[Bug c++/105584] libcxx needs using_if_exist attribute

[vital.had at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105584

--- Comment #8 from Sergey Fedorov <vital.had at gmail dot com> ---
(In reply to Sam James from comment #7)
> Nor could they really be related if it's in libstdc++. Needs to be filed
> separately.

Done: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119647