[gtest] Use `std::index_sequence` and friends instead of rolling our own

* Applies for `std::index_sequence`, `std::make_index_sequence`, and `std::index_sequence_for` replacing `IndexSequence`, `MakeIndexSequence` and IndexSequenceFor`
* Also deleted implementation helper `DoubleSequence`
* The standard interfaces [have been in the standard library since C++14](https://en.cppreference.com/w/cpp/utility/integer_sequence), which [is the minimum supported C++ version by Google Test](https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md)

PiperOrigin-RevId: 621939977
Change-Id: Id264266f08da66c0fa2a6e6fbb8f86fd3cb3a421

3 files changed, 14 insertions, 64 deletions

diff --git a/googletest/include/gtest/internal/gtest-internal.h b/googletest/include/gtest/internal/gtest-internal.h
index 4661248..cffb8e1 100644
--- a/googletest/include/gtest/internal/gtest-internal.h
+++ b/googletest/include/gtest/internal/gtest-internal.h
@@ -1137,40 +1137,6 @@ class NativeArray {
   void (NativeArray::*clone_)(const Element*, size_t);
 };
 
-// Backport of std::index_sequence.
-template <size_t... Is>
-struct IndexSequence {
-  using type = IndexSequence;
-};
-
-// Double the IndexSequence, and one if plus_one is true.
-template <bool plus_one, typename T, size_t sizeofT>
-struct DoubleSequence;
-template <size_t... I, size_t sizeofT>
-struct DoubleSequence<true, IndexSequence<I...>, sizeofT> {
-  using type = IndexSequence<I..., (sizeofT + I)..., 2 * sizeofT>;
-};
-template <size_t... I, size_t sizeofT>
-struct DoubleSequence<false, IndexSequence<I...>, sizeofT> {
-  using type = IndexSequence<I..., (sizeofT + I)...>;
-};
-
-// Backport of std::make_index_sequence.
-// It uses O(ln(N)) instantiation depth.
-template <size_t N>
-struct MakeIndexSequenceImpl
-    : DoubleSequence<N % 2 == 1, typename MakeIndexSequenceImpl<N / 2>::type,
-                     N / 2>::type {};
-
-template <>
-struct MakeIndexSequenceImpl<0> : IndexSequence<> {};
-
-template <size_t N>
-using MakeIndexSequence = typename MakeIndexSequenceImpl<N>::type;
-
-template <typename... T>
-using IndexSequenceFor = typename MakeIndexSequence<sizeof...(T)>::type;
-
 template <size_t>
 struct Ignore {
   Ignore(...);  // NOLINT
@@ -1179,7 +1145,7 @@ struct Ignore {
 template <typename>
 struct ElemFromListImpl;
 template <size_t... I>
-struct ElemFromListImpl<IndexSequence<I...>> {
+struct ElemFromListImpl<std::index_sequence<I...>> {
   // We make Ignore a template to solve a problem with MSVC.
   // A non-template Ignore would work fine with `decltype(Ignore(I))...`, but
   // MSVC doesn't understand how to deal with that pack expansion.
@@ -1190,9 +1156,8 @@ struct ElemFromListImpl<IndexSequence<I...>> {
 
 template <size_t N, typename... T>
 struct ElemFromList {
-  using type =
-      decltype(ElemFromListImpl<typename MakeIndexSequence<N>::type>::Apply(
-          static_cast<T (*)()>(nullptr)...));
+  using type = decltype(ElemFromListImpl<std::make_index_sequence<N>>::Apply(
+      static_cast<T (*)()>(nullptr)...));
 };
 
 struct FlatTupleConstructTag {};
@@ -1217,9 +1182,9 @@ template <typename Derived, typename Idx>
 struct FlatTupleBase;
 
