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 --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits
index 22c1af2..a015fd9 100644 (file)
--- 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);