[Bug libstdc++/71579] type_traits miss checks for type completeness in some traits

[antoshkka at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71579

--- Comment #6 from Antony Polukhin <antoshkka at gmail dot com> ---
C++ LWG related issue: http://cplusplus.github.io/LWG/lwg-active.html#2797