c10/util/Metaprogramming.h - platform/external/pytorch - Git at Google

 #pragma once

 #include <type_traits>
 #include <array>
 #include <functional>
 #include <c10/util/TypeList.h>
 #include <c10/util/Array.h>

 namespace c10 { namespace guts {

 /**
  * Access information about result type or arguments from a function type.
  * Example:
  * using A = function_traits<int (float, double)>::return_type // A == int
  * using A = function_traits<int (float, double)>::parameter_types::tuple_type // A == tuple<float, double>
  */
 template<class Func> struct function_traits {
   static_assert(!std::is_same<Func, Func>::value, "In function_traits<Func>, Func must be a plain function type.");
 };
 template<class Result, class... Args>
 struct function_traits<Result (Args...)> {
   using func_type = Result (Args...);
   using return_type = Result;
   using parameter_types = typelist::typelist<Args...>;
   static constexpr auto number_of_parameters = sizeof...(Args);
 };

 /**
  * infer_function_traits: creates a `function_traits` type for a simple
  * function (pointer) or functor (lambda/struct). Currently does not support
  * class methods.
  */

 template <typename Functor>
 struct infer_function_traits {
   using type = function_traits<c10::guts::detail::strip_class_t<decltype(&Functor::operator())>>;
 };

 template <typename Result, typename... Args>
 struct infer_function_traits<Result (*)(Args...)> {
   using type = function_traits<Result(Args...)>;
 };

 template <typename Result, typename... Args>
 struct infer_function_traits<Result (Args...)> {
   using type = function_traits<Result(Args...)>;
 };

 template <typename T>
 using infer_function_traits_t = typename infer_function_traits<T>::type;

 /**
  * Use extract_arg_by_filtered_index to return the i-th argument whose
  * type fulfills a given type trait. The argument itself is perfectly forwarded.
  *
  * Example:
  * std::string arg1 = "Hello";
  * std::string arg2 = "World";
  * std::string&& result = extract_arg_by_filtered_index<is_string, 1>(0, arg1, 2.0, std::move(arg2));
  *
  * Warning: Taking the result by rvalue reference can cause segfaults because ownership will not be passed on
  *          from the original reference. The original reference dies after the expression and the resulting
  */
 namespace detail {
 template<template <class> class Condition, size_t index, class Enable, class... Args> struct extract_arg_by_filtered_index_;
 template<template <class> class Condition, size_t index, class Head, class... Tail>
 struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<!Condition<Head>::value>, Head, Tail...> {
   static decltype(auto) call(Head&& /*head*/, Tail&&... tail) {
     return extract_arg_by_filtered_index_<Condition, index, void, Tail...>::call(std::forward<Tail>(tail)...);
   }
 };
 template<template <class> class Condition, size_t index, class Head, class... Tail>
 struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<Condition<Head>::value && index != 0>, Head, Tail...> {
   static decltype(auto) call(Head&& /*head*/, Tail&&... tail) {
     return extract_arg_by_filtered_index_<Condition, index-1, void, Tail...>::call(std::forward<Tail>(tail)...);
   }
 };
 template<template <class> class Condition, size_t index>
 struct extract_arg_by_filtered_index_<Condition, index, void> {
   static void call() {
     static_assert(index != index, "extract_arg_by_filtered_index out of range.");
   }
 };
 template<template <class> class Condition, size_t index, class Head, class... Tail>
 struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<Condition<Head>::value && index == 0>, Head, Tail...> {
   static decltype(auto) call(Head&& head, Tail&&... /*tail*/) {
     return std::forward<Head>(head);
   }
 };
 }
 template<template <class> class Condition, size_t index, class... Args>
 decltype(auto) extract_arg_by_filtered_index(Args&&... args) {
   static_assert(is_type_condition<Condition>::value, "In extract_arg_by_filtered_index, the Condition argument must be a condition type trait, i.e. have a static constexpr bool ::value member.");
   return detail::extract_arg_by_filtered_index_<Condition, index, void, Args...>::call(std::forward<Args>(args)...);
 }


