[libc++] Work around new GCC 15 type_traits builtins that can't be used as Clang's can (#137871)

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.

For now, just don't use the `__has_builtin(...)` branches under newer
GCC versions, so both 14 and 15 work during the transition.  This
can be cleaned up later to use all the GCC 15 builtins available.

Fixed: #137704
Fixed: #117319

Author: frobtech

libcxx/include/__type_traits/add_lvalue_reference.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__add_lvalue_reference)
+#if __has_builtin(__add_lvalue_reference) && !defined(_LIBCPP_COMPILER_GCC)
 
 template <class _Tp>
 using __add_lvalue_reference_t _LIBCPP_NODEBUG = __add_lvalue_reference(_Tp);

libcxx/include/__type_traits/add_pointer.h

@@ -20,7 +20,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer)
+#if !defined(_LIBCPP_WORKAROUND_OBJCXX_COMPILER_INTRINSICS) && __has_builtin(__add_pointer) && !defined(_LIBCPP_COMPILER_GCC)
 
 template <class _Tp>
 using __add_pointer_t _LIBCPP_NODEBUG = __add_pointer(_Tp);

libcxx/include/__type_traits/add_rvalue_reference.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__add_rvalue_reference)
+#if __has_builtin(__add_rvalue_reference) && !defined(_LIBCPP_COMPILER_GCC)
 
 template <class _Tp>
 using __add_rvalue_reference_t _LIBCPP_NODEBUG = __add_rvalue_reference(_Tp);

libcxx/include/__type_traits/decay.h

@@ -25,7 +25,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__decay)
+#if __has_builtin(__decay) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 using __decay_t _LIBCPP_NODEBUG = __decay(_Tp);
 

libcxx/include/__type_traits/remove_all_extents.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__remove_all_extents)
+#if __has_builtin(__remove_all_extents) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 struct _LIBCPP_NO_SPECIALIZATIONS remove_all_extents {
   using type _LIBCPP_NODEBUG = __remove_all_extents(_Tp);

libcxx/include/__type_traits/remove_extent.h

@@ -18,7 +18,7 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if __has_builtin(__remove_extent)
+#if __has_builtin(__remove_extent) && !defined(_LIBCPP_COMPILER_GCC)
 template <class _Tp>
 struct _LIBCPP_NO_SPECIALIZATIONS remove_extent {
   using type _LIBCPP_NODEBUG = __remove_extent(_Tp);