[Bug libstdc++/91456] std::function and std::is_invocable_r do not understand guaranteed elision

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91456

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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   Target Milestone|---                         |9.3

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Fixed on trunk so far, but I'd like to backport this for 9.3 too.