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+// 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
+//
+//      http://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/utility:utility is an extension of the <utility> header
+//
+// It provides stand-ins for C++14's std::integer_sequence and
+// related utilities. They are intended to be exactly equivalent.
+//   - integer_sequence<T, Ints...>  == std::integer_sequence<T, Ints...>
+//   - index_sequence<Ints...>       == std::index_sequence<Ints...>
+//   - make_integer_sequence<T, N>   == std::make_integer_sequence<T, N>
+//   - make_index_sequence<N>        == std::make_index_sequence<N>
+//   - index_sequence_for<Ts...>     == std::index_sequence_for<Ts...>
+//
+// It also provides the tag types in_place_t, in_place_type_t, and
+// in_place_index_t, as well as the constant in_place, and constexpr std::move
+// and std::forward impolementations in C++11.
+//
+// References:
+//  http://en.cppreference.com/w/cpp/utility/integer_sequence
+//  http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3658.html
+//
+// Example:
+//   // Unpack a tuple for use as a function call's argument list.
+//
+//   template <typename F, typename Tup, size_t... Is>
+//   auto Impl(F f, const Tup& tup, index_sequence<Is...>)
+//       -> decltype(f(std::get<Is>(tup) ...)) {
+//     return f(std::get<Is>(tup) ...);
+//   }
+//
+//   template <typename Tup>
+//   using TupIdxSeq = make_index_sequence<std::tuple_size<Tup>::value>;
+//
+//   template <typename F, typename Tup>
+//   auto ApplyFromTuple(F f, const Tup& tup)
+//       -> decltype(Impl(f, tup, TupIdxSeq<Tup>{})) {
+//     return Impl(f, tup, TupIdxSeq<Tup>{});
+//   }
+#ifndef ABSL_UTILITY_UTILITY_H_
+#define ABSL_UTILITY_UTILITY_H_
+
+#include <cstddef>
+#include <cstdlib>
+#include <utility>
+
+#include "absl/base/config.h"
+#include "absl/meta/type_traits.h"
+
+namespace absl {
+
+// Equivalent to std::integer_sequence.
+//
+// Function templates can deduce compile-time integer sequences by accepting
+// an argument of integer_sequence<T, Ints...>. This is a common need when
+// working with C++11 variadic templates.
+//
+// T       - the integer type of the sequence elements
+// ...Ints - a parameter pack of T values representing the sequence
+template <typename T, T... Ints>
+struct integer_sequence {
+  using value_type = T;
+  static constexpr size_t size() noexcept { return sizeof...(Ints); }
+};
+
+// Equivalent to std::index_sequence.
+//
+// Alias for an integer_sequence of size_t.
+template <size_t... Ints>
+using index_sequence = integer_sequence<size_t, Ints...>;
+
+namespace utility_internal {
+
+template <typename Seq, size_t SeqSize, size_t Rem>
+struct Extend;
+
+// Note that SeqSize == sizeof...(Ints). It's passed explicitly for efficiency.
+template <typename T, T... Ints, size_t SeqSize>
+struct Extend<integer_sequence<T, Ints...>, SeqSize, 0> {
+  using type = integer_sequence<T, Ints..., (Ints + SeqSize)...>;
+};
+
+template <typename T, T... Ints, size_t SeqSize>
+struct Extend<integer_sequence<T, Ints...>, SeqSize, 1> {
+  using type = integer_sequence<T, Ints..., (Ints + SeqSize)..., 2 * SeqSize>;
+};
+
+// Recursion helper for 'make_integer_sequence<T, N>'.
+// 'Gen<T, N>::type' is an alias for 'integer_sequence<T, 0, 1, ... N-1>'.
+template <typename T, size_t N>
+struct Gen {
+  using type =
+      typename Extend<typename Gen<T, N / 2>::type, N / 2, N % 2>::type;
+};
+
+template <typename T>
+struct Gen<T, 0> {
+  using type = integer_sequence<T>;
+};
+
+}  // namespace utility_internal
+
+// Compile-time sequences of integers
+
+// Equivalent to std::make_integer_sequence.
+//
+// make_integer_sequence<int, N> is integer_sequence<int, 0, 1, ..., N-1>;
+template <typename T, T N>
+using make_integer_sequence = typename utility_internal::Gen<T, N>::type;
+
+// Equivalent to std::make_index_sequence.
+//
+// make_index_sequence<N> is index_sequence<0, 1, ..., N-1>;
+template <size_t N>
+using make_index_sequence = make_integer_sequence<size_t, N>;
+
+// Equivalent to std::index_sequence_for.
+//
+// Convert a typename pack into an index sequence of the same length.
+template <typename... Ts>
+using index_sequence_for = make_index_sequence<sizeof...(Ts)>;
+
+// Tag types
+
+#ifdef ABSL_HAVE_STD_OPTIONAL
+
+using std::in_place_t;
+using std::in_place;
+
+#else  // ABSL_HAVE_STD_OPTIONAL
+
+// Tag type used in order to specify in-place construction, such as with
+// absl::optional.
+struct in_place_t {};
+extern const in_place_t in_place;
+
+#endif  // ABSL_HAVE_STD_OPTIONAL
+
+#ifdef ABSL_HAVE_STD_ANY
+using std::in_place_type_t;
+#else
+// Tag types used for in-place construction when the type to construct needs to
+// be specified, such as with absl::variant and absl::any.
+template <typename T>
+struct in_place_type_t {};
+#endif  // ABSL_HAVE_STD_ANY
+
+template <size_t I>
+struct in_place_index_t {};
+
+// Constexpr move and forward
+
+template <typename T>
+constexpr absl::remove_reference_t<T>&& move(T&& t) noexcept {
+  return static_cast<absl::remove_reference_t<T>&&>(t);
+}
+
+template <typename T>
+constexpr T&& forward(
+    absl::remove_reference_t<T>& t) noexcept {  // NOLINT(runtime/references)
+  return static_cast<T&&>(t);
+}
+
+}  // namespace absl
+
+#endif  // ABSL_UTILITY_UTILITY_H_