 /**
  * Use filter_map to map a subset of the arguments to values.
  * The subset is defined by type traits, and will be evaluated at compile time.
  * At runtime, it will just loop over the pre-filtered arguments to create an std::array.
  *
  * Example:
  *  std::array<double, 2> result = filter_map<double, std::is_integral>([] (auto a) {return (double)a;}, 3, "bla", 4);
  *  // result == {3.0, 4.0}
  */
 namespace detail {

 template<class ResultType, size_t num_results> struct filter_map_ {
    template<template <class> class Condition, class Mapper, class... Args, size_t... INDEX>
    static guts::array<ResultType, num_results> call(const Mapper& mapper, std::index_sequence<INDEX...>, Args&&... args) {
      return guts::array<ResultType, num_results> { mapper(extract_arg_by_filtered_index<Condition, INDEX>(std::forward<Args>(args)...))... };
    }
 };
 template<class ResultType> struct filter_map_<ResultType, 0> {
   template<template <class> class Condition, class Mapper, class... Args, size_t... INDEX>
   static guts::array<ResultType, 0> call(const Mapper& /*mapper*/, std::index_sequence<INDEX...>, Args&&... /*args*/) {
     return guts::array<ResultType, 0> { };
   }
 };
 }

 template<class ResultType, template <class> class Condition, class Mapper, class... Args>
 decltype(auto) filter_map(const Mapper& mapper, Args&&... args) {
   static_assert(is_type_condition<Condition>::value, "In filter_map<Result, Condition>, the Condition argument must be a condition type trait, i.e. have a static constexpr bool ::value member.");

   static constexpr size_t num_results = typelist::count_if<Condition, typelist::typelist<Args...>>::value;
   return detail::filter_map_<ResultType, num_results>::template call<Condition, Mapper, Args...>(mapper, std::make_index_sequence<num_results>(), std::forward<Args>(args)...);
 }


 /**
  * make_offset_index_sequence<Start, N>
  * Like make_index_sequence<N>, but starting from Start instead of 0.
  *
  * Example:
  *  make_offset_index_sequence<10, 3> == std::index_sequence<10, 11, 12>
  */
 template<size_t Start, size_t N, size_t... Is>
 struct make_offset_index_sequence_impl
   : make_offset_index_sequence_impl<Start, N - 1, Start + N - 1, Is...>
 {
   static_assert(static_cast<int>(Start) >= 0, "make_offset_index_sequence: Start < 0");
   static_assert(static_cast<int>(N) >= 0, "make_offset_index_sequence: N < 0");
 };

 template<size_t Start, size_t... Is>
 struct make_offset_index_sequence_impl <Start, 0, Is...> {
   typedef std::index_sequence<Is...> type;
 };

 template<size_t Start, size_t N>
 using make_offset_index_sequence = typename make_offset_index_sequence_impl<Start, N>::type;


 /**
  * Use tuple_elements to extract a position-indexed subset of elements
  * from the argument tuple into a result tuple.
  *
  * Example:
  *  std::tuple<int, const char*, double> t = std::make_tuple(0, "HEY", 2.0);
  *  std::tuple<int, double> result = tuple_elements(t, std::index_sequence<0, 2>());
  */
 template <class Tuple, size_t... Is>
 constexpr auto tuple_elements(Tuple t, std::index_sequence<Is...>) {
   return std::tuple<std::tuple_element_t<Is, Tuple>...>(std::get<Is>(t)...);
 }

 /**
  * Use tuple_take to extract the first or last n elements from the argument
  * tuple into a result tuple.
  *
  * Example:
  *  std::tuple<int, const char*, double> t = std::make_tuple(0, "HEY", 2.0);
  *  std::tuple<int, const char*> first_two = tuple_take<decltype(t), 2>(t);
  *  std::tuple<const char*, double> last_two = tuple_take<decltype(t), -2>(t);
  */
 template <class Tuple, int N, class Enable = void>
 struct TupleTake {};

 template <class Tuple, int N>
 struct TupleTake<Tuple, N, std::enable_if_t<N >= 0, void>> {
   static auto call(Tuple t) {
     constexpr size_t size = std::tuple_size<Tuple>();
     static_assert(N <= size, "tuple_take: N > size");
     return tuple_elements(t, std::make_index_sequence<N>{});
   }
 };

 template <class Tuple, int N>
 struct TupleTake<Tuple, N, std::enable_if_t<N < 0, void>> {
   static auto call(Tuple t) {
     constexpr size_t size = std::tuple_size<Tuple>();
     static_assert(-N <= size, "tuple_take: -N > size");
     return tuple_elements(t, make_offset_index_sequence<size + N, -N>{});
   }
 };

 template <class Tuple, int N>
 auto tuple_take(Tuple t) {
   return TupleTake<Tuple, N>::call(t);
 }

