[libc++] Support more GCC 15 type_traits builtins

GCC 15 has added builtins for various C++ type traits that Clang
already had.  Since __has_builtin(...) now finds these, the #if
branches previously only used for Clang are now used for GCC 15.
However, GCC 15 requires that these builtins only be used in type
aliases, not in template aliases.

These changes follow the model of llvm#81386 where a previous GCC
version added builtins for __remove_cv and __remove_cvref.

Fixed: llvm#137704
Fixed: llvm#117319

Author: frobtech

libcxx/include/__type_traits/add_lvalue_reference.h

@@ -20,8 +20,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if __has_builtin(__add_lvalue_reference)
 
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+struct __add_lvalue_reference_gcc {
+  using type = __add_lvalue_reference(_Tp);
+};
+
+template <class _Tp>
+using __add_lvalue_reference_t _LIBCPP_NODEBUG = typename __add_lvalue_reference_gcc<_Tp>::type;
+#  else
 template <class _Tp>
 using __add_lvalue_reference_t _LIBCPP_NODEBUG = __add_lvalue_reference(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
 
 #else
 

libcxx/include/__type_traits/add_pointer.h

@@ -22,8 +22,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer)
 
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+struct __add_pointer_gcc {
+  using type = __add_pointer(_Tp);
+};
+
+template <class _Tp>
+using __add_pointer_t _LIBCPP_NODEBUG = typename __add_pointer_gcc<_Tp>::type;
+#  else
 template <class _Tp>
 using __add_pointer_t _LIBCPP_NODEBUG = __add_pointer(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
 
 #else
 template <class _Tp, bool = __is_referenceable_v<_Tp> || is_void<_Tp>::value>

libcxx/include/__type_traits/add_rvalue_reference.h

@@ -20,8 +20,18 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if __has_builtin(__add_rvalue_reference)
 
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+struct __add_rvalue_reference_gcc {
+  using type = __add_rvalue_reference(_Tp);
+};
+
+template <class _Tp>
+using __add_rvalue_reference_t _LIBCPP_NODEBUG = typename __add_rvalue_reference_gcc<_Tp>::type;
+#  else
 template <class _Tp>
 using __add_rvalue_reference_t _LIBCPP_NODEBUG = __add_rvalue_reference(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
 
 #else
 

libcxx/include/__type_traits/decay.h

@@ -26,8 +26,18 @@
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if __has_builtin(__decay)
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+struct __decay_gcc {
+  using type = __decay(_Tp);
+};
+
+template <class _Tp>
+using __decay_t _LIBCPP_NODEBUG = typename __decay_gcc<_Tp>::type;
+#  else
 template <class _Tp>
 using __decay_t _LIBCPP_NODEBUG = __decay(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
 
 template <class _Tp>
 struct _LIBCPP_NO_SPECIALIZATIONS decay {

libcxx/include/__type_traits/remove_all_extents.h

@@ -24,8 +24,14 @@ struct _LIBCPP_NO_SPECIALIZATIONS remove_all_extents {
   using type _LIBCPP_NODEBUG = __remove_all_extents(_Tp);
 };
 
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+using __remove_all_extents_t = typename remove_all_extents<_Tp>::type;
+#  else
 template <class _Tp>
 using __remove_all_extents_t _LIBCPP_NODEBUG = __remove_all_extents(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
+
 #else
 template <class _Tp>
 struct remove_all_extents {

libcxx/include/__type_traits/remove_extent.h

@@ -24,8 +24,14 @@ struct _LIBCPP_NO_SPECIALIZATIONS remove_extent {
   using type _LIBCPP_NODEBUG = __remove_extent(_Tp);
 };
 
+#  if defined(_LIBCPP_COMPILER_GCC)
+template <class _Tp>
+using __remove_extent_t = typename remove_extent<_Tp>::type;
+#  else
 template <class _Tp>
 using __remove_extent_t _LIBCPP_NODEBUG = __remove_extent(_Tp);
+#  endif //  defined(_LIBCPP_COMPILER_GCC)
+
 #else
 template <class _Tp>
 struct remove_extent {