '1'}, {'6', '2'}, {'6', '3'}, {'6', '4'}, {'6', '5'},
+    {'6', '6'}, {'6', '7'}, {'6', '8'}, {'6', '9'}, {'7', '0'}, {'7', '1'},
+    {'7', '2'}, {'7', '3'}, {'7', '4'}, {'7', '5'}, {'7', '6'}, {'7', '7'},
+    {'7', '8'}, {'7', '9'}, {'8', '0'}, {'8', '1'}, {'8', '2'}, {'8', '3'},
+    {'8', '4'}, {'8', '5'}, {'8', '6'}, {'8', '7'}, {'8', '8'}, {'8', '9'},
+    {'9', '0'}, {'9', '1'}, {'9', '2'}, {'9', '3'}, {'9', '4'}, {'9', '5'},
+    {'9', '6'}, {'9', '7'}, {'9', '8'}, {'9', '9'}};
+
 bool safe_strto32_base(absl::string_view text, int32_t* value, int base) {
   return safe_int_internal<int32_t>(text, value, base);
 }
@@ -901,7 +955,11 @@
 bool safe_strtou64_base(absl::string_view text, uint64_t* value, int base) {
   return safe_uint_internal<uint64_t>(text, value, base);
 }
-}  // namespace numbers_internal
 
-}  // inline namespace lts_2019_08_08
+bool safe_strtou128_base(absl::string_view text, uint128* value, int base) {
+  return safe_uint_internal<absl::uint128>(text, value, base);
+}
+
+}  // namespace numbers_internal
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/numbers.h b/absl/strings/numbers.h
index e987801..d872cca 100644
--- a/absl/strings/numbers.h
+++ b/absl/strings/numbers.h
@@ -24,6 +24,10 @@
 #ifndef ABSL_STRINGS_NUMBERS_H_
 #define ABSL_STRINGS_NUMBERS_H_
 
+#ifdef __SSE4_2__
+#include <x86intrin.h>
+#endif
+
 #include <cstddef>
 #include <cstdlib>
 #include <cstring>
@@ -32,39 +36,54 @@
 #include <string>
 #include <type_traits>
 
+#include "absl/base/config.h"
+#include "absl/base/internal/bits.h"
+#ifdef __SSE4_2__
+// TODO(jorg): Remove this when we figure out the right way
+// to swap bytes on SSE 4.2 that works with the compilers
+// we claim to support.  Also, add tests for the compiler
+// that doesn't support the Intel _bswap64 intrinsic but
+// does support all the SSE 4.2 intrinsics
+#include "absl/base/internal/endian.h"
+#endif
 #include "absl/base/macros.h"
 #include "absl/base/port.h"
 #include "absl/numeric/int128.h"
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // SimpleAtoi()
 //
-// Converts the given string into an integer value, returning `true` if
-// successful. The string must reflect a base-10 integer (optionally followed or
-// preceded by ASCII whitespace) whose value falls within the range of the
-// integer type. If any errors are encountered, this function returns `false`,
-// leaving `out` in an unspecified state.
+// Converts the given string (optionally followed or preceded by ASCII
+// whitespace) into an integer value, returning `true` if successful. The string
+// must reflect a base-10 integer whose value falls within the range of the
+// integer type (optionally preceded by a `+` or `-`). If any errors are
+// encountered, this function returns `false`, leaving `out` in an unspecified
+// state.
 template <typename int_type>
 ABSL_MUST_USE_RESULT bool SimpleAtoi(absl::string_view str, int_type* out);
 
 // SimpleAtof()
 //
 // Converts the given string (optionally followed or preceded by ASCII
-// whitespace) into a float, which may be rounded on overflow or underflow.
+// whitespace) into a float, which may be rounded on overflow or underflow,
+// returning `true` if successful.
 // See https://en.cppreference.com/w/c/string/byte/strtof for details about the
-// allowed formats for `str`. If any errors are encountered, this function
+// allowed formats for `str`, except SimpleAtof() is locale-independent and will
+// always use the "C" locale. If any errors are encountered, this function
 // returns `false`, leaving `out` in an unspecified state.
 ABSL_MUST_USE_RESULT bool SimpleAtof(absl::string_view str, float* out);
 
 // SimpleAtod()
 //
 // Converts the given string (optionally followed or preceded by ASCII
-// whitespace) into a double, which may be rounded on overflow or underflow.
+// whitespace) into a double, which may be rounded on overflow or underflow,
+// returning `true` if successful.
 // See https://en.cppreference.com/w/c/string/byte/strtof for details about the
-// allowed formats for `str`. If any errors are encountered, this function
+// allowed formats for `str`, except SimpleAtod is locale-independent and will
+// always use the "C" locale. If any errors are encountered, this function
 // returns `false`, leaving `out` in an unspecified state.
 ABSL_MUST_USE_RESULT bool SimpleAtod(absl::string_view str, double* out);
 
@@ -78,20 +97,40 @@
 // unspecified state.
 ABSL_MUST_USE_RESULT bool SimpleAtob(absl::string_view str, bool* out);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // End of public API.  Implementation details follow.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace numbers_internal {
 
+// Digit conversion.
+ABSL_DLL extern const char kHexChar[17];  // 0123456789abcdef
+ABSL_DLL extern const char
+    kHexTable[513];  // 000102030405060708090a0b0c0d0e0f1011...
+ABSL_DLL extern const char
+    two_ASCII_digits[100][2];  // 00, 01, 02, 03...
+
+// Writes a two-character representation of 'i' to 'buf'. 'i' must be in the
+// range 0 <= i < 100, and buf must have space for two characters. Example:
+//   char buf[2];
+//   PutTwoDigits(42, buf);
+//   // buf[0] == '4'
+//   // buf[1] == '2'
+inline void PutTwoDigits(size_t i, char* buf) {
+  assert(i < 100);
+  memcpy(buf, two_ASCII_digits[i], 2);
+}
+
 // safe_strto?() functions for implementing SimpleAtoi()
 bool safe_strto32_base(absl::string_view text, int32_t* value, int base);
 bool safe_strto64_base(absl::string_view text, int64_t* value, int base);
 bool safe_strtou32_base(absl::string_view text, uint32_t* value, int base);
 bool safe_strtou64_base(absl::string_view text, uint64_t* value, int base);
+bool safe_strtou128_base(absl::string_view text, absl::uint128* value,
+                         int base);
 
 static const int kFastToBufferSize = 32;
 static const int kSixDigitsToBufferSize = 16;
@@ -173,6 +212,35 @@
   return parsed;
 }
 
+// FastHexToBufferZeroPad16()
+//
+// Outputs `val` into `out` as if by `snprintf(out, 17, "%016x", val)` but
+// without the terminating null character. Thus `out` must be of length >= 16.
+// Returns the number of non-pad digits of the output (it can never be zero
+// since 0 has one digit).
+inline size_t FastHexToBufferZeroPad16(uint64_t val, char* out) {
+#ifdef __SSE4_2__
+  uint64_t be = absl::big_endian::FromHost64(val);
+  const auto kNibbleMask = _mm_set1_epi8(0xf);
+  const auto kHexDigits = _mm_setr_epi8('0', '1', '2', '3', '4', '5', '6', '7',
+                                        '8', '9', 'a', 'b', 'c', 'd', 'e', 'f');
+  auto v = _mm_loadl_epi64(reinterpret_cast<__m128i*>(&be));  // load lo dword
+  auto v4 = _mm_srli_epi64(v, 4);                            // shift 4 right
+  auto il = _mm_unpacklo_epi8(v4, v);                        // interleave bytes
+  auto m = _mm_and_si128(il, kNibbleMask);                   // mask out nibbles
+  auto hexchars = _mm_shuffle_epi8(kHexDigits, m);           // hex chars
+  _mm_storeu_si128(reinterpret_cast<__m128i*>(out), hexchars);
+#else
+  for (int i = 0; i < 8; ++i) {
+    auto byte = (val >> (56 - 8 * i)) & 0xFF;
+    auto* hex = &absl::numbers_internal::kHexTable[byte * 2];
+    std::memcpy(out + 2 * i, hex, 2);
+  }
+#endif
+  // | 0x1 so that even 0 has 1 digit.
+  return 16 - absl::base_internal::CountLeadingZeros64(val | 0x1) / 4;
+}
+
 }  // namespace numbers_internal
 
 // SimpleAtoi()
@@ -187,7 +255,12 @@
   return numbers_internal::safe_strtoi_base(str, out, 10);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_MUST_USE_RESULT inline bool SimpleAtoi(absl::string_view str,
+                                            absl::uint128* out) {
+  return numbers_internal::safe_strtou128_base(str, out, 10);
+}
+
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_NUMBERS_H_
diff --git a/absl/strings/numbers_benchmark.cc b/absl/strings/numbers_benchmark.cc
index 54dbedd..6e79b3e 100644
--- a/absl/strings/numbers_benchmark.cc
+++ b/absl/strings/numbers_benchmark.cc
@@ -20,6 +20,8 @@
 
 #include "benchmark/benchmark.h"
 #include "absl/base/internal/raw_logging.h"
+#include "absl/random/distributions.h"
+#include "absl/random/random.h"
 #include "absl/strings/numbers.h"
 
 namespace {
@@ -260,4 +262,25 @@
     ->ArgPair(10, 4)
     ->ArgPair(10, 8);
 
+void BM_FastHexToBufferZeroPad16(benchmark::State& state) {
+  absl::BitGen rng;
+  std::vector<uint64_t> nums;
+  nums.resize(1000);
+  auto min = std::numeric_limits<uint64_t>::min();
+  auto max = std::numeric_limits<uint64_t>::max();
+  for (auto& num : nums) {
+    num = absl::LogUniform(rng, min, max);
+  }
+
+  char buf[16];
+  while (state.KeepRunningBatch(nums.size())) {
+    for (auto num : nums) {
+      auto digits = absl::numbers_internal::FastHexToBufferZeroPad16(num, buf);
+      benchmark::DoNotOptimize(digits);
+      benchmark::DoNotOptimize(buf);
+    }
+  }
+}
+BENCHMARK(BM_FastHexToBufferZeroPad16);
+
 }  // namespace
diff --git a/absl/strings/numbers_test.cc b/absl/strings/numbers_test.cc
index 77d7e78..68229b1 100644
--- a/absl/strings/numbers_test.cc
+++ b/absl/strings/numbers_test.cc
@@ -17,6 +17,7 @@
 #include "absl/strings/numbers.h"
 
 #include <sys/types.h>
+
 #include <cfenv>  // NOLINT(build/c++11)
 #include <cinttypes>
 #include <climits>
@@ -36,10 +37,11 @@
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
 #include "absl/base/internal/raw_logging.h"
-#include "absl/strings/str_cat.h"
-
+#include "absl/random/distributions.h"
+#include "absl/random/random.h"
 #include "absl/strings/internal/numbers_test_common.h"
 #include "absl/strings/internal/pow10_helper.h"
+#include "absl/strings/str_cat.h"
 
 namespace {
 
@@ -204,6 +206,9 @@
   std::string actual = absl::StrCat(absl::Hex(v, absl::kZeroPad16));
   snprintf(expected, sizeof(expected), "%016" PRIx64, static_cast<uint64_t>(v));
   EXPECT_EQ(expected, actual) << " Input " << v;
+  actual = absl::StrCat(absl::Hex(v, absl::kSpacePad16));
+  snprintf(expected, sizeof(expected), "%16" PRIx64, static_cast<uint64_t>(v));
+  EXPECT_EQ(expected, actual) << " Input " << v;
 }
 
 TEST(Numbers, TestFastPrints) {
@@ -244,7 +249,9 @@
 
 template <typename int_type, typename in_val_type>
 void VerifySimpleAtoiGood(in_val_type in_value, int_type exp_value) {
-  std::string s = absl::StrCat(in_value);
+  std::string s;
+  // uint128 can be streamed but not StrCat'd
+  absl::strings_internal::OStringStream(&s) << in_value;
   int_type x = static_cast<int_type>(~exp_value);
   EXPECT_TRUE(SimpleAtoi(s, &x))
       << "in_value=" << in_value << " s=" << s << " x=" << x;
@@ -320,6 +327,25 @@
   VerifySimpleAtoiGood<uint64_t>(std::numeric_limits<uint64_t>::max(),
                                  std::numeric_limits<uint64_t>::max());
 
+  // SimpleAtoi(absl::string_view, absl::uint128)
+  VerifySimpleAtoiGood<absl::uint128>(0, 0);
+  VerifySimpleAtoiGood<absl::uint128>(42, 42);
+  VerifySimpleAtoiBad<absl::uint128>(-42);
+
+  VerifySimpleAtoiBad<absl::uint128>(std::numeric_limits<int32_t>::min());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<int32_t>::max(),
+                                      std::numeric_limits<int32_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<uint32_t>::max(),
+                                      std::numeric_limits<uint32_t>::max());
+  VerifySimpleAtoiBad<absl::uint128>(std::numeric_limits<int64_t>::min());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<int64_t>::max(),
+                                      std::numeric_limits<int64_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(std::numeric_limits<uint64_t>::max(),
+                                      std::numeric_limits<uint64_t>::max());
+  VerifySimpleAtoiGood<absl::uint128>(
+      std::numeric_limits<absl::uint128>::max(),
+      std::numeric_limits<absl::uint128>::max());
+
   // Some other types
   VerifySimpleAtoiGood<int>(-42, -42);
   VerifySimpleAtoiGood<int32_t>(-42, -42);
@@ -652,6 +678,46 @@
 TEST(stringtest, safe_strtou64_random) {
   test_random_integer_parse_base<uint64_t>(&safe_strtou64_base);
 }
+TEST(stringtest, safe_strtou128_random) {
+  // random number generators don't work for uint128, and
+  // uint128 can be streamed but not StrCat'd, so this code must be custom
+  // implemented for uint128, but is generally the same as what's above.
+  // test_random_integer_parse_base<absl::uint128>(
+  //     &absl::numbers_internal::safe_strtou128_base);
+  using RandomEngine = std::minstd_rand0;
+  using IntType = absl::uint128;
+  constexpr auto parse_func = &absl::numbers_internal::safe_strtou128_base;
+
+  std::random_device rd;
+  RandomEngine rng(rd());
+  std::uniform_int_distribution<uint64_t> random_uint64(
+      std::numeric_limits<uint64_t>::min());
+  std::uniform_int_distribution<int> random_base(2, 35);
+
+  for (size_t i = 0; i < kNumRandomTests; i++) {
+    IntType value = random_uint64(rng);
+    value = (value << 64) + random_uint64(rng);
+    int base = random_base(rng);
+    std::string str_value;
+    EXPECT_TRUE(Itoa<IntType>(value, base, &str_value));
+    IntType parsed_value;
+
+    // Test successful parse
+    EXPECT_TRUE(parse_func(str_value, &parsed_value, base));
+    EXPECT_EQ(parsed_value, value);
+
+    // Test overflow
+    std::string s;
+    absl::strings_internal::OStringStream(&s)
+        << std::numeric_limits<IntType>::max() << value;
+    EXPECT_FALSE(parse_func(s, &parsed_value, base));
+
+    // Test underflow
+    s.clear();
+    absl::strings_internal::OStringStream(&s) << "-" << value;
+    EXPECT_FALSE(parse_func(s, &parsed_value, base));
+  }
+}
 
 TEST(stringtest, safe_strtou32_base) {
   for (int i = 0; strtouint32_test_cases()[i].str != nullptr; ++i) {
@@ -713,11 +779,11 @@
   }
 }
 
-// feenableexcept() and fedisableexcept() are missing on macOS, MSVC,
-// and WebAssembly.
-#if defined(_MSC_VER) || defined(__APPLE__) || defined(__EMSCRIPTEN__)
-#define ABSL_MISSING_FEENABLEEXCEPT 1
-#define ABSL_MISSING_FEDISABLEEXCEPT 1
+// feenableexcept() and fedisableexcept() are extensions supported by some libc
+// implementations.
+#if defined(__GLIBC__) || defined(__BIONIC__)
+#define ABSL_HAVE_FEENABLEEXCEPT 1
+#define ABSL_HAVE_FEDISABLEEXCEPT 1
 #endif
 
 class SimpleDtoaTest : public testing::Test {
@@ -725,7 +791,7 @@
   void SetUp() override {
     // Store the current floating point env & clear away any pending exceptions.
     feholdexcept(&fp_env_);
-#ifndef ABSL_MISSING_FEENABLEEXCEPT
+#ifdef ABSL_HAVE_FEENABLEEXCEPT
     // Turn on floating point exceptions.
     feenableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
 #endif
@@ -735,7 +801,7 @@
     // Restore the floating point environment to the original state.
     // In theory fedisableexcept is unnecessary; fesetenv will also do it.
     // In practice, our toolchains have subtle bugs.
-#ifndef ABSL_MISSING_FEDISABLEEXCEPT
+#ifdef ABSL_HAVE_FEDISABLEEXCEPT
     fedisableexcept(FE_DIVBYZERO | FE_INVALID | FE_OVERFLOW);
 #endif
     fesetenv(&fp_env_);
@@ -1184,4 +1250,28 @@
   }
 }
 
+void TestFastHexToBufferZeroPad16(uint64_t v) {
+  char buf[16];
+  auto digits = absl::numbers_internal::FastHexToBufferZeroPad16(v, buf);
+  absl::string_view res(buf, 16);
+  char buf2[17];
+  snprintf(buf2, sizeof(buf2), "%016" PRIx64, v);
+  EXPECT_EQ(res, buf2) << v;
+  size_t expected_digits = snprintf(buf2, sizeof(buf2), "%" PRIx64, v);
+  EXPECT_EQ(digits, expected_digits) << v;
+}
+
+TEST(FastHexToBufferZeroPad16, Smoke) {
+  TestFastHexToBufferZeroPad16(std::numeric_limits<uint64_t>::min());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<uint64_t>::max());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<int64_t>::min());
+  TestFastHexToBufferZeroPad16(std::numeric_limits<int64_t>::max());
+  absl::BitGen rng;
+  for (int i = 0; i < 100000; ++i) {
+    TestFastHexToBufferZeroPad16(
+        absl::LogUniform(rng, std::numeric_limits<uint64_t>::min(),
+                         std::numeric_limits<uint64_t>::max()));
+  }
+}
+
 }  // namespace
diff --git a/absl/strings/str_cat.cc b/absl/strings/str_cat.cc
index 73b9e0b..d9afe2f 100644
--- a/absl/strings/str_cat.cc
+++ b/absl/strings/str_cat.cc
@@ -15,35 +15,34 @@
 #include "absl/strings/str_cat.h"
 
 #include <assert.h>
+
 #include <algorithm>
 #include <cstdint>
 #include <cstring>
 
 #include "absl/strings/ascii.h"
 #include "absl/strings/internal/resize_uninitialized.h"
+#include "absl/strings/numbers.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 AlphaNum::AlphaNum(Hex hex) {
+  static_assert(numbers_internal::kFastToBufferSize >= 32,
+                "This function only works when output buffer >= 32 bytes long");
   char* const end = &digits_[numbers_internal::kFastToBufferSize];
-  char* writer = end;
-  uint64_t value = hex.value;
-  static const char hexdigits[] = "0123456789abcdef";
-  do {
-    *--writer = hexdigits[value & 0xF];
-    value >>= 4;
-  } while (value != 0);
-
-  char* beg;
-  if (end - writer < hex.width) {
-    beg = end - hex.width;
-    std::fill_n(beg, writer - beg, hex.fill);
+  auto real_width =
+      absl::numbers_internal::FastHexToBufferZeroPad16(hex.value, end - 16);
+  if (real_width >= hex.width) {
+    piece_ = absl::string_view(end - real_width, real_width);
   } else {
-    beg = writer;
+    // Pad first 16 chars because FastHexToBufferZeroPad16 pads only to 16 and
+    // max pad width can be up to 20.
+    std::memset(end - 32, hex.fill, 16);
+    // Patch up everything else up to the real_width.
+    std::memset(end - real_width - 16, hex.fill, 16);
+    piece_ = absl::string_view(end - hex.width, hex.width);
   }
-
-  piece_ = absl::string_view(beg, end - beg);
 }
 
 AlphaNum::AlphaNum(Dec dec) {
@@ -100,7 +99,7 @@
   std::string result;
   absl::strings_internal::STLStringResizeUninitialized(&result,
                                                        a.size() + b.size());
-  char* const begin = &*result.begin();
+  char* const begin = &result[0];
   char* out = begin;
   out = Append(out, a);
   out = Append(out, b);
@@ -112,7 +111,7 @@
   std::string result;
   strings_internal::STLStringResizeUninitialized(
       &result, a.size() + b.size() + c.size());
-  char* const begin = &*result.begin();
+  char* const begin = &result[0];
   char* out = begin;
   out = Append(out, a);
   out = Append(out, b);
@@ -126,7 +125,7 @@
   std::string result;
   strings_internal::STLStringResizeUninitialized(
       &result, a.size() + b.size() + c.size() + d.size());
-  char* const begin = &*result.begin();
+  char* const begin = &result[0];
   char* out = begin;
   out = Append(out, a);
   out = Append(out, b);
@@ -145,7 +144,7 @@
   for (const absl::string_view piece : pieces) total_size += piece.size();
   strings_internal::STLStringResizeUninitialized(&result, total_size);
 
-  char* const begin = &*result.begin();
+  char* const begin = &result[0];
   char* out = begin;
   for (const absl::string_view piece : pieces) {
     const size_t this_size = piece.size();
@@ -177,7 +176,7 @@
   }
   strings_internal::STLStringResizeUninitialized(dest, total_size);
 
-  char* const begin = &*dest->begin();
+  char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   for (const absl::string_view piece : pieces) {
     const size_t this_size = piece.size();
@@ -202,7 +201,7 @@
   std::string::size_type old_size = dest->size();
   strings_internal::STLStringResizeUninitialized(
       dest, old_size + a.size() + b.size());
-  char* const begin = &*dest->begin();
+  char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
   out = Append(out, b);
@@ -217,7 +216,7 @@
   std::string::size_type old_size = dest->size();
   strings_internal::STLStringResizeUninitialized(
       dest, old_size + a.size() + b.size() + c.size());
-  char* const begin = &*dest->begin();
+  char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
   out = Append(out, b);
@@ -234,7 +233,7 @@
   std::string::size_type old_size = dest->size();
   strings_internal::STLStringResizeUninitialized(
       dest, old_size + a.size() + b.size() + c.size() + d.size());
-  char* const begin = &*dest->begin();
+  char* const begin = &(*dest)[0];
   char* out = begin + old_size;
   out = Append(out, a);
   out = Append(out, b);
@@ -243,5 +242,5 @@
   assert(out == begin + dest->size());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h
index c270047..292fa23 100644
--- a/absl/strings/str_cat.h
+++ b/absl/strings/str_cat.h
@@ -64,7 +64,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace strings_internal {
 // AlphaNumBuffer allows a way to pass a string to StrCat without having to do
@@ -291,7 +291,8 @@
 // StrCat()
 // -----------------------------------------------------------------------------
 //
-// Merges given strings or numbers, using no delimiter(s).
+// Merges given strings or numbers, using no delimiter(s), returning the merged
+// result as a string.
 //
 // `StrCat()` is designed to be the fastest possible way to construct a string
 // out of a mix of raw C strings, string_views, strings, bool values,
@@ -401,7 +402,7 @@
   return result;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_CAT_H_
diff --git a/absl/strings/str_cat_test.cc b/absl/strings/str_cat_test.cc
index 29db9c0..be39880 100644
--- a/absl/strings/str_cat_test.cc
+++ b/absl/strings/str_cat_test.cc
@@ -195,6 +195,21 @@
   EXPECT_EQ(result, "12333444455555666666777777788888888999999999");
 }
 
+TEST(StrCat, CornerCases) {
+  std::string result;
+
+  result = absl::StrCat("");  // NOLINT
+  EXPECT_EQ(result, "");
+  result = absl::StrCat("", "");
+  EXPECT_EQ(result, "");
+  result = absl::StrCat("", "", "");
+  EXPECT_EQ(result, "");
+  result = absl::StrCat("", "", "", "");
+  EXPECT_EQ(result, "");
+  result = absl::StrCat("", "", "", "", "");
+  EXPECT_EQ(result, "");
+}
+
 // A minimal allocator that uses malloc().
 template <typename T>
 struct Mallocator {
@@ -433,10 +448,34 @@
 }
 #endif  // GTEST_HAS_DEATH_TEST
 
-TEST(StrAppend, EmptyString) {
-  std::string s = "";
-  absl::StrAppend(&s, s);
-  EXPECT_EQ(s, "");
+TEST(StrAppend, CornerCases) {
+  std::string result;
+  absl::StrAppend(&result, "");
+  EXPECT_EQ(result, "");
+  absl::StrAppend(&result, "", "");
+  EXPECT_EQ(result, "");
+  absl::StrAppend(&result, "", "", "");
+  EXPECT_EQ(result, "");
+  absl::StrAppend(&result, "", "", "", "");
+  EXPECT_EQ(result, "");
+  absl::StrAppend(&result, "", "", "", "", "");
+  EXPECT_EQ(result, "");
+}
+
+TEST(StrAppend, CornerCasesNonEmptyAppend) {
+  for (std::string result : {"hello", "a std::string too long to fit in the SSO"}) {
+    const std::string expected = result;
+    absl::StrAppend(&result, "");
+    EXPECT_EQ(result, expected);
+    absl::StrAppend(&result, "", "");
+    EXPECT_EQ(result, expected);
+    absl::StrAppend(&result, "", "", "");
+    EXPECT_EQ(result, expected);
+    absl::StrAppend(&result, "", "", "", "");
+    EXPECT_EQ(result, expected);
+    absl::StrAppend(&result, "", "", "", "", "");
+    EXPECT_EQ(result, expected);
+  }
 }
 
 template <typename IntType>
diff --git a/absl/strings/str_format.h b/absl/strings/str_format.h
index b4d1b7b..2f9b4b2 100644
--- a/absl/strings/str_format.h
+++ b/absl/strings/str_format.h
@@ -82,7 +82,7 @@
 #include "absl/strings/internal/str_format/parser.h"  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // UntypedFormatSpec
 //
@@ -401,6 +401,12 @@
 // This function is functionally equivalent to `std::snprintf()` (and
 // type-safe); prefer `absl::SNPrintF()` over `std::snprintf()`.
 //
+// In particular, a successful call to `absl::SNPrintF()` writes at most `size`
+// bytes of the formatted output to `output`, including a NUL-terminator, and
+// returns the number of bytes that would have been written if truncation did
+// not occur. In the event of an error, a negative value is returned and `errno`
+// is set.
+//
 // Example:
 //
 //   std::string_view s = "Ulaanbaatar";
@@ -525,7 +531,7 @@
       str_format_internal::UntypedFormatSpecImpl::Extract(format), args);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_FORMAT_H_
diff --git a/absl/strings/str_format_test.cc b/absl/strings/str_format_test.cc
index bb0f58b..acbdbf4 100644
--- a/absl/strings/str_format_test.cc
+++ b/absl/strings/str_format_test.cc
@@ -10,7 +10,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 using str_format_internal::FormatArgImpl;
 
@@ -450,7 +450,7 @@
     if (conv.precision.is_from_arg()) {
       *out += "." + std::to_string(conv.precision.get_from_arg()) + "$*";
     }
-    *out += conv.conv.Char();
+    *out += FormatConversionCharToChar(conv.conv);
     *out += "}";
     return true;
   }
@@ -623,7 +623,7 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // Some codegen thunks that we can use to easily dump the generated assembly for
diff --git a/absl/strings/str_join.h b/absl/strings/str_join.h
index c6c0c98..ae5731a 100644
--- a/absl/strings/str_join.h
+++ b/absl/strings/str_join.h
@@ -60,7 +60,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Concept: Formatter
@@ -287,7 +287,7 @@
   return strings_internal::JoinAlgorithm(value, separator, AlphaNumFormatter());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_JOIN_H_
diff --git a/absl/strings/str_replace.cc b/absl/strings/str_replace.cc
index 3bd8bae..2bd5fa9 100644
--- a/absl/strings/str_replace.cc
+++ b/absl/strings/str_replace.cc
@@ -17,7 +17,7 @@
 #include "absl/strings/str_cat.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace strings_internal {
 
 using FixedMapping =
@@ -78,5 +78,5 @@
   return StrReplaceAll<strings_internal::FixedMapping>(replacements, target);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_replace.h b/absl/strings/str_replace.h
index cb99545..273c707 100644
--- a/absl/strings/str_replace.h
+++ b/absl/strings/str_replace.h
@@ -46,7 +46,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // StrReplaceAll()
 //
@@ -213,7 +213,7 @@
   return substitutions;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_REPLACE_H_
diff --git a/absl/strings/str_split.cc b/absl/strings/str_split.cc
index f1e0371..d0f8666 100644
--- a/absl/strings/str_split.cc
+++ b/absl/strings/str_split.cc
@@ -27,7 +27,7 @@
 #include "absl/strings/ascii.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -135,5 +135,5 @@
   return absl::string_view(substr.data() + length_, 0);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/str_split.h b/absl/strings/str_split.h
index 463ca00..a79cd4a 100644
--- a/absl/strings/str_split.h
+++ b/absl/strings/str_split.h
@@ -49,7 +49,7 @@
 #include "absl/strings/strip.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // Delimiters
@@ -507,7 +507,7 @@
       std::move(text), DelimiterType(d), std::move(p));
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STR_SPLIT_H_
diff --git a/absl/strings/string_view.cc b/absl/strings/string_view.cc
index 9d241e5..c5f5de9 100644
--- a/absl/strings/string_view.cc
+++ b/absl/strings/string_view.cc
@@ -14,7 +14,7 @@
 
 #include "absl/strings/string_view.h"
 
-#ifndef ABSL_HAVE_STD_STRING_VIEW
+#ifndef ABSL_USES_STD_STRING_VIEW
 
 #include <algorithm>
 #include <climits>
@@ -24,7 +24,7 @@
 #include "absl/strings/internal/memutil.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 void WritePadding(std::ostream& o, size_t pad) {
@@ -229,7 +229,7 @@
 ABSL_STRING_VIEW_SELECTANY
 constexpr string_view::size_type string_view::kMaxSize;
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_STRING_VIEW
+#endif  // ABSL_USES_STD_STRING_VIEW
diff --git a/absl/strings/string_view.h b/absl/strings/string_view.h
index 3a0a460..1861ea6 100644
--- a/absl/strings/string_view.h
+++ b/absl/strings/string_view.h
@@ -28,20 +28,6 @@
 #define ABSL_STRINGS_STRING_VIEW_H_
 
 #include <algorithm>
-#include "absl/base/config.h"
-
-#ifdef ABSL_HAVE_STD_STRING_VIEW
-
-#include <string_view>  // IWYU pragma: export
-
-namespace absl {
-inline namespace lts_2019_08_08 {
-using std::string_view;
-}  // inline namespace lts_2019_08_08
-}  // namespace absl
-
-#else  // ABSL_HAVE_STD_STRING_VIEW
-
 #include <cassert>
 #include <cstddef>
 #include <cstring>
@@ -50,13 +36,33 @@
 #include <limits>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/base/internal/throw_delegate.h"
 #include "absl/base/macros.h"
 #include "absl/base/optimization.h"
 #include "absl/base/port.h"
 
+#ifdef ABSL_USES_STD_STRING_VIEW
+
+#include <string_view>  // IWYU pragma: export
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
+using std::string_view;
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#else  // ABSL_USES_STD_STRING_VIEW
+
+#if ABSL_HAVE_BUILTIN(__builtin_memcmp) || \
+    (defined(__GNUC__) && !defined(__clang__))
+#define ABSL_INTERNAL_STRING_VIEW_MEMCMP __builtin_memcmp
+#else  // ABSL_HAVE_BUILTIN(__builtin_memcmp)
+#define ABSL_INTERNAL_STRING_VIEW_MEMCMP memcmp
+#endif  // ABSL_HAVE_BUILTIN(__builtin_memcmp)
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
 
 // absl::string_view
 //
@@ -105,17 +111,17 @@
 // example, when splitting a string, `std::vector<absl::string_view>` is a
 // natural data type for the output.
 //
-// When constructed from a source which is nul-terminated, the `string_view`
-// itself will not include the nul-terminator unless a specific size (including
-// the nul) is passed to the constructor. As a result, common idioms that work
-// on nul-terminated strings do not work on `string_view` objects. If you write
+// When constructed from a source which is NUL-terminated, the `string_view`
+// itself will not include the NUL-terminator unless a specific size (including
+// the NUL) is passed to the constructor. As a result, common idioms that work
+// on NUL-terminated strings do not work on `string_view` objects. If you write
 // code that scans a `string_view`, you must check its length rather than test
 // for nul, for example. Note, however, that nuls may still be embedded within
 // a `string_view` explicitly.
 //
 // You may create a null `string_view` in two ways:
 //
-//   absl::string_view sv();
+//   absl::string_view sv;
 //   absl::string_view sv(nullptr, 0);
 //
 // For the above, `sv.data() == nullptr`, `sv.length() == 0`, and
@@ -171,9 +177,11 @@
   string_view(  // NOLINT(runtime/explicit)
       const std::basic_string<char, std::char_traits<char>, Allocator>&
           str) noexcept
-      : ptr_(str.data()), length_(CheckLengthInternal(str.size())) {}
+      // This is implemented in terms of `string_view(p, n)` so `str.size()`
+      // doesn't need to be reevaluated after `ptr_` is set.
+      : string_view(str.data(), str.size()) {}
 
-  // Implicit constructor of a `string_view` from nul-terminated `str`. When
+  // Implicit constructor of a `string_view` from NUL-terminated `str`. When
   // accepting possibly null strings, use `absl::NullSafeStringView(str)`
   // instead (see below).
   constexpr string_view(const char* str)  // NOLINT(runtime/explicit)
@@ -272,26 +280,45 @@
 
   // string_view::operator[]
   //
-  // Returns the ith element of an `string_view` using the array operator.
+  // Returns the ith element of the `string_view` using the array operator.
   // Note that this operator does not perform any bounds checking.
-  constexpr const_reference operator[](size_type i) const { return ptr_[i]; }
+  constexpr const_reference operator[](size_type i) const {
+    return ABSL_ASSERT(i < size()), ptr_[i];
+  }
+
+  // string_view::at()
+  //
+  // Returns the ith element of the `string_view`. Bounds checking is performed,
+  // and an exception of type `std::out_of_range` will be thrown on invalid
+  // access.
+  constexpr const_reference at(size_type i) const {
+    return ABSL_PREDICT_TRUE(i < size())
+               ? ptr_[i]
+               : ((void)base_internal::ThrowStdOutOfRange(
+                      "absl::string_view::at"),
+                  ptr_[i]);
+  }
 
   // string_view::front()
   //
   // Returns the first element of a `string_view`.
-  constexpr const_reference front() const { return ptr_[0]; }
+  constexpr const_reference front() const {
+    return ABSL_ASSERT(!empty()), ptr_[0];
+  }
 
   // string_view::back()
   //
   // Returns the last element of a `string_view`.
-  constexpr const_reference back() const { return ptr_[size() - 1]; }
+  constexpr const_reference back() const {
+    return ABSL_ASSERT(!empty()), ptr_[size() - 1];
+  }
 
   // string_view::data()
   //
   // Returns a pointer to the underlying character array (which is of course
   // stored elsewhere). Note that `string_view::data()` may contain embedded nul
-  // characters, but the returned buffer may or may not be nul-terminated;
-  // therefore, do not pass `data()` to a routine that expects a nul-terminated
+  // characters, but the returned buffer may or may not be NUL-terminated;
+  // therefore, do not pass `data()` to a routine that expects a NUL-terminated
   // std::string.
   constexpr const_pointer data() const noexcept { return ptr_; }
 
@@ -345,7 +372,7 @@
     size_type rlen = (std::min)(length_ - pos, n);
     if (rlen > 0) {
       const char* start = ptr_ + pos;
-      std::copy(start, start + rlen, buf);
+      traits_type::copy(buf, start, rlen);
     }
     return rlen;
   }
@@ -370,16 +397,12 @@
   // view. Note that in the case of data equality, a further comparison is made
   // on the respective sizes of the two `string_view`s to determine which is
   // smaller, equal, or greater.
-  int compare(string_view x) const noexcept {
-    auto min_length = (std::min)(length_, x.length_);
-    if (min_length > 0) {
-      int r = memcmp(ptr_, x.ptr_, min_length);
-      if (r < 0) return -1;
-      if (r > 0) return 1;
-    }
-    if (length_ < x.length_) return -1;
-    if (length_ > x.length_) return 1;
-    return 0;
+  constexpr int compare(string_view x) const noexcept {
+    return CompareImpl(length_, x.length_,
+                       Min(length_, x.length_) == 0
+                           ? 0
+                           : ABSL_INTERNAL_STRING_VIEW_MEMCMP(
+                                 ptr_, x.ptr_, Min(length_, x.length_)));
   }
 
   // Overload of `string_view::compare()` for comparing a substring of the
@@ -496,7 +519,7 @@
       (std::numeric_limits<difference_type>::max)();
 
   static constexpr size_type CheckLengthInternal(size_type len) {
-    return ABSL_ASSERT(len <= kMaxSize), len;
+    return (void)ABSL_ASSERT(len <= kMaxSize), len;
   }
 
   static constexpr size_type StrlenInternal(const char* str) {
@@ -517,6 +540,17 @@
 #endif
   }
 
+  static constexpr size_t Min(size_type length_a, size_type length_b) {
+    return length_a < length_b ? length_a : length_b;
+  }
+
+  static constexpr int CompareImpl(size_type length_a, size_type length_b,
+                                   int compare_result) {
+    return compare_result == 0 ? static_cast<int>(length_a > length_b) -
+                                     static_cast<int>(length_a < length_b)
+                               : (compare_result < 0 ? -1 : 1);
+  }
+
   const char* ptr_;
   size_type length_;
 };
@@ -524,46 +558,44 @@
 // This large function is defined inline so that in a fairly common case where
 // one of the arguments is a literal, the compiler can elide a lot of the
 // following comparisons.
-inline bool operator==(string_view x, string_view y) noexcept {
-  auto len = x.size();
-  if (len != y.size()) {
-    return false;
-  }
-
-  return x.data() == y.data() || len <= 0 ||
-         memcmp(x.data(), y.data(), len) == 0;
+constexpr bool operator==(string_view x, string_view y) noexcept {
+  return x.size() == y.size() &&
+         (x.empty() ||
+          ABSL_INTERNAL_STRING_VIEW_MEMCMP(x.data(), y.data(), x.size()) == 0);
 }
 
-inline bool operator!=(string_view x, string_view y) noexcept {
+constexpr bool operator!=(string_view x, string_view y) noexcept {
   return !(x == y);
 }
 
-inline bool operator<(string_view x, string_view y) noexcept {
-  auto min_size = (std::min)(x.size(), y.size());
-  const int r = min_size == 0 ? 0 : memcmp(x.data(), y.data(), min_size);
-  return (r < 0) || (r == 0 && x.size() < y.size());
+constexpr bool operator<(string_view x, string_view y) noexcept {
+  return x.compare(y) < 0;
 }
 
-inline bool operator>(string_view x, string_view y) noexcept { return y < x; }
+constexpr bool operator>(string_view x, string_view y) noexcept {
+  return y < x;
+}
 
-inline bool operator<=(string_view x, string_view y) noexcept {
+constexpr bool operator<=(string_view x, string_view y) noexcept {
   return !(y < x);
 }
 
-inline bool operator>=(string_view x, string_view y) noexcept {
+constexpr bool operator>=(string_view x, string_view y) noexcept {
   return !(x < y);
 }
 
 // IO Insertion Operator
 std::ostream& operator<<(std::ostream& o, string_view piece);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_STRING_VIEW
+#undef ABSL_INTERNAL_STRING_VIEW_MEMCMP
+
+#endif  // ABSL_USES_STD_STRING_VIEW
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // ClippedSubstr()
 //
@@ -580,11 +612,11 @@
 // Creates an `absl::string_view` from a pointer `p` even if it's null-valued.
 // This function should be used where an `absl::string_view` can be created from
 // a possibly-null pointer.
-inline string_view NullSafeStringView(const char* p) {
+constexpr string_view NullSafeStringView(const char* p) {
   return p ? string_view(p) : string_view();
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STRING_VIEW_H_
diff --git a/absl/strings/string_view_benchmark.cc b/absl/strings/string_view_benchmark.cc
index 46909cb..0d74e23 100644
--- a/absl/strings/string_view_benchmark.cc
+++ b/absl/strings/string_view_benchmark.cc
@@ -30,6 +30,24 @@
 
 namespace {
 
+void BM_StringViewFromString(benchmark::State& state) {
+  std::string s(state.range(0), 'x');
+  std::string* ps = &s;
+  struct SV {
+    SV() = default;
+    explicit SV(const std::string& s) : sv(s) {}
+    absl::string_view sv;
+  } sv;
+  SV* psv = &sv;
+  benchmark::DoNotOptimize(ps);
+  benchmark::DoNotOptimize(psv);
+  for (auto _ : state) {
+    new (psv) SV(*ps);
+    benchmark::DoNotOptimize(sv);
+  }
+}
+BENCHMARK(BM_StringViewFromString)->Arg(12)->Arg(128);
+
 // Provide a forcibly out-of-line wrapper for operator== that can be used in
 // benchmarks to measure the impact of inlining.
 ABSL_ATTRIBUTE_NOINLINE
@@ -142,11 +160,45 @@
   absl::string_view b = y;
 
   for (auto _ : state) {
+    benchmark::DoNotOptimize(a);
+    benchmark::DoNotOptimize(b);
     benchmark::DoNotOptimize(a.compare(b));
   }
 }
 BENCHMARK(BM_CompareSame)->DenseRange(0, 3)->Range(4, 1 << 10);
 
+void BM_CompareFirstOneLess(benchmark::State& state) {
+  const int len = state.range(0);
+  std::string x(len, 'a');
+  std::string y = x;
+  y.back() = 'b';
+  absl::string_view a = x;
+  absl::string_view b = y;
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(a);
+    benchmark::DoNotOptimize(b);
+    benchmark::DoNotOptimize(a.compare(b));
+  }
+}
+BENCHMARK(BM_CompareFirstOneLess)->DenseRange(1, 3)->Range(4, 1 << 10);
+
+void BM_CompareSecondOneLess(benchmark::State& state) {
+  const int len = state.range(0);
+  std::string x(len, 'a');
+  std::string y = x;
+  x.back() = 'b';
+  absl::string_view a = x;
+  absl::string_view b = y;
+
+  for (auto _ : state) {
+    benchmark::DoNotOptimize(a);
+    benchmark::DoNotOptimize(b);
+    benchmark::DoNotOptimize(a.compare(b));
+  }
+}
+BENCHMARK(BM_CompareSecondOneLess)->DenseRange(1, 3)->Range(4, 1 << 10);
+
 void BM_find_string_view_len_one(benchmark::State& state) {
   std::string haystack(state.range(0), '0');
   absl::string_view s(haystack);
diff --git a/absl/strings/string_view_test.cc b/absl/strings/string_view_test.cc
index 22d4353..7b1d56f 100644
--- a/absl/strings/string_view_test.cc
+++ b/absl/strings/string_view_test.cc
@@ -29,7 +29,8 @@
 #include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
 
-#ifdef __ANDROID__
+#if defined(ABSL_HAVE_STD_STRING_VIEW) || defined(__ANDROID__)
+// We don't control the death messaging when using std::string_view.
 // Android assert messages only go to system log, so death tests cannot inspect
 // the message for matching.
 #define ABSL_EXPECT_DEATH_IF_SUPPORTED(statement, regex) \
@@ -372,7 +373,7 @@
 #ifdef ABSL_HAVE_EXCEPTIONS
   EXPECT_THROW(a.copy(buf, 1, 27), std::out_of_range);
 #else
-  EXPECT_DEATH(a.copy(buf, 1, 27), "absl::string_view::copy");
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(a.copy(buf, 1, 27), "absl::string_view::copy");
 #endif
 }
 
@@ -686,7 +687,8 @@
 #ifdef ABSL_HAVE_EXCEPTIONS
   EXPECT_THROW((void)a.substr(99, 2), std::out_of_range);
 #else
-  EXPECT_DEATH((void)a.substr(99, 2), "absl::string_view::substr");
+  ABSL_EXPECT_DEATH_IF_SUPPORTED((void)a.substr(99, 2),
+                                 "absl::string_view::substr");
 #endif
 }
 
@@ -816,21 +818,35 @@
   EXPECT_EQ(&c, &csp.back());
 }
 
+TEST(StringViewTest, FrontBackEmpty) {
+#ifndef ABSL_USES_STD_STRING_VIEW
+#ifndef NDEBUG
+  // Abseil's string_view implementation has debug assertions that check that
+  // front() and back() are not called on an empty string_view.
+  absl::string_view sv;
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(sv.front(), "");
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(sv.back(), "");
+#endif
+#endif
+}
+
 // `std::string_view::string_view(const char*)` calls
 // `std::char_traits<char>::length(const char*)` to get the string length. In
 // libc++, it doesn't allow `nullptr` in the constexpr context, with the error
 // "read of dereferenced null pointer is not allowed in a constant expression".
 // At run time, the behavior of `std::char_traits::length()` on `nullptr` is
 // undefined by the standard and usually results in crash with libc++.
+// GCC also started rejected this in libstdc++ starting in GCC9.
 // In MSVC, creating a constexpr string_view from nullptr also triggers an
 // "unevaluable pointer value" error. This compiler implementation conforms
 // to the standard, but `absl::string_view` implements a different
 // behavior for historical reasons. We work around tests that construct
 // `string_view` from `nullptr` when using libc++.
-#if !defined(ABSL_HAVE_STD_STRING_VIEW) || \
-    (!defined(_LIBCPP_VERSION) && !defined(_MSC_VER))
+#if !defined(ABSL_USES_STD_STRING_VIEW) ||                    \
+    (!(defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE >= 9) && \
+     !defined(_LIBCPP_VERSION) && !defined(_MSC_VER))
 #define ABSL_HAVE_STRING_VIEW_FROM_NULLPTR 1
-#endif  // !defined(ABSL_HAVE_STD_STRING_VIEW) || !defined(_LIBCPP_VERSION)
+#endif
 
 TEST(StringViewTest, NULLInput) {
   absl::string_view s;
@@ -892,6 +908,18 @@
   EXPECT_LT(digits.compare(0, npos, "0123456789", 3, 5), 0);  // 6
 }
 
+TEST(StringViewTest, At) {
+  absl::string_view abc = "abc";
+  EXPECT_EQ(abc.at(0), 'a');
+  EXPECT_EQ(abc.at(1), 'b');
+  EXPECT_EQ(abc.at(2), 'c');
+#ifdef ABSL_HAVE_EXCEPTIONS
+  EXPECT_THROW(abc.at(3), std::out_of_range);
+#else
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(abc.at(3), "absl::string_view::at");
+#endif
+}
+
 struct MyCharAlloc : std::allocator<char> {};
 
 TEST(StringViewTest, ExplicitConversionOperator) {
@@ -915,6 +943,31 @@
   }
 }
 
+TEST(StringViewTest, ConstexprNullSafeStringView) {
+  {
+    constexpr absl::string_view s = absl::NullSafeStringView(nullptr);
+    EXPECT_EQ(nullptr, s.data());
+    EXPECT_EQ(0, s.size());
+    EXPECT_EQ(absl::string_view(), s);
+  }
+#if !defined(_MSC_VER) || _MSC_VER >= 1910
+  // MSVC 2017+ is required for good constexpr string_view support.
+  // See the implementation of `absl::string_view::StrlenInternal()`.
+  {
+    static constexpr char kHi[] = "hi";
+    absl::string_view s = absl::NullSafeStringView(kHi);
+    EXPECT_EQ(kHi, s.data());
+    EXPECT_EQ(strlen(kHi), s.size());
+    EXPECT_EQ(absl::string_view("hi"), s);
+  }
+  {
+    constexpr absl::string_view s = absl::NullSafeStringView("hello");
+    EXPECT_EQ(s.size(), 5);
+    EXPECT_EQ("hello", s);
+  }
+#endif
+}
+
 TEST(StringViewTest, ConstexprCompiles) {
   constexpr absl::string_view sp;
 #ifdef ABSL_HAVE_STRING_VIEW_FROM_NULLPTR
@@ -922,7 +975,7 @@
 #endif
   constexpr absl::string_view cstr_len("cstr", 4);
 
-#if defined(ABSL_HAVE_STD_STRING_VIEW)
+#if defined(ABSL_USES_STD_STRING_VIEW)
   // In libstdc++ (as of 7.2), `std::string_view::string_view(const char*)`
   // calls `std::char_traits<char>::length(const char*)` to get the std::string
   // length, but it is not marked constexpr yet. See GCC bug:
@@ -936,7 +989,7 @@
 #define ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR 1
 #endif  // !__GLIBCXX__
 
-#else  // ABSL_HAVE_STD_STRING_VIEW
+#else  // ABSL_USES_STD_STRING_VIEW
 
 // This duplicates the check for __builtin_strlen in the header.
 #if ABSL_HAVE_BUILTIN(__builtin_strlen) || \
@@ -951,13 +1004,36 @@
 #define ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR 1
 #endif
 
-#endif  // ABSL_HAVE_STD_STRING_VIEW
+#endif  // ABSL_USES_STD_STRING_VIEW
 
 #ifdef ABSL_HAVE_CONSTEXPR_STRING_VIEW_FROM_CSTR
   constexpr absl::string_view cstr_strlen("foo");
   EXPECT_EQ(cstr_strlen.length(), 3);
   constexpr absl::string_view cstr_strlen2 = "bar";
   EXPECT_EQ(cstr_strlen2, "bar");
+
+#if ABSL_HAVE_BUILTIN(__builtin_memcmp) || \
+    (defined(__GNUC__) && !defined(__clang__))
+#define ABSL_HAVE_CONSTEXPR_STRING_VIEW_COMPARISON 1
+#endif
+#ifdef ABSL_HAVE_CONSTEXPR_STRING_VIEW_COMPARISON
+  constexpr absl::string_view foo = "foo";
+  constexpr absl::string_view bar = "bar";
+  constexpr bool foo_eq_bar = foo == bar;
+  constexpr bool foo_ne_bar = foo != bar;
+  constexpr bool foo_lt_bar = foo < bar;
+  constexpr bool foo_le_bar = foo <= bar;
+  constexpr bool foo_gt_bar = foo > bar;
+  constexpr bool foo_ge_bar = foo >= bar;
+  constexpr int foo_compare_bar = foo.compare(bar);
+  EXPECT_FALSE(foo_eq_bar);
+  EXPECT_TRUE(foo_ne_bar);
+  EXPECT_FALSE(foo_lt_bar);
+  EXPECT_FALSE(foo_le_bar);
+  EXPECT_TRUE(foo_gt_bar);
+  EXPECT_TRUE(foo_ge_bar);
+  EXPECT_GT(foo_compare_bar, 0);
+#endif
 #endif
 
 #if !defined(__clang__) || 3 < __clang_major__ || \
@@ -979,8 +1055,16 @@
   constexpr absl::string_view::iterator const_end = cstr_len.end();
   constexpr absl::string_view::size_type const_size = cstr_len.size();
   constexpr absl::string_view::size_type const_length = cstr_len.length();
+  static_assert(const_begin + const_size == const_end,
+                "pointer arithmetic check");
+  static_assert(const_begin + const_length == const_end,
+                "pointer arithmetic check");
+#ifndef _MSC_VER
+  // MSVC has bugs doing constexpr pointer arithmetic.
+  // https://developercommunity.visualstudio.com/content/problem/482192/bad-pointer-arithmetic-in-constepxr-2019-rc1-svc1.html
   EXPECT_EQ(const_begin + const_size, const_end);
   EXPECT_EQ(const_begin + const_length, const_end);
+#endif
 
   constexpr bool isempty = sp.empty();
   EXPECT_TRUE(isempty);
@@ -1036,6 +1120,17 @@
   EXPECT_TRUE(noexcept(sp.find_last_not_of('f')));
 }
 
+TEST(StringViewTest, BoundsCheck) {
+#ifndef ABSL_USES_STD_STRING_VIEW
+#ifndef NDEBUG
+  // Abseil's string_view implementation has bounds-checking in debug mode.
+  absl::string_view h = "hello";
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(h[5], "");
+  ABSL_EXPECT_DEATH_IF_SUPPORTED(h[-1], "");
+#endif
+#endif
+}
+
 TEST(ComparisonOpsTest, StringCompareNotAmbiguous) {
   EXPECT_EQ("hello", std::string("hello"));
   EXPECT_LT("hello", std::string("world"));
@@ -1089,7 +1184,7 @@
 }
 #endif  // THREAD_SANITIZER
 
-#if !defined(NDEBUG) && !defined(ABSL_HAVE_STD_STRING_VIEW)
+#if !defined(NDEBUG) && !defined(ABSL_USES_STD_STRING_VIEW)
 TEST(NonNegativeLenTest, NonNegativeLen) {
   ABSL_EXPECT_DEATH_IF_SUPPORTED(absl::string_view("xyz", -1),
                                  "len <= kMaxSize");
@@ -1105,7 +1200,7 @@
   ABSL_EXPECT_DEATH_IF_SUPPORTED(absl::string_view("", max_size + 1),
                                  "len <= kMaxSize");
 }
-#endif  // !defined(NDEBUG) && !defined(ABSL_HAVE_STD_STRING_VIEW)
+#endif  // !defined(NDEBUG) && !defined(ABSL_USES_STD_STRING_VIEW)
 
 class StringViewStreamTest : public ::testing::Test {
  public:
diff --git a/absl/strings/strip.h b/absl/strings/strip.h
index 8e2ae42..111872c 100644
--- a/absl/strings/strip.h
+++ b/absl/strings/strip.h
@@ -30,7 +30,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // ConsumePrefix()
 //
@@ -85,7 +85,7 @@
   return str;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_STRIP_H_
diff --git a/absl/strings/substitute.cc b/absl/strings/substitute.cc
index 6bc9641..5b69a3e 100644
--- a/absl/strings/substitute.cc
+++ b/absl/strings/substitute.cc
@@ -23,7 +23,7 @@
 #include "absl/strings/string_view.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace substitute_internal {
 
 void SubstituteAndAppendArray(std::string* output, absl::string_view format,
@@ -36,7 +36,7 @@
       if (i + 1 >= format.size()) {
 #ifndef NDEBUG
         ABSL_RAW_LOG(FATAL,
-                     "Invalid strings::Substitute() format std::string: \"%s\".",
+                     "Invalid absl::Substitute() format std::string: \"%s\".",
                      absl::CEscape(format).c_str());
 #endif
         return;
@@ -46,7 +46,7 @@
 #ifndef NDEBUG
           ABSL_RAW_LOG(
               FATAL,
-              "Invalid strings::Substitute() format std::string: asked for \"$"
+              "Invalid absl::Substitute() format std::string: asked for \"$"
               "%d\", but only %d args were given.  Full format std::string was: "
               "\"%s\".",
               index, static_cast<int>(num_args), absl::CEscape(format).c_str());
@@ -61,7 +61,7 @@
       } else {
 #ifndef NDEBUG
         ABSL_RAW_LOG(FATAL,
-                     "Invalid strings::Substitute() format std::string: \"%s\".",
+                     "Invalid absl::Substitute() format std::string: \"%s\".",
                      absl::CEscape(format).c_str());
 #endif
         return;
@@ -95,7 +95,6 @@
   assert(target == output->data() + output->size());
 }
 
-static const char kHexDigits[] = "0123456789abcdef";
 Arg::Arg(const void* value) {
   static_assert(sizeof(scratch_) >= sizeof(value) * 2 + 2,
                 "fix sizeof(scratch_)");
@@ -105,7 +104,7 @@
     char* ptr = scratch_ + sizeof(scratch_);
     uintptr_t num = reinterpret_cast<uintptr_t>(value);
     do {
-      *--ptr = kHexDigits[num & 0xf];
+      *--ptr = absl::numbers_internal::kHexChar[num & 0xf];
       num >>= 4;
     } while (num != 0);
     *--ptr = 'x';
@@ -120,7 +119,7 @@
   char* writer = end;
   uint64_t value = hex.value;
   do {
-    *--writer = kHexDigits[value & 0xF];
+    *--writer = absl::numbers_internal::kHexChar[value & 0xF];
     value >>= 4;
   } while (value != 0);
 
@@ -168,5 +167,5 @@
 }
 
 }  // namespace substitute_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/strings/substitute.h b/absl/strings/substitute.h
index 3ba7f4d..4d0984d 100644
--- a/absl/strings/substitute.h
+++ b/absl/strings/substitute.h
@@ -86,7 +86,7 @@
 #include "absl/strings/strip.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace substitute_internal {
 
 // Arg
@@ -190,7 +190,12 @@
 
 #if defined(ABSL_BAD_CALL_IF)
 constexpr int CalculateOneBit(const char* format) {
-  return (*format < '0' || *format > '9') ? 0 : (1 << (*format - '0'));
+  // Returns:
+  // * 2^N for '$N' when N is in [0-9]
+  // * 0 for correct '$' escaping: '$$'.
+  // * -1 otherwise.
+  return (*format < '0' || *format > '9') ? (*format == '$' ? 0 : -1)
+                                          : (1 << (*format - '0'));
 }
 
 constexpr const char* SkipNumber(const char* format) {
@@ -682,7 +687,7 @@
                      "format std::string doesn't contain all of $0 through $9");
 #endif  // ABSL_BAD_CALL_IF
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_STRINGS_SUBSTITUTE_H_
diff --git a/absl/strings/substitute_test.cc b/absl/strings/substitute_test.cc
index e27abb1..450cd2b 100644
--- a/absl/strings/substitute_test.cc
+++ b/absl/strings/substitute_test.cc
@@ -189,14 +189,14 @@
 TEST(SubstituteDeathTest, SubstituteDeath) {
   EXPECT_DEBUG_DEATH(
       static_cast<void>(absl::Substitute(absl::string_view("-$2"), "a", "b")),
-      "Invalid strings::Substitute\\(\\) format std::string: asked for \"\\$2\", "
+      "Invalid absl::Substitute\\(\\) format std::string: asked for \"\\$2\", "
       "but only 2 args were given.");
   EXPECT_DEBUG_DEATH(
-      static_cast<void>(absl::Substitute("-$z-")),
-      "Invalid strings::Substitute\\(\\) format std::string: \"-\\$z-\"");
+      static_cast<void>(absl::Substitute(absl::string_view("-$z-"))),
+      "Invalid absl::Substitute\\(\\) format std::string: \"-\\$z-\"");
   EXPECT_DEBUG_DEATH(
-      static_cast<void>(absl::Substitute("-$")),
-      "Invalid strings::Substitute\\(\\) format std::string: \"-\\$\"");
+      static_cast<void>(absl::Substitute(absl::string_view("-$"))),
+      "Invalid absl::Substitute\\(\\) format std::string: \"-\\$\"");
 }
 