 /**
  * Use tuple_slice to extract a contiguous subtuple from the argument.
  *
  * Example:
  *  std::tuple<int, const char*, double, bool> t = std::make_tuple(0, "HEY", 2.0, false);
  *  std::tuple<int, const char*> middle_two = tuple_slice<decltype(t), 1, 2>(t);
  */
 template <class Tuple, size_t Start, size_t N>
 constexpr auto tuple_slice(Tuple t) {
   constexpr size_t size = std::tuple_size<Tuple>();
   static_assert(Start + N <= size, "tuple_slice: Start + N > size");
   return tuple_elements(t, make_offset_index_sequence<Start, N>{});
 }


 /**
  * Use tuple_map to run a mapping function over a tuple to get a new tuple.
  *
  * Example 1:
  *   auto result = tuple_map(std::tuple<int32_t, int32_t, int32_t>(3, 4, 5), [] (int32_t a) -> int16_t {return a+1;});
  *   // result == std::tuple<int16_t, int16_t, int16_t>(4, 5, 6)
  *
  * Example 2:
  *   struct Mapper {
  *     std::string operator()(int32_t a) const {
  *       return std::to_string(a);
  *     }
  *     int64_t operator()(const std::string& a) const {
  *        return atoi(a.c_str());
  *     }
  *   };
  *   auto result = tuple_map(std::tuple<int32_t, std::string>(3, "4"), Mapper());
  *   // result == std::tuple<std::string, int64_t>("3", 4)
  *
  * Example 3:
  *   struct A final {
  *    int32_t func() {
  *      return 5;
  *    }
  *  };
  *  struct B final {
  *    std::string func() {
  *      return "5";
  *    }
  *  };
  *  auto result = tuple_map(std::make_tuple(A(), B()), [] (auto a) { return a.func(); });
  *  // result == std::tuple<int32_t, std::string>(5, "5");
  */
 namespace detail {
   template<class Mapper, class... Args, size_t... Indices>
   auto tuple_map(std::tuple<Args...>&& tuple, const Mapper& mapper, std::index_sequence<Indices...>) {
     return std::tuple<decltype(mapper(std::forward<Args>(std::get<Indices>(tuple))))...>(
       mapper(std::forward<Args>(std::get<Indices>(tuple)))...
     );
   }
 }

 template<class Mapper, class... Args>
 auto tuple_map(std::tuple<Args...>&& tuple, const Mapper& mapper) {
   return detail::tuple_map(std::move(tuple), mapper, std::index_sequence_for<Args...>());
 }


 /**
  * tuple_concat concatenates several tuples into one.
  */

 namespace detail {
   // extract_tuple_element_by_index is a helper that takes a list of tuples and extracts the i-th element
   // in a flattened view of the tuples.
   // Example: extract_tuple_element_by_index<3>(tuple(2,3), tuple(4,5), tuple(6,7)) == 5.

   template<size_t index, class HeadTuple, class... TailTuples, std::enable_if_t<index < std::tuple_size<HeadTuple>::value, int> = 0>
   decltype(auto) extract_tuple_element_by_index(HeadTuple&& head_tuple, TailTuples&&... tail_tuples) {
     // TODO if constexpr instead of enable_if
     return std::get<index>(std::forward<HeadTuple>(head_tuple));
   }

   template<size_t index, class HeadTuple, class... TailTuples, std::enable_if_t<index >= std::tuple_size<HeadTuple>::value, int> = 0>
   decltype(auto) extract_tuple_element_by_index(HeadTuple&& head_tuple, TailTuples&&... tail_tuples) {
     // TODO if constexpr instead of enable_if
     return extract_tuple_element_by_index<index - std::tuple_size<HeadTuple>::value, TailTuples...>(std::forward<TailTuples>(tail_tuples)...);
   }

   static_assert(
     std::is_same<
       int&&,
       decltype(extract_tuple_element_by_index<2>(std::tuple<int32_t>(2), std::tuple<int32_t&&, int32_t>(std::declval<int32_t>(), 3)))
     >::value,
     "extract_tuple_element_by_index should return rvalue references if the tuple contains them. It should not move them into a value"
   );

