[Bug libstdc++/103923] is_invocable inexplicably fails

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103923

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I think libc++ just always caches the hash code, which keeps things much
simpler.