 #endif  // GTEST_HAS_DEATH_TEST
diff --git a/absl/synchronization/BUILD.bazel b/absl/synchronization/BUILD.bazel
index fca8cb6..3f876b9 100644
--- a/absl/synchronization/BUILD.bazel
+++ b/absl/synchronization/BUILD.bazel
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_binary", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -42,8 +43,25 @@
     deps = [
         "//absl/base",
         "//absl/base:base_internal",
+        "//absl/base:config",
         "//absl/base:core_headers",
         "//absl/base:malloc_internal",
+        "//absl/base:raw_logging_internal",
+    ],
+)
+
+cc_library(
+    name = "kernel_timeout_internal",
+    hdrs = ["internal/kernel_timeout.h"],
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    visibility = [
+        "//absl/synchronization:__pkg__",
+    ],
+    deps = [
+        "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
+        "//absl/time",
     ],
 )
 
@@ -63,7 +81,6 @@
         "barrier.h",
         "blocking_counter.h",
         "internal/create_thread_identity.h",
-        "internal/kernel_timeout.h",
         "internal/mutex_nonprod.inc",
         "internal/per_thread_sem.h",
         "internal/waiter.h",
@@ -77,6 +94,7 @@
     }) + ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":graphcycles_internal",
+        ":kernel_timeout_internal",
         "//absl/base",
         "//absl/base:atomic_hook",
         "//absl/base:base_internal",
@@ -84,6 +102,7 @@
         "//absl/base:core_headers",
         "//absl/base:dynamic_annotations",
         "//absl/base:malloc_internal",
+        "//absl/base:raw_logging_internal",
         "//absl/debugging:stacktrace",
         "//absl/debugging:symbolize",
         "//absl/time",
@@ -124,8 +143,8 @@
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":graphcycles_internal",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -140,7 +159,7 @@
     ],
     deps = [
         ":graphcycles_internal",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -171,6 +190,7 @@
         ":thread_pool",
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/memory",
         "//absl/time",
         "@com_google_googletest//:gtest_main",
@@ -243,7 +263,6 @@
     deps = [
         ":per_thread_sem_test_common",
         ":synchronization",
-        "//absl/base",
         "//absl/strings",
         "//absl/time",
         "@com_google_googletest//:gtest_main",
@@ -260,7 +279,7 @@
     tags = ["no_test_ios_x86_64"],
     deps = [
         ":synchronization",
-        "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
     ],
 )
diff --git a/absl/synchronization/CMakeLists.txt b/absl/synchronization/CMakeLists.txt
index 4b70882..dfe5d05 100644
--- a/absl/synchronization/CMakeLists.txt
+++ b/absl/synchronization/CMakeLists.txt
@@ -26,8 +26,23 @@
   DEPS
     absl::base
     absl::base_internal
+    absl::config
     absl::core_headers
     absl::malloc_internal
+    absl::raw_logging_internal
+)
+
+absl_cc_library(
+  NAME
+    kernel_timeout_internal
+  HDRS
+    "internal/kernel_timeout.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::core_headers
+    absl::raw_logging_internal
+    absl::time
 )
 
 absl_cc_library(
@@ -37,7 +52,6 @@
     "barrier.h"
     "blocking_counter.h"
     "internal/create_thread_identity.h"
-    "internal/kernel_timeout.h"
     "internal/mutex_nonprod.inc"
     "internal/per_thread_sem.h"
     "internal/waiter.h"
@@ -55,6 +69,7 @@
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::graphcycles_internal
+    absl::kernel_timeout_internal
     absl::atomic_hook
     absl::base
     absl::base_internal
@@ -62,6 +77,7 @@
     absl::core_headers
     absl::dynamic_annotations
     absl::malloc_internal
+    absl::raw_logging_internal
     absl::stacktrace
     absl::symbolize
     absl::time
@@ -104,8 +120,8 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::graphcycles_internal
-    absl::base
     absl::core_headers
+    absl::raw_logging_internal
     gmock_main
 )
 
@@ -135,6 +151,7 @@
     absl::base
     absl::core_headers
     absl::memory
+    absl::raw_logging_internal
     absl::time
     gmock_main
 )
@@ -178,7 +195,6 @@
   DEPS
     absl::per_thread_sem_test_common
     absl::synchronization
-    absl::base
     absl::strings
     absl::time
     gmock_main
@@ -193,6 +209,6 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::synchronization
-    absl::base
     absl::core_headers
+    absl::raw_logging_internal
 )
diff --git a/absl/synchronization/barrier.cc b/absl/synchronization/barrier.cc
index 72089c5..0dfd795 100644
--- a/absl/synchronization/barrier.cc
+++ b/absl/synchronization/barrier.cc
@@ -18,7 +18,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Return whether int *arg is zero.
 static bool IsZero(void *arg) {
@@ -48,5 +48,5 @@
   return this->num_to_exit_ == 0;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/barrier.h b/absl/synchronization/barrier.h
index 53d5ca2..d8e7544 100644
--- a/absl/synchronization/barrier.h
+++ b/absl/synchronization/barrier.h
@@ -23,7 +23,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Barrier
 //
@@ -70,10 +70,10 @@
 
  private:
   Mutex lock_;
-  int num_to_block_ GUARDED_BY(lock_);
-  int num_to_exit_ GUARDED_BY(lock_);
+  int num_to_block_ ABSL_GUARDED_BY(lock_);
+  int num_to_exit_ ABSL_GUARDED_BY(lock_);
 };
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_SYNCHRONIZATION_BARRIER_H_
diff --git a/absl/synchronization/blocking_counter.cc b/absl/synchronization/blocking_counter.cc
index c696897..3cea7ae 100644
--- a/absl/synchronization/blocking_counter.cc
+++ b/absl/synchronization/blocking_counter.cc
@@ -17,7 +17,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Return whether int *arg is zero.
 static bool IsZero(void *arg) {
@@ -53,5 +53,5 @@
   // after we return from this method.
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/blocking_counter.h b/absl/synchronization/blocking_counter.h
index 5dab5a9..1f53f9f 100644
--- a/absl/synchronization/blocking_counter.h
+++ b/absl/synchronization/blocking_counter.h
@@ -24,7 +24,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // BlockingCounter
 //
@@ -89,11 +89,11 @@
 
  private:
   Mutex lock_;
-  int count_ GUARDED_BY(lock_);
-  int num_waiting_ GUARDED_BY(lock_);
+  int count_ ABSL_GUARDED_BY(lock_);
+  int num_waiting_ ABSL_GUARDED_BY(lock_);
 };
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
diff --git a/absl/synchronization/blocking_counter_test.cc b/absl/synchronization/blocking_counter_test.cc
index 62d9873..2926224 100644
--- a/absl/synchronization/blocking_counter_test.cc
+++ b/absl/synchronization/blocking_counter_test.cc
@@ -22,7 +22,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 void PauseAndDecreaseCounter(BlockingCounter* counter, int* done) {
@@ -64,5 +64,5 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/create_thread_identity.cc b/absl/synchronization/internal/create_thread_identity.cc
index 65f6d8f..fa0070a 100644
--- a/absl/synchronization/internal/create_thread_identity.cc
+++ b/absl/synchronization/internal/create_thread_identity.cc
@@ -27,7 +27,7 @@
 #include "absl/synchronization/internal/per_thread_sem.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // ThreadIdentity storage is persistent, we maintain a free-list of previously
@@ -38,7 +38,7 @@
 
 // A per-thread destructor for reclaiming associated ThreadIdentity objects.
 // Since we must preserve their storage we cache them for re-use.
-static void ReclaimThreadIdentity(void* v) {
+void ReclaimThreadIdentity(void* v) {
   base_internal::ThreadIdentity* identity =
       static_cast<base_internal::ThreadIdentity*>(v);
 
@@ -48,6 +48,8 @@
     base_internal::LowLevelAlloc::Free(identity->per_thread_synch.all_locks);
   }
 
+  PerThreadSem::Destroy(identity);
+
   // We must explicitly clear the current thread's identity:
   // (a) Subsequent (unrelated) per-thread destructors may require an identity.
   //     We must guarantee a new identity is used in this case (this instructor
@@ -85,7 +87,6 @@
   pts->wake = false;
   pts->cond_waiter = false;
   pts->all_locks = nullptr;
-  identity->waiter_state = {};
   identity->blocked_count_ptr = nullptr;
   identity->ticker.store(0, std::memory_order_relaxed);
   identity->wait_start.store(0, std::memory_order_relaxed);
@@ -133,7 +134,7 @@
 }
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/synchronization/internal/create_thread_identity.h b/absl/synchronization/internal/create_thread_identity.h
index d743cc3..e121f68 100644
--- a/absl/synchronization/internal/create_thread_identity.h
+++ b/absl/synchronization/internal/create_thread_identity.h
@@ -29,13 +29,17 @@
 #include "absl/base/port.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // Allocates and attaches a ThreadIdentity object for the calling thread.
 // For private use only.
 base_internal::ThreadIdentity* CreateThreadIdentity();
 
+// A per-thread destructor for reclaiming associated ThreadIdentity objects.
+// For private use only.
+void ReclaimThreadIdentity(void* v);
+
 // Returns the ThreadIdentity object representing the calling thread; guaranteed
 // to be unique for its lifetime.  The returned object will remain valid for the
 // program's lifetime; although it may be re-assigned to a subsequent thread.
@@ -50,7 +54,7 @@
 }
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_CREATE_THREAD_IDENTITY_H_
diff --git a/absl/synchronization/internal/graphcycles.cc b/absl/synchronization/internal/graphcycles.cc
index f4fbead..6a2bcdf 100644
--- a/absl/synchronization/internal/graphcycles.cc
+++ b/absl/synchronization/internal/graphcycles.cc
@@ -44,7 +44,7 @@
 // Do not use STL.   This module does not use standard memory allocation.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 namespace {
@@ -691,7 +691,7 @@
 }
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_LOW_LEVEL_ALLOC_MISSING
diff --git a/absl/synchronization/internal/graphcycles.h b/absl/synchronization/internal/graphcycles.h
index 208527c..ceba33e 100644
--- a/absl/synchronization/internal/graphcycles.h
+++ b/absl/synchronization/internal/graphcycles.h
@@ -36,12 +36,14 @@
 //   InsertEdge() is very fast when the edge already exists, and reasonably fast
 //   otherwise.
 //   FindPath() is linear in the size of the graph.
-// The current implemenation uses O(|V|+|E|) space.
+// The current implementation uses O(|V|+|E|) space.
 
 #include <cstdint>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // Opaque identifier for a graph node.
@@ -133,7 +135,7 @@
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif
diff --git a/absl/synchronization/internal/graphcycles_test.cc b/absl/synchronization/internal/graphcycles_test.cc
index fca8621..74eaffe 100644
--- a/absl/synchronization/internal/graphcycles_test.cc
+++ b/absl/synchronization/internal/graphcycles_test.cc
@@ -25,7 +25,7 @@
 #include "absl/base/macros.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // We emulate a GraphCycles object with a node vector and an edge vector.
@@ -460,5 +460,5 @@
 }
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/kernel_timeout.h b/absl/synchronization/internal/kernel_timeout.h
index e0f01e0..d6ac5db 100644
--- a/absl/synchronization/internal/kernel_timeout.h
+++ b/absl/synchronization/internal/kernel_timeout.h
@@ -34,7 +34,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 class Futex;
@@ -149,7 +149,7 @@
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_KERNEL_TIMEOUT_H_
diff --git a/absl/synchronization/internal/mutex_nonprod.cc b/absl/synchronization/internal/mutex_nonprod.cc
index aa1ed83..4590b98 100644
--- a/absl/synchronization/internal/mutex_nonprod.cc
+++ b/absl/synchronization/internal/mutex_nonprod.cc
@@ -31,7 +31,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 namespace {
@@ -316,5 +316,5 @@
 
 void RegisterSymbolizer(bool (*)(const void*, char*, int)) {}
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/mutex_nonprod.inc b/absl/synchronization/internal/mutex_nonprod.inc
index ac10879..a1502e7 100644
--- a/absl/synchronization/internal/mutex_nonprod.inc
+++ b/absl/synchronization/internal/mutex_nonprod.inc
@@ -36,7 +36,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 class Condition;
 
 namespace synchronization_internal {
@@ -252,10 +252,10 @@
 
   absl::once_flag once_;
 
-  // An aligned space for T.
-  typename std::aligned_storage<sizeof(T), alignof(T)>::type space_;
+  // An aligned space for the T.
+  alignas(T) unsigned char space_[sizeof(T)];
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/per_thread_sem.cc b/absl/synchronization/internal/per_thread_sem.cc
index 284a5df..821ca9b 100644
--- a/absl/synchronization/internal/per_thread_sem.cc
+++ b/absl/synchronization/internal/per_thread_sem.cc
@@ -25,7 +25,7 @@
 #include "absl/synchronization/internal/waiter.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 void PerThreadSem::SetThreadBlockedCounter(std::atomic<int> *counter) {
@@ -41,12 +41,16 @@
 }
 
 void PerThreadSem::Init(base_internal::ThreadIdentity *identity) {
-  Waiter::GetWaiter(identity)->Init();
+  new (Waiter::GetWaiter(identity)) Waiter();
   identity->ticker.store(0, std::memory_order_relaxed);
   identity->wait_start.store(0, std::memory_order_relaxed);
   identity->is_idle.store(false, std::memory_order_relaxed);
 }
 
+void PerThreadSem::Destroy(base_internal::ThreadIdentity *identity) {
+  Waiter::GetWaiter(identity)->~Waiter();
+}
+
 void PerThreadSem::Tick(base_internal::ThreadIdentity *identity) {
   const int ticker =
       identity->ticker.fetch_add(1, std::memory_order_relaxed) + 1;
@@ -59,7 +63,7 @@
 }
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
diff --git a/absl/synchronization/internal/per_thread_sem.h b/absl/synchronization/internal/per_thread_sem.h
index 5bb0978..8ab4391 100644
--- a/absl/synchronization/internal/per_thread_sem.h
+++ b/absl/synchronization/internal/per_thread_sem.h
@@ -32,7 +32,7 @@
 #include "absl/synchronization/internal/kernel_timeout.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 class Mutex;
 
@@ -66,6 +66,10 @@
   // REQUIRES: May only be called by ThreadIdentity.
   static void Init(base_internal::ThreadIdentity* identity);
 
+  // Destroy the PerThreadSem associated with "identity".
+  // REQUIRES: May only be called by ThreadIdentity.
+  static void Destroy(base_internal::ThreadIdentity* identity);
+
   // Increments "identity"'s count.
   static inline void Post(base_internal::ThreadIdentity* identity);
 
@@ -78,10 +82,11 @@
   friend class PerThreadSemTest;
   friend class absl::Mutex;
   friend absl::base_internal::ThreadIdentity* CreateThreadIdentity();
+  friend void ReclaimThreadIdentity(void* v);
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
diff --git a/absl/synchronization/internal/per_thread_sem_test.cc b/absl/synchronization/internal/per_thread_sem_test.cc
index 93bc424..b5a2f6d 100644
--- a/absl/synchronization/internal/per_thread_sem_test.cc
+++ b/absl/synchronization/internal/per_thread_sem_test.cc
@@ -33,7 +33,7 @@
 // primitives which might use PerThreadSem, most notably absl::Mutex.
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 class SimpleSemaphore {
@@ -176,5 +176,5 @@
 }  // namespace
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/thread_pool.h b/absl/synchronization/internal/thread_pool.h
index 8941be6..0cb96da 100644
--- a/absl/synchronization/internal/thread_pool.h
+++ b/absl/synchronization/internal/thread_pool.h
@@ -26,7 +26,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // A simple ThreadPool implementation for tests.
@@ -61,7 +61,7 @@
   }
 
  private:
-  bool WorkAvailable() const EXCLUSIVE_LOCKS_REQUIRED(mu_) {
+  bool WorkAvailable() const ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
     return !queue_.empty();
   }
 
@@ -82,12 +82,12 @@
   }
 
   absl::Mutex mu_;
-  std::queue<std::function<void()>> queue_ GUARDED_BY(mu_);
+  std::queue<std::function<void()>> queue_ ABSL_GUARDED_BY(mu_);
   std::vector<std::thread> threads_;
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_THREAD_POOL_H_
diff --git a/absl/synchronization/internal/waiter.cc b/absl/synchronization/internal/waiter.cc
index 17c6a50..2949f5a 100644
--- a/absl/synchronization/internal/waiter.cc
+++ b/absl/synchronization/internal/waiter.cc
@@ -49,7 +49,7 @@
 #include "absl/synchronization/internal/kernel_timeout.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 static void MaybeBecomeIdle() {
@@ -123,16 +123,21 @@
   }
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   futex_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   // Loop until we can atomically decrement futex from a positive
   // value, waiting on a futex while we believe it is zero.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
   while (true) {
     int32_t x = futex_.load(std::memory_order_relaxed);
-    if (x != 0) {
+    while (x != 0) {
       if (!futex_.compare_exchange_weak(x, x - 1,
                                         std::memory_order_acquire,
                                         std::memory_order_relaxed)) {
@@ -141,6 +146,8 @@
       return true;  // Consumed a wakeup, we are done.
     }
 
+
+    if (!first_pass) MaybeBecomeIdle();
     const int err = Futex::WaitUntil(&futex_, 0, t);
     if (err != 0) {
       if (err == -EINTR || err == -EWOULDBLOCK) {
@@ -151,14 +158,13 @@
         ABSL_RAW_LOG(FATAL, "Futex operation failed with error %d\n", err);
       }
     }
-
-    MaybeBecomeIdle();
+    first_pass = false;
   }
 }
 
 void Waiter::Post() {
   if (futex_.fetch_add(1, std::memory_order_release) == 0) {
-    // We incremented from 0, need to wake a potential waker.
+    // We incremented from 0, need to wake a potential waiter.
     Poke();
   }
 }
@@ -196,7 +202,7 @@
   pthread_mutex_t *mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   const int err = pthread_mutex_init(&mu_, 0);
   if (err != 0) {
     ABSL_RAW_LOG(FATAL, "pthread_mutex_init failed: %d", err);
@@ -207,8 +213,20 @@
     ABSL_RAW_LOG(FATAL, "pthread_cond_init failed: %d", err2);
   }
 
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
+}
+
+Waiter::~Waiter() {
+  const int err = pthread_mutex_destroy(&mu_);
+  if (err != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_mutex_destroy failed: %d", err);
+  }
+
+  const int err2 = pthread_cond_destroy(&cv_);
+  if (err2 != 0) {
+    ABSL_RAW_LOG(FATAL, "pthread_cond_destroy failed: %d", err2);
+  }
 }
 
 bool Waiter::Wait(KernelTimeout t) {
@@ -218,21 +236,13 @@
   }
 
   PthreadMutexHolder h(&mu_);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  ++waiter_count_;
   // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
     // No wakeups available, time to wait.
     if (!t.has_timeout()) {
       const int err = pthread_cond_wait(&cv_, &mu_);
@@ -242,46 +252,56 @@
     } else {
       const int err = pthread_cond_timedwait(&cv_, &mu_, &abs_timeout);
       if (err == ETIMEDOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        --waiter_count_;
         return false;
       }
       if (err != 0) {
-        ABSL_RAW_LOG(FATAL, "pthread_cond_wait failed: %d", err);
+        ABSL_RAW_LOG(FATAL, "pthread_cond_timedwait failed: %d", err);
       }
     }
-    MaybeBecomeIdle();
+    first_pass = false;
   }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
 }
 
 void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
+  PthreadMutexHolder h(&mu_);
+  ++wakeup_count_;
+  InternalCondVarPoke();
 }
 
 void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
   PthreadMutexHolder h(&mu_);
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  const int err = pthread_cond_signal(&cv_);
-  if (err != 0) {
-    ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+  InternalCondVarPoke();
+}
+
+void Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    const int err = pthread_cond_signal(&cv_);
+    if (ABSL_PREDICT_FALSE(err != 0)) {
+      ABSL_RAW_LOG(FATAL, "pthread_cond_signal failed: %d", err);
+    }
   }
 }
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
 
-void Waiter::Init() {
+Waiter::Waiter() {
   if (sem_init(&sem_, 0, 0) != 0) {
     ABSL_RAW_LOG(FATAL, "sem_init failed with errno %d\n", errno);
   }
   wakeups_.store(0, std::memory_order_relaxed);
 }
 
+Waiter::~Waiter() {
+  if (sem_destroy(&sem_) != 0) {
+    ABSL_RAW_LOG(FATAL, "sem_destroy failed with errno %d\n", errno);
+  }
+}
+
 bool Waiter::Wait(KernelTimeout t) {
   struct timespec abs_timeout;
   if (t.has_timeout()) {
@@ -289,9 +309,12 @@
   }
 
   // Loop until we timeout or consume a wakeup.
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
   while (true) {
     int x = wakeups_.load(std::memory_order_relaxed);
-    if (x != 0) {
+    while (x != 0) {
       if (!wakeups_.compare_exchange_weak(x, x - 1,
                                           std::memory_order_acquire,
                                           std::memory_order_relaxed)) {
@@ -301,6 +324,7 @@
       return true;
     }
 
+    if (!first_pass) MaybeBecomeIdle();
     // Nothing to consume, wait (looping on EINTR).
     while (true) {
       if (!t.has_timeout()) {
@@ -314,13 +338,16 @@
         ABSL_RAW_LOG(FATAL, "sem_timedwait failed: %d", errno);
       }
     }
-    MaybeBecomeIdle();
+    first_pass = false;
   }
 }
 
 void Waiter::Post() {
-  wakeups_.fetch_add(1, std::memory_order_release);  // Post a wakeup.
-  Poke();
+  // Post a wakeup.
+  if (wakeups_.fetch_add(1, std::memory_order_release) == 0) {
+    // We incremented from 0, need to wake a potential waiter.
+    Poke();
+  }
 }
 
 void Waiter::Poke() {
@@ -341,31 +368,29 @@
     return reinterpret_cast<CONDITION_VARIABLE *>(&w->cv_storage_);
   }
 
-  static_assert(sizeof(SRWLOCK) == sizeof(Waiter::SRWLockStorage),
-                "SRWLockStorage does not have the same size as SRWLOCK");
+  static_assert(sizeof(SRWLOCK) == sizeof(void *),
+                "`mu_storage_` does not have the same size as SRWLOCK");
+  static_assert(alignof(SRWLOCK) == alignof(void *),
+                "`mu_storage_` does not have the same alignment as SRWLOCK");
+
+  static_assert(sizeof(CONDITION_VARIABLE) == sizeof(void *),
+                "`ABSL_CONDITION_VARIABLE_STORAGE` does not have the same size "
+                "as `CONDITION_VARIABLE`");
   static_assert(
-      alignof(SRWLOCK) == alignof(Waiter::SRWLockStorage),
-      "SRWLockStorage does not have the same alignment as SRWLOCK");
+      alignof(CONDITION_VARIABLE) == alignof(void *),
+      "`cv_storage_` does not have the same alignment as `CONDITION_VARIABLE`");
 
-  static_assert(sizeof(CONDITION_VARIABLE) ==
-                    sizeof(Waiter::ConditionVariableStorage),
-                "ABSL_CONDITION_VARIABLE_STORAGE does not have the same size "
-                "as CONDITION_VARIABLE");
-  static_assert(alignof(CONDITION_VARIABLE) ==
-                    alignof(Waiter::ConditionVariableStorage),
-                "ConditionVariableStorage does not have the same "
-                "alignment as CONDITION_VARIABLE");
-
-  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constuctible
+  // The SRWLOCK and CONDITION_VARIABLE types must be trivially constructible
   // and destructible because we never call their constructors or destructors.
   static_assert(std::is_trivially_constructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially constructible");
-  static_assert(std::is_trivially_constructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially constructible");
+                "The `SRWLOCK` type must be trivially constructible");
+  static_assert(
+      std::is_trivially_constructible<CONDITION_VARIABLE>::value,
+      "The `CONDITION_VARIABLE` type must be trivially constructible");
   static_assert(std::is_trivially_destructible<SRWLOCK>::value,
-                "The SRWLOCK type must be trivially destructible");
+                "The `SRWLOCK` type must be trivially destructible");
   static_assert(std::is_trivially_destructible<CONDITION_VARIABLE>::value,
-                "The CONDITION_VARIABLE type must be trivially destructible");
+                "The `CONDITION_VARIABLE` type must be trivially destructible");
 };
 
 class LockHolder {
@@ -385,36 +410,33 @@
   SRWLOCK* mu_;
 };
 
-void Waiter::Init() {
+Waiter::Waiter() {
   auto *mu = ::new (static_cast<void *>(&mu_storage_)) SRWLOCK;
   auto *cv = ::new (static_cast<void *>(&cv_storage_)) CONDITION_VARIABLE;
   InitializeSRWLock(mu);
   InitializeConditionVariable(cv);
-  waiter_count_.store(0, std::memory_order_relaxed);
-  wakeup_count_.store(0, std::memory_order_relaxed);
+  waiter_count_ = 0;
+  wakeup_count_ = 0;
 }
 
+// SRW locks and condition variables do not need to be explicitly destroyed.
+// https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-initializesrwlock
+// https://stackoverflow.com/questions/28975958/why-does-windows-have-no-deleteconditionvariable-function-to-go-together-with
+Waiter::~Waiter() = default;
+
 bool Waiter::Wait(KernelTimeout t) {
   SRWLOCK *mu = WinHelper::GetLock(this);
   CONDITION_VARIABLE *cv = WinHelper::GetCond(this);
 
   LockHolder h(mu);
-  waiter_count_.fetch_add(1, std::memory_order_relaxed);
+  ++waiter_count_;
 
   // Loop until we find a wakeup to consume or timeout.
-  while (true) {
-    int x = wakeup_count_.load(std::memory_order_relaxed);
-    if (x != 0) {
-      if (!wakeup_count_.compare_exchange_weak(x, x - 1,
-                                               std::memory_order_acquire,
-                                               std::memory_order_relaxed)) {
-        continue;  // Raced with someone, retry.
-      }
-      // Successfully consumed a wakeup, we're done.
-      waiter_count_.fetch_sub(1, std::memory_order_relaxed);
-      return true;
-    }
-
+  // Note that, since the thread ticker is just reset, we don't need to check
+  // whether the thread is idle on the very first pass of the loop.
+  bool first_pass = true;
+  while (wakeup_count_ == 0) {
+    if (!first_pass) MaybeBecomeIdle();
     // No wakeups available, time to wait.
     if (!SleepConditionVariableSRW(cv, mu, t.InMillisecondsFromNow(), 0)) {
       // GetLastError() returns a Win32 DWORD, but we assign to
@@ -422,32 +444,35 @@
       // initialization guarantees this is not a narrowing conversion.
       const unsigned long err{GetLastError()};  // NOLINT(runtime/int)
       if (err == ERROR_TIMEOUT) {
-        waiter_count_.fetch_sub(1, std::memory_order_relaxed);
+        --waiter_count_;
         return false;
       } else {
         ABSL_RAW_LOG(FATAL, "SleepConditionVariableSRW failed: %lu", err);
       }
     }
-
-    MaybeBecomeIdle();
+    first_pass = false;
   }
+  // Consume a wakeup and we're done.
+  --wakeup_count_;
+  --waiter_count_;
+  return true;
 }
 
 void Waiter::Post() {
-  wakeup_count_.fetch_add(1, std::memory_order_release);
-  Poke();
+  LockHolder h(WinHelper::GetLock(this));
+  ++wakeup_count_;
+  InternalCondVarPoke();
 }
 
 void Waiter::Poke() {
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
-  }
-  // Potentially a waker. Take the lock and check again.
   LockHolder h(WinHelper::GetLock(this));
-  if (waiter_count_.load(std::memory_order_relaxed) == 0) {
-    return;
+  InternalCondVarPoke();
+}
+
+void Waiter::InternalCondVarPoke() {
+  if (waiter_count_ != 0) {
+    WakeConditionVariable(WinHelper::GetCond(this));
   }
-  WakeConditionVariable(WinHelper::GetCond(this));
 }
 
 #else
@@ -455,5 +480,5 @@
 #endif
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/internal/waiter.h b/absl/synchronization/internal/waiter.h
index 0603264..a6e6d4c 100644
--- a/absl/synchronization/internal/waiter.h
+++ b/absl/synchronization/internal/waiter.h
@@ -18,10 +18,16 @@
 
 #include "absl/base/config.h"
 
-#ifndef _WIN32
+#ifdef _WIN32
+#include <sdkddkver.h>
+#else
 #include <pthread.h>
 #endif
 
+#ifdef __linux__
+#include <linux/futex.h>
+#endif
+
 #ifdef ABSL_HAVE_SEMAPHORE_H
 #include <semaphore.h>
 #endif
@@ -40,9 +46,14 @@
 
 #if defined(ABSL_FORCE_WAITER_MODE)
 #define ABSL_WAITER_MODE ABSL_FORCE_WAITER_MODE
-#elif defined(_WIN32)
+#elif defined(_WIN32) && _WIN32_WINNT >= _WIN32_WINNT_VISTA
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_WIN32
-#elif defined(__linux__)
+#elif defined(__BIONIC__)
+// Bionic supports all the futex operations we need even when some of the futex
+// definitions are missing.
+#define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
+#elif defined(__linux__) && defined(FUTEX_CLOCK_REALTIME)
+// FUTEX_CLOCK_REALTIME requires Linux >= 2.6.28.
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_FUTEX
 #elif defined(ABSL_HAVE_SEMAPHORE_H)
 #define ABSL_WAITER_MODE ABSL_WAITER_MODE_SEM
@@ -51,20 +62,21 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace synchronization_internal {
 
 // Waiter is an OS-specific semaphore.
 class Waiter {
  public:
-  // No constructor, instances use the reserved space in ThreadIdentity.
-  // All initialization logic belongs in `Init()`.
-  Waiter() = delete;
+  // Prepare any data to track waits.
+  Waiter();
+
+  // Not copyable or movable
   Waiter(const Waiter&) = delete;
   Waiter& operator=(const Waiter&) = delete;
 
-  // Prepare any data to track waits.
-  void Init();
+  // Destroy any data to track waits.
+  ~Waiter();
 
   // Blocks the calling thread until a matching call to `Post()` or
   // `t` has passed. Returns `true` if woken (`Post()` called),
@@ -105,10 +117,13 @@
   static_assert(sizeof(int32_t) == sizeof(futex_), "Wrong size for futex");
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_CONDVAR
+  // REQUIRES: mu_ must be held.
+  void InternalCondVarPoke();
+
   pthread_mutex_t mu_;
   pthread_cond_t cv_;
-  std::atomic<int> waiter_count_;
-  std::atomic<int> wakeup_count_;  // Unclaimed wakeups, written under lock.
+  int waiter_count_;
+  int wakeup_count_;  // Unclaimed wakeups.
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_SEM
   sem_t sem_;
@@ -118,26 +133,19 @@
   std::atomic<int> wakeups_;
 
 #elif ABSL_WAITER_MODE == ABSL_WAITER_MODE_WIN32
-  // The Windows API has lots of choices for synchronization
-  // primivitives.  We are using SRWLOCK and CONDITION_VARIABLE
-  // because they don't require a destructor to release system
-  // resources.
-  //
-  // However, we can't include Windows.h in our headers, so we use aligned
-  // storage buffers to define the storage.
-  using SRWLockStorage =
-      typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
-  using ConditionVariableStorage =
-      typename std::aligned_storage<sizeof(void*), alignof(void*)>::type;
-
   // WinHelper - Used to define utilities for accessing the lock and
   // condition variable storage once the types are complete.
   class WinHelper;
 
-  SRWLockStorage mu_storage_;
-  ConditionVariableStorage cv_storage_;
-  std::atomic<int> waiter_count_;
-  std::atomic<int> wakeup_count_;
+  // REQUIRES: WinHelper::GetLock(this) must be held.
+  void InternalCondVarPoke();
+
+  // We can't include Windows.h in our headers, so we use aligned charachter
+  // buffers to define the storage of SRWLOCK and CONDITION_VARIABLE.
+  alignas(void*) unsigned char mu_storage_[sizeof(void*)];
+  alignas(void*) unsigned char cv_storage_[sizeof(void*)];
+  int waiter_count_;
+  int wakeup_count_;
 
 #else
   #error Unknown ABSL_WAITER_MODE
@@ -145,7 +153,7 @@
 };
 
 }  // namespace synchronization_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_INTERNAL_WAITER_H_
diff --git a/absl/synchronization/lifetime_test.cc b/absl/synchronization/lifetime_test.cc
index 0279c8f..cc973a3 100644
--- a/absl/synchronization/lifetime_test.cc
+++ b/absl/synchronization/lifetime_test.cc
@@ -122,6 +122,11 @@
   Function fn_;
 };
 
+// These tests require that the compiler correctly supports C++11 constant
+// initialization... but MSVC has a known regression since v19.10:
+// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
+// TODO(epastor): Limit the affected range once MSVC fixes this bug.
+#if defined(__clang__) || !(defined(_MSC_VER) && _MSC_VER > 1900)
 // kConstInit
 // Test early usage.  (Declaration comes first; definitions must appear after
 // the test runner.)
@@ -143,14 +148,15 @@
 // constructors of globals "happen at link time"; memory is pre-initialized,
 // before the constructors of either grab_lock or check_still_locked are run.)
 extern absl::Mutex const_init_sanity_mutex;
-OnConstruction grab_lock([]() NO_THREAD_SAFETY_ANALYSIS {
+OnConstruction grab_lock([]() ABSL_NO_THREAD_SAFETY_ANALYSIS {
   const_init_sanity_mutex.Lock();
 });
 ABSL_CONST_INIT absl::Mutex const_init_sanity_mutex(absl::kConstInit);
-OnConstruction check_still_locked([]() NO_THREAD_SAFETY_ANALYSIS {
+OnConstruction check_still_locked([]() ABSL_NO_THREAD_SAFETY_ANALYSIS {
   const_init_sanity_mutex.AssertHeld();
   const_init_sanity_mutex.Unlock();
 });
+#endif  // defined(__clang__) || !(defined(_MSC_VER) && _MSC_VER > 1900)
 
 // Test shutdown usage.  (Declarations come first; definitions must appear after
 // the test runner.)
diff --git a/absl/synchronization/mutex.cc b/absl/synchronization/mutex.cc
index 07f220f..e0879b0 100644
--- a/absl/synchronization/mutex.cc
+++ b/absl/synchronization/mutex.cc
@@ -71,7 +71,7 @@
 }  // extern "C"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -107,13 +107,16 @@
     sizeof(MutexGlobals) == ABSL_CACHELINE_SIZE,
     "MutexGlobals must occupy an entire cacheline to prevent false sharing");
 
-ABSL_CONST_INIT absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
-    submit_profile_data;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
-    void (*)(const char *msg, const void *obj, int64_t wait_cycles)> mutex_tracer;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
-    void (*)(const char *msg, const void *cv)> cond_var_tracer;
-ABSL_CONST_INIT absl::base_internal::AtomicHook<
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+    absl::base_internal::AtomicHook<void (*)(int64_t wait_cycles)>
+        submit_profile_data;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<void (*)(
+    const char *msg, const void *obj, int64_t wait_cycles)>
+    mutex_tracer;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES
+    absl::base_internal::AtomicHook<void (*)(const char *msg, const void *cv)>
+        cond_var_tracer;
+ABSL_INTERNAL_ATOMIC_HOOK_ATTRIBUTES absl::base_internal::AtomicHook<
     bool (*)(const void *pc, char *out, int out_size)>
     symbolizer(absl::Symbolize);
 
@@ -208,8 +211,8 @@
     absl::base_internal::kLinkerInitialized);
 
 // graph used to detect deadlocks.
-static GraphCycles *deadlock_graph GUARDED_BY(deadlock_graph_mu)
-    PT_GUARDED_BY(deadlock_graph_mu);
+static GraphCycles *deadlock_graph ABSL_GUARDED_BY(deadlock_graph_mu)
+    ABSL_PT_GUARDED_BY(deadlock_graph_mu);
 
 //------------------------------------------------------------------
 // An event mechanism for debugging mutex use.
@@ -280,10 +283,10 @@
 
 static struct SynchEvent {     // this is a trivial hash table for the events
   // struct is freed when refcount reaches 0
-  int refcount GUARDED_BY(synch_event_mu);
+  int refcount ABSL_GUARDED_BY(synch_event_mu);
 
   // buckets have linear, 0-terminated  chains
-  SynchEvent *next GUARDED_BY(synch_event_mu);
+  SynchEvent *next ABSL_GUARDED_BY(synch_event_mu);
 
   // Constant after initialization
   uintptr_t masked_addr;  // object at this address is called "name"
@@ -296,8 +299,8 @@
   bool log;             // logging turned on
 
   // Constant after initialization
-  char name[1];         // actually longer---null-terminated std::string
-} *synch_event[kNSynchEvent] GUARDED_BY(synch_event_mu);
+  char name[1];         // actually longer---NUL-terminated std::string
+} * synch_event[kNSynchEvent] ABSL_GUARDED_BY(synch_event_mu);
 
 // Ensure that the object at "addr" has a SynchEvent struct associated with it,
 // set "bits" in the word there (waiting until lockbit is clear before doing
@@ -1144,7 +1147,7 @@
 }
 
 static GraphId GetGraphIdLocked(Mutex *mu)
-    EXCLUSIVE_LOCKS_REQUIRED(deadlock_graph_mu) {
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(deadlock_graph_mu) {
   if (!deadlock_graph) {  // (re)create the deadlock graph.
     deadlock_graph =
         new (base_internal::LowLevelAlloc::Alloc(sizeof(*deadlock_graph)))
@@ -1153,7 +1156,7 @@
   return deadlock_graph->GetId(mu);
 }
 
-static GraphId GetGraphId(Mutex *mu) LOCKS_EXCLUDED(deadlock_graph_mu) {
+static GraphId GetGraphId(Mutex *mu) ABSL_LOCKS_EXCLUDED(deadlock_graph_mu) {
   deadlock_graph_mu.Lock();
   GraphId id = GetGraphIdLocked(mu);
   deadlock_graph_mu.Unlock();
@@ -2721,5 +2724,5 @@
          a->arg_ == b->arg_ && a->method_ == b->method_;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/mutex.h b/absl/synchronization/mutex.h
index d689009..8c70c4c 100644
--- a/absl/synchronization/mutex.h
+++ b/absl/synchronization/mutex.h
@@ -82,7 +82,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 class Condition;
 struct SynchWaitParams;
@@ -136,7 +136,7 @@
 //
 // See also `MutexLock`, below, for scoped `Mutex` acquisition.
 
-class LOCKABLE Mutex {
+class ABSL_LOCKABLE Mutex {
  public:
   // Creates a `Mutex` that is not held by anyone. This constructor is
   // typically used for Mutexes allocated on the heap or the stack.
@@ -165,27 +165,27 @@
   //
   // Blocks the calling thread, if necessary, until this `Mutex` is free, and
   // then acquires it exclusively. (This lock is also known as a "write lock.")
-  void Lock() EXCLUSIVE_LOCK_FUNCTION();
+  void Lock() ABSL_EXCLUSIVE_LOCK_FUNCTION();
 
   // Mutex::Unlock()
   //
   // Releases this `Mutex` and returns it from the exclusive/write state to the
   // free state. Caller must hold the `Mutex` exclusively.
-  void Unlock() UNLOCK_FUNCTION();
+  void Unlock() ABSL_UNLOCK_FUNCTION();
 
   // Mutex::TryLock()
   //
   // If the mutex can be acquired without blocking, does so exclusively and
   // returns `true`. Otherwise, returns `false`. Returns `true` with high
   // probability if the `Mutex` was free.
-  bool TryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true);
+  bool TryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true);
 
   // Mutex::AssertHeld()
   //
   // Return immediately if this thread holds the `Mutex` exclusively (in write
   // mode). Otherwise, may report an error (typically by crashing with a
   // diagnostic), or may return immediately.
-  void AssertHeld() const ASSERT_EXCLUSIVE_LOCK();
+  void AssertHeld() const ABSL_ASSERT_EXCLUSIVE_LOCK();
 
   // ---------------------------------------------------------------------------
   // Reader-Writer Locking
@@ -226,28 +226,28 @@
   // `ReaderLock()` will block if some other thread has an exclusive/writer lock
   // on the mutex.
 
-  void ReaderLock() SHARED_LOCK_FUNCTION();
+  void ReaderLock() ABSL_SHARED_LOCK_FUNCTION();
 
   // Mutex::ReaderUnlock()
   //
   // Releases a read share of this `Mutex`. `ReaderUnlock` may return a mutex to
   // the free state if this thread holds the last reader lock on the mutex. Note
   // that you cannot call `ReaderUnlock()` on a mutex held in write mode.
-  void ReaderUnlock() UNLOCK_FUNCTION();
+  void ReaderUnlock() ABSL_UNLOCK_FUNCTION();
 
   // Mutex::ReaderTryLock()
   //
   // If the mutex can be acquired without blocking, acquires this mutex for
   // shared access and returns `true`. Otherwise, returns `false`. Returns
   // `true` with high probability if the `Mutex` was free or shared.
-  bool ReaderTryLock() SHARED_TRYLOCK_FUNCTION(true);
+  bool ReaderTryLock() ABSL_SHARED_TRYLOCK_FUNCTION(true);
 
   // Mutex::AssertReaderHeld()
   //
   // Returns immediately if this thread holds the `Mutex` in at least shared
   // mode (read mode). Otherwise, may report an error (typically by
   // crashing with a diagnostic), or may return immediately.
-  void AssertReaderHeld() const ASSERT_SHARED_LOCK();
+  void AssertReaderHeld() const ABSL_ASSERT_SHARED_LOCK();
 
   // Mutex::WriterLock()
   // Mutex::WriterUnlock()
@@ -258,11 +258,11 @@
   // These methods may be used (along with the complementary `Reader*()`
   // methods) to distingish simple exclusive `Mutex` usage (`Lock()`,
   // etc.) from reader/writer lock usage.
-  void WriterLock() EXCLUSIVE_LOCK_FUNCTION() { this->Lock(); }
+  void WriterLock() ABSL_EXCLUSIVE_LOCK_FUNCTION() { this->Lock(); }
 
-  void WriterUnlock() UNLOCK_FUNCTION() { this->Unlock(); }
+  void WriterUnlock() ABSL_UNLOCK_FUNCTION() { this->Unlock(); }
 
-  bool WriterTryLock() EXCLUSIVE_TRYLOCK_FUNCTION(true) {
+  bool WriterTryLock() ABSL_EXCLUSIVE_TRYLOCK_FUNCTION(true) {
     return this->TryLock();
   }
 
@@ -316,11 +316,11 @@
   // be acquired, then atomically acquires this `Mutex`. `LockWhen()` is
   // logically equivalent to `*Lock(); Await();` though they may have different
   // performance characteristics.
-  void LockWhen(const Condition &cond) EXCLUSIVE_LOCK_FUNCTION();
+  void LockWhen(const Condition &cond) ABSL_EXCLUSIVE_LOCK_FUNCTION();
 
-  void ReaderLockWhen(const Condition &cond) SHARED_LOCK_FUNCTION();
+  void ReaderLockWhen(const Condition &cond) ABSL_SHARED_LOCK_FUNCTION();
 
-  void WriterLockWhen(const Condition &cond) EXCLUSIVE_LOCK_FUNCTION() {
+  void WriterLockWhen(const Condition &cond) ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     this->LockWhen(cond);
   }
 
@@ -362,11 +362,11 @@
   //
   // Negative timeouts are equivalent to a zero timeout.
   bool LockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
-      EXCLUSIVE_LOCK_FUNCTION();
+      ABSL_EXCLUSIVE_LOCK_FUNCTION();
   bool ReaderLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
-      SHARED_LOCK_FUNCTION();
+      ABSL_SHARED_LOCK_FUNCTION();
   bool WriterLockWhenWithTimeout(const Condition &cond, absl::Duration timeout)
-      EXCLUSIVE_LOCK_FUNCTION() {
+      ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     return this->LockWhenWithTimeout(cond, timeout);
   }
 
@@ -382,11 +382,11 @@
   //
   // Deadlines in the past are equivalent to an immediate deadline.
   bool LockWhenWithDeadline(const Condition &cond, absl::Time deadline)
-      EXCLUSIVE_LOCK_FUNCTION();
+      ABSL_EXCLUSIVE_LOCK_FUNCTION();
   bool ReaderLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
-      SHARED_LOCK_FUNCTION();
+      ABSL_SHARED_LOCK_FUNCTION();
   bool WriterLockWhenWithDeadline(const Condition &cond, absl::Time deadline)
-      EXCLUSIVE_LOCK_FUNCTION() {
+      ABSL_EXCLUSIVE_LOCK_FUNCTION() {
     return this->LockWhenWithDeadline(cond, deadline);
   }
 
@@ -536,9 +536,9 @@
 // private:
 //   Mutex lock_;
 // };
-class SCOPED_LOCKABLE MutexLock {
+class ABSL_SCOPED_LOCKABLE MutexLock {
  public:
-  explicit MutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {
+  explicit MutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu) : mu_(mu) {
     this->mu_->Lock();
   }
 
@@ -547,7 +547,7 @@
   MutexLock& operator=(const MutexLock&) = delete;
   MutexLock& operator=(MutexLock&&) = delete;
 
-  ~MutexLock() UNLOCK_FUNCTION() { this->mu_->Unlock(); }
+  ~MutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->Unlock(); }
 
  private:
   Mutex *const mu_;
@@ -557,10 +557,9 @@
 //
 // The `ReaderMutexLock` is a helper class, like `MutexLock`, which acquires and
 // releases a shared lock on a `Mutex` via RAII.
-class SCOPED_LOCKABLE ReaderMutexLock {
+class ABSL_SCOPED_LOCKABLE ReaderMutexLock {
  public:
-  explicit ReaderMutexLock(Mutex *mu) SHARED_LOCK_FUNCTION(mu)
-      :  mu_(mu) {
+  explicit ReaderMutexLock(Mutex *mu) ABSL_SHARED_LOCK_FUNCTION(mu) : mu_(mu) {
     mu->ReaderLock();
   }
 
@@ -569,9 +568,7 @@
   ReaderMutexLock& operator=(const ReaderMutexLock&) = delete;
   ReaderMutexLock& operator=(ReaderMutexLock&&) = delete;
 
-  ~ReaderMutexLock() UNLOCK_FUNCTION() {
-    this->mu_->ReaderUnlock();
-  }
+  ~ReaderMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->ReaderUnlock(); }
 
  private:
   Mutex *const mu_;
@@ -581,9 +578,9 @@
 //
 // The `WriterMutexLock` is a helper class, like `MutexLock`, which acquires and
 // releases a write (exclusive) lock on a `Mutex` via RAII.
-class SCOPED_LOCKABLE WriterMutexLock {
+class ABSL_SCOPED_LOCKABLE WriterMutexLock {
  public:
-  explicit WriterMutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
+  explicit WriterMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
       : mu_(mu) {
     mu->WriterLock();
   }
@@ -593,9 +590,7 @@
   WriterMutexLock& operator=(const WriterMutexLock&) = delete;
   WriterMutexLock& operator=(WriterMutexLock&&) = delete;
 
-  ~WriterMutexLock() UNLOCK_FUNCTION() {
-    this->mu_->WriterUnlock();
-  }
+  ~WriterMutexLock() ABSL_UNLOCK_FUNCTION() { this->mu_->WriterUnlock(); }
 
  private:
   Mutex *const mu_;
@@ -634,7 +629,7 @@
 // Example:
 //
 //   // assume count_ is not internal reference count
-//   int count_ GUARDED_BY(mu_);
+//   int count_ ABSL_GUARDED_BY(mu_);
 //
 //   mu_.LockWhen(Condition(+[](int* count) { return *count == 0; },
 //         &count_));
@@ -861,13 +856,18 @@
 // MutexLockMaybe
 //
 // MutexLockMaybe is like MutexLock, but is a no-op when mu is null.
-class SCOPED_LOCKABLE MutexLockMaybe {
+class ABSL_SCOPED_LOCKABLE MutexLockMaybe {
  public:
-  explicit MutexLockMaybe(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
-      : mu_(mu) { if (this->mu_ != nullptr) { this->mu_->Lock(); } }
-  ~MutexLockMaybe() UNLOCK_FUNCTION() {
+  explicit MutexLockMaybe(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
+      : mu_(mu) {
+    if (this->mu_ != nullptr) {
+      this->mu_->Lock();
+    }
+  }
+  ~MutexLockMaybe() ABSL_UNLOCK_FUNCTION() {
     if (this->mu_ != nullptr) { this->mu_->Unlock(); }
   }
+
  private:
   Mutex *const mu_;
   MutexLockMaybe(const MutexLockMaybe&) = delete;
@@ -880,17 +880,17 @@
 //
 // ReleasableMutexLock is like MutexLock, but permits `Release()` of its
 // mutex before destruction. `Release()` may be called at most once.
-class SCOPED_LOCKABLE ReleasableMutexLock {
+class ABSL_SCOPED_LOCKABLE ReleasableMutexLock {
  public:
-  explicit ReleasableMutexLock(Mutex *mu) EXCLUSIVE_LOCK_FUNCTION(mu)
+  explicit ReleasableMutexLock(Mutex *mu) ABSL_EXCLUSIVE_LOCK_FUNCTION(mu)
       : mu_(mu) {
     this->mu_->Lock();
   }
-  ~ReleasableMutexLock() UNLOCK_FUNCTION() {
+  ~ReleasableMutexLock() ABSL_UNLOCK_FUNCTION() {
     if (this->mu_ != nullptr) { this->mu_->Unlock(); }
   }
 
-  void Release() UNLOCK_FUNCTION();
+  void Release() ABSL_UNLOCK_FUNCTION();
 
  private:
   Mutex *mu_;
@@ -1001,7 +1001,7 @@
 //
 // 'pc' is the program counter being symbolized, 'out' is the buffer to write
 // into, and 'out_size' is the size of the buffer.  This function can return
-// false if symbolizing failed, or true if a null-terminated symbol was written
+// false if symbolizing failed, or true if a NUL-terminated symbol was written
 // to 'out.'
 //
 // This has the same memory ordering concerns as RegisterMutexProfiler() above.
@@ -1040,7 +1040,7 @@
 // the manner chosen here.
 void SetMutexDeadlockDetectionMode(OnDeadlockCycle mode);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // In some build configurations we pass --detect-odr-violations to the
diff --git a/absl/synchronization/mutex_test.cc b/absl/synchronization/mutex_test.cc
index 9851ac1..afb363a 100644
--- a/absl/synchronization/mutex_test.cc
+++ b/absl/synchronization/mutex_test.cc
@@ -425,10 +425,10 @@
   // Use a struct so the lock annotations apply.
   struct {
     absl::Mutex barrier_mu;
-    bool barrier GUARDED_BY(barrier_mu) = false;
+    bool barrier ABSL_GUARDED_BY(barrier_mu) = false;
 
     absl::Mutex release_mu;
-    bool release GUARDED_BY(release_mu) = false;
+    bool release ABSL_GUARDED_BY(release_mu) = false;
     absl::CondVar released_cv;
   } state;
 
@@ -466,10 +466,10 @@
   // Use a struct so the lock annotations apply.
   struct {
     absl::Mutex barrier_mu;
-    bool barrier GUARDED_BY(barrier_mu) = false;
+    bool barrier ABSL_GUARDED_BY(barrier_mu) = false;
 
     absl::Mutex release_mu;
-    bool release GUARDED_BY(release_mu) = false;
+    bool release ABSL_GUARDED_BY(release_mu) = false;
     absl::CondVar released_cv;
   } state;
 
@@ -770,7 +770,7 @@
 
 // Test for reader counter being decremented incorrectly by waiter
 // with false condition.
-TEST(Mutex, MutexReaderDecrementBug) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, MutexReaderDecrementBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   ReaderDecrementBugStruct x;
   x.cond = false;
   x.waiting_on_cond = false;
@@ -819,7 +819,7 @@
 // TSAN reports errors when locked Mutexes are destroyed.
 TEST(Mutex, DISABLED_LockedMutexDestructionBug) NO_THREAD_SAFETY_ANALYSIS {
 #else
-TEST(Mutex, LockedMutexDestructionBug) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, LockedMutexDestructionBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #endif
   for (int i = 0; i != 10; i++) {
     // Create, lock and destroy 10 locks.
@@ -1101,7 +1101,7 @@
 // annotation-based static thread-safety analysis is not currently
 // predicate-aware and cannot tell if the two for-loops that acquire and
 // release the locks have the same predicates.
-TEST(Mutex, DeadlockDetectorStessTest) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, DeadlockDetectorStessTest) ABSL_NO_THREAD_SAFETY_ANALYSIS {
   // Stress test: Here we create a large number of locks and use all of them.
   // If a deadlock detector keeps a full graph of lock acquisition order,
   // it will likely be too slow for this test to pass.
@@ -1123,7 +1123,7 @@
 // TSAN reports errors when locked Mutexes are destroyed.
 TEST(Mutex, DISABLED_DeadlockIdBug) NO_THREAD_SAFETY_ANALYSIS {
 #else
-TEST(Mutex, DeadlockIdBug) NO_THREAD_SAFETY_ANALYSIS {
+TEST(Mutex, DeadlockIdBug) ABSL_NO_THREAD_SAFETY_ANALYSIS {
 #endif
   // Test a scenario where a cached deadlock graph node id in the
   // list of held locks is not invalidated when the corresponding
diff --git a/absl/synchronization/notification.cc b/absl/synchronization/notification.cc
index d691cfc..e91b903 100644
--- a/absl/synchronization/notification.cc
+++ b/absl/synchronization/notification.cc
@@ -22,7 +22,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 void Notification::Notify() {
   MutexLock l(&this->mutex_);
@@ -45,15 +45,6 @@
   MutexLock l(&this->mutex_);
 }
 
-static inline bool HasBeenNotifiedInternal(
-    const std::atomic<bool> *notified_yet) {
-  return notified_yet->load(std::memory_order_acquire);
-}
-
-bool Notification::HasBeenNotified() const {
-  return HasBeenNotifiedInternal(&this->notified_yet_);
-}
-
 void Notification::WaitForNotification() const {
   if (!HasBeenNotifiedInternal(&this->notified_yet_)) {
     this->mutex_.LockWhen(Condition(&HasBeenNotifiedInternal,
@@ -83,5 +74,5 @@
   return notified;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/synchronization/notification.h b/absl/synchronization/notification.h
index 8ed7f12..9a354ca 100644
--- a/absl/synchronization/notification.h
+++ b/absl/synchronization/notification.h
@@ -57,7 +57,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // Notification
@@ -74,7 +74,9 @@
   // Notification::HasBeenNotified()
   //
   // Returns the value of the notification's internal "notified" state.
-  bool HasBeenNotified() const;
+  bool HasBeenNotified() const {
+    return HasBeenNotifiedInternal(&this->notified_yet_);
+  }
 
   // Notification::WaitForNotification()
   //
@@ -106,11 +108,16 @@
   void Notify();
 
  private:
+  static inline bool HasBeenNotifiedInternal(
+      const std::atomic<bool>* notified_yet) {
+    return notified_yet->load(std::memory_order_acquire);
+  }
+
   mutable Mutex mutex_;
   std::atomic<bool> notified_yet_;  // written under mutex_
 };
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_SYNCHRONIZATION_NOTIFICATION_H_
diff --git a/absl/synchronization/notification_test.cc b/absl/synchronization/notification_test.cc
index a64674c..100ea76 100644
--- a/absl/synchronization/notification_test.cc
+++ b/absl/synchronization/notification_test.cc
@@ -21,7 +21,7 @@
 #include "absl/synchronization/mutex.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // A thread-safe class that holds a counter.
 class ThreadSafeCounter {
@@ -129,5 +129,5 @@
   BasicTests(true, &local_notification2);
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/BUILD.bazel b/absl/time/BUILD.bazel
index 55e83a8..9ab2adb 100644
--- a/absl/time/BUILD.bazel
+++ b/absl/time/BUILD.bazel
@@ -14,6 +14,7 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -46,6 +47,7 @@
     deps = [
         "//absl/base",
         "//absl/base:core_headers",
+        "//absl/base:raw_logging_internal",
         "//absl/numeric:int128",
         "//absl/strings",
         "//absl/time/internal/cctz:civil_time",
@@ -68,7 +70,7 @@
     ],
     deps = [
         ":time",
-        "//absl/base",
+        "//absl/base:raw_logging_internal",
         "//absl/time/internal/cctz:time_zone",
         "@com_google_googletest//:gtest",
     ],
@@ -89,9 +91,9 @@
     deps = [
         ":test_util",
         ":time",
-        "//absl/base",
         "//absl/base:config",
         "//absl/base:core_headers",
+        "//absl/numeric:int128",
         "//absl/time/internal/cctz:time_zone",
         "@com_google_googletest//:gtest_main",
     ],
diff --git a/absl/time/CMakeLists.txt b/absl/time/CMakeLists.txt
index 5909832..853563e 100644
--- a/absl/time/CMakeLists.txt
+++ b/absl/time/CMakeLists.txt
@@ -22,21 +22,22 @@
     "clock.h"
     "time.h"
   SRCS
-  "civil_time.cc"
-  "clock.cc"
-  "duration.cc"
-  "format.cc"
-  "internal/get_current_time_chrono.inc"
-  "internal/get_current_time_posix.inc"
-  "time.cc"
+    "civil_time.cc"
+    "clock.cc"
+    "duration.cc"
+    "format.cc"
+    "internal/get_current_time_chrono.inc"
+    "internal/get_current_time_posix.inc"
+    "time.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::base
+    absl::civil_time
     absl::core_headers
     absl::int128
+    absl::raw_logging_internal
     absl::strings
-    absl::civil_time
     absl::time_zone
   PUBLIC
 )
@@ -88,7 +89,7 @@
 
 absl_cc_library(
   NAME
-    test_util
+    time_internal_test_util
   HDRS
     "internal/test_util.h"
   SRCS
@@ -98,7 +99,7 @@
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::time
-    absl::base
+    absl::raw_logging_internal
     absl::time_zone
     gmock
   TESTONLY
@@ -117,9 +118,8 @@
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
-    absl::test_util
+    absl::time_internal_test_util
     absl::time
-    absl::base
     absl::config
     absl::core_headers
     absl::time_zone
diff --git a/absl/time/civil_time.cc b/absl/time/civil_time.cc
index bf7ba5a..ada82cb 100644
--- a/absl/time/civil_time.cc
+++ b/absl/time/civil_time.cc
@@ -21,7 +21,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -43,6 +43,58 @@
                 FormatTime(std::string(fmt), FromCivil(ncs, utc), utc));
 }
 
+template <typename CivilT>
+bool ParseYearAnd(string_view fmt, string_view s, CivilT* c) {
+  // Civil times support a larger year range than absl::Time, so we need to
+  // parse the year separately, normalize it, then use absl::ParseTime on the
+  // normalized std::string.
+  const std::string ss = std::string(s);  // TODO(absl-team): Avoid conversion.
+  const char* const np = ss.c_str();
+  char* endp;
+  errno = 0;
+  const civil_year_t y =
+      std::strtoll(np, &endp, 10);  // NOLINT(runtime/deprecated_fn)
+  if (endp == np || errno == ERANGE) return false;
+  const std::string norm = StrCat(NormalizeYear(y), endp);
+
+  const TimeZone utc = UTCTimeZone();
+  Time t;
+  if (ParseTime(StrCat("%Y", fmt), norm, utc, &t, nullptr)) {
+    const auto cs = ToCivilSecond(t, utc);
+    *c = CivilT(y, cs.month(), cs.day(), cs.hour(), cs.minute(), cs.second());
+    return true;
+  }
+
+  return false;
+}
+
+// Tries to parse the type as a CivilT1, but then assigns the result to the
+// argument of type CivilT2.
+template <typename CivilT1, typename CivilT2>
+bool ParseAs(string_view s, CivilT2* c) {
+  CivilT1 t1;
+  if (ParseCivilTime(s, &t1)) {
+    *c = CivilT2(t1);
+    return true;
+  }
+  return false;
+}
+
+template <typename CivilT>
+bool ParseLenient(string_view s, CivilT* c) {
+  // A fastpath for when the given std::string data parses exactly into the given
+  // type T (e.g., s="YYYY-MM-DD" and CivilT=CivilDay).
+  if (ParseCivilTime(s, c)) return true;
+  // Try parsing as each of the 6 types, trying the most common types first
+  // (based on csearch results).
+  if (ParseAs<CivilDay>(s, c)) return true;
+  if (ParseAs<CivilSecond>(s, c)) return true;
+  if (ParseAs<CivilHour>(s, c)) return true;
+  if (ParseAs<CivilMonth>(s, c)) return true;
+  if (ParseAs<CivilMinute>(s, c)) return true;
+  if (ParseAs<CivilYear>(s, c)) return true;
+  return false;
+}
 }  // namespace
 
 std::string FormatCivilTime(CivilSecond c) {
@@ -58,6 +110,44 @@
 std::string FormatCivilTime(CivilMonth c) { return FormatYearAnd("-%m", c); }
 std::string FormatCivilTime(CivilYear c) { return FormatYearAnd("", c); }
 
+bool ParseCivilTime(string_view s, CivilSecond* c) {
+  return ParseYearAnd("-%m-%dT%H:%M:%S", s, c);
+}
+bool ParseCivilTime(string_view s, CivilMinute* c) {
+  return ParseYearAnd("-%m-%dT%H:%M", s, c);
+}
+bool ParseCivilTime(string_view s, CivilHour* c) {
+  return ParseYearAnd("-%m-%dT%H", s, c);
+}
+bool ParseCivilTime(string_view s, CivilDay* c) {
+  return ParseYearAnd("-%m-%d", s, c);
+}
+bool ParseCivilTime(string_view s, CivilMonth* c) {
+  return ParseYearAnd("-%m", s, c);
+}
+bool ParseCivilTime(string_view s, CivilYear* c) {
+  return ParseYearAnd("", s, c);
+}
+
+bool ParseLenientCivilTime(string_view s, CivilSecond* c) {
+  return ParseLenient(s, c);
+}
+bool ParseLenientCivilTime(string_view s, CivilMinute* c) {
+  return ParseLenient(s, c);
+}
+bool ParseLenientCivilTime(string_view s, CivilHour* c) {
+  return ParseLenient(s, c);
+}
+bool ParseLenientCivilTime(string_view s, CivilDay* c) {
+  return ParseLenient(s, c);
+}
+bool ParseLenientCivilTime(string_view s, CivilMonth* c) {
+  return ParseLenient(s, c);
+}
+bool ParseLenientCivilTime(string_view s, CivilYear* c) {
+  return ParseLenient(s, c);
+}
+
 namespace time_internal {
 
 std::ostream& operator<<(std::ostream& os, CivilYear y) {
@@ -81,5 +171,5 @@
 
 }  // namespace time_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/civil_time.h b/absl/time/civil_time.h
index f0be303..bb46004 100644
--- a/absl/time/civil_time.h
+++ b/absl/time/civil_time.h
@@ -76,7 +76,7 @@
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace time_internal {
 struct second_tag : cctz::detail::second_tag {};
@@ -248,7 +248,7 @@
 // int          minute()
 // int          second()
 //
-// Recall that fields inferior to the type's aligment will be set to their
+// Recall that fields inferior to the type's alignment will be set to their
 // minimum valid value.
 //
 // Example:
@@ -460,6 +460,57 @@
 std::string FormatCivilTime(CivilMonth c);
 std::string FormatCivilTime(CivilYear c);
 
+// absl::ParseCivilTime()
+//
+// Parses a civil-time value from the specified `absl::string_view` into the
+// passed output parameter. Returns `true` upon successful parsing.
+//
+// The expected form of the input string is as follows:
+//
+//  Type        | Format
+//  ---------------------------------
+//  CivilSecond | YYYY-MM-DDTHH:MM:SS
+//  CivilMinute | YYYY-MM-DDTHH:MM
+//  CivilHour   | YYYY-MM-DDTHH
+//  CivilDay    | YYYY-MM-DD
+//  CivilMonth  | YYYY-MM
+//  CivilYear   | YYYY
+//
+// Example:
+//
+//   absl::CivilDay d;
+//   bool ok = absl::ParseCivilTime("2018-01-02", &d); // OK
+//
+// Note that parsing will fail if the string's format does not match the
+// expected type exactly. `ParseLenientCivilTime()` below is more lenient.
+//
+bool ParseCivilTime(absl::string_view s, CivilSecond* c);
+bool ParseCivilTime(absl::string_view s, CivilMinute* c);
+bool ParseCivilTime(absl::string_view s, CivilHour* c);
+bool ParseCivilTime(absl::string_view s, CivilDay* c);
+bool ParseCivilTime(absl::string_view s, CivilMonth* c);
+bool ParseCivilTime(absl::string_view s, CivilYear* c);
+
+// ParseLenientCivilTime()
+//
+// Parses any of the formats accepted by `absl::ParseCivilTime()`, but is more
+// lenient if the format of the string does not exactly match the associated
+// type.
+//
+// Example:
+//
+//   absl::CivilDay d;
+//   bool ok = absl::ParseLenientCivilTime("1969-07-20", &d); // OK
+//   ok = absl::ParseLenientCivilTime("1969-07-20T10", &d);   // OK: T10 floored
+//   ok = absl::ParseLenientCivilTime("1969-07", &d);   // OK: day defaults to 1
+//
+bool ParseLenientCivilTime(absl::string_view s, CivilSecond* c);
+bool ParseLenientCivilTime(absl::string_view s, CivilMinute* c);
+bool ParseLenientCivilTime(absl::string_view s, CivilHour* c);
+bool ParseLenientCivilTime(absl::string_view s, CivilDay* c);
+bool ParseLenientCivilTime(absl::string_view s, CivilMonth* c);
+bool ParseLenientCivilTime(absl::string_view s, CivilYear* c);
+
 namespace time_internal {  // For functions found via ADL on civil-time tags.
 