   template<class ConcatenatedTuple, class... Tuples, size_t... ElementIndices>
   auto tuple_concat(Tuples&&... tuples, std::index_sequence<ElementIndices...>) {
     return ConcatenatedTuple(extract_tuple_element_by_index<ElementIndices>(std::forward<Tuples>(tuples)...)...);
   }
 }

 template<class... Tuples>
   auto tuple_concat(Tuples&&... tuples) {
     using flattened_types = guts::typelist::concat_t<guts::typelist::from_tuple_t<Tuples>...>;
     using concatenated_tuple = guts::typelist::to_tuple_t<flattened_types>;
     constexpr size_t num_elements = guts::typelist::size<flattened_types>::value;
     return detail::tuple_concat<concatenated_tuple, Tuples...>(std::forward<Tuples>(tuples)..., std::make_index_sequence<num_elements>());
   }


 /**
  * Concatenate multiple integer sequences
  * Example:
  *   concat_iseq_t<std::index_sequence<2, 5, 3>, std::index_sequence<4, 2>, std::index_sequence<5>>
  *     == std::index_sequence<2, 5, 3, 4, 2, 5>
  */
 template<class... ISeqs> struct concat_iseq {
   static_assert(false_t<ISeqs...>::value, "In concat_iseq<T1, ...>, the T arguments each must be std::integer_sequence<...> with the same IntType.");
 };
 template<>
 struct concat_iseq<> {
   using type = std::index_sequence<>;
 };
 template<class IntType, IntType... Indices>
 struct concat_iseq<std::integer_sequence<IntType, Indices...>> {
   using type = std::integer_sequence<IntType, Indices...>;
 };
 template<class IntType, IntType... Head1Indices, IntType... Head2Indices, class... TailISeqs>
 struct concat_iseq<std::integer_sequence<IntType, Head1Indices...>, std::integer_sequence<IntType, Head2Indices...>, TailISeqs...> {
     using type = typename concat_iseq<std::integer_sequence<IntType, Head1Indices..., Head2Indices...>, TailISeqs...>::type;
 };
 template<class... ISeqs>
 using concat_iseq_t = typename concat_iseq<ISeqs...>::type;


 }}
	#pragma once

	#include <type_traits>
	#include <array>
	#include <functional>
	#include <c10/util/TypeList.h>
	#include <c10/util/Array.h>

	namespace c10 { namespace guts {

	/**
	* Access information about result type or arguments from a function type.
	* Example:
	* using A = function_traits<int (float, double)>::return_type // A == int
	* using A = function_traits<int (float, double)>::parameter_types::tuple_type // A == tuple<float, double>
	*/
	template<class Func> struct function_traits {
	static_assert(!std::is_same<Func, Func>::value, "In function_traits<Func>, Func must be a plain function type.");
	};
	template<class Result, class... Args>
	struct function_traits<Result (Args...)> {
	using func_type = Result (Args...);
	using return_type = Result;
	using parameter_types = typelist::typelist<Args...>;
	static constexpr auto number_of_parameters = sizeof...(Args);
	};

	/**
	* infer_function_traits: creates a `function_traits` type for a simple
	* function (pointer) or functor (lambda/struct). Currently does not support
	* class methods.
	*/

	template <typename Functor>
	struct infer_function_traits {
	using type = function_traits<c10::guts::detail::strip_class_t<decltype(&Functor::operator())>>;
	};

	template <typename Result, typename... Args>
	struct infer_function_traits<Result (*)(Args...)> {
	using type = function_traits<Result(Args...)>;
	};

	template <typename Result, typename... Args>
	struct infer_function_traits<Result (Args...)> {
	using type = function_traits<Result(Args...)>;
	};

	template <typename T>
	using infer_function_traits_t = typename infer_function_traits<T>::type;

