[gcc r13-2954] libstdc++: Guard use of new built-in with __has_builtin

[Jonathan Wakely]

https://gcc.gnu.org/g:df7f2736509cfe5e1dd7d9f263355c5043347a9c

commit r13-2954-gdf7f2736509cfe5e1dd7d9f263355c5043347a9c
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Sep 29 11:30:05 2022 +0100

    libstdc++: Guard use of new built-in with __has_builtin
    
    I forgot that non-GCC compilers don't have this built-in yet.
    
    For Clang we could do something like the check below (as described in
    P2255), but for now I'm just fixing the regression.
    
     #if __has_builtin((__reference_binds_to_temporary)
      bool _Dangle = __reference_binds_to_temporary(_Tp, _Res_t)
                     && __and_<is_reference<_Tp>,
                               __not_<is_reference<_Res_t>>,
                               is_convertible<__remove_cvref_t<_Res_t>*,
                                              __remove_cvref_t<_Tp>*>>::value
     #endif
    
    libstdc++-v3/ChangeLog:
    
            * include/std/type_traits (__is_invocable_impl): Check
            __has_builtin(__reference_converts_from_temporary) before using
            built-in.

Diff:
---
 libstdc++-v3/include/std/type_traits | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 22c1af26397..a015fd95a71 100644
--- a/libstdc++-v3/include/std/type_traits
+++ b/libstdc++-v3/include/std/type_traits
@@ -2901,7 +2901,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       template<typename _Tp,
 	       bool _Nothrow = noexcept(_S_conv<_Tp>(_S_get())),
 	       typename = decltype(_S_conv<_Tp>(_S_get())),
-	       bool _Dangle = __reference_converts_from_temporary(_Tp, _Res_t)>
+#if __has_builtin(__reference_converts_from_temporary)
+	       bool _Dangle = __reference_converts_from_temporary(_Tp, _Res_t)
+#else
+	       bool _Dangle = false
+#endif
+	      >
 	static __bool_constant<_Nothrow && !_Dangle>
 	_S_test(int);