 // Streaming Operators
@@ -469,7 +520,7 @@
 //
 // Example:
 //
-//   absl::CivilDay d = absl::CivilDay("1969-07-20");
+//   absl::CivilDay d = absl::CivilDay(1969, 7, 20);
 //   std::cout << "Date is: " << d << "\n";
 //
 std::ostream& operator<<(std::ostream& os, CivilYear y);
@@ -481,7 +532,7 @@
 
 }  // namespace time_internal
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_CIVIL_TIME_H_
diff --git a/absl/time/civil_time_benchmark.cc b/absl/time/civil_time_benchmark.cc
index 4086983..f04dbe2 100644
--- a/absl/time/civil_time_benchmark.cc
+++ b/absl/time/civil_time_benchmark.cc
@@ -66,6 +66,26 @@
 }
 BENCHMARK(BM_Format);
 
+void BM_Parse(benchmark::State& state) {
+  const std::string f = "2014-01-02T03:04:05";
+  absl::CivilSecond c;
+  while (state.KeepRunning()) {
+    const bool b = absl::ParseCivilTime(f, &c);
+    benchmark::DoNotOptimize(b);
+  }
+}
+BENCHMARK(BM_Parse);
+
+void BM_RoundTripFormatParse(benchmark::State& state) {
+  const absl::CivilSecond c(2014, 1, 2, 3, 4, 5);
+  absl::CivilSecond out;
+  while (state.KeepRunning()) {
+    const bool b = absl::ParseCivilTime(absl::FormatCivilTime(c), &out);
+    benchmark::DoNotOptimize(b);
+  }
+}
+BENCHMARK(BM_RoundTripFormatParse);
+
 template <typename T>
 void BM_CivilTimeAbslHash(benchmark::State& state) {
   const int kSize = 100000;
diff --git a/absl/time/civil_time_test.cc b/absl/time/civil_time_test.cc
index 03cd1f1..0ebd97a 100644
--- a/absl/time/civil_time_test.cc
+++ b/absl/time/civil_time_test.cc
@@ -690,6 +690,69 @@
   EXPECT_EQ("1970", absl::FormatCivilTime(y));
 }
 
+TEST(CivilTime, Parse) {
+  absl::CivilSecond ss;
+  absl::CivilMinute mm;
+  absl::CivilHour hh;
+  absl::CivilDay d;
+  absl::CivilMonth m;
+  absl::CivilYear y;
+
+  // CivilSecond OK; others fail
+  EXPECT_TRUE(absl::ParseCivilTime("2015-01-02T03:04:05", &ss));
+  EXPECT_EQ("2015-01-02T03:04:05", absl::FormatCivilTime(ss));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04:05", &mm));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04:05", &hh));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04:05", &d));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04:05", &m));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04:05", &y));
+
+  // CivilMinute OK; others fail
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04", &ss));
+  EXPECT_TRUE(absl::ParseCivilTime("2015-01-02T03:04", &mm));
+  EXPECT_EQ("2015-01-02T03:04", absl::FormatCivilTime(mm));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04", &hh));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04", &d));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04", &m));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03:04", &y));
+
+  // CivilHour OK; others fail
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03", &ss));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03", &mm));
+  EXPECT_TRUE(absl::ParseCivilTime("2015-01-02T03", &hh));
+  EXPECT_EQ("2015-01-02T03", absl::FormatCivilTime(hh));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03", &d));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03", &m));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02T03", &y));
+
+  // CivilDay OK; others fail
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02", &ss));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02", &mm));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02", &hh));
+  EXPECT_TRUE(absl::ParseCivilTime("2015-01-02", &d));
+  EXPECT_EQ("2015-01-02", absl::FormatCivilTime(d));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02", &m));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01-02", &y));
+
+  // CivilMonth OK; others fail
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01", &ss));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01", &mm));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01", &hh));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01", &d));
+  EXPECT_TRUE(absl::ParseCivilTime("2015-01", &m));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(m));
+  EXPECT_FALSE(absl::ParseCivilTime("2015-01", &y));
+
+  // CivilYear OK; others fail
+  EXPECT_FALSE(absl::ParseCivilTime("2015", &ss));
+  EXPECT_FALSE(absl::ParseCivilTime("2015", &mm));
+  EXPECT_FALSE(absl::ParseCivilTime("2015", &hh));
+  EXPECT_FALSE(absl::ParseCivilTime("2015", &d));
+  EXPECT_FALSE(absl::ParseCivilTime("2015", &m));
+  EXPECT_TRUE(absl::ParseCivilTime("2015", &y));
+  EXPECT_EQ("2015", absl::FormatCivilTime(y));
+}
+
 TEST(CivilTime, FormatAndParseLenient) {
   absl::CivilSecond ss;
   EXPECT_EQ("1970-01-01T00:00:00", absl::FormatCivilTime(ss));
@@ -708,6 +771,101 @@
 
   absl::CivilYear y;
   EXPECT_EQ("1970", absl::FormatCivilTime(y));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &ss));
+  EXPECT_EQ("2015-01-02T03:04:05", absl::FormatCivilTime(ss));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &mm));
+  EXPECT_EQ("2015-01-02T03:04", absl::FormatCivilTime(mm));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &hh));
+  EXPECT_EQ("2015-01-02T03", absl::FormatCivilTime(hh));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &d));
+  EXPECT_EQ("2015-01-02", absl::FormatCivilTime(d));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &m));
+  EXPECT_EQ("2015-01", absl::FormatCivilTime(m));
+
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-01-02T03:04:05", &y));
+  EXPECT_EQ("2015", absl::FormatCivilTime(y));
+}
+
+TEST(CivilTime, ParseEdgeCases) {
+  absl::CivilSecond ss;
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("9223372036854775807-12-31T23:59:59", &ss));
+  EXPECT_EQ("9223372036854775807-12-31T23:59:59", absl::FormatCivilTime(ss));
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("-9223372036854775808-01-01T00:00:00", &ss));
+  EXPECT_EQ("-9223372036854775808-01-01T00:00:00", absl::FormatCivilTime(ss));
+
+  absl::CivilMinute mm;
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("9223372036854775807-12-31T23:59", &mm));
+  EXPECT_EQ("9223372036854775807-12-31T23:59", absl::FormatCivilTime(mm));
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("-9223372036854775808-01-01T00:00", &mm));
+  EXPECT_EQ("-9223372036854775808-01-01T00:00", absl::FormatCivilTime(mm));
+
+  absl::CivilHour hh;
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("9223372036854775807-12-31T23", &hh));
+  EXPECT_EQ("9223372036854775807-12-31T23", absl::FormatCivilTime(hh));
+  EXPECT_TRUE(
+      absl::ParseLenientCivilTime("-9223372036854775808-01-01T00", &hh));
+  EXPECT_EQ("-9223372036854775808-01-01T00", absl::FormatCivilTime(hh));
+
+  absl::CivilDay d;
+  EXPECT_TRUE(absl::ParseLenientCivilTime("9223372036854775807-12-31", &d));
+  EXPECT_EQ("9223372036854775807-12-31", absl::FormatCivilTime(d));
+  EXPECT_TRUE(absl::ParseLenientCivilTime("-9223372036854775808-01-01", &d));
+  EXPECT_EQ("-9223372036854775808-01-01", absl::FormatCivilTime(d));
+
+  absl::CivilMonth m;
+  EXPECT_TRUE(absl::ParseLenientCivilTime("9223372036854775807-12", &m));
+  EXPECT_EQ("9223372036854775807-12", absl::FormatCivilTime(m));
+  EXPECT_TRUE(absl::ParseLenientCivilTime("-9223372036854775808-01", &m));
+  EXPECT_EQ("-9223372036854775808-01", absl::FormatCivilTime(m));
+
+  absl::CivilYear y;
+  EXPECT_TRUE(absl::ParseLenientCivilTime("9223372036854775807", &y));
+  EXPECT_EQ("9223372036854775807", absl::FormatCivilTime(y));
+  EXPECT_TRUE(absl::ParseLenientCivilTime("-9223372036854775808", &y));
+  EXPECT_EQ("-9223372036854775808", absl::FormatCivilTime(y));
+
+  // Tests some valid, but interesting, cases
+  EXPECT_TRUE(absl::ParseLenientCivilTime("0", &ss)) << ss;
+  EXPECT_EQ(absl::CivilYear(0), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime("0-1", &ss)) << ss;
+  EXPECT_EQ(absl::CivilMonth(0, 1), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime(" 2015 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilYear(2015), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime(" 2015-6 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilMonth(2015, 6), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-6-7", &ss)) << ss;
+  EXPECT_EQ(absl::CivilDay(2015, 6, 7), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime(" 2015-6-7 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilDay(2015, 6, 7), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime("2015-06-07T10:11:12 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2015, 6, 7, 10, 11, 12), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime(" 2015-06-07T10:11:12 ", &ss)) << ss;
+  EXPECT_EQ(absl::CivilSecond(2015, 6, 7, 10, 11, 12), ss);
+  EXPECT_TRUE(absl::ParseLenientCivilTime("-01-01", &ss)) << ss;
+  EXPECT_EQ(absl::CivilMonth(-1, 1), ss);
+
+  // Tests some invalid cases
+  EXPECT_FALSE(absl::ParseLenientCivilTime("01-01-2015", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015-", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("0xff-01", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015-02-30T04:05:06", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015-02-03T04:05:96", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("X2015-02-03T04:05:06", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015-02-03T04:05:003", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015 -02-03T04:05:06", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015-02-03-04:05:06", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("2015:02:03T04-05-06", &ss)) << ss;
+  EXPECT_FALSE(absl::ParseLenientCivilTime("9223372036854775808", &y)) << y;
 }
 
 TEST(CivilTime, OutputStream) {
diff --git a/absl/time/clock.cc b/absl/time/clock.cc
index 48dc445..3b895c3 100644
--- a/absl/time/clock.cc
+++ b/absl/time/clock.cc
@@ -34,7 +34,7 @@
 #include "absl/base/thread_annotations.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 Time Now() {
   // TODO(bww): Get a timespec instead so we don't have to divide.
   int64_t n = absl::GetCurrentTimeNanos();
@@ -44,7 +44,7 @@
   }
   return time_internal::FromUnixDuration(absl::Nanoseconds(n));
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // Decide if we should use the fast GetCurrentTimeNanos() algorithm
@@ -73,11 +73,11 @@
 
 #if !ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 int64_t GetCurrentTimeNanos() {
   return GET_CURRENT_TIME_NANOS_FROM_SYSTEM();
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #else  // Use the cyclecounter-based implementation below.
 
@@ -95,7 +95,7 @@
 static int64_t stats_fast_slow_paths;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 // This is a friend wrapper around UnscaledCycleClock::Now()
 // (needed to access UnscaledCycleClock).
@@ -390,7 +390,7 @@
 // TODO(absl-team): Remove this attribute when our compiler is smart enough
 // to do the right thing.
 ABSL_ATTRIBUTE_NOINLINE
-static int64_t GetCurrentTimeNanosSlowPath() LOCKS_EXCLUDED(lock) {
+static int64_t GetCurrentTimeNanosSlowPath() ABSL_LOCKS_EXCLUDED(lock) {
   // Serialize access to slow-path.  Fast-path readers are not blocked yet, and
   // code below must not modify last_sample until the seqlock is acquired.
   lock.Lock();
@@ -435,7 +435,7 @@
 static uint64_t UpdateLastSample(uint64_t now_cycles, uint64_t now_ns,
                                  uint64_t delta_cycles,
                                  const struct TimeSample *sample)
-    EXCLUSIVE_LOCKS_REQUIRED(lock) {
+    ABSL_EXCLUSIVE_LOCKS_REQUIRED(lock) {
   uint64_t estimated_base_ns = now_ns;
   uint64_t lock_value = SeqAcquire(&seq);  // acquire seqlock to block readers
 
@@ -454,7 +454,7 @@
     last_sample.min_cycles_per_sample.store(0, std::memory_order_relaxed);
     stats_initializations++;
   } else if (sample->raw_ns + 500 * 1000 * 1000 < now_ns &&
-             sample->base_cycles + 100 < now_cycles) {
+             sample->base_cycles + 50 < now_cycles) {
     // Enough time has passed to compute the cycle time.
     if (sample->nsscaled_per_cycle != 0) {  // Have a cycle time estimate.
       // Compute time from counter reading, but avoiding overflow
@@ -520,12 +520,12 @@
 
   return estimated_base_ns;
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_USE_CYCLECLOCK_FOR_GET_CURRENT_TIME_NANOS
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // Returns the maximum duration that SleepOnce() can sleep for.
@@ -553,7 +553,7 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 extern "C" {
diff --git a/absl/time/clock.h b/absl/time/clock.h
index a3b9ffe..27764a9 100644
--- a/absl/time/clock.h
+++ b/absl/time/clock.h
@@ -26,7 +26,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Now()
 //
@@ -50,7 +50,7 @@
 // * Returns immediately when passed a nonpositive duration.
 void SleepFor(absl::Duration duration);
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 // -----------------------------------------------------------------------------
diff --git a/absl/time/duration.cc b/absl/time/duration.cc
index 6a51baf..b1af840 100644
--- a/absl/time/duration.cc
+++ b/absl/time/duration.cc
@@ -71,7 +71,7 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -198,11 +198,11 @@
 // double as overflow cases.
 inline bool SafeAddRepHi(double a_hi, double b_hi, Duration* d) {
   double c = a_hi + b_hi;
-  if (c >= kint64max) {
+  if (c >= static_cast<double>(kint64max)) {
     *d = InfiniteDuration();
     return false;
   }
-  if (c <= kint64min) {
+  if (c <= static_cast<double>(kint64min)) {
     *d = -InfiniteDuration();
     return false;
   }
@@ -907,11 +907,16 @@
   return true;
 }
 
+bool AbslParseFlag(absl::string_view text, Duration* dst, std::string*) {
+  return ParseDuration(std::string(text), dst);
+}
+
+std::string AbslUnparseFlag(Duration d) { return FormatDuration(d); }
 bool ParseFlag(const std::string& text, Duration* dst, std::string* ) {
   return ParseDuration(text, dst);
 }
 
 std::string UnparseFlag(Duration d) { return FormatDuration(d); }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/duration_test.cc b/absl/time/duration_test.cc
index 5dce9ac..4d85a2c 100644
--- a/absl/time/duration_test.cc
+++ b/absl/time/duration_test.cc
@@ -762,11 +762,6 @@
   const double dbl_inf = std::numeric_limits<double>::infinity();
   const double dbl_denorm = std::numeric_limits<double>::denorm_min();
 
-  // IEEE 754 behavior
-  double z = 0.0, two = 2.0;
-  EXPECT_TRUE(std::isinf(two / z));
-  EXPECT_TRUE(std::isnan(z / z));  // We'll return inf
-
   // Operator/(Duration, double)
   EXPECT_EQ(inf, zero / 0.0);
   EXPECT_EQ(-inf, zero / -0.0);
@@ -1050,7 +1045,7 @@
   EXPECT_EQ(absl::Seconds(666666666) + absl::Nanoseconds(666666667) +
                 absl::Nanoseconds(1) / 2,
             sigfigs / 3);
-  sigfigs = absl::Seconds(7000000000LL);
+  sigfigs = absl::Seconds(int64_t{7000000000});
   EXPECT_EQ(absl::Seconds(2333333333) + absl::Nanoseconds(333333333) +
                 absl::Nanoseconds(1) / 4,
             sigfigs / 3);
diff --git a/absl/time/format.cc b/absl/time/format.cc
index 6eb83d7..ee088f3 100644
--- a/absl/time/format.cc
+++ b/absl/time/format.cc
@@ -22,13 +22,16 @@
 namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
-extern const char RFC3339_full[] = "%Y-%m-%dT%H:%M:%E*S%Ez";
-extern const char RFC3339_sec[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+ABSL_DLL extern const char RFC3339_full[] =
+    "%Y-%m-%dT%H:%M:%E*S%Ez";
+ABSL_DLL extern const char RFC3339_sec[] = "%Y-%m-%dT%H:%M:%S%Ez";
 
-extern const char RFC1123_full[] = "%a, %d %b %E4Y %H:%M:%S %z";
-extern const char RFC1123_no_wday[] =  "%d %b %E4Y %H:%M:%S %z";
+ABSL_DLL extern const char RFC1123_full[] =
+    "%a, %d %b %E4Y %H:%M:%S %z";
+ABSL_DLL extern const char RFC1123_no_wday[] =
+    "%d %b %E4Y %H:%M:%S %z";
 
 namespace {
 
@@ -130,6 +133,14 @@
 }
 
 // Functions required to support absl::Time flags.
+bool AbslParseFlag(absl::string_view text, absl::Time* t, std::string* error) {
+  return absl::ParseTime(RFC3339_full, std::string(text), absl::UTCTimeZone(),
+                         t, error);
+}
+
+std::string AbslUnparseFlag(absl::Time t) {
+  return absl::FormatTime(RFC3339_full, t, absl::UTCTimeZone());
+}
 bool ParseFlag(const std::string& text, absl::Time* t, std::string* error) {
   return absl::ParseTime(RFC3339_full, text, absl::UTCTimeZone(), t, error);
 }
@@ -138,5 +149,5 @@
   return absl::FormatTime(RFC3339_full, t, absl::UTCTimeZone());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/BUILD.bazel b/absl/time/internal/cctz/BUILD.bazel
index b05c234..7a53c81 100644
--- a/absl/time/internal/cctz/BUILD.bazel
+++ b/absl/time/internal/cctz/BUILD.bazel
@@ -12,10 +12,17 @@
 #   See the License for the specific language governing permissions and
 #   limitations under the License.
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+
 package(features = ["-parse_headers"])
 
 licenses(["notice"])  # Apache License
 
+filegroup(
+    name = "zoneinfo",
+    srcs = glob(["testdata/zoneinfo/**"]),
+)
+
 config_setting(
     name = "osx",
     constraint_values = [
@@ -33,16 +40,6 @@
 ### libraries
 
 cc_library(
-    name = "includes",
-    textual_hdrs = [
-        "include/cctz/civil_time.h",
-        "include/cctz/civil_time_detail.h",
-        "include/cctz/time_zone.h",
-    ],
-    visibility = ["//absl/time:__pkg__"],
-)
-
-cc_library(
     name = "civil_time",
     srcs = ["src/civil_time_detail.cc"],
     hdrs = [
@@ -50,6 +47,7 @@
     ],
     textual_hdrs = ["include/cctz/civil_time_detail.h"],
     visibility = ["//visibility:public"],
+    deps = ["//absl/base:config"],
 )
 
 cc_library(
@@ -86,7 +84,10 @@
         "//conditions:default": [],
     }),
     visibility = ["//visibility:public"],
-    deps = [":civil_time"],
+    deps = [
+        ":civil_time",
+        "//absl/base:config",
+    ],
 )
 
 ### tests
@@ -97,6 +98,7 @@
     srcs = ["src/civil_time_test.cc"],
     deps = [
         ":civil_time",
+        "//absl/base:config",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -114,6 +116,7 @@
     deps = [
         ":civil_time",
         ":time_zone",
+        "//absl/base:config",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -132,6 +135,7 @@
     deps = [
         ":civil_time",
         ":time_zone",
+        "//absl/base:config",
         "@com_google_googletest//:gtest_main",
     ],
 )
@@ -152,6 +156,7 @@
     deps = [
         ":civil_time",
         ":time_zone",
+        "//absl/base:config",
         "@com_github_google_benchmark//:benchmark_main",
     ],
 )
@@ -159,8 +164,3 @@
 ### examples
 
 ### binaries
-
-filegroup(
-    name = "zoneinfo",
-    srcs = glob(["testdata/zoneinfo/**"]),
-)
diff --git a/absl/time/internal/cctz/include/cctz/civil_time.h b/absl/time/internal/cctz/include/cctz/civil_time.h
index df25db0..d47ff86 100644
--- a/absl/time/internal/cctz/include/cctz/civil_time.h
+++ b/absl/time/internal/cctz/include/cctz/civil_time.h
@@ -15,10 +15,11 @@
 #ifndef ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
 #define ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time_detail.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -151,7 +152,7 @@
 //
 // All civil-time types have accessors for all six of the civil-time fields:
 // year, month, day, hour, minute, and second. Recall that fields inferior to
-// the type's aligment will be set to their minimum valid value.
+// the type's alignment will be set to their minimum valid value.
 //
 //   civil_day d(2015, 6, 28);
 //   // d.year() == 2015
@@ -325,7 +326,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_CIVIL_TIME_H_
diff --git a/absl/time/internal/cctz/include/cctz/civil_time_detail.h b/absl/time/internal/cctz/include/cctz/civil_time_detail.h
index 53e087a..4cde96f 100644
--- a/absl/time/internal/cctz/include/cctz/civil_time_detail.h
+++ b/absl/time/internal/cctz/include/cctz/civil_time_detail.h
@@ -20,6 +20,8 @@
 #include <ostream>
 #include <type_traits>
 
+#include "absl/base/config.h"
+
 // Disable constexpr support unless we are in C++14 mode.
 #if __cpp_constexpr >= 201304 || (defined(_MSC_VER) && _MSC_VER >= 1910)
 #define CONSTEXPR_D constexpr  // data
@@ -32,7 +34,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -54,8 +56,8 @@
 
 // Normalized civil-time fields: Y-M-D HH:MM:SS.
 struct fields {
-  CONSTEXPR_M fields(year_t year, month_t month, day_t day,
-                     hour_t hour, minute_t minute, second_t second)
+  CONSTEXPR_M fields(year_t year, month_t month, day_t day, hour_t hour,
+                     minute_t minute, second_t second)
       : y(year), m(month), d(day), hh(hour), mm(minute), ss(second) {}
   std::int_least64_t y;
   std::int_least8_t m;
@@ -102,8 +104,8 @@
   return k_days_per_month[m] + (m == 2 && is_leap_year(y));
 }
 
-CONSTEXPR_F fields n_day(year_t y, month_t m, diff_t d, diff_t cd,
-                         hour_t hh, minute_t mm, second_t ss) noexcept {
+CONSTEXPR_F fields n_day(year_t y, month_t m, diff_t d, diff_t cd, hour_t hh,
+                         minute_t mm, second_t ss) noexcept {
   y += (cd / 146097) * 400;
   cd %= 146097;
   if (cd < 0) {
@@ -153,8 +155,8 @@
   }
   return fields(y, m, static_cast<day_t>(d), hh, mm, ss);
 }
-CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd,
-                         hour_t hh, minute_t mm, second_t ss) noexcept {
+CONSTEXPR_F fields n_mon(year_t y, diff_t m, diff_t d, diff_t cd, hour_t hh,
+                         minute_t mm, second_t ss) noexcept {
   if (m != 12) {
     y += m / 12;
     m %= 12;
@@ -165,8 +167,8 @@
   }
   return n_day(y, static_cast<month_t>(m), d, cd, hh, mm, ss);
 }
-CONSTEXPR_F fields n_hour(year_t y, diff_t m, diff_t d, diff_t cd,
-                          diff_t hh, minute_t mm, second_t ss) noexcept {
+CONSTEXPR_F fields n_hour(year_t y, diff_t m, diff_t d, diff_t cd, diff_t hh,
+                          minute_t mm, second_t ss) noexcept {
   cd += hh / 24;
   hh %= 24;
   if (hh < 0) {
@@ -265,8 +267,8 @@
 // yet the difference between two such extreme values may actually be
 // small, so we take a little care to avoid overflow when possible by
 // exploiting the 146097-day cycle.
-CONSTEXPR_F diff_t day_difference(year_t y1, month_t m1, day_t d1,
-                                  year_t y2, month_t m2, day_t d2) noexcept {
+CONSTEXPR_F diff_t day_difference(year_t y1, month_t m1, day_t d1, year_t y2,
+                                  month_t m2, day_t d2) noexcept {
   const diff_t a_c4_off = y1 % 400;
   const diff_t b_c4_off = y2 % 400;
   diff_t c4_diff = (y1 - a_c4_off) - (y2 - b_c4_off);
@@ -306,9 +308,7 @@
 ////////////////////////////////////////////////////////////////////////
 
 // Aligns the (normalized) fields struct to the indicated field.
-CONSTEXPR_F fields align(second_tag, fields f) noexcept {
-  return f;
-}
+CONSTEXPR_F fields align(second_tag, fields f) noexcept { return f; }
 CONSTEXPR_F fields align(minute_tag, fields f) noexcept {
   return fields{f.y, f.m, f.d, f.hh, f.mm, 0};
 }
@@ -387,11 +387,11 @@
       : civil_time(ct.f_) {}
 
   // Factories for the maximum/minimum representable civil_time.
-  static CONSTEXPR_F civil_time (max)() {
+  static CONSTEXPR_F civil_time(max)() {
     const auto max_year = (std::numeric_limits<std::int_least64_t>::max)();
     return civil_time(max_year, 12, 31, 23, 59, 59);
   }
-  static CONSTEXPR_F civil_time (min)() {
+  static CONSTEXPR_F civil_time(min)() {
     const auto min_year = (std::numeric_limits<std::int_least64_t>::min)();
     return civil_time(min_year, 1, 1, 0, 0, 0);
   }
@@ -417,17 +417,13 @@
     }
     return *this;
   }
-  CONSTEXPR_M civil_time& operator++() noexcept {
-    return *this += 1;
-  }
+  CONSTEXPR_M civil_time& operator++() noexcept { return *this += 1; }
   CONSTEXPR_M civil_time operator++(int) noexcept {
     const civil_time a = *this;
     ++*this;
     return a;
   }
-  CONSTEXPR_M civil_time& operator--() noexcept {
-    return *this -= 1;
-  }
+  CONSTEXPR_M civil_time& operator--() noexcept { return *this -= 1; }
   CONSTEXPR_M civil_time operator--(int) noexcept {
     const civil_time a = *this;
     --*this;
@@ -484,17 +480,17 @@
 template <typename T1, typename T2>
 CONSTEXPR_F bool operator<(const civil_time<T1>& lhs,
                            const civil_time<T2>& rhs) noexcept {
-  return (lhs.year() < rhs.year() ||
-          (lhs.year() == rhs.year() &&
-           (lhs.month() < rhs.month() ||
-            (lhs.month() == rhs.month() &&
-             (lhs.day() < rhs.day() ||
-              (lhs.day() == rhs.day() &&
-               (lhs.hour() < rhs.hour() ||
-                (lhs.hour() == rhs.hour() &&
-                 (lhs.minute() < rhs.minute() ||
-                  (lhs.minute() == rhs.minute() &&
-                   (lhs.second() < rhs.second())))))))))));
+  return (
+      lhs.year() < rhs.year() ||
+      (lhs.year() == rhs.year() &&
+       (lhs.month() < rhs.month() ||
+        (lhs.month() == rhs.month() &&
+         (lhs.day() < rhs.day() || (lhs.day() == rhs.day() &&
+                                    (lhs.hour() < rhs.hour() ||
+                                     (lhs.hour() == rhs.hour() &&
+                                      (lhs.minute() < rhs.minute() ||
+                                       (lhs.minute() == rhs.minute() &&
+                                        (lhs.second() < rhs.second())))))))))));
 }
 template <typename T1, typename T2>
 CONSTEXPR_F bool operator<=(const civil_time<T1>& lhs,
@@ -616,7 +612,7 @@
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef CONSTEXPR_M
diff --git a/absl/time/internal/cctz/include/cctz/time_zone.h b/absl/time/internal/cctz/include/cctz/time_zone.h
index f9769c0..d05147a 100644
--- a/absl/time/internal/cctz/include/cctz/time_zone.h
+++ b/absl/time/internal/cctz/include/cctz/time_zone.h
@@ -25,10 +25,11 @@
 #include <string>
 #include <utility>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -40,8 +41,8 @@
 
 namespace detail {
 template <typename D>
-inline std::pair<time_point<seconds>, D>
-split_seconds(const time_point<D>& tp) {
+inline std::pair<time_point<seconds>, D> split_seconds(
+    const time_point<D>& tp) {
   auto sec = std::chrono::time_point_cast<seconds>(tp);
   auto sub = tp - sec;
   if (sub.count() < 0) {
@@ -50,8 +51,8 @@
   }
   return {sec, std::chrono::duration_cast<D>(sub)};
 }
-inline std::pair<time_point<seconds>, seconds>
-split_seconds(const time_point<seconds>& tp) {
+inline std::pair<time_point<seconds>, seconds> split_seconds(
+    const time_point<seconds>& tp) {
   return {tp, seconds::zero()};
 }
 }  // namespace detail
@@ -195,15 +196,13 @@
   bool next_transition(const time_point<seconds>& tp,
                        civil_transition* trans) const;
   template <typename D>
-  bool next_transition(const time_point<D>& tp,
-                       civil_transition* trans) const {
+  bool next_transition(const time_point<D>& tp, civil_transition* trans) const {
     return next_transition(detail::split_seconds(tp).first, trans);
   }
   bool prev_transition(const time_point<seconds>& tp,
                        civil_transition* trans) const;
   template <typename D>
-  bool prev_transition(const time_point<D>& tp,
-                       civil_transition* trans) const {
+  bool prev_transition(const time_point<D>& tp, civil_transition* trans) const {
     return prev_transition(detail::split_seconds(tp).first, trans);
   }
 
@@ -221,9 +220,7 @@
   friend bool operator==(time_zone lhs, time_zone rhs) {
     return &lhs.effective_impl() == &rhs.effective_impl();
   }
-  friend bool operator!=(time_zone lhs, time_zone rhs) {
-    return !(lhs == rhs);
-  }
+  friend bool operator!=(time_zone lhs, time_zone rhs) { return !(lhs == rhs); }
 
   template <typename H>
   friend H AbslHashValue(H h, time_zone tz) {
@@ -381,7 +378,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_H_
diff --git a/absl/time/internal/cctz/include/cctz/zone_info_source.h b/absl/time/internal/cctz/include/cctz/zone_info_source.h
index 7372c5d..912b44b 100644
--- a/absl/time/internal/cctz/include/cctz/zone_info_source.h
+++ b/absl/time/internal/cctz/include/cctz/zone_info_source.h
@@ -20,8 +20,10 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -31,7 +33,7 @@
   virtual ~ZoneInfoSource();
 
   virtual std::size_t Read(void* ptr, std::size_t size) = 0;  // like fread()
-  virtual int Skip(std::size_t offset) = 0;  // like fseek()
+  virtual int Skip(std::size_t offset) = 0;                   // like fseek()
 
   // Until the zoneinfo data supports versioning information, we provide
   // a way for a ZoneInfoSource to indicate it out-of-band.  The default
@@ -41,11 +43,11 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz_extension {
 
@@ -55,8 +57,8 @@
 using ZoneInfoSourceFactory =
     std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource> (*)(
         const std::string&,
-        const std::function<std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
-            const std::string&)>&);
+        const std::function<std::unique_ptr<
+            absl::time_internal::cctz::ZoneInfoSource>(const std::string&)>&);
 
 // The user can control the mapping of zone names to zoneinfo data by
 // providing a definition for cctz_extension::zone_info_source_factory.
@@ -94,7 +96,7 @@
 
 }  // namespace cctz_extension
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_ZONE_INFO_SOURCE_H_
diff --git a/absl/time/internal/cctz/src/cctz_benchmark.cc b/absl/time/internal/cctz/src/cctz_benchmark.cc
index a40f504..d30a644 100644
--- a/absl/time/internal/cctz/src/cctz_benchmark.cc
+++ b/absl/time/internal/cctz/src/cctz_benchmark.cc
@@ -105,601 +105,599 @@
 