	/**
	* Use extract_arg_by_filtered_index to return the i-th argument whose
	* type fulfills a given type trait. The argument itself is perfectly forwarded.
	*
	* Example:
	* std::string arg1 = "Hello";
	* std::string arg2 = "World";
	* std::string&& result = extract_arg_by_filtered_index<is_string, 1>(0, arg1, 2.0, std::move(arg2));
	*
	* Warning: Taking the result by rvalue reference can cause segfaults because ownership will not be passed on
	* from the original reference. The original reference dies after the expression and the resulting
	*/
	namespace detail {
	template<template <class> class Condition, size_t index, class Enable, class... Args> struct extract_arg_by_filtered_index_;
	template<template <class> class Condition, size_t index, class Head, class... Tail>
	struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<!Condition<Head>::value>, Head, Tail...> {
	static decltype(auto) call(Head&& /head/, Tail&&... tail) {
	return extract_arg_by_filtered_index_<Condition, index, void, Tail...>::call(std::forward<Tail>(tail)...);
	}
	};
	template<template <class> class Condition, size_t index, class Head, class... Tail>
	struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<Condition<Head>::value && index != 0>, Head, Tail...> {
	static decltype(auto) call(Head&& /head/, Tail&&... tail) {
	return extract_arg_by_filtered_index_<Condition, index-1, void, Tail...>::call(std::forward<Tail>(tail)...);
	}
	};
	template<template <class> class Condition, size_t index>
	struct extract_arg_by_filtered_index_<Condition, index, void> {
	static void call() {
	static_assert(index != index, "extract_arg_by_filtered_index out of range.");
	}
	};
	template<template <class> class Condition, size_t index, class Head, class... Tail>
	struct extract_arg_by_filtered_index_<Condition, index, std::enable_if_t<Condition<Head>::value && index == 0>, Head, Tail...> {
	static decltype(auto) call(Head&& head, Tail&&... /tail/) {
	return std::forward<Head>(head);
	}
	};
	}
	template<template <class> class Condition, size_t index, class... Args>
	decltype(auto) extract_arg_by_filtered_index(Args&&... args) {
	static_assert(is_type_condition<Condition>::value, "In extract_arg_by_filtered_index, the Condition argument must be a condition type trait, i.e. have a static constexpr bool ::value member.");
	return detail::extract_arg_by_filtered_index_<Condition, index, void, Args...>::call(std::forward<Args>(args)...);
	}



	/**
	* Use filter_map to map a subset of the arguments to values.
	* The subset is defined by type traits, and will be evaluated at compile time.
	* At runtime, it will just loop over the pre-filtered arguments to create an std::array.
	*
	* Example:
	* std::array<double, 2> result = filter_map<double, std::is_integral>([] (auto a) {return (double)a;}, 3, "bla", 4);
	* // result == {3.0, 4.0}
	*/
	namespace detail {

	template<class ResultType, size_t num_results> struct filter_map_ {
	template<template <class> class Condition, class Mapper, class... Args, size_t... INDEX>
	static guts::array<ResultType, num_results> call(const Mapper& mapper, std::index_sequence<INDEX...>, Args&&... args) {
	return guts::array<ResultType, num_results> { mapper(extract_arg_by_filtered_index<Condition, INDEX>(std::forward<Args>(args)...))... };
	}
	};
	template<class ResultType> struct filter_map_<ResultType, 0> {
	template<template <class> class Condition, class Mapper, class... Args, size_t... INDEX>
	static guts::array<ResultType, 0> call(const Mapper& /mapper/, std::index_sequence<INDEX...>, Args&&... /args/) {
	return guts::array<ResultType, 0> { };
	}
	};
	}

	template<class ResultType, template <class> class Condition, class Mapper, class... Args>
	decltype(auto) filter_map(const Mapper& mapper, Args&&... args) {
	static_assert(is_type_condition<Condition>::value, "In filter_map<Result, Condition>, the Condition argument must be a condition type trait, i.e. have a static constexpr bool ::value member.");

	static constexpr size_t num_results = typelist::count_if<Condition, typelist::typelist<Args...>>::value;
	return detail::filter_map_<ResultType, num_results>::template call<Condition, Mapper, Args...>(mapper, std::make_index_sequence<num_results>(), std::forward<Args>(args)...);
	}


	/**
	* make_offset_index_sequence<Start, N>
	* Like make_index_sequence<N>, but starting from Start instead of 0.
	*
	* Example:
	* make_offset_index_sequence<10, 3> == std::index_sequence<10, 11, 12>
	*/
	template<size_t Start, size_t N, size_t... Is>
	struct make_offset_index_sequence_impl
	: make_offset_index_sequence_impl<Start, N - 1, Start + N - 1, Is...>
	{
	static_assert(static_cast<int>(Start) >= 0, "make_offset_index_sequence: Start < 0");
	static_assert(static_cast<int>(N) >= 0, "make_offset_index_sequence: N < 0");
	};

	template<size_t Start, size_t... Is>
	struct make_offset_index_sequence_impl <Start, 0, Is...> {
	typedef std::index_sequence<Is...> type;
	};

	template<size_t Start, size_t N>
	using make_offset_index_sequence = typename make_offset_index_sequence_impl<Start, N>::type;


