diff --git a/include/clang/Basic/DiagnosticIDs.h b/include/clang/Basic/DiagnosticIDs.h
index 6a6a87e..e119f31 100644
--- a/include/clang/Basic/DiagnosticIDs.h
+++ b/include/clang/Basic/DiagnosticIDs.h
@@ -83,11 +83,14 @@
   unsigned Mapping : 3;
   unsigned IsUser : 1;
   unsigned IsPragma : 1;
+  unsigned HasShowInSystemHeader : 1;
+  unsigned HasNoWarningAsError : 1;
+  unsigned HasNoErrorAsFatal : 1;
 
 public:
   static DiagnosticMappingInfo MakeUnset() {
     DiagnosticMappingInfo Result;
-    Result.Mapping = Result.IsUser = Result.IsPragma = 0;
+    Result.Mapping = 0;
     return Result;
   }
 
@@ -97,14 +100,28 @@
     Result.Mapping = Mapping;
     Result.IsUser = IsUser;
     Result.IsPragma = IsPragma;
+    Result.HasShowInSystemHeader = 0;
+    Result.HasNoWarningAsError = 0;
+    Result.HasNoErrorAsFatal = 0;
     return Result;
   }
 
+  bool isUnset() const { return Mapping == 0; }
+
   diag::Mapping getMapping() const { return diag::Mapping(Mapping); }
+  void setMapping(diag::Mapping Value) { Mapping = Value; }
+
   bool isUser() const { return IsUser; }
   bool isPragma() const { return IsPragma; }
 
-  bool isUnset() const { return Mapping == 0; }
+  bool hasShowInSystemHeader() const { return HasShowInSystemHeader; }
+  void setShowInSystemHeader(bool Value) { HasShowInSystemHeader = Value; }
+
+  bool hasNoWarningAsError() const { return HasNoWarningAsError; }
+  void setNoWarningAsError(bool Value) { HasNoWarningAsError = Value; }
+
+  bool hasNoErrorAsFatal() const { return HasNoErrorAsFatal; }
+  void setNoErrorAsFatal(bool Value) { HasNoErrorAsFatal = Value; }
 };
 
 /// \brief Used for handling and querying diagnostic IDs. Can be used and shared
diff --git a/lib/Basic/Diagnostic.cpp b/lib/Basic/Diagnostic.cpp
index 0402aaa..e9abfb7 100644
--- a/lib/Basic/Diagnostic.cpp
+++ b/lib/Basic/Diagnostic.cpp
@@ -239,14 +239,56 @@
 
 bool DiagnosticsEngine::setDiagnosticGroupWarningAsError(StringRef Group,
                                                          bool Enabled) {
-  diag::Mapping Map = Enabled ? diag::MAP_ERROR : diag::MAP_WARNING_NO_WERROR;
-  return setDiagnosticGroupMapping(Group, Map);
+  // If we are enabling this feature, just set the diagnostic mappings to map to
+  // errors.
+  if (Enabled)
+    return setDiagnosticGroupMapping(Group, diag::MAP_ERROR);
+
+  // Otherwise, we want to set the diagnostic mapping's "no Werror" bit, and
+  // potentially downgrade anything already mapped to be a warning.
+
+  // Get the diagnostics in this group.
+  llvm::SmallVector<diag::kind, 8> GroupDiags;
+  if (Diags->getDiagnosticsInGroup(Group, GroupDiags))
+    return true;
+
+  // Perform the mapping change.
+  for (unsigned i = 0, e = GroupDiags.size(); i != e; ++i) {
+    DiagnosticMappingInfo &Info = GetCurDiagState()->getOrAddMappingInfo(
+      GroupDiags[i]);
+
+    Info.setMapping(diag::MAP_WARNING_NO_WERROR);
+    Info.setNoWarningAsError(true);
+  }
+
+  return false;
 }
 