 // A list of known time-zone names.
 // TODO: Refactor with src/time_zone_lookup_test.cc.
-const char* const kTimeZoneNames[] = {
-  "Africa/Abidjan",
-  "Africa/Accra",
-  "Africa/Addis_Ababa",
-  "Africa/Algiers",
-  "Africa/Asmara",
-  "Africa/Asmera",
-  "Africa/Bamako",
-  "Africa/Bangui",
-  "Africa/Banjul",
-  "Africa/Bissau",
-  "Africa/Blantyre",
-  "Africa/Brazzaville",
-  "Africa/Bujumbura",
-  "Africa/Cairo",
-  "Africa/Casablanca",
-  "Africa/Ceuta",
-  "Africa/Conakry",
-  "Africa/Dakar",
-  "Africa/Dar_es_Salaam",
-  "Africa/Djibouti",
-  "Africa/Douala",
-  "Africa/El_Aaiun",
-  "Africa/Freetown",
-  "Africa/Gaborone",
-  "Africa/Harare",
-  "Africa/Johannesburg",
-  "Africa/Juba",
-  "Africa/Kampala",
-  "Africa/Khartoum",
-  "Africa/Kigali",
-  "Africa/Kinshasa",
-  "Africa/Lagos",
-  "Africa/Libreville",
-  "Africa/Lome",
-  "Africa/Luanda",
-  "Africa/Lubumbashi",
-  "Africa/Lusaka",
-  "Africa/Malabo",
-  "Africa/Maputo",
-  "Africa/Maseru",
-  "Africa/Mbabane",
-  "Africa/Mogadishu",
-  "Africa/Monrovia",
-  "Africa/Nairobi",
-  "Africa/Ndjamena",
-  "Africa/Niamey",
-  "Africa/Nouakchott",
-  "Africa/Ouagadougou",
-  "Africa/Porto-Novo",
-  "Africa/Sao_Tome",
-  "Africa/Timbuktu",
-  "Africa/Tripoli",
-  "Africa/Tunis",
-  "Africa/Windhoek",
-  "America/Adak",
-  "America/Anchorage",
-  "America/Anguilla",
-  "America/Antigua",
-  "America/Araguaina",
-  "America/Argentina/Buenos_Aires",
-  "America/Argentina/Catamarca",
-  "America/Argentina/ComodRivadavia",
-  "America/Argentina/Cordoba",
-  "America/Argentina/Jujuy",
-  "America/Argentina/La_Rioja",
-  "America/Argentina/Mendoza",
-  "America/Argentina/Rio_Gallegos",
-  "America/Argentina/Salta",
-  "America/Argentina/San_Juan",
-  "America/Argentina/San_Luis",
-  "America/Argentina/Tucuman",
-  "America/Argentina/Ushuaia",
-  "America/Aruba",
-  "America/Asuncion",
-  "America/Atikokan",
-  "America/Atka",
-  "America/Bahia",
-  "America/Bahia_Banderas",
-  "America/Barbados",
-  "America/Belem",
-  "America/Belize",
-  "America/Blanc-Sablon",
-  "America/Boa_Vista",
-  "America/Bogota",
-  "America/Boise",
-  "America/Buenos_Aires",
-  "America/Cambridge_Bay",
-  "America/Campo_Grande",
-  "America/Cancun",
-  "America/Caracas",
-  "America/Catamarca",
-  "America/Cayenne",
-  "America/Cayman",
-  "America/Chicago",
-  "America/Chihuahua",
-  "America/Coral_Harbour",
-  "America/Cordoba",
-  "America/Costa_Rica",
-  "America/Creston",
-  "America/Cuiaba",
-  "America/Curacao",
-  "America/Danmarkshavn",
-  "America/Dawson",
-  "America/Dawson_Creek",
-  "America/Denver",
-  "America/Detroit",
-  "America/Dominica",
-  "America/Edmonton",
-  "America/Eirunepe",
-  "America/El_Salvador",
-  "America/Ensenada",
-  "America/Fort_Nelson",
-  "America/Fort_Wayne",
-  "America/Fortaleza",
-  "America/Glace_Bay",
-  "America/Godthab",
-  "America/Goose_Bay",
-  "America/Grand_Turk",
-  "America/Grenada",
-  "America/Guadeloupe",
-  "America/Guatemala",
-  "America/Guayaquil",
-  "America/Guyana",
-  "America/Halifax",
-  "America/Havana",
-  "America/Hermosillo",
-  "America/Indiana/Indianapolis",
-  "America/Indiana/Knox",
-  "America/Indiana/Marengo",
-  "America/Indiana/Petersburg",
-  "America/Indiana/Tell_City",
-  "America/Indiana/Vevay",
-  "America/Indiana/Vincennes",
-  "America/Indiana/Winamac",
-  "America/Indianapolis",
-  "America/Inuvik",
-  "America/Iqaluit",
-  "America/Jamaica",
-  "America/Jujuy",
-  "America/Juneau",
-  "America/Kentucky/Louisville",
-  "America/Kentucky/Monticello",
-  "America/Knox_IN",
-  "America/Kralendijk",
-  "America/La_Paz",
-  "America/Lima",
-  "America/Los_Angeles",
-  "America/Louisville",
-  "America/Lower_Princes",
-  "America/Maceio",
-  "America/Managua",
-  "America/Manaus",
-  "America/Marigot",
-  "America/Martinique",
-  "America/Matamoros",
-  "America/Mazatlan",
-  "America/Mendoza",
-  "America/Menominee",
-  "America/Merida",
-  "America/Metlakatla",
-  "America/Mexico_City",
-  "America/Miquelon",
-  "America/Moncton",
-  "America/Monterrey",
-  "America/Montevideo",
-  "America/Montreal",
-  "America/Montserrat",
-  "America/Nassau",
-  "America/New_York",
-  "America/Nipigon",
-  "America/Nome",
-  "America/Noronha",
-  "America/North_Dakota/Beulah",
-  "America/North_Dakota/Center",
-  "America/North_Dakota/New_Salem",
-  "America/Ojinaga",
-  "America/Panama",
-  "America/Pangnirtung",
-  "America/Paramaribo",
-  "America/Phoenix",
-  "America/Port-au-Prince",
-  "America/Port_of_Spain",
-  "America/Porto_Acre",
-  "America/Porto_Velho",
-  "America/Puerto_Rico",
-  "America/Punta_Arenas",
-  "America/Rainy_River",
-  "America/Rankin_Inlet",
-  "America/Recife",
-  "America/Regina",
-  "America/Resolute",
-  "America/Rio_Branco",
-  "America/Rosario",
-  "America/Santa_Isabel",
-  "America/Santarem",
-  "America/Santiago",
-  "America/Santo_Domingo",
-  "America/Sao_Paulo",
-  "America/Scoresbysund",
-  "America/Shiprock",
-  "America/Sitka",
-  "America/St_Barthelemy",
-  "America/St_Johns",
-  "America/St_Kitts",
-  "America/St_Lucia",
-  "America/St_Thomas",
-  "America/St_Vincent",
-  "America/Swift_Current",
-  "America/Tegucigalpa",
-  "America/Thule",
-  "America/Thunder_Bay",
-  "America/Tijuana",
-  "America/Toronto",
-  "America/Tortola",
-  "America/Vancouver",
-  "America/Virgin",
-  "America/Whitehorse",
-  "America/Winnipeg",
-  "America/Yakutat",
-  "America/Yellowknife",
-  "Antarctica/Casey",
-  "Antarctica/Davis",
-  "Antarctica/DumontDUrville",
-  "Antarctica/Macquarie",
-  "Antarctica/Mawson",
-  "Antarctica/McMurdo",
-  "Antarctica/Palmer",
-  "Antarctica/Rothera",
-  "Antarctica/South_Pole",
-  "Antarctica/Syowa",
-  "Antarctica/Troll",
-  "Antarctica/Vostok",
-  "Arctic/Longyearbyen",
-  "Asia/Aden",
-  "Asia/Almaty",
-  "Asia/Amman",
-  "Asia/Anadyr",
-  "Asia/Aqtau",
-  "Asia/Aqtobe",
-  "Asia/Ashgabat",
-  "Asia/Ashkhabad",
-  "Asia/Atyrau",
-  "Asia/Baghdad",
-  "Asia/Bahrain",
-  "Asia/Baku",
-  "Asia/Bangkok",
-  "Asia/Barnaul",
-  "Asia/Beirut",
-  "Asia/Bishkek",
-  "Asia/Brunei",
-  "Asia/Calcutta",
-  "Asia/Chita",
-  "Asia/Choibalsan",
-  "Asia/Chongqing",
-  "Asia/Chungking",
-  "Asia/Colombo",
-  "Asia/Dacca",
-  "Asia/Damascus",
-  "Asia/Dhaka",
-  "Asia/Dili",
-  "Asia/Dubai",
-  "Asia/Dushanbe",
-  "Asia/Famagusta",
-  "Asia/Gaza",
-  "Asia/Harbin",
-  "Asia/Hebron",
-  "Asia/Ho_Chi_Minh",
-  "Asia/Hong_Kong",
-  "Asia/Hovd",
-  "Asia/Irkutsk",
-  "Asia/Istanbul",
-  "Asia/Jakarta",
-  "Asia/Jayapura",
-  "Asia/Jerusalem",
-  "Asia/Kabul",
-  "Asia/Kamchatka",
-  "Asia/Karachi",
-  "Asia/Kashgar",
-  "Asia/Kathmandu",
-  "Asia/Katmandu",
-  "Asia/Khandyga",
-  "Asia/Kolkata",
-  "Asia/Krasnoyarsk",
-  "Asia/Kuala_Lumpur",
-  "Asia/Kuching",
-  "Asia/Kuwait",
-  "Asia/Macao",
-  "Asia/Macau",
-  "Asia/Magadan",
-  "Asia/Makassar",
-  "Asia/Manila",
-  "Asia/Muscat",
-  "Asia/Nicosia",
-  "Asia/Novokuznetsk",
-  "Asia/Novosibirsk",
-  "Asia/Omsk",
-  "Asia/Oral",
-  "Asia/Phnom_Penh",
-  "Asia/Pontianak",
-  "Asia/Pyongyang",
-  "Asia/Qatar",
-  "Asia/Qostanay",
-  "Asia/Qyzylorda",
-  "Asia/Rangoon",
-  "Asia/Riyadh",
-  "Asia/Saigon",
-  "Asia/Sakhalin",
-  "Asia/Samarkand",
-  "Asia/Seoul",
-  "Asia/Shanghai",
-  "Asia/Singapore",
-  "Asia/Srednekolymsk",
-  "Asia/Taipei",
-  "Asia/Tashkent",
-  "Asia/Tbilisi",
-  "Asia/Tehran",
-  "Asia/Tel_Aviv",
-  "Asia/Thimbu",
-  "Asia/Thimphu",
-  "Asia/Tokyo",
-  "Asia/Tomsk",
-  "Asia/Ujung_Pandang",
-  "Asia/Ulaanbaatar",
-  "Asia/Ulan_Bator",
-  "Asia/Urumqi",
-  "Asia/Ust-Nera",
-  "Asia/Vientiane",
-  "Asia/Vladivostok",
-  "Asia/Yakutsk",
-  "Asia/Yangon",
-  "Asia/Yekaterinburg",
-  "Asia/Yerevan",
-  "Atlantic/Azores",
-  "Atlantic/Bermuda",
-  "Atlantic/Canary",
-  "Atlantic/Cape_Verde",
-  "Atlantic/Faeroe",
-  "Atlantic/Faroe",
-  "Atlantic/Jan_Mayen",
-  "Atlantic/Madeira",
-  "Atlantic/Reykjavik",
-  "Atlantic/South_Georgia",
-  "Atlantic/St_Helena",
-  "Atlantic/Stanley",
-  "Australia/ACT",
-  "Australia/Adelaide",
-  "Australia/Brisbane",
-  "Australia/Broken_Hill",
-  "Australia/Canberra",
-  "Australia/Currie",
-  "Australia/Darwin",
-  "Australia/Eucla",
-  "Australia/Hobart",
-  "Australia/LHI",
-  "Australia/Lindeman",
-  "Australia/Lord_Howe",
-  "Australia/Melbourne",
-  "Australia/NSW",
-  "Australia/North",
-  "Australia/Perth",
-  "Australia/Queensland",
-  "Australia/South",
-  "Australia/Sydney",
-  "Australia/Tasmania",
-  "Australia/Victoria",
-  "Australia/West",
-  "Australia/Yancowinna",
-  "Brazil/Acre",
-  "Brazil/DeNoronha",
-  "Brazil/East",
-  "Brazil/West",
-  "CET",
-  "CST6CDT",
-  "Canada/Atlantic",
-  "Canada/Central",
-  "Canada/Eastern",
-  "Canada/Mountain",
-  "Canada/Newfoundland",
-  "Canada/Pacific",
-  "Canada/Saskatchewan",
-  "Canada/Yukon",
-  "Chile/Continental",
-  "Chile/EasterIsland",
-  "Cuba",
-  "EET",
-  "EST",
-  "EST5EDT",
-  "Egypt",
-  "Eire",
-  "Etc/GMT",
-  "Etc/GMT+0",
-  "Etc/GMT+1",
-  "Etc/GMT+10",
-  "Etc/GMT+11",
-  "Etc/GMT+12",
-  "Etc/GMT+2",
-  "Etc/GMT+3",
-  "Etc/GMT+4",
-  "Etc/GMT+5",
-  "Etc/GMT+6",
-  "Etc/GMT+7",
-  "Etc/GMT+8",
-  "Etc/GMT+9",
-  "Etc/GMT-0",
-  "Etc/GMT-1",
-  "Etc/GMT-10",
-  "Etc/GMT-11",
-  "Etc/GMT-12",
-  "Etc/GMT-13",
-  "Etc/GMT-14",
-  "Etc/GMT-2",
-  "Etc/GMT-3",
-  "Etc/GMT-4",
-  "Etc/GMT-5",
-  "Etc/GMT-6",
-  "Etc/GMT-7",
-  "Etc/GMT-8",
-  "Etc/GMT-9",
-  "Etc/GMT0",
-  "Etc/Greenwich",
-  "Etc/UCT",
-  "Etc/UTC",
-  "Etc/Universal",
-  "Etc/Zulu",
-  "Europe/Amsterdam",
-  "Europe/Andorra",
-  "Europe/Astrakhan",
-  "Europe/Athens",
-  "Europe/Belfast",
-  "Europe/Belgrade",
-  "Europe/Berlin",
-  "Europe/Bratislava",
-  "Europe/Brussels",
-  "Europe/Bucharest",
-  "Europe/Budapest",
-  "Europe/Busingen",
-  "Europe/Chisinau",
-  "Europe/Copenhagen",
-  "Europe/Dublin",
-  "Europe/Gibraltar",
-  "Europe/Guernsey",
-  "Europe/Helsinki",
-  "Europe/Isle_of_Man",
-  "Europe/Istanbul",
-  "Europe/Jersey",
-  "Europe/Kaliningrad",
-  "Europe/Kiev",
-  "Europe/Kirov",
-  "Europe/Lisbon",
-  "Europe/Ljubljana",
-  "Europe/London",
-  "Europe/Luxembourg",
-  "Europe/Madrid",
-  "Europe/Malta",
-  "Europe/Mariehamn",
-  "Europe/Minsk",
-  "Europe/Monaco",
-  "Europe/Moscow",
-  "Europe/Nicosia",
-  "Europe/Oslo",
-  "Europe/Paris",
-  "Europe/Podgorica",
-  "Europe/Prague",
-  "Europe/Riga",
-  "Europe/Rome",
-  "Europe/Samara",
-  "Europe/San_Marino",
-  "Europe/Sarajevo",
-  "Europe/Saratov",
-  "Europe/Simferopol",
-  "Europe/Skopje",
-  "Europe/Sofia",
-  "Europe/Stockholm",
-  "Europe/Tallinn",
-  "Europe/Tirane",
-  "Europe/Tiraspol",
-  "Europe/Ulyanovsk",
-  "Europe/Uzhgorod",
-  "Europe/Vaduz",
-  "Europe/Vatican",
-  "Europe/Vienna",
-  "Europe/Vilnius",
-  "Europe/Volgograd",
-  "Europe/Warsaw",
-  "Europe/Zagreb",
-  "Europe/Zaporozhye",
-  "Europe/Zurich",
-  "GB",
-  "GB-Eire",
-  "GMT",
-  "GMT+0",
-  "GMT-0",
-  "GMT0",
-  "Greenwich",
-  "HST",
-  "Hongkong",
-  "Iceland",
-  "Indian/Antananarivo",
-  "Indian/Chagos",
-  "Indian/Christmas",
-  "Indian/Cocos",
-  "Indian/Comoro",
-  "Indian/Kerguelen",
-  "Indian/Mahe",
-  "Indian/Maldives",
-  "Indian/Mauritius",
-  "Indian/Mayotte",
-  "Indian/Reunion",
-  "Iran",
-  "Israel",
-  "Jamaica",
-  "Japan",
-  "Kwajalein",
-  "Libya",
-  "MET",
-  "MST",
-  "MST7MDT",
-  "Mexico/BajaNorte",
-  "Mexico/BajaSur",
-  "Mexico/General",
-  "NZ",
-  "NZ-CHAT",
-  "Navajo",
-  "PRC",
-  "PST8PDT",
-  "Pacific/Apia",
-  "Pacific/Auckland",
-  "Pacific/Bougainville",
-  "Pacific/Chatham",
-  "Pacific/Chuuk",
-  "Pacific/Easter",
-  "Pacific/Efate",
-  "Pacific/Enderbury",
-  "Pacific/Fakaofo",
-  "Pacific/Fiji",
-  "Pacific/Funafuti",
-  "Pacific/Galapagos",
-  "Pacific/Gambier",
-  "Pacific/Guadalcanal",
-  "Pacific/Guam",
-  "Pacific/Honolulu",
-  "Pacific/Johnston",
-  "Pacific/Kiritimati",
-  "Pacific/Kosrae",
-  "Pacific/Kwajalein",
-  "Pacific/Majuro",
-  "Pacific/Marquesas",
-  "Pacific/Midway",
-  "Pacific/Nauru",
-  "Pacific/Niue",
-  "Pacific/Norfolk",
-  "Pacific/Noumea",
-  "Pacific/Pago_Pago",
-  "Pacific/Palau",
-  "Pacific/Pitcairn",
-  "Pacific/Pohnpei",
-  "Pacific/Ponape",
-  "Pacific/Port_Moresby",
-  "Pacific/Rarotonga",
-  "Pacific/Saipan",
-  "Pacific/Samoa",
-  "Pacific/Tahiti",
-  "Pacific/Tarawa",
-  "Pacific/Tongatapu",
-  "Pacific/Truk",
-  "Pacific/Wake",
-  "Pacific/Wallis",
-  "Pacific/Yap",
-  "Poland",
-  "Portugal",
-  "ROC",
-  "ROK",
-  "Singapore",
-  "Turkey",
-  "UCT",
-  "US/Alaska",
-  "US/Aleutian",
-  "US/Arizona",
-  "US/Central",
-  "US/East-Indiana",
-  "US/Eastern",
-  "US/Hawaii",
-  "US/Indiana-Starke",
-  "US/Michigan",
-  "US/Mountain",
-  "US/Pacific",
-  "US/Samoa",
-  "UTC",
-  "Universal",
-  "W-SU",
-  "WET",
-  "Zulu",
-  nullptr
-};
+const char* const kTimeZoneNames[] = {"Africa/Abidjan",
+                                      "Africa/Accra",
+                                      "Africa/Addis_Ababa",
+                                      "Africa/Algiers",
+                                      "Africa/Asmara",
+                                      "Africa/Asmera",
+                                      "Africa/Bamako",
+                                      "Africa/Bangui",
+                                      "Africa/Banjul",
+                                      "Africa/Bissau",
+                                      "Africa/Blantyre",
+                                      "Africa/Brazzaville",
+                                      "Africa/Bujumbura",
+                                      "Africa/Cairo",
+                                      "Africa/Casablanca",
+                                      "Africa/Ceuta",
+                                      "Africa/Conakry",
+                                      "Africa/Dakar",
+                                      "Africa/Dar_es_Salaam",
+                                      "Africa/Djibouti",
+                                      "Africa/Douala",
+                                      "Africa/El_Aaiun",
+                                      "Africa/Freetown",
+                                      "Africa/Gaborone",
+                                      "Africa/Harare",
+                                      "Africa/Johannesburg",
+                                      "Africa/Juba",
+                                      "Africa/Kampala",
+                                      "Africa/Khartoum",
+                                      "Africa/Kigali",
+                                      "Africa/Kinshasa",
+                                      "Africa/Lagos",
+                                      "Africa/Libreville",
+                                      "Africa/Lome",
+                                      "Africa/Luanda",
+                                      "Africa/Lubumbashi",
+                                      "Africa/Lusaka",
+                                      "Africa/Malabo",
+                                      "Africa/Maputo",
+                                      "Africa/Maseru",
+                                      "Africa/Mbabane",
+                                      "Africa/Mogadishu",
+                                      "Africa/Monrovia",
+                                      "Africa/Nairobi",
+                                      "Africa/Ndjamena",
+                                      "Africa/Niamey",
+                                      "Africa/Nouakchott",
+                                      "Africa/Ouagadougou",
+                                      "Africa/Porto-Novo",
+                                      "Africa/Sao_Tome",
+                                      "Africa/Timbuktu",
+                                      "Africa/Tripoli",
+                                      "Africa/Tunis",
+                                      "Africa/Windhoek",
+                                      "America/Adak",
+                                      "America/Anchorage",
+                                      "America/Anguilla",
+                                      "America/Antigua",
+                                      "America/Araguaina",
+                                      "America/Argentina/Buenos_Aires",
+                                      "America/Argentina/Catamarca",
+                                      "America/Argentina/ComodRivadavia",
+                                      "America/Argentina/Cordoba",
+                                      "America/Argentina/Jujuy",
+                                      "America/Argentina/La_Rioja",
+                                      "America/Argentina/Mendoza",
+                                      "America/Argentina/Rio_Gallegos",
+                                      "America/Argentina/Salta",
+                                      "America/Argentina/San_Juan",
+                                      "America/Argentina/San_Luis",
+                                      "America/Argentina/Tucuman",
+                                      "America/Argentina/Ushuaia",
+                                      "America/Aruba",
+                                      "America/Asuncion",
+                                      "America/Atikokan",
+                                      "America/Atka",
+                                      "America/Bahia",
+                                      "America/Bahia_Banderas",
+                                      "America/Barbados",
+                                      "America/Belem",
+                                      "America/Belize",
+                                      "America/Blanc-Sablon",
+                                      "America/Boa_Vista",
+                                      "America/Bogota",
+                                      "America/Boise",
+                                      "America/Buenos_Aires",
+                                      "America/Cambridge_Bay",
+                                      "America/Campo_Grande",
+                                      "America/Cancun",
+                                      "America/Caracas",
+                                      "America/Catamarca",
+                                      "America/Cayenne",
+                                      "America/Cayman",
+                                      "America/Chicago",
+                                      "America/Chihuahua",
+                                      "America/Coral_Harbour",
+                                      "America/Cordoba",
+                                      "America/Costa_Rica",
+                                      "America/Creston",
+                                      "America/Cuiaba",
+                                      "America/Curacao",
+                                      "America/Danmarkshavn",
+                                      "America/Dawson",
+                                      "America/Dawson_Creek",
+                                      "America/Denver",
+                                      "America/Detroit",
+                                      "America/Dominica",
+                                      "America/Edmonton",
+                                      "America/Eirunepe",
+                                      "America/El_Salvador",
+                                      "America/Ensenada",
+                                      "America/Fort_Nelson",
+                                      "America/Fort_Wayne",
+                                      "America/Fortaleza",
+                                      "America/Glace_Bay",
+                                      "America/Godthab",
+                                      "America/Goose_Bay",
+                                      "America/Grand_Turk",
+                                      "America/Grenada",
+                                      "America/Guadeloupe",
+                                      "America/Guatemala",
+                                      "America/Guayaquil",
+                                      "America/Guyana",
+                                      "America/Halifax",
+                                      "America/Havana",
+                                      "America/Hermosillo",
+                                      "America/Indiana/Indianapolis",
+                                      "America/Indiana/Knox",
+                                      "America/Indiana/Marengo",
+                                      "America/Indiana/Petersburg",
+                                      "America/Indiana/Tell_City",
+                                      "America/Indiana/Vevay",
+                                      "America/Indiana/Vincennes",
+                                      "America/Indiana/Winamac",
+                                      "America/Indianapolis",
+                                      "America/Inuvik",
+                                      "America/Iqaluit",
+                                      "America/Jamaica",
+                                      "America/Jujuy",
+                                      "America/Juneau",
+                                      "America/Kentucky/Louisville",
+                                      "America/Kentucky/Monticello",
+                                      "America/Knox_IN",
+                                      "America/Kralendijk",
+                                      "America/La_Paz",
+                                      "America/Lima",
+                                      "America/Los_Angeles",
+                                      "America/Louisville",
+                                      "America/Lower_Princes",
+                                      "America/Maceio",
+                                      "America/Managua",
+                                      "America/Manaus",
+                                      "America/Marigot",
+                                      "America/Martinique",
+                                      "America/Matamoros",
+                                      "America/Mazatlan",
+                                      "America/Mendoza",
+                                      "America/Menominee",
+                                      "America/Merida",
+                                      "America/Metlakatla",
+                                      "America/Mexico_City",
+                                      "America/Miquelon",
+                                      "America/Moncton",
+                                      "America/Monterrey",
+                                      "America/Montevideo",
+                                      "America/Montreal",
+                                      "America/Montserrat",
+                                      "America/Nassau",
+                                      "America/New_York",
+                                      "America/Nipigon",
+                                      "America/Nome",
+                                      "America/Noronha",
+                                      "America/North_Dakota/Beulah",
+                                      "America/North_Dakota/Center",
+                                      "America/North_Dakota/New_Salem",
+                                      "America/Ojinaga",
+                                      "America/Panama",
+                                      "America/Pangnirtung",
+                                      "America/Paramaribo",
+                                      "America/Phoenix",
+                                      "America/Port-au-Prince",
+                                      "America/Port_of_Spain",
+                                      "America/Porto_Acre",
+                                      "America/Porto_Velho",
+                                      "America/Puerto_Rico",
+                                      "America/Punta_Arenas",
+                                      "America/Rainy_River",
+                                      "America/Rankin_Inlet",
+                                      "America/Recife",
+                                      "America/Regina",
+                                      "America/Resolute",
+                                      "America/Rio_Branco",
+                                      "America/Rosario",
+                                      "America/Santa_Isabel",
+                                      "America/Santarem",
+                                      "America/Santiago",
+                                      "America/Santo_Domingo",
+                                      "America/Sao_Paulo",
+                                      "America/Scoresbysund",
+                                      "America/Shiprock",
+                                      "America/Sitka",
+                                      "America/St_Barthelemy",
+                                      "America/St_Johns",
+                                      "America/St_Kitts",
+                                      "America/St_Lucia",
+                                      "America/St_Thomas",
+                                      "America/St_Vincent",
+                                      "America/Swift_Current",
+                                      "America/Tegucigalpa",
+                                      "America/Thule",
+                                      "America/Thunder_Bay",
+                                      "America/Tijuana",
+                                      "America/Toronto",
+                                      "America/Tortola",
+                                      "America/Vancouver",
+                                      "America/Virgin",
+                                      "America/Whitehorse",
+                                      "America/Winnipeg",
+                                      "America/Yakutat",
+                                      "America/Yellowknife",
+                                      "Antarctica/Casey",
+                                      "Antarctica/Davis",
+                                      "Antarctica/DumontDUrville",
+                                      "Antarctica/Macquarie",
+                                      "Antarctica/Mawson",
+                                      "Antarctica/McMurdo",
+                                      "Antarctica/Palmer",
+                                      "Antarctica/Rothera",
+                                      "Antarctica/South_Pole",
+                                      "Antarctica/Syowa",
+                                      "Antarctica/Troll",
+                                      "Antarctica/Vostok",
+                                      "Arctic/Longyearbyen",
+                                      "Asia/Aden",
+                                      "Asia/Almaty",
+                                      "Asia/Amman",
+                                      "Asia/Anadyr",
+                                      "Asia/Aqtau",
+                                      "Asia/Aqtobe",
+                                      "Asia/Ashgabat",
+                                      "Asia/Ashkhabad",
+                                      "Asia/Atyrau",
+                                      "Asia/Baghdad",
+                                      "Asia/Bahrain",
+                                      "Asia/Baku",
+                                      "Asia/Bangkok",
+                                      "Asia/Barnaul",
+                                      "Asia/Beirut",
+                                      "Asia/Bishkek",
+                                      "Asia/Brunei",
+                                      "Asia/Calcutta",
+                                      "Asia/Chita",
+                                      "Asia/Choibalsan",
+                                      "Asia/Chongqing",
+                                      "Asia/Chungking",
+                                      "Asia/Colombo",
+                                      "Asia/Dacca",
+                                      "Asia/Damascus",
+                                      "Asia/Dhaka",
+                                      "Asia/Dili",
+                                      "Asia/Dubai",
+                                      "Asia/Dushanbe",
+                                      "Asia/Famagusta",
+                                      "Asia/Gaza",
+                                      "Asia/Harbin",
+                                      "Asia/Hebron",
+                                      "Asia/Ho_Chi_Minh",
+                                      "Asia/Hong_Kong",
+                                      "Asia/Hovd",
+                                      "Asia/Irkutsk",
+                                      "Asia/Istanbul",
+                                      "Asia/Jakarta",
+                                      "Asia/Jayapura",
+                                      "Asia/Jerusalem",
+                                      "Asia/Kabul",
+                                      "Asia/Kamchatka",
+                                      "Asia/Karachi",
+                                      "Asia/Kashgar",
+                                      "Asia/Kathmandu",
+                                      "Asia/Katmandu",
+                                      "Asia/Khandyga",
+                                      "Asia/Kolkata",
+                                      "Asia/Krasnoyarsk",
+                                      "Asia/Kuala_Lumpur",
+                                      "Asia/Kuching",
+                                      "Asia/Kuwait",
+                                      "Asia/Macao",
+                                      "Asia/Macau",
+                                      "Asia/Magadan",
+                                      "Asia/Makassar",
+                                      "Asia/Manila",
+                                      "Asia/Muscat",
+                                      "Asia/Nicosia",
+                                      "Asia/Novokuznetsk",
+                                      "Asia/Novosibirsk",
+                                      "Asia/Omsk",
+                                      "Asia/Oral",
+                                      "Asia/Phnom_Penh",
+                                      "Asia/Pontianak",
+                                      "Asia/Pyongyang",
+                                      "Asia/Qatar",
+                                      "Asia/Qostanay",
+                                      "Asia/Qyzylorda",
+                                      "Asia/Rangoon",
+                                      "Asia/Riyadh",
+                                      "Asia/Saigon",
+                                      "Asia/Sakhalin",
+                                      "Asia/Samarkand",
+                                      "Asia/Seoul",
+                                      "Asia/Shanghai",
+                                      "Asia/Singapore",
+                                      "Asia/Srednekolymsk",
+                                      "Asia/Taipei",
+                                      "Asia/Tashkent",
+                                      "Asia/Tbilisi",
+                                      "Asia/Tehran",
+                                      "Asia/Tel_Aviv",
+                                      "Asia/Thimbu",
+                                      "Asia/Thimphu",
+                                      "Asia/Tokyo",
+                                      "Asia/Tomsk",
+                                      "Asia/Ujung_Pandang",
+                                      "Asia/Ulaanbaatar",
+                                      "Asia/Ulan_Bator",
+                                      "Asia/Urumqi",
+                                      "Asia/Ust-Nera",
+                                      "Asia/Vientiane",
+                                      "Asia/Vladivostok",
+                                      "Asia/Yakutsk",
+                                      "Asia/Yangon",
+                                      "Asia/Yekaterinburg",
+                                      "Asia/Yerevan",
+                                      "Atlantic/Azores",
+                                      "Atlantic/Bermuda",
+                                      "Atlantic/Canary",
+                                      "Atlantic/Cape_Verde",
+                                      "Atlantic/Faeroe",
+                                      "Atlantic/Faroe",
+                                      "Atlantic/Jan_Mayen",
+                                      "Atlantic/Madeira",
+                                      "Atlantic/Reykjavik",
+                                      "Atlantic/South_Georgia",
+                                      "Atlantic/St_Helena",
+                                      "Atlantic/Stanley",
+                                      "Australia/ACT",
+                                      "Australia/Adelaide",
+                                      "Australia/Brisbane",
+                                      "Australia/Broken_Hill",
+                                      "Australia/Canberra",
+                                      "Australia/Currie",
+                                      "Australia/Darwin",
+                                      "Australia/Eucla",
+                                      "Australia/Hobart",
+                                      "Australia/LHI",
+                                      "Australia/Lindeman",
+                                      "Australia/Lord_Howe",
+                                      "Australia/Melbourne",
+                                      "Australia/NSW",
+                                      "Australia/North",
+                                      "Australia/Perth",
+                                      "Australia/Queensland",
+                                      "Australia/South",
+                                      "Australia/Sydney",
+                                      "Australia/Tasmania",
+                                      "Australia/Victoria",
+                                      "Australia/West",
+                                      "Australia/Yancowinna",
+                                      "Brazil/Acre",
+                                      "Brazil/DeNoronha",
+                                      "Brazil/East",
+                                      "Brazil/West",
+                                      "CET",
+                                      "CST6CDT",
+                                      "Canada/Atlantic",
+                                      "Canada/Central",
+                                      "Canada/Eastern",
+                                      "Canada/Mountain",
+                                      "Canada/Newfoundland",
+                                      "Canada/Pacific",
+                                      "Canada/Saskatchewan",
+                                      "Canada/Yukon",
+                                      "Chile/Continental",
+                                      "Chile/EasterIsland",
+                                      "Cuba",
+                                      "EET",
+                                      "EST",
+                                      "EST5EDT",
+                                      "Egypt",
+                                      "Eire",
+                                      "Etc/GMT",
+                                      "Etc/GMT+0",
+                                      "Etc/GMT+1",
+                                      "Etc/GMT+10",
+                                      "Etc/GMT+11",
+                                      "Etc/GMT+12",
+                                      "Etc/GMT+2",
+                                      "Etc/GMT+3",
+                                      "Etc/GMT+4",
+                                      "Etc/GMT+5",
+                                      "Etc/GMT+6",
+                                      "Etc/GMT+7",
+                                      "Etc/GMT+8",
+                                      "Etc/GMT+9",
+                                      "Etc/GMT-0",
+                                      "Etc/GMT-1",
+                                      "Etc/GMT-10",
+                                      "Etc/GMT-11",
+                                      "Etc/GMT-12",
+                                      "Etc/GMT-13",
+                                      "Etc/GMT-14",
+                                      "Etc/GMT-2",
+                                      "Etc/GMT-3",
+                                      "Etc/GMT-4",
+                                      "Etc/GMT-5",
+                                      "Etc/GMT-6",
+                                      "Etc/GMT-7",
+                                      "Etc/GMT-8",
+                                      "Etc/GMT-9",
+                                      "Etc/GMT0",
+                                      "Etc/Greenwich",
+                                      "Etc/UCT",
+                                      "Etc/UTC",
+                                      "Etc/Universal",
+                                      "Etc/Zulu",
+                                      "Europe/Amsterdam",
+                                      "Europe/Andorra",
+                                      "Europe/Astrakhan",
+                                      "Europe/Athens",
+                                      "Europe/Belfast",
+                                      "Europe/Belgrade",
+                                      "Europe/Berlin",
+                                      "Europe/Bratislava",
+                                      "Europe/Brussels",
+                                      "Europe/Bucharest",
+                                      "Europe/Budapest",
+                                      "Europe/Busingen",
+                                      "Europe/Chisinau",
+                                      "Europe/Copenhagen",
+                                      "Europe/Dublin",
+                                      "Europe/Gibraltar",
+                                      "Europe/Guernsey",
+                                      "Europe/Helsinki",
+                                      "Europe/Isle_of_Man",
+                                      "Europe/Istanbul",
+                                      "Europe/Jersey",
+                                      "Europe/Kaliningrad",
+                                      "Europe/Kiev",
+                                      "Europe/Kirov",
+                                      "Europe/Lisbon",
+                                      "Europe/Ljubljana",
+                                      "Europe/London",
+                                      "Europe/Luxembourg",
+                                      "Europe/Madrid",
+                                      "Europe/Malta",
+                                      "Europe/Mariehamn",
+                                      "Europe/Minsk",
+                                      "Europe/Monaco",
+                                      "Europe/Moscow",
+                                      "Europe/Nicosia",
+                                      "Europe/Oslo",
+                                      "Europe/Paris",
+                                      "Europe/Podgorica",
+                                      "Europe/Prague",
+                                      "Europe/Riga",
+                                      "Europe/Rome",
+                                      "Europe/Samara",
+                                      "Europe/San_Marino",
+                                      "Europe/Sarajevo",
+                                      "Europe/Saratov",
+                                      "Europe/Simferopol",
+                                      "Europe/Skopje",
+                                      "Europe/Sofia",
+                                      "Europe/Stockholm",
+                                      "Europe/Tallinn",
+                                      "Europe/Tirane",
+                                      "Europe/Tiraspol",
+                                      "Europe/Ulyanovsk",
+                                      "Europe/Uzhgorod",
+                                      "Europe/Vaduz",
+                                      "Europe/Vatican",
+                                      "Europe/Vienna",
+                                      "Europe/Vilnius",
+                                      "Europe/Volgograd",
+                                      "Europe/Warsaw",
+                                      "Europe/Zagreb",
+                                      "Europe/Zaporozhye",
+                                      "Europe/Zurich",
+                                      "GB",
+                                      "GB-Eire",
+                                      "GMT",
+                                      "GMT+0",
+                                      "GMT-0",
+                                      "GMT0",
+                                      "Greenwich",
+                                      "HST",
+                                      "Hongkong",
+                                      "Iceland",
+                                      "Indian/Antananarivo",
+                                      "Indian/Chagos",
+                                      "Indian/Christmas",
+                                      "Indian/Cocos",
+                                      "Indian/Comoro",
+                                      "Indian/Kerguelen",
+                                      "Indian/Mahe",
+                                      "Indian/Maldives",
+                                      "Indian/Mauritius",
+                                      "Indian/Mayotte",
+                                      "Indian/Reunion",
+                                      "Iran",
+                                      "Israel",
+                                      "Jamaica",
+                                      "Japan",
+                                      "Kwajalein",
+                                      "Libya",
+                                      "MET",
+                                      "MST",
+                                      "MST7MDT",
+                                      "Mexico/BajaNorte",
+                                      "Mexico/BajaSur",
+                                      "Mexico/General",
+                                      "NZ",
+                                      "NZ-CHAT",
+                                      "Navajo",
+                                      "PRC",
+                                      "PST8PDT",
+                                      "Pacific/Apia",
+                                      "Pacific/Auckland",
+                                      "Pacific/Bougainville",
+                                      "Pacific/Chatham",
+                                      "Pacific/Chuuk",
+                                      "Pacific/Easter",
+                                      "Pacific/Efate",
+                                      "Pacific/Enderbury",
+                                      "Pacific/Fakaofo",
+                                      "Pacific/Fiji",
+                                      "Pacific/Funafuti",
+                                      "Pacific/Galapagos",
+                                      "Pacific/Gambier",
+                                      "Pacific/Guadalcanal",
+                                      "Pacific/Guam",
+                                      "Pacific/Honolulu",
+                                      "Pacific/Johnston",
+                                      "Pacific/Kiritimati",
+                                      "Pacific/Kosrae",
+                                      "Pacific/Kwajalein",
+                                      "Pacific/Majuro",
+                                      "Pacific/Marquesas",
+                                      "Pacific/Midway",
+                                      "Pacific/Nauru",
+                                      "Pacific/Niue",
+                                      "Pacific/Norfolk",
+                                      "Pacific/Noumea",
+                                      "Pacific/Pago_Pago",
+                                      "Pacific/Palau",
+                                      "Pacific/Pitcairn",
+                                      "Pacific/Pohnpei",
+                                      "Pacific/Ponape",
+                                      "Pacific/Port_Moresby",
+                                      "Pacific/Rarotonga",
+                                      "Pacific/Saipan",
+                                      "Pacific/Samoa",
+                                      "Pacific/Tahiti",
+                                      "Pacific/Tarawa",
+                                      "Pacific/Tongatapu",
+                                      "Pacific/Truk",
+                                      "Pacific/Wake",
+                                      "Pacific/Wallis",
+                                      "Pacific/Yap",
+                                      "Poland",
+                                      "Portugal",
+                                      "ROC",
+                                      "ROK",
+                                      "Singapore",
+                                      "Turkey",
+                                      "UCT",
+                                      "US/Alaska",
+                                      "US/Aleutian",
+                                      "US/Arizona",
+                                      "US/Central",
+                                      "US/East-Indiana",
+                                      "US/Eastern",
+                                      "US/Hawaii",
+                                      "US/Indiana-Starke",
+                                      "US/Michigan",
+                                      "US/Mountain",
+                                      "US/Pacific",
+                                      "US/Samoa",
+                                      "UTC",
+                                      "Universal",
+                                      "W-SU",
+                                      "WET",
+                                      "Zulu",
+                                      nullptr};
 
 std::vector<std::string> AllTimeZoneNames() {
   std::vector<std::string> names;
diff --git a/absl/time/internal/cctz/src/civil_time_detail.cc b/absl/time/internal/cctz/src/civil_time_detail.cc
index 4df15d5..0b07e39 100644
--- a/absl/time/internal/cctz/src/civil_time_detail.cc
+++ b/absl/time/internal/cctz/src/civil_time_detail.cc
@@ -18,8 +18,10 @@
 #include <ostream>
 #include <sstream>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 namespace detail {
@@ -88,5 +90,5 @@
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/civil_time_test.cc b/absl/time/internal/cctz/src/civil_time_test.cc
index 7d9a183..be894d7 100644
--- a/absl/time/internal/cctz/src/civil_time_test.cc
+++ b/absl/time/internal/cctz/src/civil_time_test.cc
@@ -21,9 +21,10 @@
 #include <type_traits>
 
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -1015,19 +1016,13 @@
       int day;
     } leap_day;  // The date of the day after Feb 28.
   } kLeapYearTable[]{
-      {1900, 365, {3, 1}},
-      {1999, 365, {3, 1}},
+      {1900, 365, {3, 1}},  {1999, 365, {3, 1}},
       {2000, 366, {2, 29}},  // leap year
-      {2001, 365, {3, 1}},
-      {2002, 365, {3, 1}},
-      {2003, 365, {3, 1}},
-      {2004, 366, {2, 29}},  // leap year
-      {2005, 365, {3, 1}},
-      {2006, 365, {3, 1}},
-      {2007, 365, {3, 1}},
-      {2008, 366, {2, 29}},  // leap year
-      {2009, 365, {3, 1}},
-      {2100, 365, {3, 1}},
+      {2001, 365, {3, 1}},  {2002, 365, {3, 1}},
+      {2003, 365, {3, 1}},  {2004, 366, {2, 29}},  // leap year
+      {2005, 365, {3, 1}},  {2006, 365, {3, 1}},
+      {2007, 365, {3, 1}},  {2008, 366, {2, 29}},  // leap year
+      {2009, 365, {3, 1}},  {2100, 365, {3, 1}},
   };
 
   for (const auto& e : kLeapYearTable) {
@@ -1057,5 +1052,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_fixed.cc b/absl/time/internal/cctz/src/time_zone_fixed.cc
index aa5af02..303c024 100644
--- a/absl/time/internal/cctz/src/time_zone_fixed.cc
+++ b/absl/time/internal/cctz/src/time_zone_fixed.cc
@@ -20,8 +20,10 @@
 #include <cstring>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -60,11 +62,9 @@
   const char* const ep = kFixedZonePrefix + prefix_len;
   if (name.size() != prefix_len + 9)  // <prefix>+99:99:99
     return false;
-  if (!std::equal(kFixedZonePrefix, ep, name.begin()))
-    return false;
+  if (!std::equal(kFixedZonePrefix, ep, name.begin())) return false;
   const char* np = name.data() + prefix_len;
-  if (np[0] != '+' && np[0] != '-')
-    return false;
+  if (np[0] != '+' && np[0] != '-') return false;
   if (np[3] != ':' || np[6] != ':')  // see note below about large offsets
     return false;
 
@@ -89,29 +89,29 @@
     // offsets and to (somewhat) limit the total number of zones.
     return "UTC";
   }
-  int seconds = static_cast<int>(offset.count());
-  const char sign = (seconds < 0 ? '-' : '+');
-  int minutes = seconds / 60;
-  seconds %= 60;
+  int offset_seconds = static_cast<int>(offset.count());
+  const char sign = (offset_seconds < 0 ? '-' : '+');
+  int offset_minutes = offset_seconds / 60;
+  offset_seconds %= 60;
   if (sign == '-') {
-    if (seconds > 0) {
-      seconds -= 60;
-      minutes += 1;
+    if (offset_seconds > 0) {
+      offset_seconds -= 60;
+      offset_minutes += 1;
     }
-    seconds = -seconds;
-    minutes = -minutes;
+    offset_seconds = -offset_seconds;
+    offset_minutes = -offset_minutes;
   }
-  int hours = minutes / 60;
-  minutes %= 60;
+  int offset_hours = offset_minutes / 60;
+  offset_minutes %= 60;
   const std::size_t prefix_len = sizeof(kFixedZonePrefix) - 1;
   char buf[prefix_len + sizeof("-24:00:00")];
   char* ep = std::copy(kFixedZonePrefix, kFixedZonePrefix + prefix_len, buf);
   *ep++ = sign;
-  ep = Format02d(ep, hours);
+  ep = Format02d(ep, offset_hours);
   *ep++ = ':';
-  ep = Format02d(ep, minutes);
+  ep = Format02d(ep, offset_minutes);
   *ep++ = ':';
-  ep = Format02d(ep, seconds);
+  ep = Format02d(ep, offset_seconds);
   *ep++ = '\0';
   assert(ep == buf + sizeof(buf));
   return buf;
@@ -136,5 +136,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_fixed.h b/absl/time/internal/cctz/src/time_zone_fixed.h
index 15ea3bd..e74a0bb 100644
--- a/absl/time/internal/cctz/src/time_zone_fixed.h
+++ b/absl/time/internal/cctz/src/time_zone_fixed.h
@@ -17,10 +17,11 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -45,7 +46,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_FIXED_H_
diff --git a/absl/time/internal/cctz/src/time_zone_format.cc b/absl/time/internal/cctz/src/time_zone_format.cc
index ee2ffac..950b23a 100644
--- a/absl/time/internal/cctz/src/time_zone_format.cc
+++ b/absl/time/internal/cctz/src/time_zone_format.cc
@@ -13,17 +13,18 @@
 //   limitations under the License.
 
 #if !defined(HAS_STRPTIME)
-# if !defined(_MSC_VER) && !defined(__MINGW32__)
-#  define HAS_STRPTIME 1  // assume everyone has strptime() except windows
-# endif
+#if !defined(_MSC_VER) && !defined(__MINGW32__)
+#define HAS_STRPTIME 1  // assume everyone has strptime() except windows
+#endif
 #endif
 
 #if defined(HAS_STRPTIME) && HAS_STRPTIME
-# if !defined(_XOPEN_SOURCE)
-#  define _XOPEN_SOURCE  // Definedness suffices for strptime.
-# endif
+#if !defined(_XOPEN_SOURCE)
+#define _XOPEN_SOURCE  // Definedness suffices for strptime.
+#endif
 #endif
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 // Include time.h directly since, by C++ standards, ctime doesn't have to
@@ -48,7 +49,7 @@
 #include "time_zone_if.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 namespace detail {
@@ -503,8 +504,9 @@
           bp = ep;
           if (n > 0) {
             if (n > kDigits10_64) n = kDigits10_64;
-            bp = Format64(bp, n, (n > 15) ? fs.count() * kExp10[n - 15]
-                                          : fs.count() / kExp10[15 - n]);
+            bp = Format64(bp, n,
+                          (n > 15) ? fs.count() * kExp10[n - 15]
+                                   : fs.count() / kExp10[15 - n]);
             if (*np == 'S') *--bp = '.';
           }
           if (*np == 'S') bp = Format02d(bp, al.cs.second());
@@ -721,10 +723,9 @@
         data = ParseZone(data, &zone);
         continue;
       case 's':
-        data = ParseInt(data, 0,
-                        std::numeric_limits<std::int_fast64_t>::min(),
-                        std::numeric_limits<std::int_fast64_t>::max(),
-                        &percent_s);
+        data =
+            ParseInt(data, 0, std::numeric_limits<std::int_fast64_t>::min(),
+                     std::numeric_limits<std::int_fast64_t>::max(), &percent_s);
         if (data != nullptr) saw_percent_s = true;
         continue;
       case ':':
@@ -917,5 +918,5 @@
 }  // namespace detail
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_format_test.cc b/absl/time/internal/cctz/src/time_zone_format_test.cc
index 4973744..caebcc4 100644
--- a/absl/time/internal/cctz/src/time_zone_format_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_format_test.cc
@@ -12,21 +12,21 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "absl/time/internal/cctz/include/cctz/time_zone.h"
-
 #include <chrono>
 #include <iomanip>
 #include <sstream>
 #include <string>
 
-#include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/include/cctz/civil_time.h"
+#include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace chrono = std::chrono;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -49,10 +49,10 @@
   } while (0)
 
 const char RFC3339_full[] = "%Y-%m-%dT%H:%M:%E*S%Ez";
-const char RFC3339_sec[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+const char RFC3339_sec[] = "%Y-%m-%dT%H:%M:%S%Ez";
 
 const char RFC1123_full[] = "%a, %d %b %Y %H:%M:%S %z";
-const char RFC1123_no_wday[] =  "%d %b %Y %H:%M:%S %z";
+const char RFC1123_no_wday[] = "%d %b %Y %H:%M:%S %z";
 
 // A helper that tests the given format specifier by itself, and with leading
 // and trailing characters.  For example: TestFormatSpecifier(tp, "%a", "Thu").
@@ -89,8 +89,11 @@
       format(kFmt, chrono::time_point_cast<chrono::milliseconds>(t0), utc));
   EXPECT_EQ("03:04:05",
             format(kFmt, chrono::time_point_cast<chrono::seconds>(t0), utc));
-  EXPECT_EQ("03:04:05",
-            format(kFmt, chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0), utc));
+  EXPECT_EQ(
+      "03:04:05",
+      format(kFmt,
+             chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0),
+             utc));
   EXPECT_EQ("03:04:00",
             format(kFmt, chrono::time_point_cast<chrono::minutes>(t0), utc));
   EXPECT_EQ("03:00:00",
@@ -111,12 +114,10 @@
   EXPECT_EQ(
       "12:34:56.012345678901234",
       detail::format(kFmt, tp, detail::femtoseconds(12345678901234), utc));
-  EXPECT_EQ(
-      "12:34:56.001234567890123",
-      detail::format(kFmt, tp, detail::femtoseconds(1234567890123), utc));
-  EXPECT_EQ(
-      "12:34:56.000123456789012",
-      detail::format(kFmt, tp, detail::femtoseconds(123456789012), utc));
+  EXPECT_EQ("12:34:56.001234567890123",
+            detail::format(kFmt, tp, detail::femtoseconds(1234567890123), utc));
+  EXPECT_EQ("12:34:56.000123456789012",
+            detail::format(kFmt, tp, detail::femtoseconds(123456789012), utc));
 
   EXPECT_EQ("12:34:56.000000000000123",
             detail::format(kFmt, tp, detail::femtoseconds(123), utc));
@@ -1251,9 +1252,9 @@
         const auto expected = chrono::system_clock::from_time_t(0) +
                               chrono::nanoseconds(micros * 1000 + ns);
         for (int ps = 0; ps < 1000; ps += 250) {
-          std::ostringstream oss;
+          std::ostringstream ps_oss;
           oss << std::setfill('0') << std::setw(3) << ps;
-          const std::string input = nanos + oss.str() + "999";
+          const std::string input = nanos + ps_oss.str() + "999";
           EXPECT_TRUE(parse("%E*f", input, tz, &tp));
           EXPECT_EQ(expected + chrono::nanoseconds(ps) / 1000, tp) << input;
         }
@@ -1417,8 +1418,8 @@
       parse(RFC3339_sec, "-292277022657-01-27T08:29:51+01:00", utc, &tp));
 
   // tests max/min civil-second overflow
-  EXPECT_FALSE(parse(RFC3339_sec, "9223372036854775807-12-31T23:59:59-00:01",
-                     utc, &tp));
+  EXPECT_FALSE(
+      parse(RFC3339_sec, "9223372036854775807-12-31T23:59:59-00:01", utc, &tp));
   EXPECT_FALSE(parse(RFC3339_sec, "-9223372036854775808-01-01T00:00:00+00:01",
                      utc, &tp));
 
@@ -1475,7 +1476,8 @@
 
 TEST(FormatParse, RoundTripDistantFuture) {
   const time_zone utc = utc_time_zone();
-  const time_point<absl::time_internal::cctz::seconds> in = time_point<absl::time_internal::cctz::seconds>::max();
+  const time_point<absl::time_internal::cctz::seconds> in =
+      time_point<absl::time_internal::cctz::seconds>::max();
   const std::string s = format(RFC3339_full, in, utc);
   time_point<absl::time_internal::cctz::seconds> out;
   EXPECT_TRUE(parse(RFC3339_full, s, utc, &out)) << s;
@@ -1484,7 +1486,8 @@
 
 TEST(FormatParse, RoundTripDistantPast) {
   const time_zone utc = utc_time_zone();
-  const time_point<absl::time_internal::cctz::seconds> in = time_point<absl::time_internal::cctz::seconds>::min();
+  const time_point<absl::time_internal::cctz::seconds> in =
+      time_point<absl::time_internal::cctz::seconds>::min();
   const std::string s = format(RFC3339_full, in, utc);
   time_point<absl::time_internal::cctz::seconds> out;
   EXPECT_TRUE(parse(RFC3339_full, s, utc, &out)) << s;
@@ -1493,5 +1496,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_if.cc b/absl/time/internal/cctz/src/time_zone_if.cc
index 75d23da..0319b2f 100644
--- a/absl/time/internal/cctz/src/time_zone_if.cc
+++ b/absl/time/internal/cctz/src/time_zone_if.cc
@@ -13,11 +13,13 @@
 //   limitations under the License.
 
 #include "time_zone_if.h"
+
+#include "absl/base/config.h"
 #include "time_zone_info.h"
 #include "time_zone_libc.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -39,5 +41,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_if.h b/absl/time/internal/cctz/src/time_zone_if.h
index 0081d99..32c0891 100644
--- a/absl/time/internal/cctz/src/time_zone_if.h
+++ b/absl/time/internal/cctz/src/time_zone_if.h
@@ -20,11 +20,12 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -39,8 +40,7 @@
 
   virtual time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const = 0;
-  virtual time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const = 0;
+  virtual time_zone::civil_lookup MakeTime(const civil_second& cs) const = 0;
 
   virtual bool NextTransition(const time_point<seconds>& tp,
                               time_zone::civil_transition* trans) const = 0;
@@ -59,16 +59,18 @@
 // Unix clock are second aligned, but not that they share an epoch.
 inline std::int_fast64_t ToUnixSeconds(const time_point<seconds>& tp) {
   return (tp - std::chrono::time_point_cast<seconds>(
-                   std::chrono::system_clock::from_time_t(0))).count();
+                   std::chrono::system_clock::from_time_t(0)))
+      .count();
 }
 inline time_point<seconds> FromUnixSeconds(std::int_fast64_t t) {
   return std::chrono::time_point_cast<seconds>(
-             std::chrono::system_clock::from_time_t(0)) + seconds(t);
+             std::chrono::system_clock::from_time_t(0)) +
+         seconds(t);
 }
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_IF_H_
diff --git a/absl/time/internal/cctz/src/time_zone_impl.cc b/absl/time/internal/cctz/src/time_zone_impl.cc
index 60911f0..030ae0e 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.cc
+++ b/absl/time/internal/cctz/src/time_zone_impl.cc
@@ -14,15 +14,17 @@
 
 #include "time_zone_impl.h"
 
+#include <deque>
 #include <mutex>
 #include <string>
 #include <unordered_map>
 #include <utility>
 
+#include "absl/base/config.h"
 #include "time_zone_fixed.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -34,13 +36,16 @@
 TimeZoneImplByName* time_zone_map = nullptr;
 
 // Mutual exclusion for time_zone_map.
-std::mutex time_zone_mutex;
+std::mutex& TimeZoneMutex() {
+  // This mutex is intentionally "leaked" to avoid the static deinitialization
+  // order fiasco (std::mutex's destructor is not trivial on many platforms).
+  static std::mutex* time_zone_mutex = new std::mutex;
+  return *time_zone_mutex;
+}
 
 }  // namespace
 
-time_zone time_zone::Impl::UTC() {
-  return time_zone(UTCImpl());
-}
+time_zone time_zone::Impl::UTC() { return time_zone(UTCImpl()); }
 
 bool time_zone::Impl::LoadTimeZone(const std::string& name, time_zone* tz) {
   const time_zone::Impl* const utc_impl = UTCImpl();
@@ -55,7 +60,7 @@
   // Then check, under a shared lock, whether the time zone has already
   // been loaded. This is the common path. TODO: Move to shared_mutex.
   {
-    std::lock_guard<std::mutex> lock(time_zone_mutex);
+    std::lock_guard<std::mutex> lock(TimeZoneMutex());
     if (time_zone_map != nullptr) {
       TimeZoneImplByName::const_iterator itr = time_zone_map->find(name);
       if (itr != time_zone_map->end()) {
@@ -66,7 +71,7 @@
   }
 
   // Now check again, under an exclusive lock.
-  std::lock_guard<std::mutex> lock(time_zone_mutex);
+  std::lock_guard<std::mutex> lock(TimeZoneMutex());
   if (time_zone_map == nullptr) time_zone_map = new TimeZoneImplByName;
   const Impl*& impl = (*time_zone_map)[name];
   if (impl == nullptr) {
@@ -85,10 +90,16 @@
 }
 
 void time_zone::Impl::ClearTimeZoneMapTestOnly() {
-  std::lock_guard<std::mutex> lock(time_zone_mutex);
+  std::lock_guard<std::mutex> lock(TimeZoneMutex());
   if (time_zone_map != nullptr) {
-    // Existing time_zone::Impl* entries are in the wild, so we simply
-    // leak them.  Future requests will result in reloading the data.
+    // Existing time_zone::Impl* entries are in the wild, so we can't delete
+    // them. Instead, we move them to a private container, where they are
+    // logically unreachable but not "leaked".  Future requests will result
+    // in reloading the data.
+    static auto* cleared = new std::deque<const time_zone::Impl*>;
+    for (const auto& element : *time_zone_map) {
+      cleared->push_back(element.second);
+    }
     time_zone_map->clear();
   }
 }
@@ -106,5 +117,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_impl.h b/absl/time/internal/cctz/src/time_zone_impl.h
index 33d3f6b..69806c1 100644
--- a/absl/time/internal/cctz/src/time_zone_impl.h
+++ b/absl/time/internal/cctz/src/time_zone_impl.h
@@ -18,13 +18,14 @@
 #include <memory>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 #include "time_zone_if.h"
 #include "time_zone_info.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -86,7 +87,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_IMPL_H_
diff --git a/absl/time/internal/cctz/src/time_zone_info.cc b/absl/time/internal/cctz/src/time_zone_info.cc
index b45d82f..f1697cd 100644
--- a/absl/time/internal/cctz/src/time_zone_info.cc
+++ b/absl/time/internal/cctz/src/time_zone_info.cc
@@ -45,12 +45,13 @@
 #include <sstream>
 #include <string>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "time_zone_fixed.h"
 #include "time_zone_posix.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -66,8 +67,8 @@
 // The day offsets of the beginning of each (1-based) month in non-leap and
 // leap years respectively (e.g., 335 days before December in a leap year).
 const std::int_least16_t kMonthOffsets[2][1 + 12 + 1] = {
-  {-1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
-  {-1, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366},
+    {-1, 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
+    {-1, 0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366},
 };
 
 // We reject leap-second encoded zoneinfo and so assume 60-second minutes.
@@ -78,8 +79,8 @@
 
 // Like kDaysPerYear[] but scaled up by a factor of kSecsPerDay.
 const std::int_least32_t kSecsPerYear[2] = {
-  365 * kSecsPerDay,
-  366 * kSecsPerDay,
+    365 * kSecsPerDay,
+    366 * kSecsPerDay,
 };
 
 // Single-byte, unsigned numeric values are encoded directly.
@@ -173,8 +174,8 @@
 }
 
 inline civil_second YearShift(const civil_second& cs, year_t shift) {
-  return civil_second(cs.year() + shift, cs.month(), cs.day(),
-                      cs.hour(), cs.minute(), cs.second());
+  return civil_second(cs.year() + shift, cs.month(), cs.day(), cs.hour(),
+                      cs.minute(), cs.second());
 }
 
 }  // namespace
@@ -217,7 +218,7 @@
   default_transition_type_ = 0;
   abbreviations_ = FixedOffsetToAbbr(offset);
   abbreviations_.append(1, '\0');  // add NUL
-  future_spec_.clear();  // never needed for a fixed-offset zone
+  future_spec_.clear();            // never needed for a fixed-offset zone
   extended_ = false;
 
   tt.civil_max = LocalTime(seconds::max().count(), tt).cs;
@@ -394,31 +395,24 @@
 bool TimeZoneInfo::Load(const std::string& name, ZoneInfoSource* zip) {
   // Read and validate the header.
   tzhead tzh;
-  if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))
-    return false;
+  if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh)) return false;
   if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)
     return false;
   Header hdr;
-  if (!hdr.Build(tzh))
-    return false;
+  if (!hdr.Build(tzh)) return false;
   std::size_t time_len = 4;
   if (tzh.tzh_version[0] != '\0') {
     // Skip the 4-byte data.
-    if (zip->Skip(hdr.DataLength(time_len)) != 0)
-      return false;
+    if (zip->Skip(hdr.DataLength(time_len)) != 0) return false;
     // Read and validate the header for the 8-byte data.
-    if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh))
-      return false;
+    if (zip->Read(&tzh, sizeof(tzh)) != sizeof(tzh)) return false;
     if (strncmp(tzh.tzh_magic, TZ_MAGIC, sizeof(tzh.tzh_magic)) != 0)
       return false;
-    if (tzh.tzh_version[0] == '\0')
-      return false;
-    if (!hdr.Build(tzh))
-      return false;
+    if (tzh.tzh_version[0] == '\0') return false;
+    if (!hdr.Build(tzh)) return false;
     time_len = 8;
   }
-  if (hdr.typecnt == 0)
-    return false;
+  if (hdr.typecnt == 0) return false;
   if (hdr.leapcnt != 0) {
     // This code assumes 60-second minutes so we do not want
     // the leap-second encoded zoneinfo. We could reverse the
@@ -426,16 +420,13 @@
     // so currently we simply reject such data.
     return false;
   }
-  if (hdr.ttisstdcnt != 0 && hdr.ttisstdcnt != hdr.typecnt)
-    return false;
-  if (hdr.ttisutcnt != 0 && hdr.ttisutcnt != hdr.typecnt)
-    return false;
+  if (hdr.ttisstdcnt != 0 && hdr.ttisstdcnt != hdr.typecnt) return false;
+  if (hdr.ttisutcnt != 0 && hdr.ttisutcnt != hdr.typecnt) return false;
 
   // Read the data into a local buffer.
   std::size_t len = hdr.DataLength(time_len);
   std::vector<char> tbuf(len);
-  if (zip->Read(tbuf.data(), len) != len)
-    return false;
+  if (zip->Read(tbuf.data(), len) != len) return false;
   const char* bp = tbuf.data();
 
   // Decode and validate the transitions.
@@ -453,10 +444,8 @@
   bool seen_type_0 = false;
   for (std::size_t i = 0; i != hdr.timecnt; ++i) {
     transitions_[i].type_index = Decode8(bp++);
-    if (transitions_[i].type_index >= hdr.typecnt)
-      return false;
-    if (transitions_[i].type_index == 0)
-      seen_type_0 = true;
+    if (transitions_[i].type_index >= hdr.typecnt) return false;
+    if (transitions_[i].type_index == 0) seen_type_0 = true;
   }
 
   // Decode and validate the transition types.
@@ -470,8 +459,7 @@
     bp += 4;
     transition_types_[i].is_dst = (Decode8(bp++) != 0);
     transition_types_[i].abbr_index = Decode8(bp++);
-    if (transition_types_[i].abbr_index >= hdr.charcnt)
-      return false;
+    if (transition_types_[i].abbr_index >= hdr.charcnt) return false;
   }
 
   // Determine the before-first-transition type.
@@ -480,13 +468,10 @@
     std::uint_fast8_t index = 0;
     if (transition_types_[0].is_dst) {
       index = transitions_[0].type_index;
-      while (index != 0 && transition_types_[index].is_dst)
-        --index;
+      while (index != 0 && transition_types_[index].is_dst) --index;
     }
-    while (index != hdr.typecnt && transition_types_[index].is_dst)
-      ++index;
-    if (index != hdr.typecnt)
-      default_transition_type_ = index;
+    while (index != hdr.typecnt && transition_types_[index].is_dst) ++index;
+    if (index != hdr.typecnt) default_transition_type_ = index;
   }
 
   // Copy all the abbreviations.
@@ -510,11 +495,9 @@
       unsigned char ch;  // all non-EOF results are positive
       return (azip->Read(&ch, 1) == 1) ? ch : EOF;
     };
-    if (get_char(zip) != '\n')
-      return false;
+    if (get_char(zip) != '\n') return false;
     for (int c = get_char(zip); c != '\n'; c = get_char(zip)) {
-      if (c == EOF)
-        return false;
+      if (c == EOF) return false;
       future_spec_.push_back(static_cast<char>(c));
     }
   }
@@ -625,18 +608,18 @@
       : fp_(fp, fclose), len_(len) {}
 
  private:
-  std::unique_ptr<FILE, int(*)(FILE*)> fp_;
+  std::unique_ptr<FILE, int (*)(FILE*)> fp_;
   std::size_t len_;
 };
 
 std::unique_ptr<ZoneInfoSource> FileZoneInfoSource::Open(
     const std::string& name) {
   // Use of the "file:" prefix is intended for testing purposes only.
-  if (name.compare(0, 5, "file:") == 0) return Open(name.substr(5));
+  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
 
   // Map the time-zone name to a path name.
   std::string path;
-  if (name.empty() || name[0] != '/') {
+  if (pos == name.size() || name[pos] != '/') {
     const char* tzdir = "/usr/share/zoneinfo";
     char* tzdir_env = nullptr;
 #if defined(_MSC_VER)
@@ -651,16 +634,16 @@
     free(tzdir_env);
 #endif
   }
-  path += name;
+  path.append(name, pos, std::string::npos);
 
   // Open the zoneinfo file.
   FILE* fp = FOpen(path.c_str(), "rb");
   if (fp == nullptr) return nullptr;
   std::size_t length = 0;
   if (fseek(fp, 0, SEEK_END) == 0) {
-    long pos = ftell(fp);
-    if (pos >= 0) {
-      length = static_cast<std::size_t>(pos);
+    long offset = ftell(fp);
+    if (offset >= 0) {
+      length = static_cast<std::size_t>(offset);
     }
     rewind(fp);
   }
@@ -681,7 +664,7 @@
 std::unique_ptr<ZoneInfoSource> AndroidZoneInfoSource::Open(
     const std::string& name) {
   // Use of the "file:" prefix is intended for testing purposes only.
-  if (name.compare(0, 5, "file:") == 0) return Open(name.substr(5));
+  const std::size_t pos = (name.compare(0, 5, "file:") == 0) ? 5 : 0;
 
   // See Android's libc/tzcode/bionic.cpp for additional information.
   for (const char* tzdata : {"/data/misc/zoneinfo/current/tzdata",
@@ -710,7 +693,7 @@
       const std::int_fast32_t length = Decode32(ebuf + 44);
       if (start < 0 || length < 0) break;
       ebuf[40] = '\0';  // ensure zone name is NUL terminated
-      if (strcmp(name.c_str(), ebuf) == 0) {
+      if (strcmp(name.c_str() + pos, ebuf) == 0) {
         if (fseek(fp.get(), static_cast<long>(start), SEEK_SET) != 0) break;
         return std::unique_ptr<ZoneInfoSource>(new AndroidZoneInfoSource(
             fp.release(), static_cast<std::size_t>(length), vers));
@@ -749,13 +732,13 @@
   // A civil time in "+offset" looks like (time+offset) in UTC.
   // Note: We perform two additions in the civil_second domain to
   // sidestep the chance of overflow in (unix_time + tt.utc_offset).
-  return {(civil_second() + unix_time) + tt.utc_offset,
-          tt.utc_offset, tt.is_dst, &abbreviations_[tt.abbr_index]};
+  return {(civil_second() + unix_time) + tt.utc_offset, tt.utc_offset,
+          tt.is_dst, &abbreviations_[tt.abbr_index]};
 }
 
 // BreakTime() translation for a particular transition.
-time_zone::absolute_lookup TimeZoneInfo::LocalTime(
-    std::int_fast64_t unix_time, const Transition& tr) const {
+time_zone::absolute_lookup TimeZoneInfo::LocalTime(std::int_fast64_t unix_time,
+                                                   const Transition& tr) const {
   const TransitionType& tt = transition_types_[tr.type_index];
   // Note: (unix_time - tr.unix_time) will never overflow as we
   // have ensured that there is always a "nearby" transition.
@@ -898,9 +881,7 @@
   return MakeUnique(tr->unix_time + (cs - tr->civil_sec));
 }
 
-std::string TimeZoneInfo::Version() const {
-  return version_;
-}
+std::string TimeZoneInfo::Version() const { return version_; }
 
 std::string TimeZoneInfo::Description() const {
   std::ostringstream oss;
@@ -922,8 +903,8 @@
   }
   std::int_fast64_t unix_time = ToUnixSeconds(tp);
   const Transition target = {unix_time, 0, civil_second(), civil_second()};
-  const Transition* tr = std::upper_bound(begin, end, target,
-                                          Transition::ByUnixTime());
+  const Transition* tr =
+      std::upper_bound(begin, end, target, Transition::ByUnixTime());
   for (; tr != end; ++tr) {  // skip no-op transitions
     std::uint_fast8_t prev_type_index =
         (tr == begin) ? default_transition_type_ : tr[-1].type_index;
@@ -957,8 +938,8 @@
     unix_time += 1;  // ceils
   }
   const Transition target = {unix_time, 0, civil_second(), civil_second()};
-  const Transition* tr = std::lower_bound(begin, end, target,
-                                          Transition::ByUnixTime());
+  const Transition* tr =
+      std::lower_bound(begin, end, target, Transition::ByUnixTime());
   for (; tr != begin; --tr) {  // skip no-op transitions
     std::uint_fast8_t prev_type_index =
         (tr - 1 == begin) ? default_transition_type_ : tr[-2].type_index;
@@ -973,5 +954,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_info.h b/absl/time/internal/cctz/src/time_zone_info.h
index c724fa8..2a10c06 100644
--- a/absl/time/internal/cctz/src/time_zone_info.h
+++ b/absl/time/internal/cctz/src/time_zone_info.h
@@ -21,6 +21,7 @@
 #include <string>
 #include <vector>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 #include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
@@ -28,7 +29,7 @@
 #include "tzfile.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -73,8 +74,7 @@
   // TimeZoneIf implementations.
   time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const override;
-  time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const override;
+  time_zone::civil_lookup MakeTime(const civil_second& cs) const override;
   bool NextTransition(const time_point<seconds>& tp,
                       time_zone::civil_transition* trans) const override;
   bool PrevTransition(const time_point<seconds>& tp,
@@ -83,7 +83,7 @@
   std::string Description() const override;
 
  private:
-  struct Header {  // counts of:
+  struct Header {            // counts of:
     std::size_t timecnt;     // transition times
     std::size_t typecnt;     // transition types
     std::size_t charcnt;     // zone abbreviation characters
@@ -115,7 +115,7 @@
 
   std::vector<Transition> transitions_;  // ordered by unix_time and civil_sec
   std::vector<TransitionType> transition_types_;  // distinct transition types
-  std::uint_fast8_t default_transition_type_;  // for before first transition
+  std::uint_fast8_t default_transition_type_;     // for before first transition
   std::string abbreviations_;  // all the NUL-terminated abbreviations
 
   std::string version_;      // the tzdata version if available
@@ -132,7 +132,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_INFO_H_
diff --git a/absl/time/internal/cctz/src/time_zone_libc.cc b/absl/time/internal/cctz/src/time_zone_libc.cc
index cfe7d55..47cf84c 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.cc
+++ b/absl/time/internal/cctz/src/time_zone_libc.cc
@@ -23,11 +23,12 @@
 #include <limits>
 #include <utility>
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -85,9 +86,7 @@
 }
 #endif  // tm_gmtoff
 #if defined(tm_zone)
-auto tm_zone(const std::tm& tm) -> decltype(tm.tm_zone) {
-  return tm.tm_zone;
-}
+auto tm_zone(const std::tm& tm) -> decltype(tm.tm_zone) { return tm.tm_zone; }
 #elif defined(__tm_zone)
 auto tm_zone(const std::tm& tm) -> decltype(tm.__tm_zone) {
   return tm.__tm_zone;
@@ -104,19 +103,19 @@
 #endif  // tm_zone
 #endif
 
-inline std::tm* gm_time(const std::time_t *timep, std::tm *result) {
+inline std::tm* gm_time(const std::time_t* timep, std::tm* result) {
 #if defined(_WIN32) || defined(_WIN64)
-    return gmtime_s(result, timep) ? nullptr : result;
+  return gmtime_s(result, timep) ? nullptr : result;
 #else
-    return gmtime_r(timep, result);
+  return gmtime_r(timep, result);
 #endif
 }
 
-inline std::tm* local_time(const std::time_t *timep, std::tm *result) {
+inline std::tm* local_time(const std::time_t* timep, std::tm* result) {
 #if defined(_WIN32) || defined(_WIN64)
-    return localtime_s(result, timep) ? nullptr : result;
+  return localtime_s(result, timep) ? nullptr : result;
 #else
-    return localtime_r(timep, result);
+  return localtime_r(timep, result);
 #endif
 }
 
@@ -209,8 +208,8 @@
   }
 
   const year_t year = tmp->tm_year + year_t{1900};
-  al.cs = civil_second(year, tmp->tm_mon + 1, tmp->tm_mday,
-                       tmp->tm_hour, tmp->tm_min, tmp->tm_sec);
+  al.cs = civil_second(year, tmp->tm_mon + 1, tmp->tm_mday, tmp->tm_hour,
+                       tmp->tm_min, tmp->tm_sec);
   al.offset = static_cast<int>(tm_gmtoff(*tmp));
   al.abbr = local_ ? tm_zone(*tmp) : "UTC";  // as expected by cctz
   al.is_dst = tmp->tm_isdst > 0;
@@ -305,5 +304,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_libc.h b/absl/time/internal/cctz/src/time_zone_libc.h
index 4b1ccb0..1da9039 100644
--- a/absl/time/internal/cctz/src/time_zone_libc.h
+++ b/absl/time/internal/cctz/src/time_zone_libc.h
@@ -17,10 +17,11 @@
 
 #include <string>
 
+#include "absl/base/config.h"
 #include "time_zone_if.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -34,8 +35,7 @@
   // TimeZoneIf implementations.
   time_zone::absolute_lookup BreakTime(
       const time_point<seconds>& tp) const override;
-  time_zone::civil_lookup MakeTime(
-      const civil_second& cs) const override;
+  time_zone::civil_lookup MakeTime(const civil_second& cs) const override;
   bool NextTransition(const time_point<seconds>& tp,
                       time_zone::civil_transition* trans) const override;
   bool PrevTransition(const time_point<seconds>& tp,
@@ -49,7 +49,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_LIBC_H_
diff --git a/absl/time/internal/cctz/src/time_zone_lookup.cc b/absl/time/internal/cctz/src/time_zone_lookup.cc
index 772a937..efdea64 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup.cc
@@ -12,6 +12,7 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
+#include "absl/base/config.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 #if defined(__ANDROID__)
@@ -23,6 +24,7 @@
 
 #if defined(__APPLE__)
 #include <CoreFoundation/CFTimeZone.h>
+
 #include <vector>
 #endif
 
@@ -34,7 +36,7 @@
 #include "time_zone_impl.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -67,9 +69,7 @@
 }  // namespace
 #endif
 
-std::string time_zone::name() const {
-  return effective_impl().Name();
-}
+std::string time_zone::name() const { return effective_impl().Name(); }
 
 time_zone::absolute_lookup time_zone::lookup(
     const time_point<seconds>& tp) const {
@@ -90,9 +90,7 @@
   return effective_impl().PrevTransition(tp, trans);
 }
 
-std::string time_zone::version() const {
-  return effective_impl().Version();
-}
+std::string time_zone::version() const { return effective_impl().Version(); }
 
 std::string time_zone::description() const {
   return effective_impl().Description();
@@ -185,5 +183,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_lookup_test.cc b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
index 1bef770..99137a0 100644
--- a/absl/time/internal/cctz/src/time_zone_lookup_test.cc
+++ b/absl/time/internal/cctz/src/time_zone_lookup_test.cc
@@ -12,8 +12,6 @@
 //   See the License for the specific language governing permissions and
 //   limitations under the License.
 
