[Bug c++/118230] `std::is_invocable` gives wrong result when the tested function returns `auto`

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118230

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The C++ standard specifies the requirements on hash functions and they must
return std::size_t, so what's the benefit of the reduced return type? Why not
just return size_t?