	/**
	* Use tuple_elements to extract a position-indexed subset of elements
	* from the argument tuple into a result tuple.
	*
	* Example:
	* std::tuple<int, const char*, double> t = std::make_tuple(0, "HEY", 2.0);
	* std::tuple<int, double> result = tuple_elements(t, std::index_sequence<0, 2>());
	*/
	template <class Tuple, size_t... Is>
	constexpr auto tuple_elements(Tuple t, std::index_sequence<Is...>) {
	return std::tuple<std::tuple_element_t<Is, Tuple>...>(std::get<Is>(t)...);
	}

	/**
	* Use tuple_take to extract the first or last n elements from the argument
	* tuple into a result tuple.
	*
	* Example:
	* std::tuple<int, const char*, double> t = std::make_tuple(0, "HEY", 2.0);
	* std::tuple<int, const char*> first_two = tuple_take<decltype(t), 2>(t);
	* std::tuple<const char*, double> last_two = tuple_take<decltype(t), -2>(t);
	*/
	template <class Tuple, int N, class Enable = void>
	struct TupleTake {};

	template <class Tuple, int N>
	struct TupleTake<Tuple, N, std::enable_if_t<N >= 0, void>> {
	static auto call(Tuple t) {
	constexpr size_t size = std::tuple_size<Tuple>();
	static_assert(N <= size, "tuple_take: N > size");
	return tuple_elements(t, std::make_index_sequence<N>{});
	}
	};

	template <class Tuple, int N>
	struct TupleTake<Tuple, N, std::enable_if_t<N < 0, void>> {
	static auto call(Tuple t) {
	constexpr size_t size = std::tuple_size<Tuple>();
	static_assert(-N <= size, "tuple_take: -N > size");
	return tuple_elements(t, make_offset_index_sequence<size + N, -N>{});
	}
	};

	template <class Tuple, int N>
	auto tuple_take(Tuple t) {
	return TupleTake<Tuple, N>::call(t);
	}

	/**
	* Use tuple_slice to extract a contiguous subtuple from the argument.
	*
	* Example:
	* std::tuple<int, const char*, double, bool> t = std::make_tuple(0, "HEY", 2.0, false);
	* std::tuple<int, const char*> middle_two = tuple_slice<decltype(t), 1, 2>(t);
	*/
	template <class Tuple, size_t Start, size_t N>
	constexpr auto tuple_slice(Tuple t) {
	constexpr size_t size = std::tuple_size<Tuple>();
	static_assert(Start + N <= size, "tuple_slice: Start + N > size");
	return tuple_elements(t, make_offset_index_sequence<Start, N>{});
	}


	/**
	* Use tuple_map to run a mapping function over a tuple to get a new tuple.
	*
	* Example 1:
	* auto result = tuple_map(std::tuple<int32_t, int32_t, int32_t>(3, 4, 5), [] (int32_t a) -> int16_t {return a+1;});
	* // result == std::tuple<int16_t, int16_t, int16_t>(4, 5, 6)
	*
	* Example 2:
	* struct Mapper {
	* std::string operator()(int32_t a) const {
	* return std::to_string(a);
	* }
	* int64_t operator()(const std::string& a) const {
	* return atoi(a.c_str());
	* }
	* };
	* auto result = tuple_map(std::tuple<int32_t, std::string>(3, "4"), Mapper());
	* // result == std::tuple<std::string, int64_t>("3", 4)
	*
	* Example 3:
	* struct A final {
	* int32_t func() {
	* return 5;
	* }
	* };
	* struct B final {
	* std::string func() {
	* return "5";
	* }
	* };
	* auto result = tuple_map(std::make_tuple(A(), B()), [] (auto a) { return a.func(); });
	* // result == std::tuple<int32_t, std::string>(5, "5");
	*/
	namespace detail {
	template<class Mapper, class... Args, size_t... Indices>
	auto tuple_map(std::tuple<Args...>&& tuple, const Mapper& mapper, std::index_sequence<Indices...>) {
	return std::tuple<decltype(mapper(std::forward<Args>(std::get<Indices>(tuple))))...>(
	mapper(std::forward<Args>(std::get<Indices>(tuple)))...
	);
	}
	}

	template<class Mapper, class... Args>
	auto tuple_map(std::tuple<Args...>&& tuple, const Mapper& mapper) {
	return detail::tuple_map(std::move(tuple), mapper, std::index_sequence_for<Args...>());
	}