-#include "absl/time/internal/cctz/include/cctz/time_zone.h"
-
 #include <chrono>
 #include <cstddef>
 #include <cstdlib>
@@ -23,614 +21,614 @@
 #include <thread>
 #include <vector>
 
-#include "absl/time/internal/cctz/include/cctz/civil_time.h"
 #include "gtest/gtest.h"
+#include "absl/base/config.h"
+#include "absl/time/internal/cctz/include/cctz/civil_time.h"
+#include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace chrono = std::chrono;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
 namespace {
 
 // A list of known time-zone names.
-const char* const kTimeZoneNames[] = {
-  "Africa/Abidjan",
-  "Africa/Accra",
-  "Africa/Addis_Ababa",
-  "Africa/Algiers",
-  "Africa/Asmara",
-  "Africa/Asmera",
-  "Africa/Bamako",
-  "Africa/Bangui",
-  "Africa/Banjul",
-  "Africa/Bissau",
-  "Africa/Blantyre",
-  "Africa/Brazzaville",
-  "Africa/Bujumbura",
-  "Africa/Cairo",
-  "Africa/Casablanca",
-  "Africa/Ceuta",
-  "Africa/Conakry",
-  "Africa/Dakar",
-  "Africa/Dar_es_Salaam",
-  "Africa/Djibouti",
-  "Africa/Douala",
-  "Africa/El_Aaiun",
-  "Africa/Freetown",
-  "Africa/Gaborone",
-  "Africa/Harare",
-  "Africa/Johannesburg",
-  "Africa/Juba",
-  "Africa/Kampala",
-  "Africa/Khartoum",
-  "Africa/Kigali",
-  "Africa/Kinshasa",
-  "Africa/Lagos",
-  "Africa/Libreville",
-  "Africa/Lome",
-  "Africa/Luanda",
-  "Africa/Lubumbashi",
-  "Africa/Lusaka",
-  "Africa/Malabo",
-  "Africa/Maputo",
-  "Africa/Maseru",
-  "Africa/Mbabane",
-  "Africa/Mogadishu",
-  "Africa/Monrovia",
-  "Africa/Nairobi",
-  "Africa/Ndjamena",
-  "Africa/Niamey",
-  "Africa/Nouakchott",
-  "Africa/Ouagadougou",
-  "Africa/Porto-Novo",
-  "Africa/Sao_Tome",
-  "Africa/Timbuktu",
-  "Africa/Tripoli",
-  "Africa/Tunis",
-  "Africa/Windhoek",
-  "America/Adak",
-  "America/Anchorage",
-  "America/Anguilla",
-  "America/Antigua",
-  "America/Araguaina",
-  "America/Argentina/Buenos_Aires",
-  "America/Argentina/Catamarca",
-  "America/Argentina/ComodRivadavia",
-  "America/Argentina/Cordoba",
-  "America/Argentina/Jujuy",
-  "America/Argentina/La_Rioja",
-  "America/Argentina/Mendoza",
-  "America/Argentina/Rio_Gallegos",
-  "America/Argentina/Salta",
-  "America/Argentina/San_Juan",
-  "America/Argentina/San_Luis",
-  "America/Argentina/Tucuman",
-  "America/Argentina/Ushuaia",
-  "America/Aruba",
-  "America/Asuncion",
-  "America/Atikokan",
-  "America/Atka",
-  "America/Bahia",
-  "America/Bahia_Banderas",
-  "America/Barbados",
-  "America/Belem",
-  "America/Belize",
-  "America/Blanc-Sablon",
-  "America/Boa_Vista",
-  "America/Bogota",
-  "America/Boise",
-  "America/Buenos_Aires",
-  "America/Cambridge_Bay",
-  "America/Campo_Grande",
-  "America/Cancun",
-  "America/Caracas",
-  "America/Catamarca",
-  "America/Cayenne",
-  "America/Cayman",
-  "America/Chicago",
-  "America/Chihuahua",
-  "America/Coral_Harbour",
-  "America/Cordoba",
-  "America/Costa_Rica",
-  "America/Creston",
-  "America/Cuiaba",
-  "America/Curacao",
-  "America/Danmarkshavn",
-  "America/Dawson",
-  "America/Dawson_Creek",
-  "America/Denver",
-  "America/Detroit",
-  "America/Dominica",
-  "America/Edmonton",
-  "America/Eirunepe",
-  "America/El_Salvador",
-  "America/Ensenada",
-  "America/Fort_Nelson",
-  "America/Fort_Wayne",
-  "America/Fortaleza",
-  "America/Glace_Bay",
-  "America/Godthab",
-  "America/Goose_Bay",
-  "America/Grand_Turk",
-  "America/Grenada",
-  "America/Guadeloupe",
-  "America/Guatemala",
-  "America/Guayaquil",
-  "America/Guyana",
-  "America/Halifax",
-  "America/Havana",
-  "America/Hermosillo",
-  "America/Indiana/Indianapolis",
-  "America/Indiana/Knox",
-  "America/Indiana/Marengo",
-  "America/Indiana/Petersburg",
-  "America/Indiana/Tell_City",
-  "America/Indiana/Vevay",
-  "America/Indiana/Vincennes",
-  "America/Indiana/Winamac",
-  "America/Indianapolis",
-  "America/Inuvik",
-  "America/Iqaluit",
-  "America/Jamaica",
-  "America/Jujuy",
-  "America/Juneau",
-  "America/Kentucky/Louisville",
-  "America/Kentucky/Monticello",
-  "America/Knox_IN",
-  "America/Kralendijk",
-  "America/La_Paz",
-  "America/Lima",
-  "America/Los_Angeles",
-  "America/Louisville",
-  "America/Lower_Princes",
-  "America/Maceio",
-  "America/Managua",
-  "America/Manaus",
-  "America/Marigot",
-  "America/Martinique",
-  "America/Matamoros",
-  "America/Mazatlan",
-  "America/Mendoza",
-  "America/Menominee",
-  "America/Merida",
-  "America/Metlakatla",
-  "America/Mexico_City",
-  "America/Miquelon",
-  "America/Moncton",
-  "America/Monterrey",
-  "America/Montevideo",
-  "America/Montreal",
-  "America/Montserrat",
-  "America/Nassau",
-  "America/New_York",
-  "America/Nipigon",
-  "America/Nome",
-  "America/Noronha",
-  "America/North_Dakota/Beulah",
-  "America/North_Dakota/Center",
-  "America/North_Dakota/New_Salem",
-  "America/Ojinaga",
-  "America/Panama",
-  "America/Pangnirtung",
-  "America/Paramaribo",
-  "America/Phoenix",
-  "America/Port-au-Prince",
-  "America/Port_of_Spain",
-  "America/Porto_Acre",
-  "America/Porto_Velho",
-  "America/Puerto_Rico",
-  "America/Punta_Arenas",
-  "America/Rainy_River",
-  "America/Rankin_Inlet",
-  "America/Recife",
-  "America/Regina",
-  "America/Resolute",
-  "America/Rio_Branco",
-  "America/Rosario",
-  "America/Santa_Isabel",
-  "America/Santarem",
-  "America/Santiago",
-  "America/Santo_Domingo",
-  "America/Sao_Paulo",
-  "America/Scoresbysund",
-  "America/Shiprock",
-  "America/Sitka",
-  "America/St_Barthelemy",
-  "America/St_Johns",
-  "America/St_Kitts",
-  "America/St_Lucia",
-  "America/St_Thomas",
-  "America/St_Vincent",
-  "America/Swift_Current",
-  "America/Tegucigalpa",
-  "America/Thule",
-  "America/Thunder_Bay",
-  "America/Tijuana",
-  "America/Toronto",
-  "America/Tortola",
-  "America/Vancouver",
-  "America/Virgin",
-  "America/Whitehorse",
-  "America/Winnipeg",
-  "America/Yakutat",
-  "America/Yellowknife",
-  "Antarctica/Casey",
-  "Antarctica/Davis",
-  "Antarctica/DumontDUrville",
-  "Antarctica/Macquarie",
-  "Antarctica/Mawson",
-  "Antarctica/McMurdo",
-  "Antarctica/Palmer",
-  "Antarctica/Rothera",
-  "Antarctica/South_Pole",
-  "Antarctica/Syowa",
-  "Antarctica/Troll",
-  "Antarctica/Vostok",
-  "Arctic/Longyearbyen",
-  "Asia/Aden",
-  "Asia/Almaty",
-  "Asia/Amman",
-  "Asia/Anadyr",
-  "Asia/Aqtau",
-  "Asia/Aqtobe",
-  "Asia/Ashgabat",
-  "Asia/Ashkhabad",
-  "Asia/Atyrau",
-  "Asia/Baghdad",
-  "Asia/Bahrain",
-  "Asia/Baku",
-  "Asia/Bangkok",
-  "Asia/Barnaul",
-  "Asia/Beirut",
-  "Asia/Bishkek",
-  "Asia/Brunei",
-  "Asia/Calcutta",
-  "Asia/Chita",
-  "Asia/Choibalsan",
-  "Asia/Chongqing",
-  "Asia/Chungking",
-  "Asia/Colombo",
-  "Asia/Dacca",
-  "Asia/Damascus",
-  "Asia/Dhaka",
-  "Asia/Dili",
-  "Asia/Dubai",
-  "Asia/Dushanbe",
-  "Asia/Famagusta",
-  "Asia/Gaza",
-  "Asia/Harbin",
-  "Asia/Hebron",
-  "Asia/Ho_Chi_Minh",
-  "Asia/Hong_Kong",
-  "Asia/Hovd",
-  "Asia/Irkutsk",
-  "Asia/Istanbul",
-  "Asia/Jakarta",
-  "Asia/Jayapura",
-  "Asia/Jerusalem",
-  "Asia/Kabul",
-  "Asia/Kamchatka",
-  "Asia/Karachi",
-  "Asia/Kashgar",
-  "Asia/Kathmandu",
-  "Asia/Katmandu",
-  "Asia/Khandyga",
-  "Asia/Kolkata",
-  "Asia/Krasnoyarsk",
-  "Asia/Kuala_Lumpur",
-  "Asia/Kuching",
-  "Asia/Kuwait",
-  "Asia/Macao",
-  "Asia/Macau",
-  "Asia/Magadan",
-  "Asia/Makassar",
-  "Asia/Manila",
-  "Asia/Muscat",
-  "Asia/Nicosia",
-  "Asia/Novokuznetsk",
-  "Asia/Novosibirsk",
-  "Asia/Omsk",
-  "Asia/Oral",
-  "Asia/Phnom_Penh",
-  "Asia/Pontianak",
-  "Asia/Pyongyang",
-  "Asia/Qatar",
-  "Asia/Qostanay",
-  "Asia/Qyzylorda",
-  "Asia/Rangoon",
-  "Asia/Riyadh",
-  "Asia/Saigon",
-  "Asia/Sakhalin",
-  "Asia/Samarkand",
-  "Asia/Seoul",
-  "Asia/Shanghai",
-  "Asia/Singapore",
-  "Asia/Srednekolymsk",
-  "Asia/Taipei",
-  "Asia/Tashkent",
-  "Asia/Tbilisi",
-  "Asia/Tehran",
-  "Asia/Tel_Aviv",
-  "Asia/Thimbu",
-  "Asia/Thimphu",
-  "Asia/Tokyo",
-  "Asia/Tomsk",
-  "Asia/Ujung_Pandang",
-  "Asia/Ulaanbaatar",
-  "Asia/Ulan_Bator",
-  "Asia/Urumqi",
-  "Asia/Ust-Nera",
-  "Asia/Vientiane",
-  "Asia/Vladivostok",
-  "Asia/Yakutsk",
-  "Asia/Yangon",
-  "Asia/Yekaterinburg",
-  "Asia/Yerevan",
-  "Atlantic/Azores",
-  "Atlantic/Bermuda",
-  "Atlantic/Canary",
-  "Atlantic/Cape_Verde",
-  "Atlantic/Faeroe",
-  "Atlantic/Faroe",
-  "Atlantic/Jan_Mayen",
-  "Atlantic/Madeira",
-  "Atlantic/Reykjavik",
-  "Atlantic/South_Georgia",
-  "Atlantic/St_Helena",
-  "Atlantic/Stanley",
-  "Australia/ACT",
-  "Australia/Adelaide",
-  "Australia/Brisbane",
-  "Australia/Broken_Hill",
-  "Australia/Canberra",
-  "Australia/Currie",
-  "Australia/Darwin",
-  "Australia/Eucla",
-  "Australia/Hobart",
-  "Australia/LHI",
-  "Australia/Lindeman",
-  "Australia/Lord_Howe",
-  "Australia/Melbourne",
-  "Australia/NSW",
-  "Australia/North",
-  "Australia/Perth",
-  "Australia/Queensland",
-  "Australia/South",
-  "Australia/Sydney",
-  "Australia/Tasmania",
-  "Australia/Victoria",
-  "Australia/West",
-  "Australia/Yancowinna",
-  "Brazil/Acre",
-  "Brazil/DeNoronha",
-  "Brazil/East",
-  "Brazil/West",
-  "CET",
-  "CST6CDT",
-  "Canada/Atlantic",
-  "Canada/Central",
-  "Canada/Eastern",
-  "Canada/Mountain",
-  "Canada/Newfoundland",
-  "Canada/Pacific",
-  "Canada/Saskatchewan",
-  "Canada/Yukon",
-  "Chile/Continental",
-  "Chile/EasterIsland",
-  "Cuba",
-  "EET",
-  "EST",
-  "EST5EDT",
-  "Egypt",
-  "Eire",
-  "Etc/GMT",
-  "Etc/GMT+0",
-  "Etc/GMT+1",
-  "Etc/GMT+10",
-  "Etc/GMT+11",
-  "Etc/GMT+12",
-  "Etc/GMT+2",
-  "Etc/GMT+3",
-  "Etc/GMT+4",
-  "Etc/GMT+5",
-  "Etc/GMT+6",
-  "Etc/GMT+7",
-  "Etc/GMT+8",
-  "Etc/GMT+9",
-  "Etc/GMT-0",
-  "Etc/GMT-1",
-  "Etc/GMT-10",
-  "Etc/GMT-11",
-  "Etc/GMT-12",
-  "Etc/GMT-13",
-  "Etc/GMT-14",
-  "Etc/GMT-2",
-  "Etc/GMT-3",
-  "Etc/GMT-4",
-  "Etc/GMT-5",
-  "Etc/GMT-6",
-  "Etc/GMT-7",
-  "Etc/GMT-8",
-  "Etc/GMT-9",
-  "Etc/GMT0",
-  "Etc/Greenwich",
-  "Etc/UCT",
-  "Etc/UTC",
-  "Etc/Universal",
-  "Etc/Zulu",
-  "Europe/Amsterdam",
-  "Europe/Andorra",
-  "Europe/Astrakhan",
-  "Europe/Athens",
-  "Europe/Belfast",
-  "Europe/Belgrade",
-  "Europe/Berlin",
-  "Europe/Bratislava",
-  "Europe/Brussels",
-  "Europe/Bucharest",
-  "Europe/Budapest",
-  "Europe/Busingen",
-  "Europe/Chisinau",
-  "Europe/Copenhagen",
-  "Europe/Dublin",
-  "Europe/Gibraltar",
-  "Europe/Guernsey",
-  "Europe/Helsinki",
-  "Europe/Isle_of_Man",
-  "Europe/Istanbul",
-  "Europe/Jersey",
-  "Europe/Kaliningrad",
-  "Europe/Kiev",
-  "Europe/Kirov",
-  "Europe/Lisbon",
-  "Europe/Ljubljana",
-  "Europe/London",
-  "Europe/Luxembourg",
-  "Europe/Madrid",
-  "Europe/Malta",
-  "Europe/Mariehamn",
-  "Europe/Minsk",
-  "Europe/Monaco",
-  "Europe/Moscow",
-  "Europe/Nicosia",
-  "Europe/Oslo",
-  "Europe/Paris",
-  "Europe/Podgorica",
-  "Europe/Prague",
-  "Europe/Riga",
-  "Europe/Rome",
-  "Europe/Samara",
-  "Europe/San_Marino",
-  "Europe/Sarajevo",
-  "Europe/Saratov",
-  "Europe/Simferopol",
-  "Europe/Skopje",
-  "Europe/Sofia",
-  "Europe/Stockholm",
-  "Europe/Tallinn",
-  "Europe/Tirane",
-  "Europe/Tiraspol",
-  "Europe/Ulyanovsk",
-  "Europe/Uzhgorod",
-  "Europe/Vaduz",
-  "Europe/Vatican",
-  "Europe/Vienna",
-  "Europe/Vilnius",
-  "Europe/Volgograd",
-  "Europe/Warsaw",
-  "Europe/Zagreb",
-  "Europe/Zaporozhye",
-  "Europe/Zurich",
-  "GB",
-  "GB-Eire",
-  "GMT",
-  "GMT+0",
-  "GMT-0",
-  "GMT0",
-  "Greenwich",
-  "HST",
-  "Hongkong",
-  "Iceland",
-  "Indian/Antananarivo",
-  "Indian/Chagos",
-  "Indian/Christmas",
-  "Indian/Cocos",
-  "Indian/Comoro",
-  "Indian/Kerguelen",
-  "Indian/Mahe",
-  "Indian/Maldives",
-  "Indian/Mauritius",
-  "Indian/Mayotte",
-  "Indian/Reunion",
-  "Iran",
-  "Israel",
-  "Jamaica",
-  "Japan",
-  "Kwajalein",
-  "Libya",
-  "MET",
-  "MST",
-  "MST7MDT",
-  "Mexico/BajaNorte",
-  "Mexico/BajaSur",
-  "Mexico/General",
-  "NZ",
-  "NZ-CHAT",
-  "Navajo",
-  "PRC",
-  "PST8PDT",
-  "Pacific/Apia",
-  "Pacific/Auckland",
-  "Pacific/Bougainville",
-  "Pacific/Chatham",
-  "Pacific/Chuuk",
-  "Pacific/Easter",
-  "Pacific/Efate",
-  "Pacific/Enderbury",
-  "Pacific/Fakaofo",
-  "Pacific/Fiji",
-  "Pacific/Funafuti",
-  "Pacific/Galapagos",
-  "Pacific/Gambier",
-  "Pacific/Guadalcanal",
-  "Pacific/Guam",
-  "Pacific/Honolulu",
-  "Pacific/Johnston",
-  "Pacific/Kiritimati",
-  "Pacific/Kosrae",
-  "Pacific/Kwajalein",
-  "Pacific/Majuro",
-  "Pacific/Marquesas",
-  "Pacific/Midway",
-  "Pacific/Nauru",
-  "Pacific/Niue",
-  "Pacific/Norfolk",
-  "Pacific/Noumea",
-  "Pacific/Pago_Pago",
-  "Pacific/Palau",
-  "Pacific/Pitcairn",
-  "Pacific/Pohnpei",
-  "Pacific/Ponape",
-  "Pacific/Port_Moresby",
-  "Pacific/Rarotonga",
-  "Pacific/Saipan",
-  "Pacific/Samoa",
-  "Pacific/Tahiti",
-  "Pacific/Tarawa",
-  "Pacific/Tongatapu",
-  "Pacific/Truk",
-  "Pacific/Wake",
-  "Pacific/Wallis",
-  "Pacific/Yap",
-  "Poland",
-  "Portugal",
-  "ROC",
-  "ROK",
-  "Singapore",
-  "Turkey",
-  "UCT",
-  "US/Alaska",
-  "US/Aleutian",
-  "US/Arizona",
-  "US/Central",
-  "US/East-Indiana",
-  "US/Eastern",
-  "US/Hawaii",
-  "US/Indiana-Starke",
-  "US/Michigan",
-  "US/Mountain",
-  "US/Pacific",
-  "US/Samoa",
-  "UTC",
-  "Universal",
-  "W-SU",
-  "WET",
-  "Zulu",
-  nullptr
-};
+const char* const kTimeZoneNames[] = {"Africa/Abidjan",
+                                      "Africa/Accra",
+                                      "Africa/Addis_Ababa",
+                                      "Africa/Algiers",
+                                      "Africa/Asmara",
+                                      "Africa/Asmera",
+                                      "Africa/Bamako",
+                                      "Africa/Bangui",
+                                      "Africa/Banjul",
+                                      "Africa/Bissau",
+                                      "Africa/Blantyre",
+                                      "Africa/Brazzaville",
+                                      "Africa/Bujumbura",
+                                      "Africa/Cairo",
+                                      "Africa/Casablanca",
+                                      "Africa/Ceuta",
+                                      "Africa/Conakry",
+                                      "Africa/Dakar",
+                                      "Africa/Dar_es_Salaam",
+                                      "Africa/Djibouti",
+                                      "Africa/Douala",
+                                      "Africa/El_Aaiun",
+                                      "Africa/Freetown",
+                                      "Africa/Gaborone",
+                                      "Africa/Harare",
+                                      "Africa/Johannesburg",
+                                      "Africa/Juba",
+                                      "Africa/Kampala",
+                                      "Africa/Khartoum",
+                                      "Africa/Kigali",
+                                      "Africa/Kinshasa",
+                                      "Africa/Lagos",
+                                      "Africa/Libreville",
+                                      "Africa/Lome",
+                                      "Africa/Luanda",
+                                      "Africa/Lubumbashi",
+                                      "Africa/Lusaka",
+                                      "Africa/Malabo",
+                                      "Africa/Maputo",
+                                      "Africa/Maseru",
+                                      "Africa/Mbabane",
+                                      "Africa/Mogadishu",
+                                      "Africa/Monrovia",
+                                      "Africa/Nairobi",
+                                      "Africa/Ndjamena",
+                                      "Africa/Niamey",
+                                      "Africa/Nouakchott",
+                                      "Africa/Ouagadougou",
+                                      "Africa/Porto-Novo",
+                                      "Africa/Sao_Tome",
+                                      "Africa/Timbuktu",
+                                      "Africa/Tripoli",
+                                      "Africa/Tunis",
+                                      "Africa/Windhoek",
+                                      "America/Adak",
+                                      "America/Anchorage",
+                                      "America/Anguilla",
+                                      "America/Antigua",
+                                      "America/Araguaina",
+                                      "America/Argentina/Buenos_Aires",
+                                      "America/Argentina/Catamarca",
+                                      "America/Argentina/ComodRivadavia",
+                                      "America/Argentina/Cordoba",
+                                      "America/Argentina/Jujuy",
+                                      "America/Argentina/La_Rioja",
+                                      "America/Argentina/Mendoza",
+                                      "America/Argentina/Rio_Gallegos",
+                                      "America/Argentina/Salta",
+                                      "America/Argentina/San_Juan",
+                                      "America/Argentina/San_Luis",
+                                      "America/Argentina/Tucuman",
+                                      "America/Argentina/Ushuaia",
+                                      "America/Aruba",
+                                      "America/Asuncion",
+                                      "America/Atikokan",
+                                      "America/Atka",
+                                      "America/Bahia",
+                                      "America/Bahia_Banderas",
+                                      "America/Barbados",
+                                      "America/Belem",
+                                      "America/Belize",
+                                      "America/Blanc-Sablon",
+                                      "America/Boa_Vista",
+                                      "America/Bogota",
+                                      "America/Boise",
+                                      "America/Buenos_Aires",
+                                      "America/Cambridge_Bay",
+                                      "America/Campo_Grande",
+                                      "America/Cancun",
+                                      "America/Caracas",
+                                      "America/Catamarca",
+                                      "America/Cayenne",
+                                      "America/Cayman",
+                                      "America/Chicago",
+                                      "America/Chihuahua",
+                                      "America/Coral_Harbour",
+                                      "America/Cordoba",
+                                      "America/Costa_Rica",
+                                      "America/Creston",
+                                      "America/Cuiaba",
+                                      "America/Curacao",
+                                      "America/Danmarkshavn",
+                                      "America/Dawson",
+                                      "America/Dawson_Creek",
+                                      "America/Denver",
+                                      "America/Detroit",
+                                      "America/Dominica",
+                                      "America/Edmonton",
+                                      "America/Eirunepe",
+                                      "America/El_Salvador",
+                                      "America/Ensenada",
+                                      "America/Fort_Nelson",
+                                      "America/Fort_Wayne",
+                                      "America/Fortaleza",
+                                      "America/Glace_Bay",
+                                      "America/Godthab",
+                                      "America/Goose_Bay",
+                                      "America/Grand_Turk",
+                                      "America/Grenada",
+                                      "America/Guadeloupe",
+                                      "America/Guatemala",
+                                      "America/Guayaquil",
+                                      "America/Guyana",
+                                      "America/Halifax",
+                                      "America/Havana",
+                                      "America/Hermosillo",
+                                      "America/Indiana/Indianapolis",
+                                      "America/Indiana/Knox",
+                                      "America/Indiana/Marengo",
+                                      "America/Indiana/Petersburg",
+                                      "America/Indiana/Tell_City",
+                                      "America/Indiana/Vevay",
+                                      "America/Indiana/Vincennes",
+                                      "America/Indiana/Winamac",
+                                      "America/Indianapolis",
+                                      "America/Inuvik",
+                                      "America/Iqaluit",
+                                      "America/Jamaica",
+                                      "America/Jujuy",
+                                      "America/Juneau",
+                                      "America/Kentucky/Louisville",
+                                      "America/Kentucky/Monticello",
+                                      "America/Knox_IN",
+                                      "America/Kralendijk",
+                                      "America/La_Paz",
+                                      "America/Lima",
+                                      "America/Los_Angeles",
+                                      "America/Louisville",
+                                      "America/Lower_Princes",
+                                      "America/Maceio",
+                                      "America/Managua",
+                                      "America/Manaus",
+                                      "America/Marigot",
+                                      "America/Martinique",
+                                      "America/Matamoros",
+                                      "America/Mazatlan",
+                                      "America/Mendoza",
+                                      "America/Menominee",
+                                      "America/Merida",
+                                      "America/Metlakatla",
+                                      "America/Mexico_City",
+                                      "America/Miquelon",
+                                      "America/Moncton",
+                                      "America/Monterrey",
+                                      "America/Montevideo",
+                                      "America/Montreal",
+                                      "America/Montserrat",
+                                      "America/Nassau",
+                                      "America/New_York",
+                                      "America/Nipigon",
+                                      "America/Nome",
+                                      "America/Noronha",
+                                      "America/North_Dakota/Beulah",
+                                      "America/North_Dakota/Center",
+                                      "America/North_Dakota/New_Salem",
+                                      "America/Ojinaga",
+                                      "America/Panama",
+                                      "America/Pangnirtung",
+                                      "America/Paramaribo",
+                                      "America/Phoenix",
+                                      "America/Port-au-Prince",
+                                      "America/Port_of_Spain",
+                                      "America/Porto_Acre",
+                                      "America/Porto_Velho",
+                                      "America/Puerto_Rico",
+                                      "America/Punta_Arenas",
+                                      "America/Rainy_River",
+                                      "America/Rankin_Inlet",
+                                      "America/Recife",
+                                      "America/Regina",
+                                      "America/Resolute",
+                                      "America/Rio_Branco",
+                                      "America/Rosario",
+                                      "America/Santa_Isabel",
+                                      "America/Santarem",
+                                      "America/Santiago",
+                                      "America/Santo_Domingo",
+                                      "America/Sao_Paulo",
+                                      "America/Scoresbysund",
+                                      "America/Shiprock",
+                                      "America/Sitka",
+                                      "America/St_Barthelemy",
+                                      "America/St_Johns",
+                                      "America/St_Kitts",
+                                      "America/St_Lucia",
+                                      "America/St_Thomas",
+                                      "America/St_Vincent",
+                                      "America/Swift_Current",
+                                      "America/Tegucigalpa",
+                                      "America/Thule",
+                                      "America/Thunder_Bay",
+                                      "America/Tijuana",
+                                      "America/Toronto",
+                                      "America/Tortola",
+                                      "America/Vancouver",
+                                      "America/Virgin",
+                                      "America/Whitehorse",
+                                      "America/Winnipeg",
+                                      "America/Yakutat",
+                                      "America/Yellowknife",
+                                      "Antarctica/Casey",
+                                      "Antarctica/Davis",
+                                      "Antarctica/DumontDUrville",
+                                      "Antarctica/Macquarie",
+                                      "Antarctica/Mawson",
+                                      "Antarctica/McMurdo",
+                                      "Antarctica/Palmer",
+                                      "Antarctica/Rothera",
+                                      "Antarctica/South_Pole",
+                                      "Antarctica/Syowa",
+                                      "Antarctica/Troll",
+                                      "Antarctica/Vostok",
+                                      "Arctic/Longyearbyen",
+                                      "Asia/Aden",
+                                      "Asia/Almaty",
+                                      "Asia/Amman",
+                                      "Asia/Anadyr",
+                                      "Asia/Aqtau",
+                                      "Asia/Aqtobe",
+                                      "Asia/Ashgabat",
+                                      "Asia/Ashkhabad",
+                                      "Asia/Atyrau",
+                                      "Asia/Baghdad",
+                                      "Asia/Bahrain",
+                                      "Asia/Baku",
+                                      "Asia/Bangkok",
+                                      "Asia/Barnaul",
+                                      "Asia/Beirut",
+                                      "Asia/Bishkek",
+                                      "Asia/Brunei",
+                                      "Asia/Calcutta",
+                                      "Asia/Chita",
+                                      "Asia/Choibalsan",
+                                      "Asia/Chongqing",
+                                      "Asia/Chungking",
+                                      "Asia/Colombo",
+                                      "Asia/Dacca",
+                                      "Asia/Damascus",
+                                      "Asia/Dhaka",
+                                      "Asia/Dili",
+                                      "Asia/Dubai",
+                                      "Asia/Dushanbe",
+                                      "Asia/Famagusta",
+                                      "Asia/Gaza",
+                                      "Asia/Harbin",
+                                      "Asia/Hebron",
+                                      "Asia/Ho_Chi_Minh",
+                                      "Asia/Hong_Kong",
+                                      "Asia/Hovd",
+                                      "Asia/Irkutsk",
+                                      "Asia/Istanbul",
+                                      "Asia/Jakarta",
+                                      "Asia/Jayapura",
+                                      "Asia/Jerusalem",
+                                      "Asia/Kabul",
+                                      "Asia/Kamchatka",
+                                      "Asia/Karachi",
+                                      "Asia/Kashgar",
+                                      "Asia/Kathmandu",
+                                      "Asia/Katmandu",
+                                      "Asia/Khandyga",
+                                      "Asia/Kolkata",
+                                      "Asia/Krasnoyarsk",
+                                      "Asia/Kuala_Lumpur",
+                                      "Asia/Kuching",
+                                      "Asia/Kuwait",
+                                      "Asia/Macao",
+                                      "Asia/Macau",
+                                      "Asia/Magadan",
+                                      "Asia/Makassar",
+                                      "Asia/Manila",
+                                      "Asia/Muscat",
+                                      "Asia/Nicosia",
+                                      "Asia/Novokuznetsk",
+                                      "Asia/Novosibirsk",
+                                      "Asia/Omsk",
+                                      "Asia/Oral",
+                                      "Asia/Phnom_Penh",
+                                      "Asia/Pontianak",
+                                      "Asia/Pyongyang",
+                                      "Asia/Qatar",
+                                      "Asia/Qostanay",
+                                      "Asia/Qyzylorda",
+                                      "Asia/Rangoon",
+                                      "Asia/Riyadh",
+                                      "Asia/Saigon",
+                                      "Asia/Sakhalin",
+                                      "Asia/Samarkand",
+                                      "Asia/Seoul",
+                                      "Asia/Shanghai",
+                                      "Asia/Singapore",
+                                      "Asia/Srednekolymsk",
+                                      "Asia/Taipei",
+                                      "Asia/Tashkent",
+                                      "Asia/Tbilisi",
+                                      "Asia/Tehran",
+                                      "Asia/Tel_Aviv",
+                                      "Asia/Thimbu",
+                                      "Asia/Thimphu",
+                                      "Asia/Tokyo",
+                                      "Asia/Tomsk",
+                                      "Asia/Ujung_Pandang",
+                                      "Asia/Ulaanbaatar",
+                                      "Asia/Ulan_Bator",
+                                      "Asia/Urumqi",
+                                      "Asia/Ust-Nera",
+                                      "Asia/Vientiane",
+                                      "Asia/Vladivostok",
+                                      "Asia/Yakutsk",
+                                      "Asia/Yangon",
+                                      "Asia/Yekaterinburg",
+                                      "Asia/Yerevan",
+                                      "Atlantic/Azores",
+                                      "Atlantic/Bermuda",
+                                      "Atlantic/Canary",
+                                      "Atlantic/Cape_Verde",
+                                      "Atlantic/Faeroe",
+                                      "Atlantic/Faroe",
+                                      "Atlantic/Jan_Mayen",
+                                      "Atlantic/Madeira",
+                                      "Atlantic/Reykjavik",
+                                      "Atlantic/South_Georgia",
+                                      "Atlantic/St_Helena",
+                                      "Atlantic/Stanley",
+                                      "Australia/ACT",
+                                      "Australia/Adelaide",
+                                      "Australia/Brisbane",
+                                      "Australia/Broken_Hill",
+                                      "Australia/Canberra",
+                                      "Australia/Currie",
+                                      "Australia/Darwin",
+                                      "Australia/Eucla",
+                                      "Australia/Hobart",
+                                      "Australia/LHI",
+                                      "Australia/Lindeman",
+                                      "Australia/Lord_Howe",
+                                      "Australia/Melbourne",
+                                      "Australia/NSW",
+                                      "Australia/North",
+                                      "Australia/Perth",
+                                      "Australia/Queensland",
+                                      "Australia/South",
+                                      "Australia/Sydney",
+                                      "Australia/Tasmania",
+                                      "Australia/Victoria",
+                                      "Australia/West",
+                                      "Australia/Yancowinna",
+                                      "Brazil/Acre",
+                                      "Brazil/DeNoronha",
+                                      "Brazil/East",
+                                      "Brazil/West",
+                                      "CET",
+                                      "CST6CDT",
+                                      "Canada/Atlantic",
+                                      "Canada/Central",
+                                      "Canada/Eastern",
+                                      "Canada/Mountain",
+                                      "Canada/Newfoundland",
+                                      "Canada/Pacific",
+                                      "Canada/Saskatchewan",
+                                      "Canada/Yukon",
+                                      "Chile/Continental",
+                                      "Chile/EasterIsland",
+                                      "Cuba",
+                                      "EET",
+                                      "EST",
+                                      "EST5EDT",
+                                      "Egypt",
+                                      "Eire",
+                                      "Etc/GMT",
+                                      "Etc/GMT+0",
+                                      "Etc/GMT+1",
+                                      "Etc/GMT+10",
+                                      "Etc/GMT+11",
+                                      "Etc/GMT+12",
+                                      "Etc/GMT+2",
+                                      "Etc/GMT+3",
+                                      "Etc/GMT+4",
+                                      "Etc/GMT+5",
+                                      "Etc/GMT+6",
+                                      "Etc/GMT+7",
+                                      "Etc/GMT+8",
+                                      "Etc/GMT+9",
+                                      "Etc/GMT-0",
+                                      "Etc/GMT-1",
+                                      "Etc/GMT-10",
+                                      "Etc/GMT-11",
+                                      "Etc/GMT-12",
+                                      "Etc/GMT-13",
+                                      "Etc/GMT-14",
+                                      "Etc/GMT-2",
+                                      "Etc/GMT-3",
+                                      "Etc/GMT-4",
+                                      "Etc/GMT-5",
+                                      "Etc/GMT-6",
+                                      "Etc/GMT-7",
+                                      "Etc/GMT-8",
+                                      "Etc/GMT-9",
+                                      "Etc/GMT0",
+                                      "Etc/Greenwich",
+                                      "Etc/UCT",
+                                      "Etc/UTC",
+                                      "Etc/Universal",
+                                      "Etc/Zulu",
+                                      "Europe/Amsterdam",
+                                      "Europe/Andorra",
+                                      "Europe/Astrakhan",
+                                      "Europe/Athens",
+                                      "Europe/Belfast",
+                                      "Europe/Belgrade",
+                                      "Europe/Berlin",
+                                      "Europe/Bratislava",
+                                      "Europe/Brussels",
+                                      "Europe/Bucharest",
+                                      "Europe/Budapest",
+                                      "Europe/Busingen",
+                                      "Europe/Chisinau",
+                                      "Europe/Copenhagen",
+                                      "Europe/Dublin",
+                                      "Europe/Gibraltar",
+                                      "Europe/Guernsey",
+                                      "Europe/Helsinki",
+                                      "Europe/Isle_of_Man",
+                                      "Europe/Istanbul",
+                                      "Europe/Jersey",
+                                      "Europe/Kaliningrad",
+                                      "Europe/Kiev",
+                                      "Europe/Kirov",
+                                      "Europe/Lisbon",
+                                      "Europe/Ljubljana",
+                                      "Europe/London",
+                                      "Europe/Luxembourg",
+                                      "Europe/Madrid",
+                                      "Europe/Malta",
+                                      "Europe/Mariehamn",
+                                      "Europe/Minsk",
+                                      "Europe/Monaco",
+                                      "Europe/Moscow",
+                                      "Europe/Nicosia",
+                                      "Europe/Oslo",
+                                      "Europe/Paris",
+                                      "Europe/Podgorica",
+                                      "Europe/Prague",
+                                      "Europe/Riga",
+                                      "Europe/Rome",
+                                      "Europe/Samara",
+                                      "Europe/San_Marino",
+                                      "Europe/Sarajevo",
+                                      "Europe/Saratov",
+                                      "Europe/Simferopol",
+                                      "Europe/Skopje",
+                                      "Europe/Sofia",
+                                      "Europe/Stockholm",
+                                      "Europe/Tallinn",
+                                      "Europe/Tirane",
+                                      "Europe/Tiraspol",
+                                      "Europe/Ulyanovsk",
+                                      "Europe/Uzhgorod",
+                                      "Europe/Vaduz",
+                                      "Europe/Vatican",
+                                      "Europe/Vienna",
+                                      "Europe/Vilnius",
+                                      "Europe/Volgograd",
+                                      "Europe/Warsaw",
+                                      "Europe/Zagreb",
+                                      "Europe/Zaporozhye",
+                                      "Europe/Zurich",
+                                      "GB",
+                                      "GB-Eire",
+                                      "GMT",
+                                      "GMT+0",
+                                      "GMT-0",
+                                      "GMT0",
+                                      "Greenwich",
+                                      "HST",
+                                      "Hongkong",
+                                      "Iceland",
+                                      "Indian/Antananarivo",
+                                      "Indian/Chagos",
+                                      "Indian/Christmas",
+                                      "Indian/Cocos",
+                                      "Indian/Comoro",
+                                      "Indian/Kerguelen",
+                                      "Indian/Mahe",
+                                      "Indian/Maldives",
+                                      "Indian/Mauritius",
+                                      "Indian/Mayotte",
+                                      "Indian/Reunion",
+                                      "Iran",
+                                      "Israel",
+                                      "Jamaica",
+                                      "Japan",
+                                      "Kwajalein",
+                                      "Libya",
+                                      "MET",
+                                      "MST",
+                                      "MST7MDT",
+                                      "Mexico/BajaNorte",
+                                      "Mexico/BajaSur",
+                                      "Mexico/General",
+                                      "NZ",
+                                      "NZ-CHAT",
+                                      "Navajo",
+                                      "PRC",
+                                      "PST8PDT",
+                                      "Pacific/Apia",
+                                      "Pacific/Auckland",
+                                      "Pacific/Bougainville",
+                                      "Pacific/Chatham",
+                                      "Pacific/Chuuk",
+                                      "Pacific/Easter",
+                                      "Pacific/Efate",
+                                      "Pacific/Enderbury",
+                                      "Pacific/Fakaofo",
+                                      "Pacific/Fiji",
+                                      "Pacific/Funafuti",
+                                      "Pacific/Galapagos",
+                                      "Pacific/Gambier",
+                                      "Pacific/Guadalcanal",
+                                      "Pacific/Guam",
+                                      "Pacific/Honolulu",
+                                      "Pacific/Johnston",
+                                      "Pacific/Kiritimati",
+                                      "Pacific/Kosrae",
+                                      "Pacific/Kwajalein",
+                                      "Pacific/Majuro",
+                                      "Pacific/Marquesas",
+                                      "Pacific/Midway",
+                                      "Pacific/Nauru",
+                                      "Pacific/Niue",
+                                      "Pacific/Norfolk",
+                                      "Pacific/Noumea",
+                                      "Pacific/Pago_Pago",
+                                      "Pacific/Palau",
+                                      "Pacific/Pitcairn",
+                                      "Pacific/Pohnpei",
+                                      "Pacific/Ponape",
+                                      "Pacific/Port_Moresby",
+                                      "Pacific/Rarotonga",
+                                      "Pacific/Saipan",
+                                      "Pacific/Samoa",
+                                      "Pacific/Tahiti",
+                                      "Pacific/Tarawa",
+                                      "Pacific/Tongatapu",
+                                      "Pacific/Truk",
+                                      "Pacific/Wake",
+                                      "Pacific/Wallis",
+                                      "Pacific/Yap",
+                                      "Poland",
+                                      "Portugal",
+                                      "ROC",
+                                      "ROK",
+                                      "Singapore",
+                                      "Turkey",
+                                      "UCT",
+                                      "US/Alaska",
+                                      "US/Aleutian",
+                                      "US/Arizona",
+                                      "US/Central",
+                                      "US/East-Indiana",
+                                      "US/Eastern",
+                                      "US/Hawaii",
+                                      "US/Indiana-Starke",
+                                      "US/Michigan",
+                                      "US/Mountain",
+                                      "US/Pacific",
+                                      "US/Samoa",
+                                      "UTC",
+                                      "Universal",
+                                      "W-SU",
+                                      "WET",
+                                      "Zulu",
+                                      nullptr};
 
 // Helper to return a loaded time zone by value (UTC on error).
 time_zone LoadZone(const std::string& name) {
@@ -725,7 +723,8 @@
   EXPECT_EQ("America/New_York", nyc.name());
   const time_zone syd = LoadZone("Australia/Sydney");
   EXPECT_EQ("Australia/Sydney", syd.name());
-  const time_zone fixed0 = fixed_time_zone(absl::time_internal::cctz::seconds::zero());
+  const time_zone fixed0 =
+      fixed_time_zone(absl::time_internal::cctz::seconds::zero());
   EXPECT_EQ("UTC", fixed0.name());
   const time_zone fixed_pos = fixed_time_zone(
       chrono::hours(3) + chrono::minutes(25) + chrono::seconds(45));
@@ -768,7 +767,8 @@
   EXPECT_EQ(implicit_utc, explicit_utc);
   EXPECT_EQ(implicit_utc.name(), explicit_utc.name());
 
-  const time_zone fixed_zero = fixed_time_zone(absl::time_internal::cctz::seconds::zero());
+  const time_zone fixed_zero =
+      fixed_time_zone(absl::time_internal::cctz::seconds::zero());
   EXPECT_EQ(fixed_zero, LoadZone(fixed_zero.name()));
   EXPECT_EQ(fixed_zero, explicit_utc);
 
@@ -807,8 +807,8 @@
 TEST(StdChronoTimePoint, TimeTAlignment) {
   // Ensures that the Unix epoch and the system clock epoch are an integral
   // number of seconds apart. This simplifies conversions to/from time_t.
-  auto diff = chrono::system_clock::time_point() -
-              chrono::system_clock::from_time_t(0);
+  auto diff =
+      chrono::system_clock::time_point() - chrono::system_clock::from_time_t(0);
   EXPECT_EQ(chrono::system_clock::time_point::duration::zero(),
             diff % chrono::seconds(1));
 }
@@ -817,20 +817,20 @@
   const time_zone utc = utc_time_zone();
   const auto t0 = chrono::system_clock::from_time_t(0);
 
-  ExpectTime(chrono::time_point_cast<chrono::nanoseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::microseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::milliseconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::seconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::minutes>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
-  ExpectTime(chrono::time_point_cast<chrono::hours>(t0), utc,
-             1970, 1, 1, 0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::nanoseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::microseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::milliseconds>(t0), utc, 1970, 1, 1,
+             0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::seconds>(t0), utc, 1970, 1, 1, 0,
+             0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<absl::time_internal::cctz::seconds>(t0),
+             utc, 1970, 1, 1, 0, 0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::minutes>(t0), utc, 1970, 1, 1, 0,
+             0, 0, 0, false, "UTC");
+  ExpectTime(chrono::time_point_cast<chrono::hours>(t0), utc, 1970, 1, 1, 0, 0,
+             0, 0, false, "UTC");
 }
 
 TEST(BreakTime, LocalTimeInUTC) {
@@ -912,9 +912,8 @@
       chrono::time_point_cast<chrono::minutes>(
           convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
   EXPECT_EQ("04:00", format("%M:%E*S", tp_m, utc));
-  const time_point<chrono::hours> tp_h =
-      chrono::time_point_cast<chrono::hours>(
-          convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
+  const time_point<chrono::hours> tp_h = chrono::time_point_cast<chrono::hours>(
+      convert(civil_second(2015, 1, 2, 3, 4, 5), utc));
   EXPECT_EQ("00:00", format("%M:%E*S", tp_h, utc));
 }
 
@@ -933,7 +932,7 @@
 
 // NOTE: Run this with -ftrapv to detect overflow problems.
 TEST(MakeTime, SysSecondsLimits) {
-  const char RFC3339[] =  "%Y-%m-%dT%H:%M:%S%Ez";
+  const char RFC3339[] = "%Y-%m-%dT%H:%M:%S%Ez";
   const time_zone utc = utc_time_zone();
   const time_zone east = fixed_time_zone(chrono::hours(14));
   const time_zone west = fixed_time_zone(-chrono::hours(14));
@@ -1029,17 +1028,17 @@
     ASSERT_EQ(0, setenv("TZ", *np, 1));  // change what "localtime" means
     const auto zi = local_time_zone();
     const auto lc = LoadZone("libc:localtime");
-    time_zone::civil_transition trans;
+    time_zone::civil_transition transition;
     for (auto tp = zi.lookup(civil_second()).trans;
-         zi.next_transition(tp, &trans);
-         tp = zi.lookup(trans.to).trans) {
-      const auto fcl = zi.lookup(trans.from);
-      const auto tcl = zi.lookup(trans.to);
+         zi.next_transition(tp, &transition);
+         tp = zi.lookup(transition.to).trans) {
+      const auto fcl = zi.lookup(transition.from);
+      const auto tcl = zi.lookup(transition.to);
       civil_second cs;  // compare cs in zi and lc
       if (fcl.kind == time_zone::civil_lookup::UNIQUE) {
         if (tcl.kind == time_zone::civil_lookup::UNIQUE) {
           // Both unique; must be an is_dst or abbr change.
-          ASSERT_EQ(trans.from, trans.to);
+          ASSERT_EQ(transition.from, transition.to);
           const auto trans = fcl.trans;
           const auto tal = zi.lookup(trans);
           const auto tprev = trans - absl::time_internal::cctz::seconds(1);
@@ -1050,11 +1049,11 @@
           continue;
         }
         ASSERT_EQ(time_zone::civil_lookup::REPEATED, tcl.kind);
-        cs = trans.to;
+        cs = transition.to;
       } else {
         ASSERT_EQ(time_zone::civil_lookup::UNIQUE, tcl.kind);
         ASSERT_EQ(time_zone::civil_lookup::SKIPPED, fcl.kind);
-        cs = trans.from;
+        cs = transition.from;
       }
       if (cs.year() > 2037) break;  // limit test time (and to 32-bit time_t)
       const auto cl_zi = zi.lookup(cs);
@@ -1434,5 +1433,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_posix.cc b/absl/time/internal/cctz/src/time_zone_posix.cc
index d1f8ecb..5cdd09e 100644
--- a/absl/time/internal/cctz/src/time_zone_posix.cc
+++ b/absl/time/internal/cctz/src/time_zone_posix.cc
@@ -19,8 +19,10 @@
 #include <limits>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -153,5 +155,5 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/src/time_zone_posix.h b/absl/time/internal/cctz/src/time_zone_posix.h
index 07e3427..0cf2905 100644
--- a/absl/time/internal/cctz/src/time_zone_posix.h
+++ b/absl/time/internal/cctz/src/time_zone_posix.h
@@ -55,8 +55,10 @@
 #include <cstdint>
 #include <string>
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -124,7 +126,7 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_CCTZ_TIME_ZONE_POSIX_H_
diff --git a/absl/time/internal/cctz/src/tzfile.h b/absl/time/internal/cctz/src/tzfile.h
index 51b1f1f..1ed55e0 100644
--- a/absl/time/internal/cctz/src/tzfile.h
+++ b/absl/time/internal/cctz/src/tzfile.h
@@ -22,36 +22,35 @@
 */
 
 #ifndef TZDIR
-#define TZDIR	"/usr/share/zoneinfo" /* Time zone object file directory */
-#endif /* !defined TZDIR */
+#define TZDIR "/usr/share/zoneinfo" /* Time zone object file directory */
+#endif                              /* !defined TZDIR */
 
 #ifndef TZDEFAULT
-#define TZDEFAULT	"/etc/localtime"
+#define TZDEFAULT "/etc/localtime"
 #endif /* !defined TZDEFAULT */
 
 #ifndef TZDEFRULES
-#define TZDEFRULES	"posixrules"
+#define TZDEFRULES "posixrules"
 #endif /* !defined TZDEFRULES */
 
-
 /* See Internet RFC 8536 for more details about the following format.  */
 
 /*
 ** Each file begins with. . .
 */
 
-#define	TZ_MAGIC	"TZif"
+#define TZ_MAGIC "TZif"
 
 struct tzhead {
-	char	tzh_magic[4];		/* TZ_MAGIC */
-	char	tzh_version[1];		/* '\0' or '2' or '3' as of 2013 */
-	char	tzh_reserved[15];	/* reserved; must be zero */
-	char	tzh_ttisutcnt[4];	/* coded number of trans. time flags */
-	char	tzh_ttisstdcnt[4];	/* coded number of trans. time flags */
-	char	tzh_leapcnt[4];		/* coded number of leap seconds */
-	char	tzh_timecnt[4];		/* coded number of transition times */
-	char	tzh_typecnt[4];		/* coded number of local time types */
-	char	tzh_charcnt[4];		/* coded number of abbr. chars */
+  char tzh_magic[4];      /* TZ_MAGIC */
+  char tzh_version[1];    /* '\0' or '2' or '3' as of 2013 */
+  char tzh_reserved[15];  /* reserved; must be zero */
+  char tzh_ttisutcnt[4];  /* coded number of trans. time flags */
+  char tzh_ttisstdcnt[4]; /* coded number of trans. time flags */
+  char tzh_leapcnt[4];    /* coded number of leap seconds */
+  char tzh_timecnt[4];    /* coded number of transition times */
+  char tzh_typecnt[4];    /* coded number of local time types */
+  char tzh_charcnt[4];    /* coded number of abbr. chars */
 };
 
 /*
@@ -103,21 +102,21 @@
 */
 
 #ifndef TZ_MAX_TIMES
-#define TZ_MAX_TIMES	2000
+#define TZ_MAX_TIMES 2000
 #endif /* !defined TZ_MAX_TIMES */
 
 #ifndef TZ_MAX_TYPES
 /* This must be at least 17 for Europe/Samara and Europe/Vilnius.  */
-#define TZ_MAX_TYPES	256 /* Limited by what (unsigned char)'s can hold */
-#endif /* !defined TZ_MAX_TYPES */
+#define TZ_MAX_TYPES 256 /* Limited by what (unsigned char)'s can hold */
+#endif                   /* !defined TZ_MAX_TYPES */
 
 #ifndef TZ_MAX_CHARS
-#define TZ_MAX_CHARS	50	/* Maximum number of abbreviation characters */
-				/* (limited by what unsigned chars can hold) */
-#endif /* !defined TZ_MAX_CHARS */
+#define TZ_MAX_CHARS 50 /* Maximum number of abbreviation characters */
+                        /* (limited by what unsigned chars can hold) */
+#endif                  /* !defined TZ_MAX_CHARS */
 
 #ifndef TZ_MAX_LEAPS
-#define TZ_MAX_LEAPS	50	/* Maximum number of leap second corrections */
-#endif /* !defined TZ_MAX_LEAPS */
+#define TZ_MAX_LEAPS 50 /* Maximum number of leap second corrections */
+#endif                  /* !defined TZ_MAX_LEAPS */
 
 #endif /* !defined TZFILE_H */
diff --git a/absl/time/internal/cctz/src/zone_info_source.cc b/absl/time/internal/cctz/src/zone_info_source.cc
index b64950c..98ea161 100644
--- a/absl/time/internal/cctz/src/zone_info_source.cc
+++ b/absl/time/internal/cctz/src/zone_info_source.cc
@@ -14,8 +14,10 @@
 
 #include "absl/time/internal/cctz/include/cctz/zone_info_source.h"
 
+#include "absl/base/config.h"
+
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz {
 
@@ -25,11 +27,11 @@
 
 }  // namespace cctz
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz_extension {
 
@@ -39,8 +41,9 @@
 // defers to the fallback factory.
 std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource> DefaultFactory(
     const std::string& name,
-    const std::function<std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
-        const std::string& name)>& fallback_factory) {
+    const std::function<
+        std::unique_ptr<absl::time_internal::cctz::ZoneInfoSource>(
+            const std::string& name)>& fallback_factory) {
   return fallback_factory(name);
 }
 
@@ -52,21 +55,52 @@
 #if !defined(__has_attribute)
 #define __has_attribute(x) 0
 #endif
-#if __has_attribute(weak) || defined(__GNUC__)
-ZoneInfoSourceFactory zone_info_source_factory
-    __attribute__((weak)) = DefaultFactory;
-#elif defined(_MSC_VER) && !defined(_LIBCPP_VERSION)
+// MinGW is GCC on Windows, so while it asserts __has_attribute(weak), the
+// Windows linker cannot handle that. Nor does the MinGW compiler know how to
+// pass "#pragma comment(linker, ...)" to the Windows linker.
+#if (__has_attribute(weak) || defined(__GNUC__)) && !defined(__MINGW32__)
+ZoneInfoSourceFactory zone_info_source_factory __attribute__((weak)) =
+    DefaultFactory;
+#elif defined(_MSC_VER) && !defined(__MINGW32__) && !defined(_LIBCPP_VERSION)
 extern ZoneInfoSourceFactory zone_info_source_factory;
 extern ZoneInfoSourceFactory default_factory;
 ZoneInfoSourceFactory default_factory = DefaultFactory;
 #if defined(_M_IX86)
-#pragma comment( \
-    linker,      \
-    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@lts_2019_08_08@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@6@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@6@@ZA=?default_factory@cctz_extension@time_internal@lts_2019_08_08@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@6@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@6@@ZA")
+#pragma comment(                                                                                                         \
+    linker,                                                                                                              \
+    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                    \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                 \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZA=?default_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                                       \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                 \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@ABV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                   \
+    "@@@std@@@std@@ABV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZA")
 #elif defined(_M_IA_64) || defined(_M_AMD64) || defined(_M_ARM64)
-#pragma comment( \
-    linker,      \
-    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@lts_2019_08_08@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@6@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@6@@ZEA=?default_factory@cctz_extension@time_internal@lts_2019_08_08@absl@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@6@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@U?$default_delete@VZoneInfoSource@cctz@time_internal@lts_2019_08_08@absl@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@6@@ZEA")
+#pragma comment(                                                                                                          \
+    linker,                                                                                                               \
+    "/alternatename:?zone_info_source_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                     \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                  \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZEA=?default_factory@cctz_extension@time_internal@" ABSL_INTERNAL_MANGLED_NS                                       \
+    "@@3P6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                  \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@" ABSL_INTERNAL_MANGLED_BACKREFERENCE      \
+    "@AEBV?$function@$$A6A?AV?$unique_ptr@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                   \
+    "@@U?$default_delete@VZoneInfoSource@cctz@time_internal@" ABSL_INTERNAL_MANGLED_NS                                    \
+    "@@@std@@@std@@AEBV?$basic_string@DU?$char_traits@D@std@@V?$allocator@D@2@@2@@Z@" ABSL_INTERNAL_MANGLED_BACKREFERENCE \
+    "@@ZEA")
 #else
 #error Unsupported MSVC platform
 #endif  // _M_<PLATFORM>
@@ -77,5 +111,5 @@
 
 }  // namespace cctz_extension
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/cctz/testdata/version b/absl/time/internal/cctz/testdata/version
index 8d4a43b..db18f83 100644
--- a/absl/time/internal/cctz/testdata/version
+++ b/absl/time/internal/cctz/testdata/version
@@ -1 +1 @@
-2019b
+2019c
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Detroit b/absl/time/internal/cctz/testdata/zoneinfo/America/Detroit
index 5e02260..e104faa 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Detroit
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Detroit
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Edmonton b/absl/time/internal/cctz/testdata/zoneinfo/America/Edmonton
index 3fa0579..cd78a6f 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Edmonton
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Edmonton
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Indiana/Tell_City b/absl/time/internal/cctz/testdata/zoneinfo/America/Indiana/Tell_City
index 4ce95c1..7bbb653 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Indiana/Tell_City
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Indiana/Tell_City
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Kentucky/Louisville b/absl/time/internal/cctz/testdata/zoneinfo/America/Kentucky/Louisville
index f4c4cf9..177836e 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Kentucky/Louisville
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Kentucky/Louisville
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Louisville b/absl/time/internal/cctz/testdata/zoneinfo/America/Louisville
index f4c4cf9..177836e 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Louisville
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Louisville
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/America/Vancouver b/absl/time/internal/cctz/testdata/zoneinfo/America/Vancouver
index 0f9f832..bb60cbc 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/America/Vancouver
+++ b/absl/time/internal/cctz/testdata/zoneinfo/America/Vancouver
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Hong_Kong b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Hong_Kong
index 378a37f..23d0375 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Hong_Kong
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Hong_Kong
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Istanbul b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Istanbul
index 10d4b21..508446b 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Istanbul
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Istanbul
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Seoul b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Seoul
index 73182cf..96199e7 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Asia/Seoul
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Asia/Seoul
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Canada/Mountain b/absl/time/internal/cctz/testdata/zoneinfo/Canada/Mountain
index 3fa0579..cd78a6f 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Canada/Mountain
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Canada/Mountain
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Canada/Pacific b/absl/time/internal/cctz/testdata/zoneinfo/Canada/Pacific
index 0f9f832..bb60cbc 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Canada/Pacific
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Canada/Pacific
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Brussels b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Brussels
index 9613c98..40d7124 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Brussels
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Brussels
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Istanbul b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Istanbul
index 10d4b21..508446b 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Istanbul
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Istanbul
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Kaliningrad b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Kaliningrad
index f774ffd..cc99bea 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Kaliningrad
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Kaliningrad
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Vienna b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Vienna
index 696f359..3582bb1 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Europe/Vienna
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Europe/Vienna
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Hongkong b/absl/time/internal/cctz/testdata/zoneinfo/Hongkong
index 378a37f..23d0375 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Hongkong
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Hongkong
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Fiji b/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Fiji
index 6bf667d..d39bf53 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Fiji
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Fiji
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Norfolk b/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Norfolk
index 1f6d610..53c1aad 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Norfolk
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Pacific/Norfolk
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/ROK b/absl/time/internal/cctz/testdata/zoneinfo/ROK
index 73182cf..96199e7 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/ROK
+++ b/absl/time/internal/cctz/testdata/zoneinfo/ROK
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/Turkey b/absl/time/internal/cctz/testdata/zoneinfo/Turkey
index 10d4b21..508446b 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/Turkey
+++ b/absl/time/internal/cctz/testdata/zoneinfo/Turkey
Binary files differ
diff --git a/absl/time/internal/cctz/testdata/zoneinfo/US/Michigan b/absl/time/internal/cctz/testdata/zoneinfo/US/Michigan
index 5e02260..e104faa 100644
--- a/absl/time/internal/cctz/testdata/zoneinfo/US/Michigan
+++ b/absl/time/internal/cctz/testdata/zoneinfo/US/Michigan
Binary files differ
diff --git a/absl/time/internal/get_current_time_chrono.inc b/absl/time/internal/get_current_time_chrono.inc
index d294b3a..5eeb640 100644
--- a/absl/time/internal/get_current_time_chrono.inc
+++ b/absl/time/internal/get_current_time_chrono.inc
@@ -16,7 +16,7 @@
 #include <cstdint>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 static int64_t GetCurrentTimeNanosFromSystem() {
@@ -27,5 +27,5 @@
 }
 
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/get_current_time_posix.inc b/absl/time/internal/get_current_time_posix.inc
index 0ce2a9a..4207200 100644
--- a/absl/time/internal/get_current_time_posix.inc
+++ b/absl/time/internal/get_current_time_posix.inc
@@ -7,7 +7,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 static int64_t GetCurrentTimeNanosFromSystem() {
@@ -20,5 +20,5 @@
 }
 
