[libc++] Use __integer_pack to implement integer_sequence on GCC (#82983)

This significantly simplifies the implementation.

Author: philnik777

libcxx/include/__utility/integer_sequence.h

@@ -31,65 +31,16 @@ struct __integer_sequence {
   using __to_tuple_indices = __tuple_indices<(_Values + _Sp)...>;
 };
 
-#if !__has_builtin(__make_integer_seq) || defined(_LIBCPP_TESTING_FALLBACK_MAKE_INTEGER_SEQUENCE)
-
-namespace __detail {
-
-template <typename _Tp, size_t... _Extra>
-struct __repeat;
-template <typename _Tp, _Tp... _Np, size_t... _Extra>
-struct __repeat<__integer_sequence<_Tp, _Np...>, _Extra...> {
-  typedef _LIBCPP_NODEBUG __integer_sequence<
-      _Tp,
-      _Np...,
-      sizeof...(_Np) + _Np...,
-      2 * sizeof...(_Np) + _Np...,
-      3 * sizeof...(_Np) + _Np...,
-      4 * sizeof...(_Np) + _Np...,
-      5 * sizeof...(_Np) + _Np...,
-      6 * sizeof...(_Np) + _Np...,
-      7 * sizeof...(_Np) + _Np...,
-      _Extra...>
-      type;
-};
-
-template <size_t _Np>
-struct __parity;
-template <size_t _Np>
-struct __make : __parity<_Np % 8>::template __pmake<_Np> {};
-
-// clang-format off
-template<> struct __make<0> { typedef __integer_sequence<size_t> type; };
-template<> struct __make<1> { typedef __integer_sequence<size_t, 0> type; };
-template<> struct __make<2> { typedef __integer_sequence<size_t, 0, 1> type; };
-template<> struct __make<3> { typedef __integer_sequence<size_t, 0, 1, 2> type; };
-template<> struct __make<4> { typedef __integer_sequence<size_t, 0, 1, 2, 3> type; };
-template<> struct __make<5> { typedef __integer_sequence<size_t, 0, 1, 2, 3, 4> type; };
-template<> struct __make<6> { typedef __integer_sequence<size_t, 0, 1, 2, 3, 4, 5> type; };
-template<> struct __make<7> { typedef __integer_sequence<size_t, 0, 1, 2, 3, 4, 5, 6> type; };
-
-template<> struct __parity<0> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type> {}; };
-template<> struct __parity<1> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 1> {}; };
-template<> struct __parity<2> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 2, _Np - 1> {}; };
-template<> struct __parity<3> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 3, _Np - 2, _Np - 1> {}; };
-template<> struct __parity<4> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 4, _Np - 3, _Np - 2, _Np - 1> {}; };
-template<> struct __parity<5> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 5, _Np - 4, _Np - 3, _Np - 2, _Np - 1> {}; };
-template<> struct __parity<6> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 6, _Np - 5, _Np - 4, _Np - 3, _Np - 2, _Np - 1> {}; };
-template<> struct __parity<7> { template<size_t _Np> struct __pmake : __repeat<typename __make<_Np / 8>::type, _Np - 7, _Np - 6, _Np - 5, _Np - 4, _Np - 3, _Np - 2, _Np - 1> {}; };
-// clang-format on
-
-} // namespace __detail
-
-#endif
-
 #if __has_builtin(__make_integer_seq)
 template <size_t _Ep, size_t _Sp>
 using __make_indices_imp =
     typename __make_integer_seq<__integer_sequence, size_t, _Ep - _Sp>::template __to_tuple_indices<_Sp>;
-#else
+#elif __has_builtin(__integer_pack)
 template <size_t _Ep, size_t _Sp>
-using __make_indices_imp = typename __detail::__make<_Ep - _Sp>::type::template __to_tuple_indices<_Sp>;
-
+using __make_indices_imp =
+    typename __integer_sequence<size_t, __integer_pack(_Ep - _Sp)...>::template __to_tuple_indices<_Sp>;
+#else
+#  error "No known way to get an integer pack from the compiler"
 #endif
 
 #if _LIBCPP_STD_VER >= 14
@@ -104,34 +55,20 @@ struct _LIBCPP_TEMPLATE_VIS integer_sequence {
 template <size_t... _Ip>
 using index_sequence = integer_sequence<size_t, _Ip...>;
 
-#  if __has_builtin(__make_integer_seq) && !defined(_LIBCPP_TESTING_FALLBACK_MAKE_INTEGER_SEQUENCE)
+#  if __has_builtin(__make_integer_seq)
 
 template <class _Tp, _Tp _Ep>
-using __make_integer_sequence _LIBCPP_NODEBUG = __make_integer_seq<integer_sequence, _Tp, _Ep>;
-
-#  else
+using make_integer_sequence _LIBCPP_NODEBUG = __make_integer_seq<integer_sequence, _Tp, _Ep>;
 
-template <typename _Tp, _Tp _Np>
-using __make_integer_sequence_unchecked _LIBCPP_NODEBUG =
-    typename __detail::__make<_Np>::type::template __convert<integer_sequence, _Tp>;
+#  elif __has_builtin(__integer_pack)
 
-template <class _Tp, _Tp _Ep>
-struct __make_integer_sequence_checked {
-  static_assert(is_integral<_Tp>::value, "std::make_integer_sequence can only be instantiated with an integral type");
-  static_assert(0 <= _Ep, "std::make_integer_sequence must have a non-negative sequence length");
-  // Workaround GCC bug by preventing bad installations when 0 <= _Ep
-  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68929
-  typedef _LIBCPP_NODEBUG __make_integer_sequence_unchecked<_Tp, 0 <= _Ep ? _Ep : 0> type;
-};
-
-template <class _Tp, _Tp _Ep>
-using __make_integer_sequence _LIBCPP_NODEBUG = typename __make_integer_sequence_checked<_Tp, _Ep>::type;
+template <class _Tp, _Tp _SequenceSize>
+using make_integer_sequence _LIBCPP_NODEBUG = integer_sequence<_Tp, __integer_pack(_SequenceSize)...>;
 
+#  else
+#    error "No known way to get an integer pack from the compiler"
 #  endif
 
-template <class _Tp, _Tp _Np>
-using make_integer_sequence = __make_integer_sequence<_Tp, _Np>;
-
 template <size_t _Np>
 using make_index_sequence = make_integer_sequence<size_t, _Np>;