	/**
	* tuple_concat concatenates several tuples into one.
	*/

	namespace detail {
	// extract_tuple_element_by_index is a helper that takes a list of tuples and extracts the i-th element
	// in a flattened view of the tuples.
	// Example: extract_tuple_element_by_index<3>(tuple(2,3), tuple(4,5), tuple(6,7)) == 5.

	template<size_t index, class HeadTuple, class... TailTuples, std::enable_if_t<index < std::tuple_size<HeadTuple>::value, int> = 0>
	decltype(auto) extract_tuple_element_by_index(HeadTuple&& head_tuple, TailTuples&&... tail_tuples) {
	// TODO if constexpr instead of enable_if
	return std::get<index>(std::forward<HeadTuple>(head_tuple));
	}

	template<size_t index, class HeadTuple, class... TailTuples, std::enable_if_t<index >= std::tuple_size<HeadTuple>::value, int> = 0>
	decltype(auto) extract_tuple_element_by_index(HeadTuple&& head_tuple, TailTuples&&... tail_tuples) {
	// TODO if constexpr instead of enable_if
	return extract_tuple_element_by_index<index - std::tuple_size<HeadTuple>::value, TailTuples...>(std::forward<TailTuples>(tail_tuples)...);
	}

	static_assert(
	std::is_same<
	int&&,
	decltype(extract_tuple_element_by_index<2>(std::tuple<int32_t>(2), std::tuple<int32_t&&, int32_t>(std::declval<int32_t>(), 3)))
	>::value,
	"extract_tuple_element_by_index should return rvalue references if the tuple contains them. It should not move them into a value"
	);

	template<class ConcatenatedTuple, class... Tuples, size_t... ElementIndices>
	auto tuple_concat(Tuples&&... tuples, std::index_sequence<ElementIndices...>) {
	return ConcatenatedTuple(extract_tuple_element_by_index<ElementIndices>(std::forward<Tuples>(tuples)...)...);
	}
	}

	template<class... Tuples>
	auto tuple_concat(Tuples&&... tuples) {
	using flattened_types = guts::typelist::concat_t<guts::typelist::from_tuple_t<Tuples>...>;
	using concatenated_tuple = guts::typelist::to_tuple_t<flattened_types>;
	constexpr size_t num_elements = guts::typelist::size<flattened_types>::value;
	return detail::tuple_concat<concatenated_tuple, Tuples...>(std::forward<Tuples>(tuples)..., std::make_index_sequence<num_elements>());
	}


	/**
	* Concatenate multiple integer sequences
	* Example:
	* concat_iseq_t<std::index_sequence<2, 5, 3>, std::index_sequence<4, 2>, std::index_sequence<5>>
	* == std::index_sequence<2, 5, 3, 4, 2, 5>
	*/
	template<class... ISeqs> struct concat_iseq {
	static_assert(false_t<ISeqs...>::value, "In concat_iseq<T1, ...>, the T arguments each must be std::integer_sequence<...> with the same IntType.");
	};
	template<>
	struct concat_iseq<> {
	using type = std::index_sequence<>;
	};
	template<class IntType, IntType... Indices>
	struct concat_iseq<std::integer_sequence<IntType, Indices...>> {
	using type = std::integer_sequence<IntType, Indices...>;
	};
	template<class IntType, IntType... Head1Indices, IntType... Head2Indices, class... TailISeqs>
	struct concat_iseq<std::integer_sequence<IntType, Head1Indices...>, std::integer_sequence<IntType, Head2Indices...>, TailISeqs...> {
	using type = typename concat_iseq<std::integer_sequence<IntType, Head1Indices..., Head2Indices...>, TailISeqs...>::type;
	};
	template<class... ISeqs>
	using concat_iseq_t = typename concat_iseq<ISeqs...>::type;


	}}