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/test_util.cc b/absl/time/internal/test_util.cc
index 2bc449f..9bffe12 100644
--- a/absl/time/internal/test_util.cc
+++ b/absl/time/internal/test_util.cc
@@ -24,7 +24,7 @@
 namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 TimeZone LoadTimeZone(const std::string& name) {
@@ -34,11 +34,11 @@
 }
 
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 namespace cctz_extension {
 namespace {
@@ -119,9 +119,12 @@
 
 }  // namespace
 
+#if !defined(__MINGW32__)
+// MinGW does not support the weak symbol extension mechanism.
 ZoneInfoSourceFactory zone_info_source_factory = TestFactory;
+#endif
 
 }  // namespace cctz_extension
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/internal/test_util.h b/absl/time/internal/test_util.h
index 1a320ca..5c4bf1f 100644
--- a/absl/time/internal/test_util.h
+++ b/absl/time/internal/test_util.h
@@ -20,14 +20,14 @@
 #include "absl/time/time.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace time_internal {
 
 // Loads the named timezone, but dies on any failure.
 absl::TimeZone LoadTimeZone(const std::string& name);
 
 }  // namespace time_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_INTERNAL_TEST_UTIL_H_
diff --git a/absl/time/time.cc b/absl/time/time.cc
index 2c7da3a..6bb36cb 100644
--- a/absl/time/time.cc
+++ b/absl/time/time.cc
@@ -47,7 +47,7 @@
 namespace cctz = absl::time_internal::cctz;
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -431,9 +431,17 @@
 }
 
 absl::Time FromTM(const struct tm& tm, absl::TimeZone tz) {
-  const CivilSecond cs(tm.tm_year + 1900, tm.tm_mon + 1, tm.tm_mday,
-                       tm.tm_hour, tm.tm_min, tm.tm_sec);
-  const auto ti = tz.At(cs);
+  civil_year_t tm_year = tm.tm_year;
+  // Avoids years that are too extreme for CivilSecond to normalize.
+  if (tm_year > 300000000000ll) return InfiniteFuture();
+  if (tm_year < -300000000000ll) return InfinitePast();
+  int tm_mon = tm.tm_mon;
+  if (tm_mon == std::numeric_limits<int>::max()) {
+    tm_mon -= 12;
+    tm_year += 1;
+  }
+  const auto ti = tz.At(CivilSecond(tm_year + 1900, tm_mon + 1, tm.tm_mday,
+                                    tm.tm_hour, tm.tm_min, tm.tm_sec));
   return tm.tm_isdst == 0 ? ti.post : ti.pre;
 }
 
@@ -487,5 +495,5 @@
   return tm;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/time/time.h b/absl/time/time.h
index 1f6500e..33a4a63 100644
--- a/absl/time/time.h
+++ b/absl/time/time.h
@@ -83,12 +83,13 @@
 #include <type_traits>
 #include <utility>
 
+#include "absl/base/macros.h"
 #include "absl/strings/string_view.h"
 #include "absl/time/civil_time.h"
 #include "absl/time/internal/cctz/include/cctz/time_zone.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 class Duration;  // Defined below
 class Time;      // Defined below
@@ -381,11 +382,11 @@
 // of the unit indicated by the factory function's name. The number must be
 // representable as int64_t.
 //
-// Note: no "Days()" factory function exists because "a day" is ambiguous.
+// NOTE: no "Days()" factory function exists because "a day" is ambiguous.
 // Civil days are not always 24 hours long, and a 24-hour duration often does
 // not correspond with a civil day. If a 24-hour duration is needed, use
-// `absl::Hours(24)`. (If you actually want a civil day, use absl::CivilDay
-// from civil_time.h.)
+// `absl::Hours(24)`. If you actually want a civil day, use absl::CivilDay
+// from civil_time.h.
 //
 // Example:
 //
@@ -422,7 +423,9 @@
 template <typename T, time_internal::EnableIfFloat<T> = 0>
 Duration Seconds(T n) {
   if (n >= 0) {  // Note: `NaN >= 0` is false.
-    if (n >= (std::numeric_limits<int64_t>::max)()) return InfiniteDuration();
+    if (n >= static_cast<T>((std::numeric_limits<int64_t>::max)())) {
+      return InfiniteDuration();
+    }
     return time_internal::MakePosDoubleDuration(n);
   } else {
     if (std::isnan(n))
@@ -546,7 +549,11 @@
 
 // Support for flag values of type Duration. Duration flags must be specified
 // in a format that is valid input for absl::ParseDuration().
+bool AbslParseFlag(absl::string_view text, Duration* dst, std::string* error);
+std::string AbslUnparseFlag(Duration d);
+ABSL_DEPRECATED("Use AbslParseFlag() instead.")
 bool ParseFlag(const std::string& text, Duration* dst, std::string* error);
+ABSL_DEPRECATED("Use AbslUnparseFlag() instead.")
 std::string UnparseFlag(Duration d);
 
 // Time
@@ -816,7 +823,11 @@
 // Additionally, if you'd like to specify a time as a count of
 // seconds/milliseconds/etc from the Unix epoch, use an absl::Duration flag
 // and add that duration to absl::UnixEpoch() to get an absl::Time.
+bool AbslParseFlag(absl::string_view text, Time* t, std::string* error);
+std::string AbslUnparseFlag(Time t);
+ABSL_DEPRECATED("Use AbslParseFlag() instead.")
 bool ParseFlag(const std::string& text, Time* t, std::string* error);
+ABSL_DEPRECATED("Use AbslUnparseFlag() instead.")
 std::string UnparseFlag(Time t);
 
 // TimeZone
@@ -1104,7 +1115,7 @@
 // An `absl::TimeConversion` represents the conversion of year, month, day,
 // hour, minute, and second values (i.e., a civil time), in a particular
 // `absl::TimeZone`, to a time instant (an absolute time), as returned by
-// `absl::ConvertDateTime()`. Lecacy version of `absl::TimeZone::TimeInfo`.
+// `absl::ConvertDateTime()`. Legacy version of `absl::TimeZone::TimeInfo`.
 //
 // Deprecated. Use `absl::TimeZone::TimeInfo`.
 struct
@@ -1192,15 +1203,18 @@
 // time with UTC offset.  Also note the use of "%Y": RFC3339 mandates that
 // years have exactly four digits, but we allow them to take their natural
 // width.
-extern const char RFC3339_full[];  // %Y-%m-%dT%H:%M:%E*S%Ez
-extern const char RFC3339_sec[];   // %Y-%m-%dT%H:%M:%S%Ez
+ABSL_DLL extern const char
+    RFC3339_full[];  // %Y-%m-%dT%H:%M:%E*S%Ez
+ABSL_DLL extern const char RFC3339_sec[];  // %Y-%m-%dT%H:%M:%S%Ez
 
 // RFC1123_full
 // RFC1123_no_wday
 //
 // FormatTime()/ParseTime() format specifiers for RFC1123 date/time strings.
-extern const char RFC1123_full[];     // %a, %d %b %E4Y %H:%M:%S %z
-extern const char RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
+ABSL_DLL extern const char
+    RFC1123_full[];  // %a, %d %b %E4Y %H:%M:%S %z
+ABSL_DLL extern const char
+    RFC1123_no_wday[];  // %d %b %E4Y %H:%M:%S %z
 
 // FormatTime()
 //
@@ -1402,8 +1416,7 @@
 // IsValidRep64<T>(0) is true if the expression `int64_t{std::declval<T>()}` is
 // valid. That is, if a T can be assigned to an int64_t without narrowing.
 template <typename T>
-constexpr auto IsValidRep64(int)
-    -> decltype(int64_t{std::declval<T>()}, bool()) {
+constexpr auto IsValidRep64(int) -> decltype(int64_t{std::declval<T>()} == 0) {
   return true;
 }
 template <typename T>
@@ -1565,7 +1578,7 @@
   return time_internal::FromUnixDuration(Seconds(t));
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TIME_TIME_H_
diff --git a/absl/time/time_test.cc b/absl/time/time_test.cc
index 37af39d..6f89672 100644
--- a/absl/time/time_test.cc
+++ b/absl/time/time_test.cc
@@ -27,6 +27,7 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "absl/numeric/int128.h"
 #include "absl/time/clock.h"
 #include "absl/time/internal/test_util.h"
 
@@ -575,6 +576,50 @@
             absl::ToChronoTime(absl::UnixEpoch() - tick));
 }
 
+// Check that absl::int128 works as a std::chrono::duration representation.
+TEST(Time, Chrono128) {
+  // Define a std::chrono::time_point type whose time[sic]_since_epoch() is
+  // a signed 128-bit count of attoseconds. This has a range and resolution
+  // (currently) beyond those of absl::Time, and undoubtedly also beyond those
+  // of std::chrono::system_clock::time_point.
+  //
+  // Note: The to/from-chrono support should probably be updated to handle
+  // such wide representations.
+  using Timestamp =
+      std::chrono::time_point<std::chrono::system_clock,
+                              std::chrono::duration<absl::int128, std::atto>>;
+
+  // Expect that we can round-trip the std::chrono::system_clock::time_point
+  // extremes through both absl::Time and Timestamp, and that Timestamp can
+  // handle the (current) absl::Time extremes.
+  //
+  // Note: We should use std::chrono::floor() instead of time_point_cast(),
+  // but floor() is only available since c++17.
+  for (const auto tp : {std::chrono::system_clock::time_point::min(),
+                        std::chrono::system_clock::time_point::max()}) {
+    EXPECT_EQ(tp, absl::ToChronoTime(absl::FromChrono(tp)));
+    EXPECT_EQ(tp, std::chrono::time_point_cast<
+                      std::chrono::system_clock::time_point::duration>(
+                      std::chrono::time_point_cast<Timestamp::duration>(tp)));
+  }
+  Timestamp::duration::rep v = std::numeric_limits<int64_t>::min();
+  v *= Timestamp::duration::period::den;
+  auto ts = Timestamp(Timestamp::duration(v));
+  ts += std::chrono::duration<int64_t, std::atto>(0);
+  EXPECT_EQ(std::numeric_limits<int64_t>::min(),
+            ts.time_since_epoch().count() / Timestamp::duration::period::den);
+  EXPECT_EQ(0,
+            ts.time_since_epoch().count() % Timestamp::duration::period::den);
+  v = std::numeric_limits<int64_t>::max();
+  v *= Timestamp::duration::period::den;
+  ts = Timestamp(Timestamp::duration(v));
+  ts += std::chrono::duration<int64_t, std::atto>(999999999750000000);
+  EXPECT_EQ(std::numeric_limits<int64_t>::max(),
+            ts.time_since_epoch().count() / Timestamp::duration::period::den);
+  EXPECT_EQ(999999999750000000,
+            ts.time_since_epoch().count() % Timestamp::duration::period::den);
+}
+
 TEST(Time, TimeZoneAt) {
   const absl::TimeZone nyc =
       absl::time_internal::LoadTimeZone("America/New_York");
@@ -795,6 +840,30 @@
   tm.tm_isdst = 1;
   t = FromTM(tm, nyc);
   EXPECT_EQ("2014-03-09T03:30:42-04:00", absl::FormatTime(t, nyc));  // DST
+
+  // Adjusts tm to refer to a time with a year larger than 2147483647.
+  tm.tm_year = 2147483647 - 1900 + 1;
+  tm.tm_mon = 6 - 1;
+  tm.tm_mday = 28;
+  tm.tm_hour = 1;
+  tm.tm_min = 2;
+  tm.tm_sec = 3;
+  tm.tm_isdst = -1;
+  t = FromTM(tm, absl::UTCTimeZone());
+  EXPECT_EQ("2147483648-06-28T01:02:03+00:00",
+            absl::FormatTime(t, absl::UTCTimeZone()));
+
+  // Adjusts tm to refer to a time with a very large month.
+  tm.tm_year = 2019 - 1900;
+  tm.tm_mon = 2147483647;
+  tm.tm_mday = 28;
+  tm.tm_hour = 1;
+  tm.tm_min = 2;
+  tm.tm_sec = 3;
+  tm.tm_isdst = -1;
+  t = FromTM(tm, absl::UTCTimeZone());
+  EXPECT_EQ("178958989-08-28T01:02:03+00:00",
+            absl::FormatTime(t, absl::UTCTimeZone()));
 }
 
 TEST(Time, TMRoundTrip) {
diff --git a/absl/types/BUILD.bazel b/absl/types/BUILD.bazel
index 66ecb04..f2ea9f3 100644
--- a/absl/types/BUILD.bazel
+++ b/absl/types/BUILD.bazel
@@ -14,12 +14,11 @@
 # limitations under the License.
 #
 
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
     "ABSL_DEFAULT_LINKOPTS",
-    "ABSL_EXCEPTIONS_FLAG",
-    "ABSL_EXCEPTIONS_FLAG_LINKOPTS",
     "ABSL_TEST_COPTS",
 )
 
@@ -58,12 +57,12 @@
         "bad_any_cast.cc",
         "bad_any_cast.h",
     ],
-    copts = ABSL_EXCEPTIONS_FLAG + ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     visibility = ["//visibility:private"],
     deps = [
-        "//absl/base",
         "//absl/base:config",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -73,31 +72,13 @@
     srcs = [
         "any_test.cc",
     ],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":any",
-        "//absl/base",
-        "//absl/base:config",
-        "//absl/base:exception_testing",
-        "//absl/container:test_instance_tracker",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "any_test_noexceptions",
-    size = "small",
-    srcs = [
-        "any_test.cc",
-    ],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":any",
-        "//absl/base",
         "//absl/base:config",
         "//absl/base:exception_testing",
+        "//absl/base:raw_logging_internal",
         "//absl/container:test_instance_tracker",
         "@com_google_googletest//:gtest_main",
     ],
@@ -106,10 +87,11 @@
 cc_test(
     name = "any_exception_safety_test",
     srcs = ["any_exception_safety_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":any",
+        "//absl/base:config",
         "//absl/base:exception_safety_testing",
         "@com_google_googletest//:gtest_main",
     ],
@@ -137,25 +119,6 @@
     name = "span_test",
     size = "small",
     srcs = ["span_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
-    deps = [
-        ":span",
-        "//absl/base:config",
-        "//absl/base:core_headers",
-        "//absl/base:exception_testing",
-        "//absl/container:fixed_array",
-        "//absl/container:inlined_vector",
-        "//absl/hash:hash_testing",
-        "//absl/strings",
-        "@com_google_googletest//:gtest_main",
-    ],
-)
-
-cc_test(
-    name = "span_test_noexceptions",
-    size = "small",
-    srcs = ["span_test.cc"],
     copts = ABSL_TEST_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
@@ -192,11 +155,11 @@
     name = "bad_optional_access",
     srcs = ["bad_optional_access.cc"],
     hdrs = ["bad_optional_access.h"],
-    copts = ABSL_DEFAULT_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        "//absl/base",
         "//absl/base:config",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -204,11 +167,11 @@
     name = "bad_variant_access",
     srcs = ["bad_variant_access.cc"],
     hdrs = ["bad_variant_access.h"],
-    copts = ABSL_EXCEPTIONS_FLAG + ABSL_DEFAULT_COPTS,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_DEFAULT_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
-        "//absl/base",
         "//absl/base:config",
+        "//absl/base:raw_logging_internal",
     ],
 )
 
@@ -218,12 +181,12 @@
     srcs = [
         "optional_test.cc",
     ],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":optional",
-        "//absl/base",
         "//absl/base:config",
+        "//absl/base:raw_logging_internal",
         "//absl/meta:type_traits",
         "//absl/strings",
         "@com_google_googletest//:gtest_main",
@@ -235,16 +198,51 @@
     srcs = [
         "optional_exception_safety_test.cc",
     ],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":optional",
+        "//absl/base:config",
         "//absl/base:exception_safety_testing",
         "@com_google_googletest//:gtest_main",
     ],
 )
 
 cc_library(
+    name = "conformance_testing",
+    testonly = 1,
+    hdrs = [
+        "internal/conformance_aliases.h",
+        "internal/conformance_archetype.h",
+        "internal/conformance_profile.h",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        "//absl/debugging:demangle_internal",
+        "//absl/meta:type_traits",
+        "//absl/strings",
+        "//absl/utility",
+        "@com_google_googletest//:gtest",
+    ],
+)
+
+cc_test(
+    name = "conformance_testing_test",
+    size = "small",
+    srcs = [
+        "internal/conformance_testing_test.cc",
+    ],
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
+    deps = [
+        ":conformance_testing",
+        "//absl/meta:type_traits",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
+cc_library(
     name = "variant",
     srcs = ["internal/variant.h"],
     hdrs = ["variant.h"],
@@ -264,8 +262,8 @@
     name = "variant_test",
     size = "small",
     srcs = ["variant_test.cc"],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":variant",
         "//absl/base:config",
@@ -298,8 +296,8 @@
     srcs = [
         "variant_exception_safety_test.cc",
     ],
-    copts = ABSL_TEST_COPTS + ABSL_EXCEPTIONS_FLAG,
-    linkopts = ABSL_EXCEPTIONS_FLAG_LINKOPTS + ABSL_DEFAULT_LINKOPTS,
+    copts = ABSL_TEST_COPTS,
+    linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":variant",
         "//absl/base:config",
diff --git a/absl/types/CMakeLists.txt b/absl/types/CMakeLists.txt
index 4ce685d..c7c8825 100644
--- a/absl/types/CMakeLists.txt
+++ b/absl/types/CMakeLists.txt
@@ -50,12 +50,9 @@
    "bad_any_cast.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
-    absl::base
     absl::config
+    absl::raw_logging_internal
 )
 
 absl_cc_test(
@@ -65,14 +62,11 @@
     "any_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::any
-    absl::base
     absl::config
     absl::exception_testing
+    absl::raw_logging_internal
     absl::test_instance_tracker
     gmock_main
 )
@@ -86,9 +80,9 @@
     ${ABSL_TEST_COPTS}
   DEPS
     absl::any
-    absl::base
     absl::config
     absl::exception_testing
+    absl::raw_logging_internal
     absl::test_instance_tracker
     gmock_main
 )
@@ -100,11 +94,9 @@
     "any_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::any
+    absl::config
     absl::exception_safety_testing
     gmock_main
 )
@@ -133,9 +125,6 @@
     "span_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::span
     absl::base
@@ -198,12 +187,9 @@
     "bad_optional_access.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
-    absl::base
     absl::config
+    absl::raw_logging_internal
   PUBLIC
 )
 
@@ -216,12 +202,9 @@
     "bad_variant_access.cc"
   COPTS
     ${ABSL_DEFAULT_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
-    absl::base
     absl::config
+    absl::raw_logging_internal
   PUBLIC
 )
 
@@ -232,15 +215,12 @@
     "optional_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::optional
-    absl::base
     absl::config
-    absl::type_traits
+    absl::raw_logging_internal
     absl::strings
+    absl::type_traits
     gmock_main
 )
 
@@ -251,12 +231,56 @@
     "optional_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
+  DEPS
+    absl::optional
+    absl::config
+    absl::exception_safety_testing
+    gmock_main
+)
+
+absl_cc_library(
+  NAME
+    conformance_testing
+  HDRS
+    "internal/conformance_aliases.h"
+    "internal/conformance_archetype.h"
+    "internal/conformance_profile.h"
+  COPTS
+    ${ABSL_DEFAULT_COPTS}
+  DEPS
+    absl::debugging
+    absl::type_traits
+    absl::strings
+    absl::utility
+    gmock_main
+  PUBLIC
+)
+
+absl_cc_test(
+  NAME
+    conformance_testing_test
+  SRCS
+    "internal/conformance_testing_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
     ${ABSL_EXCEPTIONS_FLAG}
   LINKOPTS
     ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
-    absl::optional
-    absl::exception_safety_testing
+    absl::conformance_testing
+    absl::type_traits
+    gmock_main
+)
+
+absl_cc_test(
+  NAME
+    conformance_testing_test_no_exceptions
+  SRCS
+    "internal/conformance_testing_test.cc"
+  COPTS
+    ${ABSL_TEST_COPTS}
+  DEPS
+    absl::conformance_testing
     gmock_main
 )
 
@@ -286,9 +310,6 @@
     "variant_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::variant
     absl::config
@@ -335,9 +356,6 @@
     "variant_exception_safety_test.cc"
   COPTS
     ${ABSL_TEST_COPTS}
-    ${ABSL_EXCEPTIONS_FLAG}
-  LINKOPTS
-    ${ABSL_EXCEPTIONS_FLAG_LINKOPTS}
   DEPS
     absl::variant
     absl::config
diff --git a/absl/types/any.h b/absl/types/any.h
index 7cae9dd..16bda79 100644
--- a/absl/types/any.h
+++ b/absl/types/any.h
@@ -56,20 +56,20 @@
 #include "absl/base/config.h"
 #include "absl/utility/utility.h"
 
-#ifdef ABSL_HAVE_STD_ANY
+#ifdef ABSL_USES_STD_ANY
 
 #include <any>  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::any;
 using std::any_cast;
 using std::bad_any_cast;
 using std::make_any;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_ANY
+#else  // ABSL_USES_STD_ANY
 
 #include <algorithm>
 #include <cstddef>
@@ -93,7 +93,7 @@
 #endif  // !defined(__GNUC__) || defined(__GXX_RTTI)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace any_internal {
 
@@ -185,7 +185,7 @@
 // object, when containing a value, must contain a value type; storing a
 // reference type is neither desired nor supported.
 //
-// An `absl::any` can only store a type that is copy-constructable; move-only
+// An `absl::any` can only store a type that is copy-constructible; move-only
 // types are not allowed within an `any` object.
 //
 // Example:
@@ -193,7 +193,7 @@
 //   auto a = absl::any(65);                 // Literal, copyable
 //   auto b = absl::any(std::vector<int>()); // Default-initialized, copyable
 //   std::unique_ptr<Foo> my_foo;
-//   auto c = absl::any(std::move(my_foo));  // Error, not copy-constructable
+//   auto c = absl::any(std::move(my_foo));  // Error, not copy-constructible
 //
 // Note that `absl::any` makes use of decayed types (`absl::decay_t` in this
 // context) to remove const-volatile qualifiers (known as "cv qualifiers"),
@@ -518,26 +518,30 @@
 // Description at the declaration site (top of file).
 template <typename T>
 const T* any_cast(const any* operand) noexcept {
-  return operand && operand->GetObjTypeId() == any::IdForType<T>()
+  using U =
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+  return operand && operand->GetObjTypeId() == any::IdForType<U>()
              ? std::addressof(
-                   static_cast<const any::Obj<T>*>(operand->obj_.get())->value)
+                   static_cast<const any::Obj<U>*>(operand->obj_.get())->value)
              : nullptr;
 }
 
 // Description at the declaration site (top of file).
 template <typename T>
 T* any_cast(any* operand) noexcept {
-  return operand && operand->GetObjTypeId() == any::IdForType<T>()
+  using U =
+      typename std::remove_cv<typename std::remove_reference<T>::type>::type;
+  return operand && operand->GetObjTypeId() == any::IdForType<U>()
              ? std::addressof(
-                   static_cast<any::Obj<T>*>(operand->obj_.get())->value)
+                   static_cast<any::Obj<U>*>(operand->obj_.get())->value)
              : nullptr;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_ANY_DETAIL_HAS_RTTI
 
-#endif  // ABSL_HAVE_STD_ANY
+#endif  // ABSL_USES_STD_ANY
 
 #endif  // ABSL_TYPES_ANY_H_
diff --git a/absl/types/any_exception_safety_test.cc b/absl/types/any_exception_safety_test.cc
index 5d7d8a5..31c1140 100644
--- a/absl/types/any_exception_safety_test.cc
+++ b/absl/types/any_exception_safety_test.cc
@@ -14,6 +14,12 @@
 
 #include "absl/types/any.h"
 
+#include "absl/base/config.h"
+
+// This test is a no-op when absl::any is an alias for std::any and when
+// exceptions are not enabled.
+#if !defined(ABSL_USES_STD_ANY) && defined(ABSL_HAVE_EXCEPTIONS)
+
 #include <typeinfo>
 #include <vector>
 
@@ -136,8 +142,6 @@
   EXPECT_TRUE(strong_empty_any_tester.Test(move));
 }
 
-// libstdc++ std::any fails this test
-#if !defined(ABSL_HAVE_STD_ANY)
 TEST(AnyExceptionSafety, Emplace) {
   auto initial_val =
       absl::any{absl::in_place_type_t<Thrower>(), 1, testing::nothrow_ctor};
@@ -163,6 +167,7 @@
   EXPECT_TRUE(empty_tester.Test(emp_thrower));
   EXPECT_TRUE(empty_tester.Test(emp_throwervec));
 }
-#endif  // ABSL_HAVE_STD_ANY
 
 }  // namespace
+
+#endif  // #if !defined(ABSL_USES_STD_ANY) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/any_test.cc b/absl/types/any_test.cc
index 8710472..70e4ba2 100644
--- a/absl/types/any_test.cc
+++ b/absl/types/any_test.cc
@@ -14,6 +14,9 @@
 
 #include "absl/types/any.h"
 
+// This test is a no-op when absl::any is an alias for std::any.
+#if !defined(ABSL_USES_STD_ANY)
+
 #include <initializer_list>
 #include <type_traits>
 #include <utility>
@@ -639,7 +642,7 @@
 // Tests for Exception Behavior //
 //////////////////////////////////
 
-#if defined(ABSL_HAVE_STD_ANY)
+#if defined(ABSL_USES_STD_ANY)
 
 // If using a std `any` implementation, we can't check for a specific message.
 #define ABSL_ANY_TEST_EXPECT_BAD_ANY_CAST(...)                      \
@@ -653,7 +656,7 @@
   ABSL_BASE_INTERNAL_EXPECT_FAIL((__VA_ARGS__), absl::bad_any_cast, \
                                  "Bad any cast")
 
-#endif  // defined(ABSL_HAVE_STD_ANY)
+#endif  // defined(ABSL_USES_STD_ANY)
 
 TEST(AnyTest, ThrowBadAlloc) {
   {
@@ -761,7 +764,7 @@
     BadCopyable bad;
     absl::any target(absl::in_place_type<int>);
     ABSL_ANY_TEST_EXPECT_BAD_COPY(target.emplace<BadCopyable>(bad));
-#if defined(ABSL_HAVE_STD_ANY) && defined(__GLIBCXX__)
+#if defined(ABSL_USES_STD_ANY) && defined(__GLIBCXX__)
     // libstdc++ std::any::emplace() implementation (as of 7.2) has a bug: if an
     // exception is thrown, *this contains a value.
 #define ABSL_GLIBCXX_ANY_EMPLACE_EXCEPTION_BUG 1
@@ -774,3 +777,5 @@
 }
 
 }  // namespace
+
+#endif  // #if !defined(ABSL_USES_STD_ANY)
diff --git a/absl/types/bad_any_cast.cc b/absl/types/bad_any_cast.cc
index 062675d..b0592cc 100644
--- a/absl/types/bad_any_cast.cc
+++ b/absl/types/bad_any_cast.cc
@@ -14,7 +14,7 @@
 
 #include "absl/types/bad_any_cast.h"
 
-#ifndef ABSL_HAVE_STD_ANY
+#ifndef ABSL_USES_STD_ANY
 
 #include <cstdlib>
 
@@ -22,7 +22,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 bad_any_cast::~bad_any_cast() = default;
 
@@ -40,7 +40,7 @@
 }
 
 }  // namespace any_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_ANY
+#endif  // ABSL_USES_STD_ANY
diff --git a/absl/types/bad_any_cast.h b/absl/types/bad_any_cast.h
index 8a8476c..114cef8 100644
--- a/absl/types/bad_any_cast.h
+++ b/absl/types/bad_any_cast.h
@@ -25,20 +25,20 @@
 
 #include "absl/base/config.h"
 
-#ifdef ABSL_HAVE_STD_ANY
+#ifdef ABSL_USES_STD_ANY
 
 #include <any>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::bad_any_cast;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_ANY
+#else  // ABSL_USES_STD_ANY
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // bad_any_cast
@@ -67,9 +67,9 @@
 [[noreturn]] void ThrowBadAnyCast();
 
 }  // namespace any_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_ANY
+#endif  // ABSL_USES_STD_ANY
 
 #endif  // ABSL_TYPES_BAD_ANY_CAST_H_
diff --git a/absl/types/bad_optional_access.cc b/absl/types/bad_optional_access.cc
index 440adac..26aca70 100644
--- a/absl/types/bad_optional_access.cc
+++ b/absl/types/bad_optional_access.cc
@@ -14,7 +14,7 @@
 
 #include "absl/types/bad_optional_access.h"
 
-#ifndef ABSL_HAVE_STD_OPTIONAL
+#ifndef ABSL_USES_STD_OPTIONAL
 
 #include <cstdlib>
 
@@ -22,7 +22,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 bad_optional_access::~bad_optional_access() = default;
 
@@ -42,7 +42,7 @@
 }
 
 }  // namespace optional_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
diff --git a/absl/types/bad_optional_access.h b/absl/types/bad_optional_access.h
index 585c7c7..a500286 100644
--- a/absl/types/bad_optional_access.h
+++ b/absl/types/bad_optional_access.h
@@ -25,20 +25,20 @@
 
 #include "absl/base/config.h"
 
-#ifdef ABSL_HAVE_STD_OPTIONAL
+#ifdef ABSL_USES_STD_OPTIONAL
 
 #include <optional>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::bad_optional_access;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_OPTIONAL
+#else  // ABSL_USES_STD_OPTIONAL
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // bad_optional_access
@@ -70,9 +70,9 @@
 [[noreturn]] void throw_bad_optional_access();
 
 }  // namespace optional_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
 
 #endif  // ABSL_TYPES_BAD_OPTIONAL_ACCESS_H_
diff --git a/absl/types/bad_variant_access.cc b/absl/types/bad_variant_access.cc
index d60dae9..3dc88cc 100644
--- a/absl/types/bad_variant_access.cc
+++ b/absl/types/bad_variant_access.cc
@@ -14,7 +14,7 @@
 
 #include "absl/types/bad_variant_access.h"
 
-#ifndef ABSL_HAVE_STD_VARIANT
+#ifndef ABSL_USES_STD_VARIANT
 
 #include <cstdlib>
 #include <stdexcept>
@@ -23,7 +23,7 @@
 #include "absl/base/internal/raw_logging.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 //////////////////////////
 // [variant.bad.access] //
@@ -58,7 +58,7 @@
 }
 
 }  // namespace variant_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_VARIANT
diff --git a/absl/types/bad_variant_access.h b/absl/types/bad_variant_access.h
index 8d635b5..095969f 100644
--- a/absl/types/bad_variant_access.h
+++ b/absl/types/bad_variant_access.h
@@ -25,20 +25,20 @@
 
 #include "absl/base/config.h"
 
-#ifdef ABSL_HAVE_STD_VARIANT
+#ifdef ABSL_USES_STD_VARIANT
 
 #include <variant>
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::bad_variant_access;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_VARIANT
+#else  // ABSL_USES_STD_VARIANT
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // bad_variant_access
@@ -74,9 +74,9 @@
 [[noreturn]] void Rethrow();
 
 }  // namespace variant_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_VARIANT
 
 #endif  // ABSL_TYPES_BAD_VARIANT_ACCESS_H_
diff --git a/absl/types/compare.h b/absl/types/compare.h
index 6f371be..62ca70f 100644
--- a/absl/types/compare.h
+++ b/absl/types/compare.h
@@ -39,7 +39,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace compare_internal {
 
 using value_type = int8_t;
@@ -80,79 +80,72 @@
 
 enum class ncmp : value_type { unordered = -127 };
 
+// Define macros to allow for creation or emulation of C++17 inline variables
+// based on whether the feature is supported. Note: we can't use
+// ABSL_INTERNAL_INLINE_CONSTEXPR here because the variables here are of
+// incomplete types so they need to be defined after the types are complete.
+#ifdef __cpp_inline_variables
+
+#define ABSL_COMPARE_INLINE_BASECLASS_DECL(name)
+
+#define ABSL_COMPARE_INLINE_SUBCLASS_DECL(type, name) \
+  static const type name
+
+#define ABSL_COMPARE_INLINE_INIT(type, name, init) \
+  inline constexpr type type::name(init)
+
+#else  // __cpp_inline_variables
+
+#define ABSL_COMPARE_INLINE_BASECLASS_DECL(name) \
+  ABSL_CONST_INIT static const T name
+
+#define ABSL_COMPARE_INLINE_SUBCLASS_DECL(type, name)
+
+#define ABSL_COMPARE_INLINE_INIT(type, name, init) \
+  template <typename T>                            \
+  const T compare_internal::type##_base<T>::name(init)
+
+#endif  // __cpp_inline_variables
+
 // These template base classes allow for defining the values of the constants
 // in the header file (for performance) without using inline variables (which
 // aren't available in C++11).
 template <typename T>
 struct weak_equality_base {
-  ABSL_CONST_INIT static const T equivalent;
-  ABSL_CONST_INIT static const T nonequivalent;
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
 };
-template <typename T>
-const T weak_equality_base<T>::equivalent(eq::equivalent);
-template <typename T>
-const T weak_equality_base<T>::nonequivalent(eq::nonequivalent);
 
 template <typename T>
 struct strong_equality_base {
-  ABSL_CONST_INIT static const T equal;
-  ABSL_CONST_INIT static const T nonequal;
-  ABSL_CONST_INIT static const T equivalent;
-  ABSL_CONST_INIT static const T nonequivalent;
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equal);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequal);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(nonequivalent);
 };
-template <typename T>
-const T strong_equality_base<T>::equal(eq::equal);
-template <typename T>
-const T strong_equality_base<T>::nonequal(eq::nonequal);
-template <typename T>
-const T strong_equality_base<T>::equivalent(eq::equivalent);
-template <typename T>
-const T strong_equality_base<T>::nonequivalent(eq::nonequivalent);
 
 template <typename T>
 struct partial_ordering_base {
-  ABSL_CONST_INIT static const T less;
-  ABSL_CONST_INIT static const T equivalent;
-  ABSL_CONST_INIT static const T greater;
-  ABSL_CONST_INIT static const T unordered;
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(unordered);
 };
-template <typename T>
-const T partial_ordering_base<T>::less(ord::less);
-template <typename T>
-const T partial_ordering_base<T>::equivalent(eq::equivalent);
-template <typename T>
-const T partial_ordering_base<T>::greater(ord::greater);
-template <typename T>
-const T partial_ordering_base<T>::unordered(ncmp::unordered);
 
 template <typename T>
 struct weak_ordering_base {
-  ABSL_CONST_INIT static const T less;
-  ABSL_CONST_INIT static const T equivalent;
-  ABSL_CONST_INIT static const T greater;
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
 };
-template <typename T>
-const T weak_ordering_base<T>::less(ord::less);
-template <typename T>
-const T weak_ordering_base<T>::equivalent(eq::equivalent);
-template <typename T>
-const T weak_ordering_base<T>::greater(ord::greater);
 
 template <typename T>
 struct strong_ordering_base {
-  ABSL_CONST_INIT static const T less;
-  ABSL_CONST_INIT static const T equal;
-  ABSL_CONST_INIT static const T equivalent;
-  ABSL_CONST_INIT static const T greater;
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(less);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equal);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(equivalent);
+  ABSL_COMPARE_INLINE_BASECLASS_DECL(greater);
 };
-template <typename T>
-const T strong_ordering_base<T>::less(ord::less);
-template <typename T>
-const T strong_ordering_base<T>::equal(eq::equal);
-template <typename T>
-const T strong_ordering_base<T>::equivalent(eq::equivalent);
-template <typename T>
-const T strong_ordering_base<T>::greater(ord::greater);
 
 }  // namespace compare_internal
 
@@ -163,6 +156,9 @@
   friend struct compare_internal::weak_equality_base<weak_equality>;
 
  public:
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, equivalent);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_equality, nonequivalent);
+
   // Comparisons
   friend constexpr bool operator==(
       weak_equality v, compare_internal::OnlyLiteralZero<>) noexcept {
@@ -180,10 +176,22 @@
                                    weak_equality v) noexcept {
     return 0 != v.value_;
   }
+  friend constexpr bool operator==(weak_equality v1,
+                                   weak_equality v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(weak_equality v1,
+                                   weak_equality v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
+ABSL_COMPARE_INLINE_INIT(weak_equality, equivalent,
+                         compare_internal::eq::equivalent);
+ABSL_COMPARE_INLINE_INIT(weak_equality, nonequivalent,
+                         compare_internal::eq::nonequivalent);
 
 class strong_equality
     : public compare_internal::strong_equality_base<strong_equality> {
@@ -192,6 +200,11 @@
   friend struct compare_internal::strong_equality_base<strong_equality>;
 
  public:
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equal);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequal);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, equivalent);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_equality, nonequivalent);
+
   // Conversion
   constexpr operator weak_equality() const noexcept {  // NOLINT
     return value_ == 0 ? weak_equality::equivalent
@@ -214,10 +227,25 @@
                                    strong_equality v) noexcept {
     return 0 != v.value_;
   }
+  friend constexpr bool operator==(strong_equality v1,
+                                   strong_equality v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(strong_equality v1,
+                                   strong_equality v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
+ABSL_COMPARE_INLINE_INIT(strong_equality, equal, compare_internal::eq::equal);
+ABSL_COMPARE_INLINE_INIT(strong_equality, nonequal,
+                         compare_internal::eq::nonequal);
+ABSL_COMPARE_INLINE_INIT(strong_equality, equivalent,
+                         compare_internal::eq::equivalent);
+ABSL_COMPARE_INLINE_INIT(strong_equality, nonequivalent,
+                         compare_internal::eq::nonequivalent);
 
 class partial_ordering
     : public compare_internal::partial_ordering_base<partial_ordering> {
@@ -235,6 +263,11 @@
   }
 
  public:
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, less);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, equivalent);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, greater);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(partial_ordering, unordered);
+
   // Conversion
   constexpr operator weak_equality() const noexcept {  // NOLINT
     return value_ == 0 ? weak_equality::equivalent
@@ -289,10 +322,25 @@
                                    partial_ordering v) noexcept {
     return v.is_ordered() && 0 >= v.value_;
   }
+  friend constexpr bool operator==(partial_ordering v1,
+                                   partial_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(partial_ordering v1,
+                                   partial_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
+ABSL_COMPARE_INLINE_INIT(partial_ordering, less, compare_internal::ord::less);
+ABSL_COMPARE_INLINE_INIT(partial_ordering, equivalent,
+                         compare_internal::eq::equivalent);
+ABSL_COMPARE_INLINE_INIT(partial_ordering, greater,
+                         compare_internal::ord::greater);
+ABSL_COMPARE_INLINE_INIT(partial_ordering, unordered,
+                         compare_internal::ncmp::unordered);
 
 class weak_ordering
     : public compare_internal::weak_ordering_base<weak_ordering> {
@@ -303,6 +351,10 @@
   friend struct compare_internal::weak_ordering_base<weak_ordering>;
 
  public:
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, less);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, equivalent);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(weak_ordering, greater);
+
   // Conversions
   constexpr operator weak_equality() const noexcept {  // NOLINT
     return value_ == 0 ? weak_equality::equivalent
@@ -362,10 +414,23 @@
                                    weak_ordering v) noexcept {
     return 0 >= v.value_;
   }
+  friend constexpr bool operator==(weak_ordering v1,
+                                   weak_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(weak_ordering v1,
+                                   weak_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
+ABSL_COMPARE_INLINE_INIT(weak_ordering, less, compare_internal::ord::less);
+ABSL_COMPARE_INLINE_INIT(weak_ordering, equivalent,
+                         compare_internal::eq::equivalent);
+ABSL_COMPARE_INLINE_INIT(weak_ordering, greater,
+                         compare_internal::ord::greater);
 
 class strong_ordering
     : public compare_internal::strong_ordering_base<strong_ordering> {
@@ -376,6 +441,11 @@
   friend struct compare_internal::strong_ordering_base<strong_ordering>;
 
  public:
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, less);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, equal);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, equivalent);
+  ABSL_COMPARE_INLINE_SUBCLASS_DECL(strong_ordering, greater);
+
   // Conversions
   constexpr operator weak_equality() const noexcept {  // NOLINT
     return value_ == 0 ? weak_equality::equivalent
@@ -443,10 +513,28 @@
                                    strong_ordering v) noexcept {
     return 0 >= v.value_;
   }
+  friend constexpr bool operator==(strong_ordering v1,
+                                   strong_ordering v2) noexcept {
+    return v1.value_ == v2.value_;
+  }
+  friend constexpr bool operator!=(strong_ordering v1,
+                                   strong_ordering v2) noexcept {
+    return v1.value_ != v2.value_;
+  }
 
  private:
   compare_internal::value_type value_;
 };
+ABSL_COMPARE_INLINE_INIT(strong_ordering, less, compare_internal::ord::less);
+ABSL_COMPARE_INLINE_INIT(strong_ordering, equal, compare_internal::eq::equal);
+ABSL_COMPARE_INLINE_INIT(strong_ordering, equivalent,
+                         compare_internal::eq::equivalent);
+ABSL_COMPARE_INLINE_INIT(strong_ordering, greater,
+                         compare_internal::ord::greater);
+
+#undef ABSL_COMPARE_INLINE_BASECLASS_DECL
+#undef ABSL_COMPARE_INLINE_SUBCLASS_DECL
+#undef ABSL_COMPARE_INLINE_INIT
 
 namespace compare_internal {
 // We also provide these comparator adapter functions for internal absl use.
@@ -504,7 +592,7 @@
 }
 
 }  // namespace compare_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_COMPARE_H_
diff --git a/absl/types/compare_test.cc b/absl/types/compare_test.cc
index 11b3ad4..8095baf 100644
--- a/absl/types/compare_test.cc
+++ b/absl/types/compare_test.cc
@@ -18,7 +18,7 @@
 #include "absl/base/casts.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 // This is necessary to avoid a bunch of lint warnings suggesting that we use
@@ -31,6 +31,15 @@
   EXPECT_TRUE(Identity(0 == weak_equality::equivalent));
   EXPECT_TRUE(Identity(weak_equality::nonequivalent != 0));
   EXPECT_TRUE(Identity(0 != weak_equality::nonequivalent));
+  const weak_equality values[] = {weak_equality::equivalent,
+                                  weak_equality::nonequivalent};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
 }
 
 TEST(Compare, StrongEquality) {
@@ -42,6 +51,18 @@
   EXPECT_TRUE(Identity(0 == strong_equality::equivalent));
   EXPECT_TRUE(Identity(strong_equality::nonequivalent != 0));
   EXPECT_TRUE(Identity(0 != strong_equality::nonequivalent));
+  const strong_equality values[] = {strong_equality::equal,
+                                    strong_equality::nonequal};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
+  EXPECT_TRUE(Identity(strong_equality::equivalent == strong_equality::equal));
+  EXPECT_TRUE(
+      Identity(strong_equality::nonequivalent == strong_equality::nonequal));
 }
 
 TEST(Compare, PartialOrdering) {
@@ -65,6 +86,16 @@
   EXPECT_FALSE(Identity(0 > partial_ordering::unordered));
   EXPECT_FALSE(Identity(partial_ordering::unordered >= 0));
   EXPECT_FALSE(Identity(0 >= partial_ordering::unordered));
+  const partial_ordering values[] = {
+      partial_ordering::less, partial_ordering::equivalent,
+      partial_ordering::greater, partial_ordering::unordered};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
 }
 
 TEST(Compare, WeakOrdering) {
@@ -78,6 +109,15 @@
   EXPECT_TRUE(Identity(0 < weak_ordering::greater));
   EXPECT_TRUE(Identity(weak_ordering::greater >= 0));
   EXPECT_TRUE(Identity(0 <= weak_ordering::greater));
+  const weak_ordering values[] = {
+      weak_ordering::less, weak_ordering::equivalent, weak_ordering::greater};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
 }
 
 TEST(Compare, StrongOrdering) {
@@ -93,6 +133,16 @@
   EXPECT_TRUE(Identity(0 < strong_ordering::greater));
   EXPECT_TRUE(Identity(strong_ordering::greater >= 0));
   EXPECT_TRUE(Identity(0 <= strong_ordering::greater));
+  const strong_ordering values[] = {
+      strong_ordering::less, strong_ordering::equal, strong_ordering::greater};
+  for (const auto& lhs : values) {
+    for (const auto& rhs : values) {
+      const bool are_equal = &lhs == &rhs;
+      EXPECT_EQ(lhs == rhs, are_equal);
+      EXPECT_EQ(lhs != rhs, !are_equal);
+    }
+  }
+  EXPECT_TRUE(Identity(strong_ordering::equivalent == strong_ordering::equal));
 }
 
 TEST(Compare, Conversions) {
@@ -190,7 +240,7 @@
 
 struct WeakOrderingLess {
   template <typename T>
-  absl::weak_ordering operator()(const T &a, const T &b) const {
+  absl::weak_ordering operator()(const T& a, const T& b) const {
     return a < b ? absl::weak_ordering::less
                  : a == b ? absl::weak_ordering::equivalent
                           : absl::weak_ordering::greater;
@@ -224,10 +274,10 @@
 }
 
 TEST(CompareResultAsOrdering, SanityTest) {
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(-1) < 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(-1) == 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(-1) < 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(-1) == 0));
   EXPECT_FALSE(
       Identity(absl::compare_internal::compare_result_as_ordering(-1) > 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
@@ -237,31 +287,31 @@
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::less) > 0));
 
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) < 0));
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) == 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(0) > 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) < 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) == 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(0) > 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
-                           weak_ordering::equivalent) < 0));
+                            weak_ordering::equivalent) < 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
-                            weak_ordering::equivalent) == 0));
+                           weak_ordering::equivalent) == 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::equivalent) > 0));
 
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) < 0));
-  EXPECT_FALSE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) == 0));
-  EXPECT_TRUE(Identity(
-      absl::compare_internal::compare_result_as_ordering(1) > 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) < 0));
+  EXPECT_FALSE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) == 0));
+  EXPECT_TRUE(
+      Identity(absl::compare_internal::compare_result_as_ordering(1) > 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
-                           weak_ordering::greater) < 0));
+                            weak_ordering::greater) < 0));
   EXPECT_FALSE(Identity(absl::compare_internal::compare_result_as_ordering(
                             weak_ordering::greater) == 0));
   EXPECT_TRUE(Identity(absl::compare_internal::compare_result_as_ordering(
-                            weak_ordering::greater) > 0));
+                           weak_ordering::greater) > 0));
 }
 
 TEST(DoThreeWayComparison, SanityTest) {
@@ -308,6 +358,32 @@
       absl::compare_internal::do_three_way_comparison(weak, 10, 5) > 0));
 }
 