 bool DiagnosticsEngine::setDiagnosticGroupErrorAsFatal(StringRef Group,
                                                        bool Enabled) {
-  diag::Mapping Map = Enabled ? diag::MAP_FATAL : diag::MAP_ERROR_NO_WFATAL;
-  return setDiagnosticGroupMapping(Group, Map);
+  // If we are enabling this feature, just set the diagnostic mappings to map to
+  // fatal errors.
+  if (Enabled)
+    return setDiagnosticGroupMapping(Group, diag::MAP_FATAL);
+
+  // Otherwise, we want to set the diagnostic mapping's "no Werror" bit, and
+  // potentially downgrade anything already mapped to be an error.
+
+  // Get the diagnostics in this group.
+  llvm::SmallVector<diag::kind, 8> GroupDiags;
+  if (Diags->getDiagnosticsInGroup(Group, GroupDiags))
+    return true;
+
+  // Perform the mapping change.
+  for (unsigned i = 0, e = GroupDiags.size(); i != e; ++i) {
+    DiagnosticMappingInfo &Info = GetCurDiagState()->getOrAddMappingInfo(
+      GroupDiags[i]);
+
+    Info.setMapping(diag::MAP_ERROR_NO_WFATAL);
+    Info.setNoErrorAsFatal(true);
+  }
+
+  return false;
 }
 
 void DiagnosticsEngine::Report(const StoredDiagnostic &storedDiag) {
diff --git a/lib/Basic/DiagnosticIDs.cpp b/lib/Basic/DiagnosticIDs.cpp
index 3129f73..65e5c63 100644
--- a/lib/Basic/DiagnosticIDs.cpp
+++ b/lib/Basic/DiagnosticIDs.cpp
@@ -184,26 +184,29 @@
   return Found;
 }
 
-static diag::Mapping GetDefaultDiagMapping(unsigned DiagID) {
-  if (const StaticDiagInfoRec *Info = GetDiagInfo(DiagID)) {
-    // Compute the effective mapping based on the extra bits.
-    diag::Mapping Mapping = (diag::Mapping) Info->Mapping;
+static DiagnosticMappingInfo GetDefaultDiagMappingInfo(unsigned DiagID) {
+  DiagnosticMappingInfo Info = DiagnosticMappingInfo::MakeInfo(
+    diag::MAP_FATAL, /*IsUser=*/false, /*IsPragma=*/false);
 
-    if (Info->WarnNoWerror) {
-      assert(Mapping == diag::MAP_WARNING &&
+  if (const StaticDiagInfoRec *StaticInfo = GetDiagInfo(DiagID)) {
+    Info.setMapping((diag::Mapping) StaticInfo->Mapping);
+
+    if (StaticInfo->WarnNoWerror) {
+      assert(Info.getMapping() == diag::MAP_WARNING &&
              "Unexpected mapping with no-Werror bit!");
-      Mapping = diag::MAP_WARNING_NO_WERROR;
+      Info.setMapping(diag::MAP_WARNING_NO_WERROR);
+      Info.setNoWarningAsError(true);
     }
 
-    if (Info->WarnShowInSystemHeader) {
-      assert(Mapping == diag::MAP_WARNING &&
+    if (StaticInfo->WarnShowInSystemHeader) {
+      assert(Info.getMapping() == diag::MAP_WARNING &&
              "Unexpected mapping with show-in-system-header bit!");
-      Mapping = diag::MAP_WARNING_SHOW_IN_SYSTEM_HEADER;
+      Info.setMapping(diag::MAP_WARNING_SHOW_IN_SYSTEM_HEADER);
+      Info.setShowInSystemHeader(true);
     }
-
-    return Mapping;
   }
-  return diag::MAP_FATAL;
+
+  return Info;
 }
 
 /// getWarningOptionForDiag - Return the lowest-level warning option that
@@ -247,8 +250,7 @@
   // Initialize the entry if we added it.
   if (Result.second) {
     assert(Result.first->second.isUnset() && "unexpected unset entry");
-    Result.first->second = DiagnosticMappingInfo::MakeInfo(
-      GetDefaultDiagMapping(Diag), /*IsUser=*/false, /*IsPragma=*/false);
+    Result.first->second = GetDefaultDiagMappingInfo(Diag);
   }
 
   return Result.first->second;
@@ -474,7 +476,8 @@
       getBuiltinDiagClass(DiagID) != CLASS_EXTENSION)
     return false;
   
-  EnabledByDefault = GetDefaultDiagMapping(DiagID) != diag::MAP_IGNORE;
+  EnabledByDefault =
+    GetDefaultDiagMappingInfo(DiagID).getMapping() != diag::MAP_IGNORE;
   return true;
 }
 
@@ -482,7 +485,7 @@
   if (DiagID >= diag::DIAG_UPPER_LIMIT)
     return false;
 
-  return GetDefaultDiagMapping(DiagID) == diag::MAP_ERROR;
+  return GetDefaultDiagMappingInfo(DiagID).getMapping() == diag::MAP_ERROR;
 }
 
 /// getDescription - Given a diagnostic ID, return a description of the