 template <size_t... Idx, typename... T>
-struct FlatTupleBase<FlatTuple<T...>, IndexSequence<Idx...>>
+struct FlatTupleBase<FlatTuple<T...>, std::index_sequence<Idx...>>
     : FlatTupleElemBase<FlatTuple<T...>, Idx>... {
-  using Indices = IndexSequence<Idx...>;
+  using Indices = std::index_sequence<Idx...>;
   FlatTupleBase() = default;
   template <typename... Args>
   explicit FlatTupleBase(FlatTupleConstructTag, Args&&... args)
@@ -1254,14 +1219,15 @@ struct FlatTupleBase<FlatTuple<T...>, IndexSequence<Idx...>>
 // implementations.
 // FlatTuple and ElemFromList are not recursive and have a fixed depth
 // regardless of T...
-// MakeIndexSequence, on the other hand, it is recursive but with an
+// std::make_index_sequence, on the other hand, it is recursive but with an
 // instantiation depth of O(ln(N)).
 template <typename... T>
 class FlatTuple
     : private FlatTupleBase<FlatTuple<T...>,
-                            typename MakeIndexSequence<sizeof...(T)>::type> {
-  using Indices = typename FlatTupleBase<
-      FlatTuple<T...>, typename MakeIndexSequence<sizeof...(T)>::type>::Indices;
+                            std::make_index_sequence<sizeof...(T)>> {
+  using Indices =
+      typename FlatTupleBase<FlatTuple<T...>,
+                             std::make_index_sequence<sizeof...(T)>>::Indices;
 
  public:
   FlatTuple() = default;
diff --git a/googletest/include/gtest/internal/gtest-param-util.h b/googletest/include/gtest/internal/gtest-param-util.h
index 1fc500f..cc7ea53 100644
--- a/googletest/include/gtest/internal/gtest-param-util.h
+++ b/googletest/include/gtest/internal/gtest-param-util.h
@@ -807,12 +807,12 @@ class ValueArray {
 
   template <typename T>
   operator ParamGenerator<T>() const {  // NOLINT
-    return ValuesIn(MakeVector<T>(MakeIndexSequence<sizeof...(Ts)>()));
+    return ValuesIn(MakeVector<T>(std::make_index_sequence<sizeof...(Ts)>()));
   }
 
  private:
   template <typename T, size_t... I>
-  std::vector<T> MakeVector(IndexSequence<I...>) const {
+  std::vector<T> MakeVector(std::index_sequence<I...>) const {
     return std::vector<T>{static_cast<T>(v_.template Get<I>())...};
   }
 
@@ -842,7 +842,7 @@ class CartesianProductGenerator
   template <class I>
   class IteratorImpl;
   template <size_t... I>
-  class IteratorImpl<IndexSequence<I...>>
+  class IteratorImpl<std::index_sequence<I...>>
       : public ParamIteratorInterface<ParamType> {
    public:
     IteratorImpl(const ParamGeneratorInterface<ParamType>* base,
@@ -933,7 +933,7 @@ class CartesianProductGenerator
     std::shared_ptr<ParamType> current_value_;
   };
 
-  using Iterator = IteratorImpl<typename MakeIndexSequence<sizeof...(T)>::type>;
+  using Iterator = IteratorImpl<std::make_index_sequence<sizeof...(T)>>;
 
   std::tuple<ParamGenerator<T>...> generators_;
 };
diff --git a/googletest/test/gtest_unittest.cc b/googletest/test/gtest_unittest.cc
index 5d7cfb0..edbe2ea 100644
--- a/googletest/test/gtest_unittest.cc
+++ b/googletest/test/gtest_unittest.cc
@@ -7482,22 +7482,6 @@ TEST(NativeArrayTest, WorksForTwoDimensionalArray) {
   EXPECT_EQ(a, na.begin());
 }
 
-// IndexSequence
-TEST(IndexSequence, MakeIndexSequence) {
-  using testing::internal::IndexSequence;
-  using testing::internal::MakeIndexSequence;
-  EXPECT_TRUE(
-      (std::is_same<IndexSequence<>, MakeIndexSequence<0>::type>::value));
-  EXPECT_TRUE(
-      (std::is_same<IndexSequence<0>, MakeIndexSequence<1>::type>::value));
-  EXPECT_TRUE(
-      (std::is_same<IndexSequence<0, 1>, MakeIndexSequence<2>::type>::value));
-  EXPECT_TRUE((
-      std::is_same<IndexSequence<0, 1, 2>, MakeIndexSequence<3>::type>::value));
-  EXPECT_TRUE(
-      (std::is_base_of<IndexSequence<0, 1, 2>, MakeIndexSequence<3>>::value));
-}
-
 // ElemFromList
 TEST(ElemFromList, Basic) {
   using testing::internal::ElemFromList;