[committed] libstdc++: Guard use of new built-in with __has_builtin

[Jonathan Wakely <jwakely@redhat.com>]

This fixes libstdc++ for Clang and other non-GCC compilers.

Tested powerpc64le-linux. Pushed to trunk as r13-2954-gdf7f2736509cfe.

-- >8 --

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.
---
 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);
 
-- 
2.37.3