+#ifdef __cpp_inline_variables
+TEST(Compare, StaticAsserts) {
+  static_assert(weak_equality::equivalent == 0, "");
+  static_assert(weak_equality::nonequivalent != 0, "");
+
+  static_assert(strong_equality::equal == 0, "");
+  static_assert(strong_equality::nonequal != 0, "");
+  static_assert(strong_equality::equivalent == 0, "");
+  static_assert(strong_equality::nonequivalent != 0, "");
+
+  static_assert(partial_ordering::less < 0, "");
+  static_assert(partial_ordering::equivalent == 0, "");
+  static_assert(partial_ordering::greater > 0, "");
+  static_assert(partial_ordering::unordered != 0, "");
+
+  static_assert(weak_ordering::less < 0, "");
+  static_assert(weak_ordering::equivalent == 0, "");
+  static_assert(weak_ordering::greater > 0, "");
+
+  static_assert(strong_ordering::less < 0, "");
+  static_assert(strong_ordering::equal == 0, "");
+  static_assert(strong_ordering::equivalent == 0, "");
+  static_assert(strong_ordering::greater > 0, "");
+}
+#endif  // __cpp_inline_variables
+
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/types/internal/conformance_aliases.h b/absl/types/internal/conformance_aliases.h
new file mode 100644
index 0000000..0cc6884
--- /dev/null
+++ b/absl/types/internal/conformance_aliases.h
@@ -0,0 +1,447 @@
+// Copyright 2018 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// regularity_aliases.h
+// -----------------------------------------------------------------------------
+//
+// This file contains type aliases of common ConformanceProfiles and Archetypes
+// so that they can be directly used by name without creating them from scratch.
+
+#ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_ALIASES_H_
+#define ABSL_TYPES_INTERNAL_CONFORMANCE_ALIASES_H_
+
+#include "absl/types/internal/conformance_archetype.h"
+#include "absl/types/internal/conformance_profile.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+
+// Creates both a Profile and a corresponding Archetype with root name "name".
+#define ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(name, ...)                \
+  struct name##Profile : __VA_ARGS__ {};                                    \
+                                                                            \
+  using name##Archetype = ::absl::types_internal::Archetype<name##Profile>; \
+                                                                            \
+  template <class AbslInternalProfileTag>                                   \
+  using name##Archetype##_ = ::absl::types_internal::Archetype<             \
+      ::absl::types_internal::StrongProfileTypedef<name##Profile,           \
+                                                   AbslInternalProfileTag>>
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialDefaultConstructor,
+    ConformanceProfile<default_constructible::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowDefaultConstructor,
+    ConformanceProfile<default_constructible::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasDefaultConstructor, ConformanceProfile<default_constructible::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialMoveConstructor, ConformanceProfile<default_constructible::maybe,
+                                                  move_constructible::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowMoveConstructor, ConformanceProfile<default_constructible::maybe,
+                                                  move_constructible::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasMoveConstructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialCopyConstructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowCopyConstructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasCopyConstructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialMoveAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowMoveAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasMoveAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialCopyAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowCopyAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasCopyAssign,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasTrivialDestructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::trivial>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowDestructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasDestructor,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowEquality,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasEquality,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowInequality,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe,
+                       inequality_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasInequality,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe, inequality_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowLessThan,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe, inequality_comparable::maybe,
+                       less_than_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasLessThan,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe, inequality_comparable::maybe,
+                       less_than_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowLessEqual,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe, inequality_comparable::maybe,
+                       less_than_comparable::maybe,
+                       less_equal_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasLessEqual,
+    ConformanceProfile<default_constructible::maybe, move_constructible::maybe,
+                       copy_constructible::maybe, move_assignable::maybe,
+                       copy_assignable::maybe, destructible::maybe,
+                       equality_comparable::maybe, inequality_comparable::maybe,
+                       less_than_comparable::maybe,
+                       less_equal_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowGreaterEqual,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasGreaterEqual,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowGreaterThan,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::maybe,
+        greater_than_comparable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasGreaterThan,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::maybe,
+        greater_than_comparable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasNothrowSwap,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::maybe,
+        greater_than_comparable::maybe, swappable::nothrow>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasSwap,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::maybe,
+        greater_than_comparable::maybe, swappable::yes>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HasStdHashSpecialization,
+    ConformanceProfile<
+        default_constructible::maybe, move_constructible::maybe,
+        copy_constructible::maybe, move_assignable::maybe,
+        copy_assignable::maybe, destructible::maybe, equality_comparable::maybe,
+        inequality_comparable::maybe, less_than_comparable::maybe,
+        less_equal_comparable::maybe, greater_equal_comparable::maybe,
+        greater_than_comparable::maybe, swappable::maybe, hashable::yes>);
+
+////////////////////////////////////////////////////////////////////////////////
+////     The remaining aliases are combinations of the previous aliases.    ////
+////////////////////////////////////////////////////////////////////////////////
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    Equatable, CombineProfiles<HasEqualityProfile, HasInequalityProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    Comparable,
+    CombineProfiles<EquatableProfile, HasLessThanProfile, HasLessEqualProfile,
+                    HasGreaterEqualProfile, HasGreaterThanProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    NothrowEquatable,
+    CombineProfiles<HasNothrowEqualityProfile, HasNothrowInequalityProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    NothrowComparable,
+    CombineProfiles<NothrowEquatableProfile, HasNothrowLessThanProfile,
+                    HasNothrowLessEqualProfile, HasNothrowGreaterEqualProfile,
+                    HasNothrowGreaterThanProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    Value,
+    CombineProfiles<HasNothrowMoveConstructorProfile, HasCopyConstructorProfile,
+                    HasNothrowMoveAssignProfile, HasCopyAssignProfile,
+                    HasNothrowDestructorProfile, HasNothrowSwapProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    EquatableValue, CombineProfiles<EquatableProfile, ValueProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    ComparableValue, CombineProfiles<ComparableProfile, ValueProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    DefaultConstructibleValue,
+    CombineProfiles<HasDefaultConstructorProfile, ValueProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    NothrowMoveConstructible, CombineProfiles<HasNothrowMoveConstructorProfile,
+                                              HasNothrowDestructorProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    EquatableNothrowMoveConstructible,
+    CombineProfiles<EquatableProfile, NothrowMoveConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    ComparableNothrowMoveConstructible,
+    CombineProfiles<ComparableProfile, NothrowMoveConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    DefaultConstructibleNothrowMoveConstructible,
+    CombineProfiles<HasDefaultConstructorProfile,
+                    NothrowMoveConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    CopyConstructible,
+    CombineProfiles<HasNothrowMoveConstructorProfile, HasCopyConstructorProfile,
+                    HasNothrowDestructorProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    EquatableCopyConstructible,
+    CombineProfiles<EquatableProfile, CopyConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    ComparableCopyConstructible,
+    CombineProfiles<ComparableProfile, CopyConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    DefaultConstructibleCopyConstructible,
+    CombineProfiles<HasDefaultConstructorProfile, CopyConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    NothrowMovable,
+    CombineProfiles<HasNothrowMoveConstructorProfile,
+                    HasNothrowMoveAssignProfile, HasNothrowDestructorProfile,
+                    HasNothrowSwapProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    EquatableNothrowMovable,
+    CombineProfiles<EquatableProfile, NothrowMovableProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    ComparableNothrowMovable,
+    CombineProfiles<ComparableProfile, NothrowMovableProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    DefaultConstructibleNothrowMovable,
+    CombineProfiles<HasDefaultConstructorProfile, NothrowMovableProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    TrivialSpecialMemberFunctions,
+    CombineProfiles<HasTrivialDefaultConstructorProfile,
+                    HasTrivialMoveConstructorProfile,
+                    HasTrivialCopyConstructorProfile,
+                    HasTrivialMoveAssignProfile, HasTrivialCopyAssignProfile,
+                    HasTrivialDestructorProfile, HasNothrowSwapProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    TriviallyComplete,
+    CombineProfiles<TrivialSpecialMemberFunctionsProfile, ComparableProfile,
+                    HasStdHashSpecializationProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HashableNothrowMoveConstructible,
+    CombineProfiles<HasStdHashSpecializationProfile,
+                    NothrowMoveConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HashableCopyConstructible,
+    CombineProfiles<HasStdHashSpecializationProfile, CopyConstructibleProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HashableNothrowMovable,
+    CombineProfiles<HasStdHashSpecializationProfile, NothrowMovableProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    HashableValue,
+    CombineProfiles<HasStdHashSpecializationProfile, ValueProfile>);
+
+ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS(
+    ComparableHashableValue,
+    CombineProfiles<HashableValueProfile, ComparableProfile>);
+
+// The "preferred" profiles that we support in Abseil.
+template <template <class...> class Receiver>
+using ExpandBasicProfiles =
+    Receiver<NothrowMoveConstructibleProfile, CopyConstructibleProfile,
+             NothrowMovableProfile, ValueProfile>;
+
+// The basic profiles except that they are also all Equatable.
+template <template <class...> class Receiver>
+using ExpandBasicEquatableProfiles =
+    Receiver<EquatableNothrowMoveConstructibleProfile,
+             EquatableCopyConstructibleProfile, EquatableNothrowMovableProfile,
+             EquatableValueProfile>;
+
+// The basic profiles except that they are also all Comparable.
+template <template <class...> class Receiver>
+using ExpandBasicComparableProfiles =
+    Receiver<ComparableNothrowMoveConstructibleProfile,
+             ComparableCopyConstructibleProfile,
+             ComparableNothrowMovableProfile, ComparableValueProfile>;
+
+// The basic profiles except that they are also all Hashable.
+template <template <class...> class Receiver>
+using ExpandBasicHashableProfiles =
+    Receiver<HashableNothrowMoveConstructibleProfile,
+             HashableCopyConstructibleProfile, HashableNothrowMovableProfile,
+             HashableValueProfile>;
+
+// The basic profiles except that they are also all DefaultConstructible.
+template <template <class...> class Receiver>
+using ExpandBasicDefaultConstructibleProfiles =
+    Receiver<DefaultConstructibleNothrowMoveConstructibleProfile,
+             DefaultConstructibleCopyConstructibleProfile,
+             DefaultConstructibleNothrowMovableProfile,
+             DefaultConstructibleValueProfile>;
+
+// The type profiles that we support in Abseil (all of the previous lists).
+template <template <class...> class Receiver>
+using ExpandSupportedProfiles = Receiver<
+    NothrowMoveConstructibleProfile, CopyConstructibleProfile,
+    NothrowMovableProfile, ValueProfile,
+    EquatableNothrowMoveConstructibleProfile, EquatableCopyConstructibleProfile,
+    EquatableNothrowMovableProfile, EquatableValueProfile,
+    ComparableNothrowMoveConstructibleProfile,
+    ComparableCopyConstructibleProfile, ComparableNothrowMovableProfile,
+    ComparableValueProfile, DefaultConstructibleNothrowMoveConstructibleProfile,
+    DefaultConstructibleCopyConstructibleProfile,
+    DefaultConstructibleNothrowMovableProfile, DefaultConstructibleValueProfile,
+    HashableNothrowMoveConstructibleProfile, HashableCopyConstructibleProfile,
+    HashableNothrowMovableProfile, HashableValueProfile>;
+
+// TODO(calabrese) Include types that have throwing move constructors, since in
+// practice we still need to support them because of standard library types with
+// (potentially) non-noexcept moves.
+
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#undef ABSL_INTERNAL_PROFILE_AND_ARCHETYPE_ALIAS
+
+#endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_ALIASES_H_
diff --git a/absl/types/internal/conformance_archetype.h b/absl/types/internal/conformance_archetype.h
new file mode 100644
index 0000000..2349e0f
--- /dev/null
+++ b/absl/types/internal/conformance_archetype.h
@@ -0,0 +1,978 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// conformance_archetype.h
+// -----------------------------------------------------------------------------
+//
+// This file contains a facility for generating "archetypes" of out of
+// "Conformance Profiles" (see "conformance_profiles.h" for more information
+// about Conformance Profiles). An archetype is a type that aims to support the
+// bare minimum requirements of a given Conformance Profile. For instance, an
+// archetype that corresponds to an ImmutableProfile has exactly a nothrow
+// move-constructor, a potentially-throwing copy constructor, a nothrow
+// destructor, with all other special-member-functions deleted. These archetypes
+// are useful for testing to make sure that templates are able to work with the
+// kinds of types that they claim to support (i.e. that they do not accidentally
+// under-constrain),
+//
+// The main type template in this file is the Archetype template, which takes
+// a Conformance Profile as a template argument and its instantiations are a
+// minimum-conforming model of that profile.
+
+#ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_ARCHETYPE_H_
+#define ABSL_TYPES_INTERNAL_CONFORMANCE_ARCHETYPE_H_
+
+#include <cstddef>
+#include <functional>
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+#include "absl/types/internal/conformance_profile.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+
+// A minimum-conforming implementation of a type with properties specified in
+// `Prof`, where `Prof` is a valid Conformance Profile.
+template <class Prof, class /*Enabler*/ = void>
+class Archetype;
+
+// Given an Archetype, obtain the properties of the profile associated with that
+// archetype.
+template <class Archetype>
+struct PropertiesOfArchetype;
+
+template <class Prof>
+struct PropertiesOfArchetype<Archetype<Prof>> {
+  using type = PropertiesOfT<Prof>;
+};
+
+template <class Archetype>
+using PropertiesOfArchetypeT = typename PropertiesOfArchetype<Archetype>::type;
+
+// A metafunction to determine if a type is an `Archetype`.
+template <class T>
+struct IsArchetype : std::false_type {};
+
+template <class Prof>
+struct IsArchetype<Archetype<Prof>> : std::true_type {};
+
+// A constructor tag type used when creating an Archetype with internal state.
+struct MakeArchetypeState {};
+
+// Data stored within an archetype that is copied/compared/hashed when the
+// corresponding operations are used.
+using ArchetypeState = std::size_t;
+
+////////////////////////////////////////////////////////////////////////////////
+//   This section of the file defines a chain of base classes for Archetype,  //
+//   where each base defines a specific special member function with the      //
+//   appropriate properties (deleted, noexcept(false), noexcept, or trivial). //
+////////////////////////////////////////////////////////////////////////////////
+
+// The bottom-most base, which contains the state and the default constructor.
+template <default_constructible DefaultConstructibleValue>
+struct ArchetypeStateBase {
+  static_assert(DefaultConstructibleValue == default_constructible::yes ||
+                    DefaultConstructibleValue == default_constructible::nothrow,
+                "");
+
+  ArchetypeStateBase() noexcept(
+      DefaultConstructibleValue ==
+      default_constructible::
+          nothrow) /*Vacuous archetype_state initialization*/ {}
+  explicit ArchetypeStateBase(MakeArchetypeState, ArchetypeState state) noexcept
+      : archetype_state(state) {}
+
+  ArchetypeState archetype_state;
+};
+
+template <>
+struct ArchetypeStateBase<default_constructible::maybe> {
+  explicit ArchetypeStateBase() = delete;
+  explicit ArchetypeStateBase(MakeArchetypeState, ArchetypeState state) noexcept
+      : archetype_state(state) {}
+
+  ArchetypeState archetype_state;
+};
+
+template <>
+struct ArchetypeStateBase<default_constructible::trivial> {
+  ArchetypeStateBase() = default;
+  explicit ArchetypeStateBase(MakeArchetypeState, ArchetypeState state) noexcept
+      : archetype_state(state) {}
+
+  ArchetypeState archetype_state;
+};
+
+// The move-constructor base
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue>
+struct ArchetypeMoveConstructor
+    : ArchetypeStateBase<DefaultConstructibleValue> {
+  static_assert(MoveConstructibleValue == move_constructible::yes ||
+                    MoveConstructibleValue == move_constructible::nothrow,
+                "");
+
+  explicit ArchetypeMoveConstructor(MakeArchetypeState,
+                                    ArchetypeState state) noexcept
+      : ArchetypeStateBase<DefaultConstructibleValue>(MakeArchetypeState(),
+                                                      state) {}
+
+  ArchetypeMoveConstructor() = default;
+  ArchetypeMoveConstructor(ArchetypeMoveConstructor&& other) noexcept(
+      MoveConstructibleValue == move_constructible::nothrow)
+      : ArchetypeStateBase<DefaultConstructibleValue>(MakeArchetypeState(),
+                                                      other.archetype_state) {}
+  ArchetypeMoveConstructor(const ArchetypeMoveConstructor&) = default;
+  ArchetypeMoveConstructor& operator=(ArchetypeMoveConstructor&&) = default;
+  ArchetypeMoveConstructor& operator=(const ArchetypeMoveConstructor&) =
+      default;
+};
+
+template <default_constructible DefaultConstructibleValue>
+struct ArchetypeMoveConstructor<DefaultConstructibleValue,
+                                move_constructible::trivial>
+    : ArchetypeStateBase<DefaultConstructibleValue> {
+  explicit ArchetypeMoveConstructor(MakeArchetypeState,
+                                    ArchetypeState state) noexcept
+      : ArchetypeStateBase<DefaultConstructibleValue>(MakeArchetypeState(),
+                                                      state) {}
+
+  ArchetypeMoveConstructor() = default;
+};
+
+// The copy-constructor base
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue>
+struct ArchetypeCopyConstructor
+    : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                               MoveConstructibleValue> {
+  static_assert(CopyConstructibleValue == copy_constructible::yes ||
+                    CopyConstructibleValue == copy_constructible::nothrow,
+                "");
+  explicit ArchetypeCopyConstructor(MakeArchetypeState,
+                                    ArchetypeState state) noexcept
+      : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue>(MakeArchetypeState(),
+                                                         state) {}
+
+  ArchetypeCopyConstructor() = default;
+  ArchetypeCopyConstructor(ArchetypeCopyConstructor&&) = default;
+  ArchetypeCopyConstructor(const ArchetypeCopyConstructor& other) noexcept(
+      CopyConstructibleValue == copy_constructible::nothrow)
+      : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue>(
+            MakeArchetypeState(), other.archetype_state) {}
+  ArchetypeCopyConstructor& operator=(ArchetypeCopyConstructor&&) = default;
+  ArchetypeCopyConstructor& operator=(const ArchetypeCopyConstructor&) =
+      default;
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue>
+struct ArchetypeCopyConstructor<DefaultConstructibleValue,
+                                MoveConstructibleValue,
+                                copy_constructible::maybe>
+    : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                               MoveConstructibleValue> {
+  explicit ArchetypeCopyConstructor(MakeArchetypeState,
+                                    ArchetypeState state) noexcept
+      : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue>(MakeArchetypeState(),
+                                                         state) {}
+
+  ArchetypeCopyConstructor() = default;
+  ArchetypeCopyConstructor(ArchetypeCopyConstructor&&) = default;
+  ArchetypeCopyConstructor(const ArchetypeCopyConstructor&) = delete;
+  ArchetypeCopyConstructor& operator=(ArchetypeCopyConstructor&&) = default;
+  ArchetypeCopyConstructor& operator=(const ArchetypeCopyConstructor&) =
+      default;
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue>
+struct ArchetypeCopyConstructor<DefaultConstructibleValue,
+                                MoveConstructibleValue,
+                                copy_constructible::trivial>
+    : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                               MoveConstructibleValue> {
+  explicit ArchetypeCopyConstructor(MakeArchetypeState,
+                                    ArchetypeState state) noexcept
+      : ArchetypeMoveConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue>(MakeArchetypeState(),
+                                                         state) {}
+
+  ArchetypeCopyConstructor() = default;
+};
+
+// The move-assign base
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue>
+struct ArchetypeMoveAssign
+    : ArchetypeCopyConstructor<DefaultConstructibleValue,
+                               MoveConstructibleValue, CopyConstructibleValue> {
+  static_assert(MoveAssignableValue == move_assignable::yes ||
+                    MoveAssignableValue == move_assignable::nothrow,
+                "");
+  explicit ArchetypeMoveAssign(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeCopyConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue,
+                                 CopyConstructibleValue>(MakeArchetypeState(),
+                                                         state) {}
+
+  ArchetypeMoveAssign() = default;
+  ArchetypeMoveAssign(ArchetypeMoveAssign&&) = default;
+  ArchetypeMoveAssign(const ArchetypeMoveAssign&) = default;
+  ArchetypeMoveAssign& operator=(ArchetypeMoveAssign&& other) noexcept(
+      MoveAssignableValue == move_assignable::nothrow) {
+    this->archetype_state = other.archetype_state;
+    return *this;
+  }
+
+  ArchetypeMoveAssign& operator=(const ArchetypeMoveAssign&) = default;
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue>
+struct ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                           CopyConstructibleValue, move_assignable::trivial>
+    : ArchetypeCopyConstructor<DefaultConstructibleValue,
+                               MoveConstructibleValue, CopyConstructibleValue> {
+  explicit ArchetypeMoveAssign(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeCopyConstructor<DefaultConstructibleValue,
+                                 MoveConstructibleValue,
+                                 CopyConstructibleValue>(MakeArchetypeState(),
+                                                         state) {}
+
+  ArchetypeMoveAssign() = default;
+};
+
+// The copy-assign base
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue,
+          copy_assignable CopyAssignableValue>
+struct ArchetypeCopyAssign
+    : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                          CopyConstructibleValue, MoveAssignableValue> {
+  static_assert(CopyAssignableValue == copy_assignable::yes ||
+                    CopyAssignableValue == copy_assignable::nothrow,
+                "");
+  explicit ArchetypeCopyAssign(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                            CopyConstructibleValue, MoveAssignableValue>(
+            MakeArchetypeState(), state) {}
+
+  ArchetypeCopyAssign() = default;
+  ArchetypeCopyAssign(ArchetypeCopyAssign&&) = default;
+  ArchetypeCopyAssign(const ArchetypeCopyAssign&) = default;
+  ArchetypeCopyAssign& operator=(ArchetypeCopyAssign&&) = default;
+
+  ArchetypeCopyAssign& operator=(const ArchetypeCopyAssign& other) noexcept(
+      CopyAssignableValue == copy_assignable::nothrow) {
+    this->archetype_state = other.archetype_state;
+    return *this;
+  }
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue>
+struct ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                           CopyConstructibleValue, MoveAssignableValue,
+                           copy_assignable::maybe>
+    : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                          CopyConstructibleValue, MoveAssignableValue> {
+  explicit ArchetypeCopyAssign(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                            CopyConstructibleValue, MoveAssignableValue>(
+            MakeArchetypeState(), state) {}
+
+  ArchetypeCopyAssign() = default;
+  ArchetypeCopyAssign(ArchetypeCopyAssign&&) = default;
+  ArchetypeCopyAssign(const ArchetypeCopyAssign&) = default;
+  ArchetypeCopyAssign& operator=(ArchetypeCopyAssign&&) = default;
+  ArchetypeCopyAssign& operator=(const ArchetypeCopyAssign&) = delete;
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue>
+struct ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                           CopyConstructibleValue, MoveAssignableValue,
+                           copy_assignable::trivial>
+    : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                          CopyConstructibleValue, MoveAssignableValue> {
+  explicit ArchetypeCopyAssign(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeMoveAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                            CopyConstructibleValue, MoveAssignableValue>(
+            MakeArchetypeState(), state) {}
+
+  ArchetypeCopyAssign() = default;
+};
+
+// The destructor base
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue,
+          copy_assignable CopyAssignableValue, destructible DestructibleValue>
+struct ArchetypeDestructor
+    : ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                          CopyConstructibleValue, MoveAssignableValue,
+                          CopyAssignableValue> {
+  static_assert(DestructibleValue == destructible::yes ||
+                    DestructibleValue == destructible::nothrow,
+                "");
+
+  explicit ArchetypeDestructor(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                            CopyConstructibleValue, MoveAssignableValue,
+                            CopyAssignableValue>(MakeArchetypeState(), state) {}
+
+  ArchetypeDestructor() = default;
+  ArchetypeDestructor(ArchetypeDestructor&&) = default;
+  ArchetypeDestructor(const ArchetypeDestructor&) = default;
+  ArchetypeDestructor& operator=(ArchetypeDestructor&&) = default;
+  ArchetypeDestructor& operator=(const ArchetypeDestructor&) = default;
+  ~ArchetypeDestructor() noexcept(DestructibleValue == destructible::nothrow) {}
+};
+
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue,
+          copy_assignable CopyAssignableValue>
+struct ArchetypeDestructor<DefaultConstructibleValue, MoveConstructibleValue,
+                           CopyConstructibleValue, MoveAssignableValue,
+                           CopyAssignableValue, destructible::trivial>
+    : ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                          CopyConstructibleValue, MoveAssignableValue,
+                          CopyAssignableValue> {
+  explicit ArchetypeDestructor(MakeArchetypeState,
+                               ArchetypeState state) noexcept
+      : ArchetypeCopyAssign<DefaultConstructibleValue, MoveConstructibleValue,
+                            CopyConstructibleValue, MoveAssignableValue,
+                            CopyAssignableValue>(MakeArchetypeState(), state) {}
+
+  ArchetypeDestructor() = default;
+};
+
+// An alias to the top of the chain of bases for special-member functions.
+// NOTE: move_constructible::maybe, move_assignable::maybe, and
+// destructible::maybe are handled in the top-level type by way of SFINAE.
+// Because of this, we never instantiate the base classes with
+// move_constructible::maybe, move_assignable::maybe, or destructible::maybe so
+// that we minimize the number of different possible type-template
+// instantiations.
+template <default_constructible DefaultConstructibleValue,
+          move_constructible MoveConstructibleValue,
+          copy_constructible CopyConstructibleValue,
+          move_assignable MoveAssignableValue,
+          copy_assignable CopyAssignableValue, destructible DestructibleValue>
+using ArchetypeSpecialMembersBase = ArchetypeDestructor<
+    DefaultConstructibleValue,
+    MoveConstructibleValue != move_constructible::maybe
+        ? MoveConstructibleValue
+        : move_constructible::nothrow,
+    CopyConstructibleValue,
+    MoveAssignableValue != move_assignable::maybe ? MoveAssignableValue
+                                                  : move_assignable::nothrow,
+    CopyAssignableValue,
+    DestructibleValue != destructible::maybe ? DestructibleValue
+                                             : destructible::nothrow>;
+
+// A function that is used to create an archetype with some associated state.
+template <class Arch>
+Arch MakeArchetype(ArchetypeState state) noexcept {
+  static_assert(IsArchetype<Arch>::value,
+                "The explicit template argument to MakeArchetype is required "
+                "to be an Archetype.");
+  return Arch(MakeArchetypeState(), state);
+}
+
+// This is used to conditionally delete "copy" and "move" constructors in a way
+// that is consistent with what the ConformanceProfile requires and that also
+// strictly enforces the arguments to the copy/move to not come from implicit
+// conversions when dealing with the Archetype.
+template <class Prof, class T>
+constexpr bool ShouldDeleteConstructor() {
+  return !((PropertiesOfT<Prof>::move_constructible_support !=
+                move_constructible::maybe &&
+            std::is_same<T, Archetype<Prof>>::value) ||
+           (PropertiesOfT<Prof>::copy_constructible_support !=
+                copy_constructible::maybe &&
+            (std::is_same<T, const Archetype<Prof>&>::value ||
+             std::is_same<T, Archetype<Prof>&>::value ||
+             std::is_same<T, const Archetype<Prof>>::value)));
+}
+
+// This is used to conditionally delete "copy" and "move" assigns in a way
+// that is consistent with what the ConformanceProfile requires and that also
+// strictly enforces the arguments to the copy/move to not come from implicit
+// conversions when dealing with the Archetype.
+template <class Prof, class T>
+constexpr bool ShouldDeleteAssign() {
+  return !(
+      (PropertiesOfT<Prof>::move_assignable_support != move_assignable::maybe &&
+       std::is_same<T, Archetype<Prof>>::value) ||
+      (PropertiesOfT<Prof>::copy_assignable_support != copy_assignable::maybe &&
+       (std::is_same<T, const Archetype<Prof>&>::value ||
+        std::is_same<T, Archetype<Prof>&>::value ||
+        std::is_same<T, const Archetype<Prof>>::value)));
+}
+
+// TODO(calabrese) Inherit from a chain of secondary bases to pull in the
+// associated functions of other concepts.
+template <class Prof, class Enabler>
+class Archetype : ArchetypeSpecialMembersBase<
+                      PropertiesOfT<Prof>::default_constructible_support,
+                      PropertiesOfT<Prof>::move_constructible_support,
+                      PropertiesOfT<Prof>::copy_constructible_support,
+                      PropertiesOfT<Prof>::move_assignable_support,
+                      PropertiesOfT<Prof>::copy_assignable_support,
+                      PropertiesOfT<Prof>::destructible_support> {
+  static_assert(std::is_same<Enabler, void>::value,
+                "An explicit type must not be passed as the second template "
+                "argument to 'Archetype`.");
+
+  // The cases mentioned in these static_asserts are expected to be handled in
+  // the partial template specializations of Archetype that follow this
+  // definition.
+  static_assert(PropertiesOfT<Prof>::destructible_support !=
+                    destructible::maybe,
+                "");
+  static_assert(PropertiesOfT<Prof>::move_constructible_support !=
+                        move_constructible::maybe ||
+                    PropertiesOfT<Prof>::copy_constructible_support ==
+                        copy_constructible::maybe,
+                "");
+  static_assert(PropertiesOfT<Prof>::move_assignable_support !=
+                        move_assignable::maybe ||
+                    PropertiesOfT<Prof>::copy_assignable_support ==
+                        copy_assignable::maybe,
+                "");
+
+ public:
+  Archetype() = default;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support !=
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support ==
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support !=
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = default;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = delete;
+  Archetype& operator=(const Archetype&) = default;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support ==
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support ==
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support !=
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = delete;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = delete;
+  Archetype& operator=(const Archetype&) = default;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support ==
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support !=
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support !=
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = delete;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = default;
+  Archetype& operator=(const Archetype&) = default;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support !=
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support ==
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support ==
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = default;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = delete;
+  Archetype& operator=(const Archetype&) = default;
+  ~Archetype() = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support ==
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support ==
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support ==
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = delete;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = delete;
+  Archetype& operator=(const Archetype&) = default;
+  ~Archetype() = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+template <class Prof>
+class Archetype<Prof, typename std::enable_if<
+                          PropertiesOfT<Prof>::move_constructible_support ==
+                              move_constructible::maybe &&
+                          PropertiesOfT<Prof>::move_assignable_support !=
+                              move_assignable::maybe &&
+                          PropertiesOfT<Prof>::destructible_support ==
+                              destructible::maybe>::type>
+    : ArchetypeSpecialMembersBase<
+          PropertiesOfT<Prof>::default_constructible_support,
+          PropertiesOfT<Prof>::move_constructible_support,
+          PropertiesOfT<Prof>::copy_constructible_support,
+          PropertiesOfT<Prof>::move_assignable_support,
+          PropertiesOfT<Prof>::copy_assignable_support,
+          PropertiesOfT<Prof>::destructible_support> {
+ public:
+  Archetype() = default;
+  Archetype(Archetype&&) = delete;
+  Archetype(const Archetype&) = default;
+  Archetype& operator=(Archetype&&) = default;
+  Archetype& operator=(const Archetype&) = default;
+  ~Archetype() = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteConstructor<Prof, T>()>::type* = nullptr>
+  Archetype(T&&) = delete;
+
+  // Disallow moves when requested, and disallow implicit conversions.
+  template <class T, typename std::enable_if<
+                         ShouldDeleteAssign<Prof, T>()>::type* = nullptr>
+  Archetype& operator=(T&&) = delete;
+
+  using ArchetypeSpecialMembersBase<
+      PropertiesOfT<Prof>::default_constructible_support,
+      PropertiesOfT<Prof>::move_constructible_support,
+      PropertiesOfT<Prof>::copy_constructible_support,
+      PropertiesOfT<Prof>::move_assignable_support,
+      PropertiesOfT<Prof>::copy_assignable_support,
+      PropertiesOfT<Prof>::destructible_support>::archetype_state;
+
+ private:
+  explicit Archetype(MakeArchetypeState, ArchetypeState state) noexcept
+      : ArchetypeSpecialMembersBase<
+            PropertiesOfT<Prof>::default_constructible_support,
+            PropertiesOfT<Prof>::move_constructible_support,
+            PropertiesOfT<Prof>::copy_constructible_support,
+            PropertiesOfT<Prof>::move_assignable_support,
+            PropertiesOfT<Prof>::copy_assignable_support,
+            PropertiesOfT<Prof>::destructible_support>(MakeArchetypeState(),
+                                                       state) {}
+
+  friend Archetype MakeArchetype<Archetype>(ArchetypeState) noexcept;
+};
+
+// Explicitly deleted swap for Archetype if the profile does not require swap.
+// It is important to delete it rather than simply leave it out so that the
+// "using std::swap;" idiom will result in this deleted overload being picked.
+template <class Prof,
+          absl::enable_if_t<!PropertiesOfT<Prof>::is_swappable, int> = 0>
+void swap(Archetype<Prof>&, Archetype<Prof>&) = delete;  // NOLINT
+
+// A conditionally-noexcept swap implementation for Archetype when the profile
+// supports swap.
+template <class Prof,
+          absl::enable_if_t<PropertiesOfT<Prof>::is_swappable, int> = 0>
+void swap(Archetype<Prof>& lhs, Archetype<Prof>& rhs)  // NOLINT
+    noexcept(PropertiesOfT<Prof>::swappable_support != swappable::yes) {
+  std::swap(lhs.archetype_state, rhs.archetype_state);
+}
+
+// A convertible-to-bool type that is used as the return type of comparison
+// operators since the standard doesn't always require exactly bool.
+struct NothrowBool {
+  explicit NothrowBool() = delete;
+  ~NothrowBool() = default;
+
+  // TODO(calabrese) Delete the copy constructor in C++17 mode since guaranteed
+  // elision makes it not required when returning from a function.
+  // NothrowBool(NothrowBool const&) = delete;
+
+  NothrowBool& operator=(NothrowBool const&) = delete;
+
+  explicit operator bool() const noexcept { return value; }
+
+  static NothrowBool make(bool const value) noexcept {
+    return NothrowBool(value);
+  }
+
+ private:
+  explicit NothrowBool(bool const value) noexcept : value(value) {}
+
+  bool value;
+};
+
+// A convertible-to-bool type that is used as the return type of comparison
+// operators since the standard doesn't always require exactly bool.
+// Note: ExceptionalBool has a conversion operator that is not noexcept, so
+// that even when a comparison operator is noexcept, that operation may still
+// potentially throw when converted to bool.
+struct ExceptionalBool {
+  explicit ExceptionalBool() = delete;
+  ~ExceptionalBool() = default;
+
+  // TODO(calabrese) Delete the copy constructor in C++17 mode since guaranteed
+  // elision makes it not required when returning from a function.
+  // ExceptionalBool(ExceptionalBool const&) = delete;
+
+  ExceptionalBool& operator=(ExceptionalBool const&) = delete;
+
+  explicit operator bool() const { return value; }  // NOLINT
+
+  static ExceptionalBool make(bool const value) noexcept {
+    return ExceptionalBool(value);
+  }
+
+ private:
+  explicit ExceptionalBool(bool const value) noexcept : value(value) {}
+
+  bool value;
+};
+
+// The following macro is only used as a helper in this file to stamp out
+// comparison operator definitions. It is undefined after usage.
+//
+// NOTE: Non-nothrow operators throw via their result's conversion to bool even
+// though the operation itself is noexcept.
+#define ABSL_TYPES_INTERNAL_OP(enum_name, op)                                \
+  template <class Prof>                                                      \
+  absl::enable_if_t<!PropertiesOfT<Prof>::is_##enum_name, bool> operator op( \
+      const Archetype<Prof>&, const Archetype<Prof>&) = delete;              \
+                                                                             \
+  template <class Prof>                                                      \
+  typename absl::enable_if_t<                                                \
+      PropertiesOfT<Prof>::is_##enum_name,                                   \
+      std::conditional<PropertiesOfT<Prof>::enum_name##_support ==           \
+                           enum_name::nothrow,                               \
+                       NothrowBool, ExceptionalBool>>::type                  \
+  operator op(const Archetype<Prof>& lhs,                                    \
+              const Archetype<Prof>& rhs) noexcept {                         \
+    return absl::conditional_t<                                              \
+        PropertiesOfT<Prof>::enum_name##_support == enum_name::nothrow,      \
+        NothrowBool, ExceptionalBool>::make(lhs.archetype_state op           \
+                                                rhs.archetype_state);        \
+  }
+
+ABSL_TYPES_INTERNAL_OP(equality_comparable, ==);
+ABSL_TYPES_INTERNAL_OP(inequality_comparable, !=);
+ABSL_TYPES_INTERNAL_OP(less_than_comparable, <);
+ABSL_TYPES_INTERNAL_OP(less_equal_comparable, <=);
+ABSL_TYPES_INTERNAL_OP(greater_equal_comparable, >=);
+ABSL_TYPES_INTERNAL_OP(greater_than_comparable, >);
+
+#undef ABSL_TYPES_INTERNAL_OP
+
+// Base class for std::hash specializations when an Archetype doesn't support
+// hashing.
+struct PoisonedHash {
+  PoisonedHash() = delete;
+  PoisonedHash(const PoisonedHash&) = delete;
+  PoisonedHash& operator=(const PoisonedHash&) = delete;
+};
+
+// Base class for std::hash specializations when an Archetype supports hashing.
+template <class Prof>
+struct EnabledHash {
+  using argument_type = Archetype<Prof>;
+  using result_type = std::size_t;
+  result_type operator()(const argument_type& arg) const {
+    return std::hash<ArchetypeState>()(arg.archetype_state);
+  }
+};
+
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+namespace std {
+
+template <class Prof>  // NOLINT
+struct hash<::absl::types_internal::Archetype<Prof>>
+    : conditional<::absl::types_internal::PropertiesOfT<Prof>::is_hashable,
+                  ::absl::types_internal::EnabledHash<Prof>,
+                  ::absl::types_internal::PoisonedHash>::type {};
+
+}  // namespace std
+
+#endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_ARCHETYPE_H_
diff --git a/absl/types/internal/conformance_profile.h b/absl/types/internal/conformance_profile.h
new file mode 100644
index 0000000..e62004f
--- /dev/null
+++ b/absl/types/internal/conformance_profile.h
@@ -0,0 +1,376 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// conformance_profiles.h
+// -----------------------------------------------------------------------------
+//
+// This file contains templates for representing "Regularity Profiles" and
+// concisely-named versions of commonly used Regularity Profiles.
+//
+// A Regularity Profile is a compile-time description of the types of operations
+// that a given type supports, along with properties of those operations when
+// they do exist. For instance, a Regularity Profile may describe a type that
+// has a move-constructor that is noexcept and a copy constructor that is not
+// noexcept. This description can then be examined and passed around to other
+// templates for the purposes of asserting expectations on user-defined types
+// via a series trait checks, or for determining what kinds of run-time tests
+// are able to be performed.
+//
+// Regularity Profiles are also used when creating "archetypes," which are
+// minimum-conforming types that meet all of the requirements of a given
+// Regularity Profile. For more information regarding archetypes, see
+// "conformance_archetypes.h".
+
+#ifndef ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_
+#define ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_
+
+#include <type_traits>
+#include <utility>
+
+#include "absl/meta/type_traits.h"
+
+// TODO(calabrese) Add support for extending profiles.
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace types_internal {
+
+template <class T, class /*Enabler*/ = void>
+struct PropertiesOfImpl {};
+
+template <class T>
+struct PropertiesOfImpl<T, absl::void_t<typename T::properties>> {
+  using type = typename T::properties;
+};
+
+template <class T>
+struct PropertiesOfImpl<T, absl::void_t<typename T::profile_alias_of>> {
+  using type = typename PropertiesOfImpl<typename T::profile_alias_of>::type;
+};
+
+template <class T>
+struct PropertiesOf : PropertiesOfImpl<T> {};
+
+template <class T>
+using PropertiesOfT = typename PropertiesOf<T>::type;
+
+// NOTE: These enums use this naming convention to be consistent with the
+// standard trait names, which is useful since it allows us to match up each
+// enum name with a corresponding trait name in macro definitions.
+
+enum class function_kind { maybe, yes, nothrow, trivial };
+
+#define ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(name) \
+  enum class name { maybe, yes, nothrow, trivial }
+
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(default_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(move_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(copy_constructible);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(move_assignable);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(copy_assignable);
+ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM(destructible);
+
+#undef ABSL_INTERNAL_SPECIAL_MEMBER_FUNCTION_ENUM
+
+#define ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(name) \
+  enum class name { maybe, yes, nothrow }
+
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(equality_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(inequality_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(less_than_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(less_equal_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(greater_equal_comparable);
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(greater_than_comparable);
+
+ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM(swappable);
+
+#undef ABSL_INTERNAL_INTRINSIC_FUNCTION_ENUM
+
+enum class hashable { maybe, yes };
+
+constexpr const char* PropertyName(hashable v) {
+  return "support for std::hash";
+}
+
+template <
+    default_constructible DefaultConstructibleValue =
+        default_constructible::maybe,
+    move_constructible MoveConstructibleValue = move_constructible::maybe,
+    copy_constructible CopyConstructibleValue = copy_constructible::maybe,
+    move_assignable MoveAssignableValue = move_assignable::maybe,
+    copy_assignable CopyAssignableValue = copy_assignable::maybe,
+    destructible DestructibleValue = destructible::maybe,
+    equality_comparable EqualityComparableValue = equality_comparable::maybe,
+    inequality_comparable InequalityComparableValue =
+        inequality_comparable::maybe,
+    less_than_comparable LessThanComparableValue = less_than_comparable::maybe,
+    less_equal_comparable LessEqualComparableValue =
+        less_equal_comparable::maybe,
+    greater_equal_comparable GreaterEqualComparableValue =
+        greater_equal_comparable::maybe,
+    greater_than_comparable GreaterThanComparableValue =
+        greater_than_comparable::maybe,
+    swappable SwappableValue = swappable::maybe,
+    hashable HashableValue = hashable::maybe>
+struct ConformanceProfile {
+  using properties = ConformanceProfile;
+
+  static constexpr default_constructible
+      default_constructible_support =  // NOLINT
+      DefaultConstructibleValue;
+
+  static constexpr move_constructible move_constructible_support =  // NOLINT
+      MoveConstructibleValue;
+
+  static constexpr copy_constructible copy_constructible_support =  // NOLINT
+      CopyConstructibleValue;
+
+  static constexpr move_assignable move_assignable_support =  // NOLINT
+      MoveAssignableValue;
+
+  static constexpr copy_assignable copy_assignable_support =  // NOLINT
+      CopyAssignableValue;
+
+  static constexpr destructible destructible_support =  // NOLINT
+      DestructibleValue;
+
+  static constexpr equality_comparable equality_comparable_support =  // NOLINT
+      EqualityComparableValue;
+
+  static constexpr inequality_comparable
+      inequality_comparable_support =  // NOLINT
+      InequalityComparableValue;
+
+  static constexpr less_than_comparable
+      less_than_comparable_support =  // NOLINT
+      LessThanComparableValue;
+
+  static constexpr less_equal_comparable
+      less_equal_comparable_support =  // NOLINT
+      LessEqualComparableValue;
+
+  static constexpr greater_equal_comparable
+      greater_equal_comparable_support =  // NOLINT
+      GreaterEqualComparableValue;
+
+  static constexpr greater_than_comparable
+      greater_than_comparable_support =  // NOLINT
+      GreaterThanComparableValue;
+
+  static constexpr swappable swappable_support = SwappableValue;  // NOLINT
+
+  static constexpr hashable hashable_support = HashableValue;  // NOLINT
+
+  static constexpr bool is_default_constructible =  // NOLINT
+      DefaultConstructibleValue != default_constructible::maybe;
+
+  static constexpr bool is_move_constructible =  // NOLINT
+      MoveConstructibleValue != move_constructible::maybe;
+
+  static constexpr bool is_copy_constructible =  // NOLINT
+      CopyConstructibleValue != copy_constructible::maybe;
+
+  static constexpr bool is_move_assignable =  // NOLINT
+      MoveAssignableValue != move_assignable::maybe;
+
+  static constexpr bool is_copy_assignable =  // NOLINT
+      CopyAssignableValue != copy_assignable::maybe;
+
+  static constexpr bool is_destructible =  // NOLINT
+      DestructibleValue != destructible::maybe;
+
+  static constexpr bool is_equality_comparable =  // NOLINT
+      EqualityComparableValue != equality_comparable::maybe;
+
+  static constexpr bool is_inequality_comparable =  // NOLINT
+      InequalityComparableValue != inequality_comparable::maybe;
+
+  static constexpr bool is_less_than_comparable =  // NOLINT
+      LessThanComparableValue != less_than_comparable::maybe;
+
+  static constexpr bool is_less_equal_comparable =  // NOLINT
+      LessEqualComparableValue != less_equal_comparable::maybe;
+
+  static constexpr bool is_greater_equal_comparable =  // NOLINT
+      GreaterEqualComparableValue != greater_equal_comparable::maybe;
+
+  static constexpr bool is_greater_than_comparable =  // NOLINT
+      GreaterThanComparableValue != greater_than_comparable::maybe;
+
+  static constexpr bool is_swappable =  // NOLINT
+      SwappableValue != swappable::maybe;
+
+  static constexpr bool is_hashable =  // NOLINT
+      HashableValue != hashable::maybe;
+};
+
+#define ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL(type, name)     \
+  template <default_constructible DefaultConstructibleValue,                   \
+            move_constructible MoveConstructibleValue,                         \
+            copy_constructible CopyConstructibleValue,                         \
+            move_assignable MoveAssignableValue,                               \
+            copy_assignable CopyAssignableValue,                               \
+            destructible DestructibleValue,                                    \
+            equality_comparable EqualityComparableValue,                       \
+            inequality_comparable InequalityComparableValue,                   \
+            less_than_comparable LessThanComparableValue,                      \
+            less_equal_comparable LessEqualComparableValue,                    \
+            greater_equal_comparable GreaterEqualComparableValue,              \
+            greater_than_comparable GreaterThanComparableValue,                \
+            swappable SwappableValue, hashable HashableValue>                  \
+  constexpr type ConformanceProfile<                                           \
+      DefaultConstructibleValue, MoveConstructibleValue,                       \
+      CopyConstructibleValue, MoveAssignableValue, CopyAssignableValue,        \
+      DestructibleValue, EqualityComparableValue, InequalityComparableValue,   \
+      LessThanComparableValue, LessEqualComparableValue,                       \
+      GreaterEqualComparableValue, GreaterThanComparableValue, SwappableValue, \
+      HashableValue>::name
+
+#define ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(type)           \
+  ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL(type,            \
+                                                         type##_support); \
+  ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL(bool, is_##type)
+
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(default_constructible);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(move_constructible);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(copy_constructible);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(move_assignable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(copy_assignable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(destructible);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(equality_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(inequality_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(less_than_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(less_equal_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(greater_equal_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(greater_than_comparable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(swappable);
+ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF(hashable);
+
+#undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF
+#undef ABSL_INTERNAL_CONFORMANCE_TESTING_DATA_MEMBER_DEF_IMPL
+
+// Converts an enum to its underlying integral value.
+template <class Enum>
+constexpr absl::underlying_type_t<Enum> UnderlyingValue(Enum value) {
+  return static_cast<absl::underlying_type_t<Enum>>(value);
+}
+
+// Retrieve the enum with the greatest underlying value.
+// Note: std::max is not constexpr in C++11, which is why this is necessary.
+template <class H>
+constexpr H MaxEnum(H head) {
+  return head;
+}
+
+template <class H, class N, class... T>
+constexpr H MaxEnum(H head, N next, T... tail) {
+  return (UnderlyingValue)(next) < (UnderlyingValue)(head)
+             ? (MaxEnum)(head, tail...)
+             : (MaxEnum)(next, tail...);
+}
+
+template <class... Profs>
+struct CombineProfilesImpl {
+  static constexpr default_constructible
+      default_constructible_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::default_constructible_support...);
+
+  static constexpr move_constructible move_constructible_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::move_constructible_support...);
+
+  static constexpr copy_constructible copy_constructible_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::copy_constructible_support...);
+
+  static constexpr move_assignable move_assignable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::move_assignable_support...);
+
+  static constexpr copy_assignable copy_assignable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::copy_assignable_support...);
+
+  static constexpr destructible destructible_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::destructible_support...);
+
+  static constexpr equality_comparable equality_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::equality_comparable_support...);
+
+  static constexpr inequality_comparable
+      inequality_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::inequality_comparable_support...);
+
+  static constexpr less_than_comparable
+      less_than_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::less_than_comparable_support...);
+
+  static constexpr less_equal_comparable
+      less_equal_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::less_equal_comparable_support...);
+
+  static constexpr greater_equal_comparable
+      greater_equal_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::greater_equal_comparable_support...);
+
+  static constexpr greater_than_comparable
+      greater_than_comparable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::greater_than_comparable_support...);
+
+  static constexpr swappable swappable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::swappable_support...);
+
+  static constexpr hashable hashable_support =  // NOLINT
+      (MaxEnum)(PropertiesOfT<Profs>::hashable_support...);
+
+  using properties = ConformanceProfile<
+      default_constructible_support, move_constructible_support,
+      copy_constructible_support, move_assignable_support,
+      copy_assignable_support, destructible_support,
+      equality_comparable_support, inequality_comparable_support,
+      less_than_comparable_support, less_equal_comparable_support,
+      greater_equal_comparable_support, greater_than_comparable_support,
+      swappable_support, hashable_support>;
+};
+
+// NOTE: We use this as opposed to a direct alias of CombineProfilesImpl so that
+// when named aliases of CombineProfiles are created (such as in
+// conformance_aliases.h), we only pay for the combination algorithm on the
+// profiles that are actually used.
+template <class... Profs>
+struct CombineProfiles {
+  using profile_alias_of = CombineProfilesImpl<Profs...>;
+};
+
+template <>
+struct CombineProfiles<> {
+  using properties = ConformanceProfile<>;
+};
+
+template <class Profile, class Tag>
+struct StrongProfileTypedef {
+  using properties = PropertiesOfT<Profile>;
+};
+
+template <class T, class /*Enabler*/ = void>
+struct IsProfileImpl : std::false_type {};
+
+template <class T>
+struct IsProfileImpl<T, absl::void_t<PropertiesOfT<T>>> : std::true_type {};
+
+template <class T>
+struct IsProfile : IsProfileImpl<T>::type {};
+
+}  // namespace types_internal
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_TYPES_INTERNAL_CONFORMANCE_PROFILE_H_
diff --git a/absl/types/internal/conformance_testing_test.cc b/absl/types/internal/conformance_testing_test.cc
new file mode 100644
index 0000000..3dcf530
--- /dev/null
+++ b/absl/types/internal/conformance_testing_test.cc
@@ -0,0 +1,1186 @@
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include <new>
+#include <type_traits>
+#include <utility>
+
+#include "gtest/gtest.h"
+#include "absl/meta/type_traits.h"
+#include "absl/types/internal/conformance_aliases.h"
+
+namespace {
+
+namespace ti = absl::types_internal;
+
+template <class T>
+using DefaultConstructibleWithNewImpl = decltype(::new (std::nothrow) T);
+
+template <class T>
+using DefaultConstructibleWithNew =
+    absl::type_traits_internal::is_detected<DefaultConstructibleWithNewImpl, T>;
+
+template <class T>
+using MoveConstructibleWithNewImpl =
+    decltype(::new (std::nothrow) T(std::declval<T>()));
+
+template <class T>
+using MoveConstructibleWithNew =
+    absl::type_traits_internal::is_detected<MoveConstructibleWithNewImpl, T>;
+
+template <class T>
+using CopyConstructibleWithNewImpl =
+    decltype(::new (std::nothrow) T(std::declval<const T&>()));
+
+template <class T>
+using CopyConstructibleWithNew =
+    absl::type_traits_internal::is_detected<CopyConstructibleWithNewImpl, T>;
+
+template <class T,
+          class Result =
+              std::integral_constant<bool, noexcept(::new (std::nothrow) T)>>
+using NothrowDefaultConstructibleWithNewImpl =
+    typename std::enable_if<Result::value>::type;
+
+template <class T>
+using NothrowDefaultConstructibleWithNew =
+    absl::type_traits_internal::is_detected<
+        NothrowDefaultConstructibleWithNewImpl, T>;
+
+template <class T,
+          class Result = std::integral_constant<
+              bool, noexcept(::new (std::nothrow) T(std::declval<T>()))>>
+using NothrowMoveConstructibleWithNewImpl =
+    typename std::enable_if<Result::value>::type;
+
+template <class T>
+using NothrowMoveConstructibleWithNew =
+    absl::type_traits_internal::is_detected<NothrowMoveConstructibleWithNewImpl,
+                                            T>;
+
+template <class T,
+          class Result = std::integral_constant<
+              bool, noexcept(::new (std::nothrow) T(std::declval<const T&>()))>>
+using NothrowCopyConstructibleWithNewImpl =
+    typename std::enable_if<Result::value>::type;
+
+template <class T>
+using NothrowCopyConstructibleWithNew =
+    absl::type_traits_internal::is_detected<NothrowCopyConstructibleWithNewImpl,
+                                            T>;
+
+// NOTE: ?: is used to verify contextually-convertible to bool and not simply
+//       implicit or explicit convertibility.
+#define ABSL_INTERNAL_COMPARISON_OP_EXPR(op) \
+  ((std::declval<const T&>() op std::declval<const T&>()) ? true : true)
+
+#define ABSL_INTERNAL_COMPARISON_OP_TRAIT(name, op)                         \
+  template <class T>                                                        \
+  using name##Impl = decltype(ABSL_INTERNAL_COMPARISON_OP_EXPR(op));        \
+                                                                            \
+  template <class T>                                                        \
+  using name = absl::type_traits_internal::is_detected<name##Impl, T>;      \
+                                                                            \
+  template <class T,                                                        \
+            class Result = std::integral_constant<                          \
+                bool, noexcept(ABSL_INTERNAL_COMPARISON_OP_EXPR(op))>>      \
+  using Nothrow##name##Impl = typename std::enable_if<Result::value>::type; \
+                                                                            \
+  template <class T>                                                        \
+  using Nothrow##name =                                                     \
+      absl::type_traits_internal::is_detected<Nothrow##name##Impl, T>
+
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(EqualityComparable, ==);
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(InequalityComparable, !=);
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(LessThanComparable, <);
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(LessEqualComparable, <=);
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(GreaterEqualComparable, >=);
+ABSL_INTERNAL_COMPARISON_OP_TRAIT(GreaterThanComparable, >);
+
+#undef ABSL_INTERNAL_COMPARISON_OP_TRAIT
+
+template <class T>
+class ProfileTest : public ::testing::Test {};
+
+TYPED_TEST_SUITE_P(ProfileTest);
+
+TYPED_TEST_P(ProfileTest, HasAppropriateConstructionProperties) {
+  using profile = typename TypeParam::profile;
+  using arch = typename TypeParam::arch;
+  using expected_profile = typename TypeParam::expected_profile;
+
+  using props = ti::PropertiesOfT<profile>;
+  using arch_props = ti::PropertiesOfArchetypeT<arch>;
+  using expected_props = ti::PropertiesOfT<expected_profile>;
+
+  // Make sure all of the properties are as expected.
+  // There are seemingly redundant tests here to make it easier to diagnose
+  // the specifics of the failure if something were to go wrong.
+  EXPECT_TRUE((std::is_same<props, arch_props>::value));
+  EXPECT_TRUE((std::is_same<props, expected_props>::value));
+  EXPECT_TRUE((std::is_same<arch_props, expected_props>::value));
+
+  EXPECT_EQ(props::default_constructible_support,
+            expected_props::default_constructible_support);
+
+  EXPECT_EQ(props::move_constructible_support,
+            expected_props::move_constructible_support);
+
+  EXPECT_EQ(props::copy_constructible_support,
+            expected_props::copy_constructible_support);
+
+  EXPECT_EQ(props::destructible_support, expected_props::destructible_support);
+
+  // Avoid additional error message noise when profile and archetype match with
+  // each other but were not what was expected.
+  if (!std::is_same<props, arch_props>::value) {
+    EXPECT_EQ(arch_props::default_constructible_support,
+              expected_props::default_constructible_support);
+
+    EXPECT_EQ(arch_props::move_constructible_support,
+              expected_props::move_constructible_support);
+
+    EXPECT_EQ(arch_props::copy_constructible_support,
+              expected_props::copy_constructible_support);
+
+    EXPECT_EQ(arch_props::destructible_support,
+              expected_props::destructible_support);
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                       Default constructor checks                         //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::default_constructible_support,
+            expected_props::default_constructible_support);
+
+  switch (expected_props::default_constructible_support) {
+    case ti::default_constructible::maybe:
+      EXPECT_FALSE(DefaultConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowDefaultConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_FALSE(std::is_default_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_default_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_default_constructible<arch>::value);
+      }
+      break;
+    case ti::default_constructible::yes:
+      EXPECT_TRUE(DefaultConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowDefaultConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_default_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_default_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_default_constructible<arch>::value);
+      }
+      break;
+    case ti::default_constructible::nothrow:
+      EXPECT_TRUE(DefaultConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowDefaultConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_default_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_default_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_default_constructible<arch>::value);
+
+        // Constructor traits also check the destructor.
+        if (std::is_nothrow_destructible<arch>::value) {
+          EXPECT_TRUE(std::is_nothrow_default_constructible<arch>::value);
+        }
+      }
+      break;
+    case ti::default_constructible::trivial:
+      EXPECT_TRUE(DefaultConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowDefaultConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_default_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_default_constructible<arch>::value);
+
+        // Constructor triviality traits require trivially destructible types.
+        if (absl::is_trivially_destructible<arch>::value) {
+          EXPECT_TRUE(absl::is_trivially_default_constructible<arch>::value);
+        }
+      }
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                         Move constructor checks                          //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::move_constructible_support,
+            expected_props::move_constructible_support);
+
+  switch (expected_props::move_constructible_support) {
+    case ti::move_constructible::maybe:
+      EXPECT_FALSE(MoveConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowMoveConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_FALSE(std::is_move_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_move_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_move_constructible<arch>::value);
+      }
+      break;
+    case ti::move_constructible::yes:
+      EXPECT_TRUE(MoveConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowMoveConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_move_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_move_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_move_constructible<arch>::value);
+      }
+      break;
+    case ti::move_constructible::nothrow:
+      EXPECT_TRUE(MoveConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowMoveConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_move_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_move_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_move_constructible<arch>::value);
+
+        // Constructor traits also check the destructor.
+        if (std::is_nothrow_destructible<arch>::value) {
+          EXPECT_TRUE(std::is_nothrow_move_constructible<arch>::value);
+        }
+      }
+      break;
+    case ti::move_constructible::trivial:
+      EXPECT_TRUE(MoveConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowMoveConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_move_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_move_constructible<arch>::value);
+
+        // Constructor triviality traits require trivially destructible types.
+        if (absl::is_trivially_destructible<arch>::value) {
+          EXPECT_TRUE(absl::is_trivially_move_constructible<arch>::value);
+        }
+      }
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                         Copy constructor checks                          //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::copy_constructible_support,
+            expected_props::copy_constructible_support);
+
+  switch (expected_props::copy_constructible_support) {
+    case ti::copy_constructible::maybe:
+      EXPECT_FALSE(CopyConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowCopyConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_FALSE(std::is_copy_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_copy_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_copy_constructible<arch>::value);
+      }
+      break;
+    case ti::copy_constructible::yes:
+      EXPECT_TRUE(CopyConstructibleWithNew<arch>::value);
+      EXPECT_FALSE(NothrowCopyConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_copy_constructible<arch>::value);
+        EXPECT_FALSE(std::is_nothrow_copy_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_copy_constructible<arch>::value);
+      }
+      break;
+    case ti::copy_constructible::nothrow:
+      EXPECT_TRUE(CopyConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowCopyConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_copy_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_copy_constructible<arch>::value);
+        EXPECT_FALSE(absl::is_trivially_copy_constructible<arch>::value);
+
+        // Constructor traits also check the destructor.
+        if (std::is_nothrow_destructible<arch>::value) {
+          EXPECT_TRUE(std::is_nothrow_copy_constructible<arch>::value);
+        }
+      }
+      break;
+    case ti::copy_constructible::trivial:
+      EXPECT_TRUE(CopyConstructibleWithNew<arch>::value);
+      EXPECT_TRUE(NothrowCopyConstructibleWithNew<arch>::value);
+
+      // Standard constructible traits depend on the destructor.
+      if (std::is_destructible<arch>::value) {
+        EXPECT_TRUE(std::is_copy_constructible<arch>::value);
+        EXPECT_TRUE(std::is_nothrow_copy_constructible<arch>::value);
+
+        // Constructor triviality traits require trivially destructible types.
+        if (absl::is_trivially_destructible<arch>::value) {
+          EXPECT_TRUE(absl::is_trivially_copy_constructible<arch>::value);
+        }
+      }
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                           Destructible checks                            //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::destructible_support, expected_props::destructible_support);
+
+  switch (expected_props::destructible_support) {
+    case ti::destructible::maybe:
+      EXPECT_FALSE(std::is_destructible<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_destructible<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_destructible<arch>::value);
+      break;
+    case ti::destructible::yes:
+      EXPECT_TRUE(std::is_destructible<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_destructible<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_destructible<arch>::value);
+      break;
+    case ti::destructible::nothrow:
+      EXPECT_TRUE(std::is_destructible<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_destructible<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_destructible<arch>::value);
+      break;
+    case ti::destructible::trivial:
+      EXPECT_TRUE(std::is_destructible<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_destructible<arch>::value);
+      EXPECT_TRUE(absl::is_trivially_destructible<arch>::value);
+      break;
+  }
+}
+
+TYPED_TEST_P(ProfileTest, HasAppropriateAssignmentProperties) {
+  using profile = typename TypeParam::profile;
+  using arch = typename TypeParam::arch;
+  using expected_profile = typename TypeParam::expected_profile;
+
+  using props = ti::PropertiesOfT<profile>;
+  using arch_props = ti::PropertiesOfArchetypeT<arch>;
+  using expected_props = ti::PropertiesOfT<expected_profile>;
+
+  // Make sure all of the properties are as expected.
+  // There are seemingly redundant tests here to make it easier to diagnose
+  // the specifics of the failure if something were to go wrong.
+  EXPECT_TRUE((std::is_same<props, arch_props>::value));
+  EXPECT_TRUE((std::is_same<props, expected_props>::value));
+  EXPECT_TRUE((std::is_same<arch_props, expected_props>::value));
+
+  EXPECT_EQ(props::move_assignable_support,
+            expected_props::move_assignable_support);
+
+  EXPECT_EQ(props::copy_assignable_support,
+            expected_props::copy_assignable_support);
+
+  // Avoid additional error message noise when profile and archetype match with
+  // each other but were not what was expected.
+  if (!std::is_same<props, arch_props>::value) {
+    EXPECT_EQ(arch_props::move_assignable_support,
+              expected_props::move_assignable_support);
+
+    EXPECT_EQ(arch_props::copy_assignable_support,
+              expected_props::copy_assignable_support);
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                          Move assignment checks                          //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::move_assignable_support,
+            expected_props::move_assignable_support);
+
+  switch (expected_props::move_assignable_support) {
+    case ti::move_assignable::maybe:
+      EXPECT_FALSE(std::is_move_assignable<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_move_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_move_assignable<arch>::value);
+      break;
+    case ti::move_assignable::yes:
+      EXPECT_TRUE(std::is_move_assignable<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_move_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_move_assignable<arch>::value);
+      break;
+    case ti::move_assignable::nothrow:
+      EXPECT_TRUE(std::is_move_assignable<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_move_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_move_assignable<arch>::value);
+      break;
+    case ti::move_assignable::trivial:
+      EXPECT_TRUE(std::is_move_assignable<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_move_assignable<arch>::value);
+      EXPECT_TRUE(absl::is_trivially_move_assignable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                          Copy assignment checks                          //
+  //////////////////////////////////////////////////////////////////////////////
+  EXPECT_EQ(props::copy_assignable_support,
+            expected_props::copy_assignable_support);
+
+  switch (expected_props::copy_assignable_support) {
+    case ti::copy_assignable::maybe:
+      EXPECT_FALSE(std::is_copy_assignable<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_copy_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_copy_assignable<arch>::value);
+      break;
+    case ti::copy_assignable::yes:
+      EXPECT_TRUE(std::is_copy_assignable<arch>::value);
+      EXPECT_FALSE(std::is_nothrow_copy_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_copy_assignable<arch>::value);
+      break;
+    case ti::copy_assignable::nothrow:
+      EXPECT_TRUE(std::is_copy_assignable<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_copy_assignable<arch>::value);
+      EXPECT_FALSE(absl::is_trivially_copy_assignable<arch>::value);
+      break;
+    case ti::copy_assignable::trivial:
+      EXPECT_TRUE(std::is_copy_assignable<arch>::value);
+      EXPECT_TRUE(std::is_nothrow_copy_assignable<arch>::value);
+      EXPECT_TRUE(absl::is_trivially_copy_assignable<arch>::value);
+      break;
+  }
+}
+
+TYPED_TEST_P(ProfileTest, HasAppropriateComparisonProperties) {
+  using profile = typename TypeParam::profile;
+  using arch = typename TypeParam::arch;
+  using expected_profile = typename TypeParam::expected_profile;
+
+  using props = ti::PropertiesOfT<profile>;
+  using arch_props = ti::PropertiesOfArchetypeT<arch>;
+  using expected_props = ti::PropertiesOfT<expected_profile>;
+
+  // Make sure all of the properties are as expected.
+  // There are seemingly redundant tests here to make it easier to diagnose
+  // the specifics of the failure if something were to go wrong.
+  EXPECT_TRUE((std::is_same<props, arch_props>::value));
+  EXPECT_TRUE((std::is_same<props, expected_props>::value));
+  EXPECT_TRUE((std::is_same<arch_props, expected_props>::value));
+
+  EXPECT_EQ(props::equality_comparable_support,
+            expected_props::equality_comparable_support);
+
+  EXPECT_EQ(props::inequality_comparable_support,
+            expected_props::inequality_comparable_support);
+
+  EXPECT_EQ(props::less_than_comparable_support,
+            expected_props::less_than_comparable_support);
+
+  EXPECT_EQ(props::less_equal_comparable_support,
+            expected_props::less_equal_comparable_support);
+
+  EXPECT_EQ(props::greater_equal_comparable_support,
+            expected_props::greater_equal_comparable_support);
+
+  EXPECT_EQ(props::greater_than_comparable_support,
+            expected_props::greater_than_comparable_support);
+
+  // Avoid additional error message noise when profile and archetype match with
+  // each other but were not what was expected.
+  if (!std::is_same<props, arch_props>::value) {
+    EXPECT_EQ(arch_props::equality_comparable_support,
+              expected_props::equality_comparable_support);
+
+    EXPECT_EQ(arch_props::inequality_comparable_support,
+              expected_props::inequality_comparable_support);
+
+    EXPECT_EQ(arch_props::less_than_comparable_support,
+              expected_props::less_than_comparable_support);
+
+    EXPECT_EQ(arch_props::less_equal_comparable_support,
+              expected_props::less_equal_comparable_support);
+
+    EXPECT_EQ(arch_props::greater_equal_comparable_support,
+              expected_props::greater_equal_comparable_support);
+
+    EXPECT_EQ(arch_props::greater_than_comparable_support,
+              expected_props::greater_than_comparable_support);
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                        Equality comparable checks                        //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::equality_comparable_support) {
+    case ti::equality_comparable::maybe:
+      EXPECT_FALSE(EqualityComparable<arch>::value);
+      EXPECT_FALSE(NothrowEqualityComparable<arch>::value);
+      break;
+    case ti::equality_comparable::yes:
+      EXPECT_TRUE(EqualityComparable<arch>::value);
+      EXPECT_FALSE(NothrowEqualityComparable<arch>::value);
+      break;
+    case ti::equality_comparable::nothrow:
+      EXPECT_TRUE(EqualityComparable<arch>::value);
+      EXPECT_TRUE(NothrowEqualityComparable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                       Inequality comparable checks                       //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::inequality_comparable_support) {
+    case ti::inequality_comparable::maybe:
+      EXPECT_FALSE(InequalityComparable<arch>::value);
+      EXPECT_FALSE(NothrowInequalityComparable<arch>::value);
+      break;
+    case ti::inequality_comparable::yes:
+      EXPECT_TRUE(InequalityComparable<arch>::value);
+      EXPECT_FALSE(NothrowInequalityComparable<arch>::value);
+      break;
+    case ti::inequality_comparable::nothrow:
+      EXPECT_TRUE(InequalityComparable<arch>::value);
+      EXPECT_TRUE(NothrowInequalityComparable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                       Less than comparable checks                        //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::less_than_comparable_support) {
+    case ti::less_than_comparable::maybe:
+      EXPECT_FALSE(LessThanComparable<arch>::value);
+      EXPECT_FALSE(NothrowLessThanComparable<arch>::value);
+      break;
+    case ti::less_than_comparable::yes:
+      EXPECT_TRUE(LessThanComparable<arch>::value);
+      EXPECT_FALSE(NothrowLessThanComparable<arch>::value);
+      break;
+    case ti::less_than_comparable::nothrow:
+      EXPECT_TRUE(LessThanComparable<arch>::value);
+      EXPECT_TRUE(NothrowLessThanComparable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                      Less equal comparable checks                        //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::less_equal_comparable_support) {
+    case ti::less_equal_comparable::maybe:
+      EXPECT_FALSE(LessEqualComparable<arch>::value);
+      EXPECT_FALSE(NothrowLessEqualComparable<arch>::value);
+      break;
+    case ti::less_equal_comparable::yes:
+      EXPECT_TRUE(LessEqualComparable<arch>::value);
+      EXPECT_FALSE(NothrowLessEqualComparable<arch>::value);
+      break;
+    case ti::less_equal_comparable::nothrow:
+      EXPECT_TRUE(LessEqualComparable<arch>::value);
+      EXPECT_TRUE(NothrowLessEqualComparable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                     Greater equal comparable checks                      //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::greater_equal_comparable_support) {
+    case ti::greater_equal_comparable::maybe:
+      EXPECT_FALSE(GreaterEqualComparable<arch>::value);
+      EXPECT_FALSE(NothrowGreaterEqualComparable<arch>::value);
+      break;
+    case ti::greater_equal_comparable::yes:
+      EXPECT_TRUE(GreaterEqualComparable<arch>::value);
+      EXPECT_FALSE(NothrowGreaterEqualComparable<arch>::value);
+      break;
+    case ti::greater_equal_comparable::nothrow:
+      EXPECT_TRUE(GreaterEqualComparable<arch>::value);
+      EXPECT_TRUE(NothrowGreaterEqualComparable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                     Greater than comparable checks                       //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::greater_than_comparable_support) {
+    case ti::greater_than_comparable::maybe:
+      EXPECT_FALSE(GreaterThanComparable<arch>::value);
+      EXPECT_FALSE(NothrowGreaterThanComparable<arch>::value);
+      break;
+    case ti::greater_than_comparable::yes:
+      EXPECT_TRUE(GreaterThanComparable<arch>::value);
+      EXPECT_FALSE(NothrowGreaterThanComparable<arch>::value);
+      break;
+    case ti::greater_than_comparable::nothrow:
+      EXPECT_TRUE(GreaterThanComparable<arch>::value);
+      EXPECT_TRUE(NothrowGreaterThanComparable<arch>::value);
+      break;
+  }
+}
+
+TYPED_TEST_P(ProfileTest, HasAppropriateAuxilliaryProperties) {
+  using profile = typename TypeParam::profile;
+  using arch = typename TypeParam::arch;
+  using expected_profile = typename TypeParam::expected_profile;
+
+  using props = ti::PropertiesOfT<profile>;
+  using arch_props = ti::PropertiesOfArchetypeT<arch>;
+  using expected_props = ti::PropertiesOfT<expected_profile>;
+
+  // Make sure all of the properties are as expected.
+  // There are seemingly redundant tests here to make it easier to diagnose
+  // the specifics of the failure if something were to go wrong.
+  EXPECT_TRUE((std::is_same<props, arch_props>::value));
+  EXPECT_TRUE((std::is_same<props, expected_props>::value));
+  EXPECT_TRUE((std::is_same<arch_props, expected_props>::value));
+
+  EXPECT_EQ(props::swappable_support, expected_props::swappable_support);
+
+  EXPECT_EQ(props::hashable_support, expected_props::hashable_support);
+
+  // Avoid additional error message noise when profile and archetype match with
+  // each other but were not what was expected.
+  if (!std::is_same<props, arch_props>::value) {
+    EXPECT_EQ(arch_props::swappable_support, expected_props::swappable_support);
+
+    EXPECT_EQ(arch_props::hashable_support, expected_props::hashable_support);
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                            Swappable checks                              //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::swappable_support) {
+    case ti::swappable::maybe:
+      EXPECT_FALSE(absl::type_traits_internal::IsSwappable<arch>::value);
+      EXPECT_FALSE(absl::type_traits_internal::IsNothrowSwappable<arch>::value);
+      break;
+    case ti::swappable::yes:
+      EXPECT_TRUE(absl::type_traits_internal::IsSwappable<arch>::value);
+      EXPECT_FALSE(absl::type_traits_internal::IsNothrowSwappable<arch>::value);
+      break;
+    case ti::swappable::nothrow:
+      EXPECT_TRUE(absl::type_traits_internal::IsSwappable<arch>::value);
+      EXPECT_TRUE(absl::type_traits_internal::IsNothrowSwappable<arch>::value);
+      break;
+  }
+
+  //////////////////////////////////////////////////////////////////////////////
+  //                             Hashable checks                              //
+  //////////////////////////////////////////////////////////////////////////////
+  switch (expected_props::hashable_support) {
+    case ti::hashable::maybe:
+#if ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+      EXPECT_FALSE(absl::type_traits_internal::IsHashable<arch>::value);
+#endif  // ABSL_META_INTERNAL_STD_HASH_SFINAE_FRIENDLY_
+      break;
+    case ti::hashable::yes:
+      EXPECT_TRUE(absl::type_traits_internal::IsHashable<arch>::value);
+      break;
+  }
+}
+
+REGISTER_TYPED_TEST_SUITE_P(ProfileTest, HasAppropriateConstructionProperties,
+                            HasAppropriateAssignmentProperties,
+                            HasAppropriateComparisonProperties,
+                            HasAppropriateAuxilliaryProperties);
+
+template <class Profile, class Arch, class ExpectedProfile>
+struct ProfileAndExpectation {
+  using profile = Profile;
+  using arch = Arch;
+  using expected_profile = ExpectedProfile;
+};
+
+using CoreProfilesToTest = ::testing::Types<
+    // The terminating case of combine (all properties are "maybe").
+    ProfileAndExpectation<ti::CombineProfiles<>,
+                          ti::Archetype<ti::CombineProfiles<>>,
+                          ti::ConformanceProfile<>>,
+
+    // Core default constructor profiles
+    ProfileAndExpectation<
+        ti::HasDefaultConstructorProfile, ti::HasDefaultConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowDefaultConstructorProfile,
+        ti::HasNothrowDefaultConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialDefaultConstructorProfile,
+        ti::HasTrivialDefaultConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::trivial>>,
+
+    // Core move constructor profiles
+    ProfileAndExpectation<
+        ti::HasMoveConstructorProfile, ti::HasMoveConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowMoveConstructorProfile,
+        ti::HasNothrowMoveConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialMoveConstructorProfile,
+        ti::HasTrivialMoveConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::trivial>>,
+
+    // Core copy constructor profiles
+    ProfileAndExpectation<
+        ti::HasCopyConstructorProfile, ti::HasCopyConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::maybe,
+                               ti::copy_constructible::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowCopyConstructorProfile,
+        ti::HasNothrowCopyConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::maybe,
+                               ti::copy_constructible::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialCopyConstructorProfile,
+        ti::HasTrivialCopyConstructorArchetype,
+        ti::ConformanceProfile<ti::default_constructible::maybe,
+                               ti::move_constructible::maybe,
+                               ti::copy_constructible::trivial>>,
+
+    // Core move assignment profiles
+    ProfileAndExpectation<
+        ti::HasMoveAssignProfile, ti::HasMoveAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowMoveAssignProfile, ti::HasNothrowMoveAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialMoveAssignProfile, ti::HasTrivialMoveAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::trivial>>,
+
+    // Core copy assignment profiles
+    ProfileAndExpectation<
+        ti::HasCopyAssignProfile, ti::HasCopyAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowCopyAssignProfile, ti::HasNothrowCopyAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialCopyAssignProfile, ti::HasTrivialCopyAssignArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::trivial>>,
+
+    // Core destructor profiles
+    ProfileAndExpectation<
+        ti::HasDestructorProfile, ti::HasDestructorArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowDestructorProfile, ti::HasNothrowDestructorArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow>>,
+    ProfileAndExpectation<
+        ti::HasTrivialDestructorProfile, ti::HasTrivialDestructorArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::trivial>>,
+
+    // Core equality comparable profiles
+    ProfileAndExpectation<
+        ti::HasEqualityProfile, ti::HasEqualityArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowEqualityProfile, ti::HasNothrowEqualityArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::nothrow>>,
+
+    // Core inequality comparable profiles
+    ProfileAndExpectation<
+        ti::HasInequalityProfile, ti::HasInequalityArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowInequalityProfile, ti::HasNothrowInequalityArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe,
+            ti::inequality_comparable::nothrow>>,
+
+    // Core less than comparable profiles
+    ProfileAndExpectation<
+        ti::HasLessThanProfile, ti::HasLessThanArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowLessThanProfile, ti::HasNothrowLessThanArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::nothrow>>,
+
+    // Core less equal comparable profiles
+    ProfileAndExpectation<
+        ti::HasLessEqualProfile, ti::HasLessEqualArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowLessEqualProfile, ti::HasNothrowLessEqualArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe,
+            ti::less_equal_comparable::nothrow>>,
+
+    // Core greater equal comparable profiles
+    ProfileAndExpectation<
+        ti::HasGreaterEqualProfile, ti::HasGreaterEqualArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowGreaterEqualProfile, ti::HasNothrowGreaterEqualArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::nothrow>>,
+
+    // Core greater than comparable profiles
+    ProfileAndExpectation<
+        ti::HasGreaterThanProfile, ti::HasGreaterThanArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowGreaterThanProfile, ti::HasNothrowGreaterThanArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::nothrow>>,
+
+    // Core swappable profiles
+    ProfileAndExpectation<
+        ti::HasSwapProfile, ti::HasSwapArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::yes>>,
+    ProfileAndExpectation<
+        ti::HasNothrowSwapProfile, ti::HasNothrowSwapArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>,
+
+    // Core hashable profiles
+    ProfileAndExpectation<
+        ti::HasStdHashSpecializationProfile,
+        ti::HasStdHashSpecializationArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::maybe,
+            ti::hashable::yes>>>;
+
+using CommonProfilesToTest = ::testing::Types<
+    // NothrowMoveConstructible
+    ProfileAndExpectation<
+        ti::NothrowMoveConstructibleProfile,
+        ti::NothrowMoveConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow>>,
+
+    // CopyConstructible
+    ProfileAndExpectation<
+        ti::CopyConstructibleProfile, ti::CopyConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow>>,
+
+    // NothrowMovable
+    ProfileAndExpectation<
+        ti::NothrowMovableProfile, ti::NothrowMovableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::nothrow,
+            ti::copy_assignable::maybe, ti::destructible::nothrow,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>,
+
+    // Value
+    ProfileAndExpectation<
+        ti::ValueProfile, ti::ValueArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::nothrow,
+            ti::copy_assignable::yes, ti::destructible::nothrow,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>,
+
+    ////////////////////////////////////////////////////////////////////////////
+    //                  Common but also DefaultConstructible                  //
+    ////////////////////////////////////////////////////////////////////////////
+
+    // DefaultConstructibleNothrowMoveConstructible
+    ProfileAndExpectation<
+        ti::DefaultConstructibleNothrowMoveConstructibleProfile,
+        ti::DefaultConstructibleNothrowMoveConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::yes, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow>>,
+
+    // DefaultConstructibleCopyConstructible
+    ProfileAndExpectation<
+        ti::DefaultConstructibleCopyConstructibleProfile,
+        ti::DefaultConstructibleCopyConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::yes, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow>>,
+
+    // DefaultConstructibleNothrowMovable
+    ProfileAndExpectation<
+        ti::DefaultConstructibleNothrowMovableProfile,
+        ti::DefaultConstructibleNothrowMovableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::yes, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::nothrow,
+            ti::copy_assignable::maybe, ti::destructible::nothrow,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>,
+
+    // DefaultConstructibleValue
+    ProfileAndExpectation<
+        ti::DefaultConstructibleValueProfile,
+        ti::DefaultConstructibleValueArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::yes, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::nothrow,
+            ti::copy_assignable::yes, ti::destructible::nothrow,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>>;
+
+using ComparableHelpersProfilesToTest = ::testing::Types<
+    // Equatable
+    ProfileAndExpectation<
+        ti::EquatableProfile, ti::EquatableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes>>,
+
+    // Comparable
+    ProfileAndExpectation<
+        ti::ComparableProfile, ti::ComparableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes,
+            ti::greater_than_comparable::yes>>,
+
+    // NothrowEquatable
+    ProfileAndExpectation<
+        ti::NothrowEquatableProfile, ti::NothrowEquatableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::nothrow,
+            ti::inequality_comparable::nothrow>>,
+
+    // NothrowComparable
+    ProfileAndExpectation<
+        ti::NothrowComparableProfile, ti::NothrowComparableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::maybe,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::maybe,
+            ti::equality_comparable::nothrow,
+            ti::inequality_comparable::nothrow,
+            ti::less_than_comparable::nothrow,
+            ti::less_equal_comparable::nothrow,
+            ti::greater_equal_comparable::nothrow,
+            ti::greater_than_comparable::nothrow>>>;
+
+using CommonComparableProfilesToTest = ::testing::Types<
+    // ComparableNothrowMoveConstructible
+    ProfileAndExpectation<
+        ti::ComparableNothrowMoveConstructibleProfile,
+        ti::ComparableNothrowMoveConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes,
+            ti::greater_than_comparable::yes>>,
+
+    // ComparableCopyConstructible
+    ProfileAndExpectation<
+        ti::ComparableCopyConstructibleProfile,
+        ti::ComparableCopyConstructibleArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::maybe,
+            ti::copy_assignable::maybe, ti::destructible::nothrow,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes,
+            ti::greater_than_comparable::yes>>,
+
+    // ComparableNothrowMovable
+    ProfileAndExpectation<
+        ti::ComparableNothrowMovableProfile,
+        ti::ComparableNothrowMovableArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::maybe, ti::move_assignable::nothrow,
+            ti::copy_assignable::maybe, ti::destructible::nothrow,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes, ti::greater_than_comparable::yes,
+            ti::swappable::nothrow>>,
+
+    // ComparableValue
+    ProfileAndExpectation<
+        ti::ComparableValueProfile, ti::ComparableValueArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::maybe, ti::move_constructible::nothrow,
+            ti::copy_constructible::yes, ti::move_assignable::nothrow,
+            ti::copy_assignable::yes, ti::destructible::nothrow,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes, ti::greater_than_comparable::yes,
+            ti::swappable::nothrow>>>;
+
+using TrivialProfilesToTest = ::testing::Types<
+    ProfileAndExpectation<
+        ti::TrivialSpecialMemberFunctionsProfile,
+        ti::TrivialSpecialMemberFunctionsArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::trivial, ti::move_constructible::trivial,
+            ti::copy_constructible::trivial, ti::move_assignable::trivial,
+            ti::copy_assignable::trivial, ti::destructible::trivial,
+            ti::equality_comparable::maybe, ti::inequality_comparable::maybe,
+            ti::less_than_comparable::maybe, ti::less_equal_comparable::maybe,
+            ti::greater_equal_comparable::maybe,
+            ti::greater_than_comparable::maybe, ti::swappable::nothrow>>,
+
+    ProfileAndExpectation<
+        ti::TriviallyCompleteProfile, ti::TriviallyCompleteArchetype,
+        ti::ConformanceProfile<
+            ti::default_constructible::trivial, ti::move_constructible::trivial,
+            ti::copy_constructible::trivial, ti::move_assignable::trivial,
+            ti::copy_assignable::trivial, ti::destructible::trivial,
+            ti::equality_comparable::yes, ti::inequality_comparable::yes,
+            ti::less_than_comparable::yes, ti::less_equal_comparable::yes,
+            ti::greater_equal_comparable::yes, ti::greater_than_comparable::yes,
+            ti::swappable::nothrow, ti::hashable::yes>>>;
+
+INSTANTIATE_TYPED_TEST_SUITE_P(Core, ProfileTest, CoreProfilesToTest);
+INSTANTIATE_TYPED_TEST_SUITE_P(Common, ProfileTest, CommonProfilesToTest);
+INSTANTIATE_TYPED_TEST_SUITE_P(ComparableHelpers, ProfileTest,
+                               ComparableHelpersProfilesToTest);
+INSTANTIATE_TYPED_TEST_SUITE_P(CommonComparable, ProfileTest,
+                               CommonComparableProfilesToTest);
+INSTANTIATE_TYPED_TEST_SUITE_P(Trivial, ProfileTest, TrivialProfilesToTest);
+
+// TODO(calabrese) Test runtime results
+
+}  // namespace
diff --git a/absl/types/internal/optional.h b/absl/types/internal/optional.h
index 3c8e7cc..92932b6 100644
--- a/absl/types/internal/optional.h
+++ b/absl/types/internal/optional.h
@@ -54,7 +54,7 @@
 #endif
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // Forward declaration
 template <typename T>
@@ -85,8 +85,8 @@
   bool engaged_;
   // Data storage
   union {
-    dummy_type dummy_;
     T data_;
+    dummy_type dummy_;
   };
 
