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

Another case where I forgot that non-GCC compilers don't have this
built-in yet.

libstdc++-v3/ChangeLog:

	* include/bits/invoke.h (__invoke_r): Check
	__has_builtin(__reference_converts_from_temporary) before using
	built-in.

diff --git a/libstdc++-v3/include/bits/invoke.h b/libstdc++-v3/include/bits/invoke.h
index 8724a764f7390e42a437753d6855ca166af31ed7..8fa8cf876d8d78106260501156929abedc896818 100644 (file)
--- a/libstdc++-v3/include/bits/invoke.h
+++ b/libstdc++-v3/include/bits/invoke.h
@@ -130,8 +130,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       using __result = __invoke_result<_Callable, _Args...>;
       using __type = typename __result::type;
+#if __has_builtin(__reference_converts_from_temporary)
       static_assert(!__reference_converts_from_temporary(_Res, __type),
                    "INVOKE<R> must not create a dangling reference");
+#endif
       using __tag = typename __result::__invoke_type;
       return std::__invoke_impl<__type>(__tag{}, std::forward<_Callable>(__fn),
                                        std::forward<_Args>(__args)...);