[Bug libstdc++/91243] is_invocable mishandles functions returning indestructible types by value

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91243

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Oh that's not the problem here though. Hmm, maybe the specification of
is_invocable is bad then :-)