[PATCH] c++: Enable __has_builtin for new reference binding built-ins

[Jonathan Wakely via Gcc-patches]

Tested x86_64-linux, OK for trunk?

-- >8 --

The new built-ins need to be detectable using __has_builtin, and the
library should use that to check for them.

This fixes an error with Clang when C++23 is enabled.

gcc/cp/ChangeLog:

        * cp-objcp-common.cc (names_builtin_p): Return true for
        RID_REF_CONSTRUCTS_FROM_TEMPORARY and
        RID_REF_CONVERTS_FROM_TEMPORARY.

libstdc++-v3/ChangeLog:

        * include/std/type_traits (__cpp_lib_reference_from_temporary)
        (reference_constructs_from_temporary)
        (reference_converts_from_temporary): Only define when the
        built-ins are available.
---
 gcc/cp/cp-objcp-common.cc            | 2 ++
 libstdc++-v3/include/std/type_traits | 4 ++++
 2 files changed, 6 insertions(+)

diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
index 0b70d5567e4..4079a4b4aec 100644
--- a/gcc/cp/cp-objcp-common.cc
+++ b/gcc/cp/cp-objcp-common.cc
@@ -461,6 +461,8 @@ names_builtin_p (const char *name)
     case RID_IS_ASSIGNABLE:
     case RID_IS_CONSTRUCTIBLE:
     case RID_UNDERLYING_TYPE:
+    case RID_REF_CONSTRUCTS_FROM_TEMPORARY:
+    case RID_REF_CONVERTS_FROM_TEMPORARY:
       return true;
     default:
       break;
diff --git a/libstdc++-v3/include/std/type_traits 
b/libstdc++-v3/include/std/type_traits
index b1a1deecf66..14b029cec64 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -3505,6 +3505,9 @@ template<typename _Ret, typename _Fn, typename... _Args>
   template<typename _Tp>
     inline constexpr bool is_scoped_enum_v = is_scoped_enum<_Tp>::value;
 
+#if __has_builtin(__reference_constructs_from_temporary) \
+  && __has_builtin(__reference_converts_from_temporary)
+
 #define __cpp_lib_reference_from_temporary 202202L
 
   /// True if _Tp is a reference type, a _Up value can be bound to _Tp in
@@ -3544,6 +3547,7 @@ template<typename _Ret, typename _Fn, typename... _Args>
   template<typename _Tp, typename _Up>
     inline constexpr bool reference_converts_from_temporary_v
       = reference_converts_from_temporary<_Tp, _Up>::value;
+#endif // __has_builtin for reference_from_temporary
 #endif // C++23
 
 #if _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
-- 
2.34.3