   void destruct() noexcept {
@@ -120,8 +120,8 @@
   bool engaged_;
   // Data storage
   union {
-    dummy_type dummy_;
     T data_;
+    dummy_type dummy_;
   };
   void destruct() noexcept { engaged_ = false; }
 
@@ -388,7 +388,7 @@
 };
 
 }  // namespace optional_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
diff --git a/absl/types/internal/span.h b/absl/types/internal/span.h
index 873ae16..112612f 100644
--- a/absl/types/internal/span.h
+++ b/absl/types/internal/span.h
@@ -26,7 +26,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace span_internal {
 // A constexpr min function
@@ -122,7 +122,7 @@
 using EnableIfConvertibleTo =
     typename std::enable_if<IsConvertible<From, To>::value>::type;
 }  // namespace span_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_INTERNAL_SPAN_H_
diff --git a/absl/types/internal/variant.h b/absl/types/internal/variant.h
index 4f29f61..71bd3ad 100644
--- a/absl/types/internal/variant.h
+++ b/absl/types/internal/variant.h
@@ -37,10 +37,10 @@
 #include "absl/types/bad_variant_access.h"
 #include "absl/utility/utility.h"
 
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 template <class... Types>
 class variant;
@@ -1639,8 +1639,8 @@
 };
 
 }  // namespace variant_internal
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#endif  // !defined(ABSL_HAVE_STD_VARIANT)
+#endif  // !defined(ABSL_USES_STD_VARIANT)
 #endif  // ABSL_TYPES_variant_internal_H_
diff --git a/absl/types/optional.h b/absl/types/optional.h
index 6614d7b..2025e29 100644
--- a/absl/types/optional.h
+++ b/absl/types/optional.h
@@ -38,21 +38,21 @@
 #include "absl/base/config.h"   // TODO(calabrese) IWYU removal?
 #include "absl/utility/utility.h"
 
-#ifdef ABSL_HAVE_STD_OPTIONAL
+#ifdef ABSL_USES_STD_OPTIONAL
 
 #include <optional>  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::bad_optional_access;
 using std::optional;
 using std::make_optional;
 using std::nullopt_t;
 using std::nullopt;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_OPTIONAL
+#else  // ABSL_USES_STD_OPTIONAL
 
 #include <cassert>
 #include <functional>
@@ -67,7 +67,7 @@
 #include "absl/types/internal/optional.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // nullopt_t
 //
@@ -757,7 +757,7 @@
   return static_cast<bool>(x) ? static_cast<bool>(v >= *x) : true;
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace std {
@@ -771,6 +771,6 @@
 
 #undef ABSL_MSVC_CONSTEXPR_BUG_IN_UNION_LIKE_CLASS
 
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
 
 #endif  // ABSL_TYPES_OPTIONAL_H_
diff --git a/absl/types/optional_exception_safety_test.cc b/absl/types/optional_exception_safety_test.cc
index 056ced4..8e5fe85 100644
--- a/absl/types/optional_exception_safety_test.cc
+++ b/absl/types/optional_exception_safety_test.cc
@@ -14,11 +14,17 @@
 
 #include "absl/types/optional.h"
 
+#include "absl/base/config.h"
+
+// This test is a no-op when absl::optional is an alias for std::optional and
+// when exceptions are not enabled.
+#if !defined(ABSL_USES_STD_OPTIONAL) && defined(ABSL_HAVE_EXCEPTIONS)
+
 #include "gtest/gtest.h"
 #include "absl/base/internal/exception_safety_testing.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 namespace {
 
@@ -280,5 +286,7 @@
 
 }  // namespace
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
+
+#endif  // #if !defined(ABSL_USES_STD_OPTIONAL) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/optional_test.cc b/absl/types/optional_test.cc
index e6a36eb..47d5c8a 100644
--- a/absl/types/optional_test.cc
+++ b/absl/types/optional_test.cc
@@ -14,6 +14,9 @@
 
 #include "absl/types/optional.h"
 
+// This test is a no-op when absl::optional is an alias for std::optional.
+#if !defined(ABSL_USES_STD_OPTIONAL)
+
 #include <string>
 #include <type_traits>
 #include <utility>
@@ -221,7 +224,7 @@
 
   EXPECT_FALSE(
       absl::is_trivially_copy_constructible<absl::optional<Copyable>>::value);
-#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(__GLIBCXX__)
+#if defined(ABSL_USES_STD_OPTIONAL) && defined(__GLIBCXX__)
   // libstdc++ std::optional implementation (as of 7.2) has a bug: when T is
   // trivially copyable, optional<T> is not trivially copyable (due to one of
   // its base class is unconditionally nontrivial).
@@ -276,7 +279,7 @@
     // std::optional when T is volatile-qualified. So skipping this test.
     // Bug report:
     // https://connect.microsoft.com/VisualStudio/feedback/details/3142534
-#if defined(ABSL_HAVE_STD_OPTIONAL) && defined(_MSC_VER) && _MSC_VER >= 1911
+#if defined(ABSL_USES_STD_OPTIONAL) && defined(_MSC_VER) && _MSC_VER >= 1911
 #define ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG 1
 #endif
 #ifndef ABSL_MSVC_OPTIONAL_VOLATILE_COPY_BUG
@@ -302,7 +305,7 @@
   EXPECT_FALSE(std::is_move_constructible<absl::optional<NonMovable>>::value);
   // test noexcept
   EXPECT_TRUE(std::is_nothrow_move_constructible<absl::optional<int>>::value);
-#ifndef ABSL_HAVE_STD_OPTIONAL
+#ifndef ABSL_USES_STD_OPTIONAL
   EXPECT_EQ(
       absl::default_allocator_is_nothrow::value,
       std::is_nothrow_move_constructible<absl::optional<MoveableThrow>>::value);
@@ -636,7 +639,7 @@
   EXPECT_FALSE(absl::is_trivially_copy_assignable<NonTrivial>::value);
 
   // std::optional doesn't support volatile nontrivial types.
-#ifndef ABSL_HAVE_STD_OPTIONAL
+#ifndef ABSL_USES_STD_OPTIONAL
   {
     StructorListener listener;
     Listenable::listener = &listener;
@@ -655,7 +658,7 @@
     EXPECT_EQ(1, listener.destruct);
     EXPECT_EQ(1, listener.volatile_copy_assign);
   }
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
 }
 
 TEST(optionalTest, MoveAssignment) {
@@ -679,7 +682,7 @@
     EXPECT_EQ(1, listener.move_assign);
   }
   // std::optional doesn't support volatile nontrivial types.
-#ifndef ABSL_HAVE_STD_OPTIONAL
+#ifndef ABSL_USES_STD_OPTIONAL
   {
     StructorListener listener;
     Listenable::listener = &listener;
@@ -699,7 +702,7 @@
     EXPECT_EQ(1, listener.destruct);
     EXPECT_EQ(1, listener.volatile_move_assign);
   }
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
   EXPECT_FALSE(absl::is_move_assignable<absl::optional<const int>>::value);
   EXPECT_TRUE(absl::is_move_assignable<absl::optional<Copyable>>::value);
   EXPECT_TRUE(absl::is_move_assignable<absl::optional<MoveableThrow>>::value);
@@ -941,7 +944,7 @@
 
 template <int v>
 struct DeletedOpAddr {
-  constexpr static const int value = v;
+  int value = v;
   constexpr DeletedOpAddr() = default;
   constexpr const DeletedOpAddr<v>* operator&() const = delete;  // NOLINT
   DeletedOpAddr<v>* operator&() = delete;                        // NOLINT
@@ -951,9 +954,9 @@
 // to document the fact that the current implementation of absl::optional<T>
 // expects such usecases to be malformed and not compile.
 TEST(optionalTest, OperatorAddr) {
-  constexpr const int v = -1;
+  constexpr int v = -1;
   {  // constexpr
-    constexpr const absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
+    constexpr absl::optional<DeletedOpAddr<v>> opt(absl::in_place_t{});
     static_assert(opt.has_value(), "");
     // static_assert(opt->value == v, "");
     static_assert((*opt).value == v, "");
@@ -1557,7 +1560,7 @@
   static_assert(
       std::is_nothrow_move_constructible<absl::optional<MoveMeNoThrow>>::value,
       "");
-#ifndef ABSL_HAVE_STD_OPTIONAL
+#ifndef ABSL_USES_STD_OPTIONAL
   static_assert(absl::default_allocator_is_nothrow::value ==
                     std::is_nothrow_move_constructible<
                         absl::optional<MoveMeThrow>>::value,
@@ -1654,3 +1657,5 @@
 #endif  // !defined(__EMSCRIPTEN__)
 
 }  // namespace
+
+#endif  // #if !defined(ABSL_USES_STD_OPTIONAL)
diff --git a/absl/types/span.h b/absl/types/span.h
index 98c6cdc..3283145 100644
--- a/absl/types/span.h
+++ b/absl/types/span.h
@@ -71,7 +71,7 @@
 #include "absl/types/internal/span.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 //------------------------------------------------------------------------------
 // Span
@@ -708,6 +708,6 @@
 constexpr Span<const T> MakeConstSpan(const T (&array)[N]) noexcept {
   return Span<const T>(array, N);
 }
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 #endif  // ABSL_TYPES_SPAN_H_
diff --git a/absl/types/span_test.cc b/absl/types/span_test.cc
index 9269f91..22467a0 100644
--- a/absl/types/span_test.cc
+++ b/absl/types/span_test.cc
@@ -35,7 +35,7 @@
 namespace {
 
 MATCHER_P(DataIs, data,
-          absl::StrCat("data() is ", negation ? "is " : "isn't ",
+          absl::StrCat("data() ", negation ? "isn't " : "is ",
                        testing::PrintToString(data))) {
   return arg.data() == data;
 }
diff --git a/absl/types/variant.h b/absl/types/variant.h
index 1c1962b..776d19a 100644
--- a/absl/types/variant.h
+++ b/absl/types/variant.h
@@ -24,7 +24,7 @@
 // should always hold a value of one of its alternative types (except in the
 // "valueless by exception state" -- see below). A default-constructed
 // `absl::variant` will hold the value of its first alternative type, provided
-// it is default-constructable.
+// it is default-constructible.
 //
 // In exceptional cases due to error, an `absl::variant` can hold no
 // value (known as a "valueless by exception" state), though this is not the
@@ -45,12 +45,12 @@
 #include "absl/base/config.h"
 #include "absl/utility/utility.h"
 
-#ifdef ABSL_HAVE_STD_VARIANT
+#ifdef ABSL_USES_STD_VARIANT
 
 #include <variant>  // IWYU pragma: export
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 using std::bad_variant_access;
 using std::get;
 using std::get_if;
@@ -63,10 +63,10 @@
 using std::variant_size;
 using std::variant_size_v;
 using std::visit;
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
-#else  // ABSL_HAVE_STD_VARIANT
+#else  // ABSL_USES_STD_VARIANT
 
 #include <functional>
 #include <new>
@@ -79,7 +79,7 @@
 #include "absl/types/internal/variant.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // -----------------------------------------------------------------------------
 // absl::variant
@@ -95,7 +95,7 @@
 //   // assign it to a std::string.
 //   absl::variant<int, std::string> v = std::string("abc");
 //
-//   // A default-contructed variant will hold a value-initialized value of
+//   // A default-constructed variant will hold a value-initialized value of
 //   // the first alternative type.
 //   auto a = absl::variant<int, std::string>();   // Holds an int of value '0'.
 //
@@ -798,7 +798,7 @@
                    a.index());
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 namespace std {
@@ -816,10 +816,10 @@
 
 }  // namespace std
 
-#endif  // ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_VARIANT
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace variant_internal {
 
 // Helper visitor for converting a variant<Ts...>` into another type (mostly
@@ -855,7 +855,7 @@
                      std::forward<Variant>(variant));
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_TYPES_VARIANT_H_
diff --git a/absl/types/variant_benchmark.cc b/absl/types/variant_benchmark.cc
index efe0231..350b175 100644
--- a/absl/types/variant_benchmark.cc
+++ b/absl/types/variant_benchmark.cc
@@ -28,7 +28,7 @@
 #include "absl/utility/utility.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 template <std::size_t I>
@@ -218,5 +218,5 @@
     ->DenseRange(0, integral_pow(4, 2) - 1);
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
diff --git a/absl/types/variant_exception_safety_test.cc b/absl/types/variant_exception_safety_test.cc
index 3166254..439c6e1 100644
--- a/absl/types/variant_exception_safety_test.cc
+++ b/absl/types/variant_exception_safety_test.cc
@@ -14,6 +14,12 @@
 
 #include "absl/types/variant.h"
 
+#include "absl/base/config.h"
+
+// This test is a no-op when absl::variant is an alias for std::variant and when
+// exceptions are not enabled.
+#if !defined(ABSL_USES_STD_VARIANT) && defined(ABSL_HAVE_EXCEPTIONS)
+
 #include <iostream>
 #include <memory>
 #include <utility>
@@ -21,7 +27,6 @@
 
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
-#include "absl/base/config.h"
 #include "absl/base/internal/exception_safety_testing.h"
 #include "absl/memory/memory.h"
 
@@ -29,7 +34,7 @@
 #if !defined(ABSL_INTERNAL_MSVC_2017_DBG_MODE)
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 using ::testing::MakeExceptionSafetyTester;
@@ -233,7 +238,7 @@
   }
   // libstdc++ std::variant has bugs on copy assignment regarding exception
   // safety.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#if !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
   // index() != j
   // if is_nothrow_copy_constructible_v<Tj> or
   // !is_nothrow_move_constructible<Tj> is true, equivalent to
@@ -264,7 +269,7 @@
                     .Test());
     EXPECT_FALSE(tester.WithContracts(strong_guarantee).Test());
   }
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#endif  // !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
   {
     // is_nothrow_copy_constructible_v<Tj> == false &&
     // is_nothrow_move_constructible_v<Tj> == true
@@ -321,7 +326,7 @@
     // The fix is targeted for gcc-9.
     // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431#c7
     // https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=267614
-#if !(defined(ABSL_HAVE_STD_VARIANT) && \
+#if !(defined(ABSL_USES_STD_VARIANT) && \
       defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 8)
     // - otherwise (index() != j), equivalent to
     // emplace<j>(get<j>(std::move(rhs)))
@@ -338,7 +343,7 @@
                       auto copy = rhs;
                       *lhs = std::move(copy);
                     }));
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) &&
+#endif  // !(defined(ABSL_USES_STD_VARIANT) &&
         //   defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE == 8)
   }
 }
@@ -441,7 +446,7 @@
   // and operator=(variant&&) invokes Tj's move ctor which doesn't throw.
   // libstdc++ std::variant has bugs on conversion assignment regarding
   // exception safety.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#if !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
   {
     MoveNothrow rhs;
     EXPECT_TRUE(MakeExceptionSafetyTester()
@@ -449,7 +454,7 @@
                     .WithContracts(VariantInvariants, strong_guarantee)
                     .Test([&rhs](ThrowingVariant* lhs) { *lhs = rhs; }));
   }
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#endif  // !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
 }
 
 TEST(VariantExceptionSafetyTest, Emplace) {
@@ -519,7 +524,9 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // !defined(ABSL_INTERNAL_MSVC_2017_DBG_MODE)
+
+#endif  // #if !defined(ABSL_USES_STD_VARIANT) && defined(ABSL_HAVE_EXCEPTIONS)
diff --git a/absl/types/variant_test.cc b/absl/types/variant_test.cc
index ff0f187..9639333 100644
--- a/absl/types/variant_test.cc
+++ b/absl/types/variant_test.cc
@@ -19,6 +19,9 @@
 
 #include "absl/types/variant.h"
 
+// This test is a no-op when absl::variant is an alias for std::variant.
+#if !defined(ABSL_USES_STD_VARIANT)
+
 #include <algorithm>
 #include <cstddef>
 #include <functional>
@@ -67,7 +70,7 @@
 struct NonHashable {};
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 namespace {
 
 using ::testing::DoubleEq;
@@ -840,7 +843,7 @@
   }
 
   // libstdc++ doesn't pass this test
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#if !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
   EXPECT_EQ(3, counter[0]);
   EXPECT_EQ(2, counter[1]);
   EXPECT_EQ(2, counter[2]);
@@ -1040,8 +1043,6 @@
   using V = variant<MoveCanThrow, std::string, int>;
   int i = 33;
   std::string s = "abc";
-  V valueless(in_place_index<0>);
-  ToValuelessByException(valueless);
   {
     // lhs and rhs holds different alternative
     V lhs(i), rhs(s);
@@ -1049,6 +1050,9 @@
     EXPECT_THAT(lhs, VariantWith<std::string>(s));
     EXPECT_THAT(rhs, VariantWith<int>(i));
   }
+#ifdef ABSL_HAVE_EXCEPTIONS
+  V valueless(in_place_index<0>);
+  ToValuelessByException(valueless);
   {
     // lhs is valueless
     V lhs(valueless), rhs(i);
@@ -1070,6 +1074,7 @@
     EXPECT_TRUE(lhs.valueless_by_exception());
     EXPECT_TRUE(rhs.valueless_by_exception());
   }
+#endif  // ABSL_HAVE_EXCEPTIONS
 }
 
 //////////////////////
@@ -1929,7 +1934,7 @@
 }
 
 // libstdc++ std::variant doesn't support the INVOKE semantics.
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#if !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
 TEST(VariantTest, VisitMemberFunction) {
   absl::variant<std::unique_ptr<Class>> p(absl::make_unique<Class>());
   absl::variant<std::unique_ptr<const Class>> cp(
@@ -1953,7 +1958,7 @@
 
   EXPECT_EQ(42, absl::visit(&Class::member, cp));
 }
-#endif  // !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#endif  // !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
 
 /////////////////////////
 // [variant.monostate] //
@@ -2031,7 +2036,7 @@
   std::swap(a, b);
   EXPECT_THAT(a, VariantWith<SpecialSwap>(v2));
   EXPECT_THAT(b, VariantWith<SpecialSwap>(v1));
-#ifndef ABSL_HAVE_STD_VARIANT
+#ifndef ABSL_USES_STD_VARIANT
   EXPECT_FALSE(absl::get<SpecialSwap>(a).special_swap);
 #endif
 
@@ -2079,7 +2084,7 @@
 
 // MSVC std::hash<std::variant> does not use the index, thus produce the same
 // result on the same value as different alternative.
-#if !(defined(_MSC_VER) && defined(ABSL_HAVE_STD_VARIANT))
+#if !(defined(_MSC_VER) && defined(ABSL_USES_STD_VARIANT))
   {
     // same value as different alternative
     variant<int, int> v0(in_place_index<0>, 42);
@@ -2087,7 +2092,7 @@
     std::hash<variant<int, int>> hash;
     EXPECT_NE(hash(v0), hash(v1));
   }
-#endif  // !(defined(_MSC_VER) && defined(ABSL_HAVE_STD_VARIANT))
+#endif  // !(defined(_MSC_VER) && defined(ABSL_USES_STD_VARIANT))
 
   {
     std::hash<variant<int>> hash;
@@ -2114,7 +2119,7 @@
 ////////////////////////////////////////
 
 // Test that a set requiring a basic type conversion works correctly
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
 TEST(VariantTest, TestConvertingSet) {
   typedef variant<double> Variant;
   Variant v(1.0);
@@ -2124,7 +2129,7 @@
   ASSERT_TRUE(nullptr != absl::get_if<double>(&v));
   EXPECT_DOUBLE_EQ(2, absl::get<double>(v));
 }
-#endif  // ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_VARIANT
 
 // Test that a vector of variants behaves reasonably.
 TEST(VariantTest, Container) {
@@ -2276,7 +2281,7 @@
 };
 
 TEST(VariantTest, TestRvalueConversion) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
   variant<double, std::string> var(
       ConvertVariantTo<variant<double, std::string>>(
           variant<std::string, int>(0)));
@@ -2309,7 +2314,7 @@
   variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(variant<uint32_t>(42));
   ASSERT_TRUE(absl::holds_alternative<uint32_t>(variant2));
   EXPECT_EQ(42, absl::get<uint32_t>(variant2));
-#endif  // !ABSL_HAVE_STD_VARIANT
+#endif  // !ABSL_USES_STD_VARIANT
 
   variant<Convertible1, Convertible2> variant3(
       ConvertVariantTo<variant<Convertible1, Convertible2>>(
@@ -2322,7 +2327,7 @@
 }
 
 TEST(VariantTest, TestLvalueConversion) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
   variant<std::string, int> source1 = 0;
   variant<double, std::string> destination(
       ConvertVariantTo<variant<double, std::string>>(source1));
@@ -2424,7 +2429,7 @@
 }
 
 TEST(VariantTest, TestRvalueConversionViaConvertVariantTo) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
   variant<double, std::string> var(
       ConvertVariantTo<variant<double, std::string>>(
           variant<std::string, int>(3)));
@@ -2463,7 +2468,7 @@
 }
 
 TEST(VariantTest, TestLvalueConversionViaConvertVariantTo) {
-#if !defined(ABSL_HAVE_STD_VARIANT)
+#if !defined(ABSL_USES_STD_VARIANT)
   variant<std::string, int> source1 = 3;
   variant<double, std::string> destination(
       ConvertVariantTo<variant<double, std::string>>(source1));
@@ -2495,7 +2500,7 @@
   variant<uint32_t> source6(42);
   variant2 = ConvertVariantTo<variant<int32_t, uint32_t>>(source6);
   EXPECT_THAT(absl::get_if<uint32_t>(&variant2), Pointee(42));
-#endif  // !ABSL_HAVE_STD_VARIANT
+#endif  // !ABSL_USES_STD_VARIANT
 
   variant<Convertible2, Convertible1> source7((Convertible1()));
   variant<Convertible1, Convertible2> variant3(
@@ -2529,7 +2534,7 @@
 // standard and we know that libstdc++ variant doesn't have this feature.
 // For more details see the paper:
 // http://open-std.org/JTC1/SC22/WG21/docs/papers/2017/p0602r0.html
-#if !(defined(ABSL_HAVE_STD_VARIANT) && defined(__GLIBCXX__))
+#if !(defined(ABSL_USES_STD_VARIANT) && defined(__GLIBCXX__))
 #define ABSL_VARIANT_PROPAGATE_COPY_MOVE_TRIVIALITY 1
 #endif
 
@@ -2705,5 +2710,7 @@
 }
 
 }  // namespace
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
+
+#endif  // #if !defined(ABSL_USES_STD_VARIANT)
diff --git a/absl/utility/BUILD.bazel b/absl/utility/BUILD.bazel
index d41317e..6881f93 100644
--- a/absl/utility/BUILD.bazel
+++ b/absl/utility/BUILD.bazel
@@ -1,3 +1,20 @@
+#
+# Copyright 2019 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
 load(
     "//absl:copts/configure_copts.bzl",
     "ABSL_DEFAULT_COPTS",
@@ -11,7 +28,9 @@
 
 cc_library(
     name = "utility",
-    hdrs = ["utility.h"],
+    hdrs = [
+        "utility.h",
+    ],
     copts = ABSL_DEFAULT_COPTS,
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
diff --git a/absl/utility/utility.h b/absl/utility/utility.h
index bc9af04..e6647c7 100644
--- a/absl/utility/utility.h
+++ b/absl/utility/utility.h
@@ -51,7 +51,7 @@
 #include "absl/meta/type_traits.h"
 
 namespace absl {
-inline namespace lts_2019_08_08 {
+ABSL_NAMESPACE_BEGIN
 
 // integer_sequence
 //
@@ -159,12 +159,12 @@
 
 // Tag types
 
-#ifdef ABSL_HAVE_STD_OPTIONAL
+#ifdef ABSL_USES_STD_OPTIONAL
 
 using std::in_place_t;
 using std::in_place;
 
-#else  // ABSL_HAVE_STD_OPTIONAL
+#else  // ABSL_USES_STD_OPTIONAL
 
 // in_place_t
 //
@@ -175,9 +175,9 @@
 
 ABSL_INTERNAL_INLINE_CONSTEXPR(in_place_t, in_place, {});
 
-#endif  // ABSL_HAVE_STD_OPTIONAL
+#endif  // ABSL_USES_STD_OPTIONAL
 
-#if defined(ABSL_HAVE_STD_ANY) || defined(ABSL_HAVE_STD_VARIANT)
+#if defined(ABSL_USES_STD_ANY) || defined(ABSL_USES_STD_VARIANT)
 using std::in_place_type;
 using std::in_place_type_t;
 #else
@@ -192,9 +192,9 @@
 
 template <typename T>
 void in_place_type(utility_internal::InPlaceTypeTag<T>) {}
-#endif  // ABSL_HAVE_STD_ANY || ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_ANY || ABSL_USES_STD_VARIANT
 
-#ifdef ABSL_HAVE_STD_VARIANT
+#ifdef ABSL_USES_STD_VARIANT
 using std::in_place_index;
 using std::in_place_index_t;
 #else
@@ -209,7 +209,7 @@
 
 template <size_t I>
 void in_place_index(utility_internal::InPlaceIndexTag<I>) {}
-#endif  // ABSL_HAVE_STD_VARIANT
+#endif  // ABSL_USES_STD_VARIANT
 
 // Constexpr move and forward
 
@@ -344,7 +344,7 @@
           std::tuple_size<absl::decay_t<Tuple>>::value>{});
 }
 
-}  // inline namespace lts_2019_08_08
+ABSL_NAMESPACE_END
 }  // namespace absl
 
 #endif  // ABSL_UTILITY_UTILITY_H_
diff --git a/ci/absl_alternate_options.h b/ci/absl_alternate_options.h
new file mode 100644
index 0000000..f0c21fe
--- /dev/null
+++ b/ci/absl_alternate_options.h
@@ -0,0 +1,28 @@
+#ifndef ABSL_BASE_OPTIONS_H_
+#define ABSL_BASE_OPTIONS_H_
+
+// Copyright 2019 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Alternate options.h file, used in continuous integration testing to exercise
+// option settings not used by default.
+
+#define ABSL_OPTION_USE_STD_ANY 0
+#define ABSL_OPTION_USE_STD_OPTIONAL 0
+#define ABSL_OPTION_USE_STD_STRING_VIEW 0
+#define ABSL_OPTION_USE_STD_VARIANT 0
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME ns
+
+#endif  // ABSL_BASE_OPTIONS_H_
diff --git a/ci/cmake_install_test.sh b/ci/cmake_install_test.sh
index 03eb043..55fb4f1 100755
--- a/ci/cmake_install_test.sh
+++ b/ci/cmake_install_test.sh
@@ -28,6 +28,5 @@
     --rm \
     -e CFLAGS="-Werror" \
     -e CXXFLAGS="-Werror" \
-    gcr.io/google.com/absl-177019/linux_gcc-latest:20190703 \
+    gcr.io/google.com/absl-177019/linux_gcc-latest:20200106 \
     /bin/bash CMake/install_test_project/test.sh $@
-
diff --git a/ci/linux_clang-latest_libcxx_asan_bazel.sh b/ci/linux_clang-latest_libcxx_asan_bazel.sh
index 07af64d..24efe3b 100755
--- a/ci/linux_clang-latest_libcxx_asan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_asan_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190701"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200102"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,42 +52,43 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/opt/llvm/clang/bin/clang" \
-      -e BAZEL_COMPILER="llvm" \
-      -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-      -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
-      -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --compilation_mode=${compilation_mode} \
-        --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
-        --copt="-DADDRESS_SANITIZER" \
-        --copt="-DUNDEFINED_BEHAVIOR_SANITIZER" \
-        --copt="-fsanitize=address" \
-        --copt="-fsanitize=float-divide-by-zero" \
-        --copt="-fsanitize=nullability" \
-        --copt="-fsanitize=undefined" \
-        --copt="-fno-sanitize=vptr" \
-        --copt=-Werror \
-        --keep_going \
-        --linkopt="-fsanitize=address" \
-        --linkopt="-fsanitize-link-c++-runtime" \
-        --show_timestamps \
-        --test_env="ASAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_env="UBSAN_OPTIONS=print_stacktrace=1" \
-        --test_env="UBSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
-        --test_output=errors \
-        --test_tag_filters="-benchmark,-noasan" \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/opt/llvm/clang/bin/clang" \
+        -e BAZEL_COMPILER="llvm" \
+        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
+        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
+        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /usr/local/bin/bazel test ... \
+          --compilation_mode="${compilation_mode}" \
+          --copt="${exceptions_mode}" \
+          --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
+          --copt="-DADDRESS_SANITIZER" \
+          --copt="-DUNDEFINED_BEHAVIOR_SANITIZER" \
+          --copt="-fsanitize=address" \
+          --copt="-fsanitize=float-divide-by-zero" \
+          --copt="-fsanitize=nullability" \
+          --copt="-fsanitize=undefined" \
+          --copt="-fno-sanitize-blacklist" \
+          --copt=-Werror \
+          --keep_going \
+          --linkopt="-fsanitize=address" \
+          --linkopt="-fsanitize-link-c++-runtime" \
+          --show_timestamps \
+          --test_env="ASAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
+          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_env="UBSAN_OPTIONS=print_stacktrace=1" \
+          --test_env="UBSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
+          --test_output=errors \
+          --test_tag_filters="-benchmark,-noasan" \
+          ${BAZEL_EXTRA_ARGS:-}
+    done
   done
 done
diff --git a/ci/linux_clang-latest_libcxx_bazel.sh b/ci/linux_clang-latest_libcxx_bazel.sh
index 1197369..127e7bc 100755
--- a/ci/linux_clang-latest_libcxx_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190701"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200102"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,31 +52,38 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/opt/llvm/clang/bin/clang" \
-      -e BAZEL_COMPILER="llvm" \
-      -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-      -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
-      -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --compilation_mode=${compilation_mode} \
-        --copt=-Werror \
-        --define="absl=1" \
-        --keep_going \
-        --show_timestamps \
-        --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_output=errors \
-        --test_tag_filters=-benchmark \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp-ro:ro" \
+        --tmpfs=/abseil-cpp \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/opt/llvm/clang/bin/clang" \
+        -e BAZEL_COMPILER="llvm" \
+        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
+        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx/lib" \
+        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx/include/c++/v1" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /bin/sh -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --keep_going \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
+    done
   done
 done
diff --git a/ci/linux_clang-latest_libcxx_tsan_bazel.sh b/ci/linux_clang-latest_libcxx_tsan_bazel.sh
index 04171df..00257b3 100755
--- a/ci/linux_clang-latest_libcxx_tsan_bazel.sh
+++ b/ci/linux_clang-latest_libcxx_tsan_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190701"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200102"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,36 +52,38 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/opt/llvm/clang/bin/clang" \
-      -e BAZEL_COMPILER="llvm" \
-      -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
-      -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx-tsan/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx-tsan/lib" \
-      -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx-tsan/include/c++/v1" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --build_tag_filters="-notsan" \
-        --compilation_mode=${compilation_mode} \
-        --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
-        --copt="-DTHREAD_SANITIZER" \
-        --copt="-fsanitize=thread" \
-        --copt=-Werror \
-        --keep_going \
-        --linkopt="-fsanitize=thread" \
-        --show_timestamps \
-        --test_env="TSAN_OPTIONS=report_atomic_races=0" \
-        --test_env="TSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_output=errors \
-        --test_tag_filters="-benchmark,-notsan" \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/opt/llvm/clang/bin/clang" \
+        -e BAZEL_COMPILER="llvm" \
+        -e BAZEL_CXXOPTS="-std=${std}:-nostdinc++" \
+        -e BAZEL_LINKOPTS="-L/opt/llvm/libcxx-tsan/lib:-lc++:-lc++abi:-lm:-Wl,-rpath=/opt/llvm/libcxx-tsan/lib" \
+        -e CPLUS_INCLUDE_PATH="/opt/llvm/libcxx-tsan/include/c++/v1" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /usr/local/bin/bazel test ... \
+          --build_tag_filters="-notsan" \
+          --compilation_mode="${compilation_mode}" \
+          --copt="${exceptions_mode}" \
+          --copt="-DDYNAMIC_ANNOTATIONS_ENABLED=1" \
+          --copt="-DTHREAD_SANITIZER" \
+          --copt="-fsanitize=thread" \
+          --copt="-fno-sanitize-blacklist" \
+          --copt=-Werror \
+          --keep_going \
+          --linkopt="-fsanitize=thread" \
+          --show_timestamps \
+          --test_env="TSAN_OPTIONS=report_atomic_races=0" \
+          --test_env="TSAN_SYMBOLIZER_PATH=/opt/llvm/clang/bin/llvm-symbolizer" \
+          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_output=errors \
+          --test_tag_filters="-benchmark,-notsan" \
+          ${BAZEL_EXTRA_ARGS:-}
+    done
   done
 done
diff --git a/ci/linux_clang-latest_libstdcxx_bazel.sh b/ci/linux_clang-latest_libstdcxx_bazel.sh
index c2217a0..9fe71d3 100755
--- a/ci/linux_clang-latest_libstdcxx_bazel.sh
+++ b/ci/linux_clang-latest_libstdcxx_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20190701"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_clang-latest:20200102"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,30 +52,31 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/opt/llvm/clang/bin/clang" \
-      -e BAZEL_COMPILER="llvm" \
-      -e BAZEL_CXXOPTS="-std=${std}" \
-      -e CPLUS_INCLUDE_PATH="/usr/include/c++/6" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --compilation_mode=${compilation_mode} \
-        --copt=-Werror \
-        --define="absl=1" \
-        --keep_going \
-        --show_timestamps \
-        --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_output=errors \
-        --test_tag_filters=-benchmark \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/opt/llvm/clang/bin/clang" \
+        -e BAZEL_COMPILER="llvm" \
+        -e BAZEL_CXXOPTS="-std=${std}" \
+        -e CPLUS_INCLUDE_PATH="/usr/include/c++/6" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /usr/local/bin/bazel test ... \
+          --compilation_mode="${compilation_mode}" \
+          --copt="${exceptions_mode}" \
+          --copt=-Werror \
+          --define="absl=1" \
+          --keep_going \
+          --show_timestamps \
+          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
+          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_output=errors \
+          --test_tag_filters=-benchmark \
+          ${BAZEL_EXTRA_ARGS:-}
+    done
   done
 done
diff --git a/ci/linux_gcc-4.9_libstdcxx_bazel.sh b/ci/linux_gcc-4.9_libstdcxx_bazel.sh
index bdd3e46..f8102cc 100755
--- a/ci/linux_gcc-4.9_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-4.9_libstdcxx_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-4.9:20190702"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-4.9:20191018"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,28 +52,29 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/usr/bin/gcc-4.9" \
-      -e BAZEL_CXXOPTS="-std=${std}" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --compilation_mode=${compilation_mode} \
-        --copt=-Werror \
-        --define="absl=1" \
-        --keep_going \
-        --show_timestamps \
-        --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_output=errors \
-        --test_tag_filters=-benchmark \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/usr/bin/gcc-4.9" \
+        -e BAZEL_CXXOPTS="-std=${std}" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /usr/local/bin/bazel test ... \
+          --compilation_mode="${compilation_mode}" \
+          --copt="${exceptions_mode}" \
+          --copt=-Werror \
+          --define="absl=1" \
+          --keep_going \
+          --show_timestamps \
+          --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
+          --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
+          --test_output=errors \
+          --test_tag_filters=-benchmark \
+          ${BAZEL_EXTRA_ARGS:-}
+    done
   done
 done
diff --git a/ci/linux_gcc-latest_libstdcxx_bazel.sh b/ci/linux_gcc-latest_libstdcxx_bazel.sh
index 92933e3..5964703 100755
--- a/ci/linux_gcc-latest_libstdcxx_bazel.sh
+++ b/ci/linux_gcc-latest_libstdcxx_bazel.sh
@@ -32,7 +32,11 @@
   COMPILATION_MODE="fastbuild opt"
 fi
 
-readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-latest:20190703"
+if [ -z ${EXCEPTIONS_MODE:-} ]; then
+  EXCEPTIONS_MODE="-fno-exceptions -fexceptions"
+fi
+
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/linux_gcc-latest:20200106"
 
 # USE_BAZEL_CACHE=1 only works on Kokoro.
 # Without access to the credentials this won't work.
@@ -48,28 +52,35 @@
 
 for std in ${STD}; do
   for compilation_mode in ${COMPILATION_MODE}; do
-    echo "--------------------------------------------------------------------"
-    echo "Testing with --compilation_mode=${compilation_mode} and --std=${std}"
-
-    time docker run \
-      --volume="${ABSEIL_ROOT}:/abseil-cpp:ro" \
-      --workdir=/abseil-cpp \
-      --cap-add=SYS_PTRACE \
-      --rm \
-      -e CC="/usr/local/bin/gcc" \
-      -e BAZEL_CXXOPTS="-std=${std}" \
-      ${DOCKER_EXTRA_ARGS:-} \
-      ${DOCKER_CONTAINER} \
-      /usr/local/bin/bazel test ... \
-        --compilation_mode=${compilation_mode} \
-        --copt=-Werror \
-        --define="absl=1" \
-        --keep_going \
-        --show_timestamps \
-        --test_env="GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1" \
-        --test_env="TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo" \
-        --test_output=errors \
-        --test_tag_filters=-benchmark \
-        ${BAZEL_EXTRA_ARGS:-}
+    for exceptions_mode in ${EXCEPTIONS_MODE}; do
+      echo "--------------------------------------------------------------------"
+      time docker run \
+        --volume="${ABSEIL_ROOT}:/abseil-cpp-ro:ro" \
+        --tmpfs=/abseil-cpp \
+        --workdir=/abseil-cpp \
+        --cap-add=SYS_PTRACE \
+        --rm \
+        -e CC="/usr/local/bin/gcc" \
+        -e BAZEL_CXXOPTS="-std=${std}" \
+        ${DOCKER_EXTRA_ARGS:-} \
+        ${DOCKER_CONTAINER} \
+        /bin/sh -c "
+          cp -r /abseil-cpp-ro/* /abseil-cpp/
+          if [ -n \"${ALTERNATE_OPTIONS:-}\" ]; then
+            cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+          fi
+          /usr/local/bin/bazel test ... \
+            --compilation_mode=\"${compilation_mode}\" \
+            --copt=\"${exceptions_mode}\" \
+            --copt=-Werror \
+            --define=\"absl=1\" \
+            --keep_going \
+            --show_timestamps \
+            --test_env=\"GTEST_INSTALL_FAILURE_SIGNAL_HANDLER=1\" \
+            --test_env=\"TZDIR=/abseil-cpp/absl/time/internal/cctz/testdata/zoneinfo\" \
+            --test_output=errors \
+            --test_tag_filters=-benchmark \
+            ${BAZEL_EXTRA_ARGS:-}"
+    done
   done
 done
diff --git a/ci/linux_gcc-latest_libstdcxx_cmake.sh b/ci/linux_gcc-latest_libstdcxx_cmake.sh
index 3e831c1..38ad99f 100755
--- a/ci/linux_gcc-latest_libstdcxx_cmake.sh
+++ b/ci/linux_gcc-latest_libstdcxx_cmake.sh
@@ -47,7 +47,7 @@
       --rm \
       -e CFLAGS="-Werror" \
       -e CXXFLAGS="-Werror" \
-      gcr.io/google.com/absl-177019/linux_gcc-latest:20190703 \
+      gcr.io/google.com/absl-177019/linux_gcc-latest:20200106 \
       /bin/bash -c "
         cd /buildfs && \
         cmake /abseil-cpp \
diff --git a/ci/linux_gcc-4.8_libstdcxx_cmake.sh b/ci/linux_gcc_alpine_cmake.sh
similarity index 90%
rename from ci/linux_gcc-4.8_libstdcxx_cmake.sh
rename to ci/linux_gcc_alpine_cmake.sh
index 4f964e2..830a136 100755
--- a/ci/linux_gcc-4.8_libstdcxx_cmake.sh
+++ b/ci/linux_gcc_alpine_cmake.sh
@@ -27,13 +27,15 @@
 fi
 
 if [ -z ${ABSL_CMAKE_CXX_STANDARDS:-} ]; then
-  ABSL_CMAKE_CXX_STANDARDS="11 14"
+  ABSL_CMAKE_CXX_STANDARDS="11 14 17"
 fi
 
 if [ -z ${ABSL_CMAKE_BUILD_TYPES:-} ]; then
   ABSL_CMAKE_BUILD_TYPES="Debug Release"
 fi
 
+readonly DOCKER_CONTAINER="gcr.io/google.com/absl-177019/alpine:20191016"
+
 for std in ${ABSL_CMAKE_CXX_STANDARDS}; do
   for compilation_mode in ${ABSL_CMAKE_BUILD_TYPES}; do
     echo "--------------------------------------------------------------------"
@@ -47,15 +49,15 @@
       --rm \
       -e CFLAGS="-Werror" \
       -e CXXFLAGS="-Werror" \
-      gcr.io/google.com/absl-177019/linux_gcc-4.8:20190316 \
-      /bin/bash -c "
+      "${DOCKER_CONTAINER}" \
+      /bin/sh -c "
         cd /buildfs && \
         cmake /abseil-cpp \
           -DABSL_USE_GOOGLETEST_HEAD=ON \
           -DABSL_RUN_TESTS=ON \
           -DCMAKE_BUILD_TYPE=${compilation_mode} \
           -DCMAKE_CXX_STANDARD=${std} && \
-        make -j$(nproc) VERBOSE=1 && \
+        make -j$(nproc) && \
         ctest -j$(nproc) --output-on-failure"
   done
 done
diff --git a/ci/macos_xcode_bazel.sh b/ci/macos_xcode_bazel.sh
index f0f1428..f5f2d75 100755
--- a/ci/macos_xcode_bazel.sh
+++ b/ci/macos_xcode_bazel.sh
@@ -23,16 +23,29 @@
   ABSEIL_ROOT="$(realpath $(dirname ${0})/..)"
 fi
 
-# Print the default compiler and Bazel versions.
+# If we are running on Kokoro, check for a versioned Bazel binary.
+KOKORO_GFILE_BAZEL_BIN="bazel-2.0.0-darwin-x86_64"
+if [ ${KOKORO_GFILE_DIR:-} ] && [ -f ${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN} ]; then
+  BAZEL_BIN="${KOKORO_GFILE_DIR}/${KOKORO_GFILE_BAZEL_BIN}"
+  chmod +x ${BAZEL_BIN}
+else
+  BAZEL_BIN="bazel"
+fi
+
+# Print the compiler and Bazel versions.
 echo "---------------"
 gcc -v
 echo "---------------"
-bazel version
+${BAZEL_BIN} version
 echo "---------------"
 
 cd ${ABSEIL_ROOT}
 
-bazel test ... \
+if [ -n "${ALTERNATE_OPTIONS:-}" ]; then
+  cp ${ALTERNATE_OPTIONS:-} absl/base/options.h || exit 1
+fi
+
+${BAZEL_BIN} test ... \
   --copt=-Werror \
   --keep_going \
   --show_timestamps \
diff --git a/ci/macos_xcode_cmake.sh b/ci/macos_xcode_cmake.sh
index 75b0f2d..a1f4a85 100755
--- a/ci/macos_xcode_cmake.sh
+++ b/ci/macos_xcode_cmake.sh
@@ -36,6 +36,7 @@
   time cmake ${ABSEIL_ROOT} \
     -GXcode \
     -DCMAKE_BUILD_TYPE=${compilation_mode} \
+    -DCMAKE_CXX_STANDARD=11 \
     -DABSL_USE_GOOGLETEST_HEAD=ON \
     -DABSL_RUN_TESTS=ON
   time cmake --build .
diff --git a/conanfile.py b/conanfile.py
index cd124aa..926ec5c 100644
--- a/conanfile.py
+++ b/conanfile.py
@@ -30,7 +30,7 @@
             raise ConanInvalidConfiguration("Abseil does not support MSVC < 14")
 
     def build(self):
-        tools.replace_in_file("CMakeLists.txt", "project(absl)", "project(absl)\ninclude(conanbuildinfo.cmake)\nconan_basic_setup()")
+        tools.replace_in_file("CMakeLists.txt", "project(absl CXX)", "project(absl CXX)\ninclude(conanbuildinfo.cmake)\nconan_basic_setup()")
         cmake = CMake(self)
         cmake.definitions["BUILD_TESTING"] = False
         cmake.configure()