[Bug libstdc++/72745] Please static assert in std::get to check out of range

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72745

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is harder than you think, because the incomplete type error happens in the
return type, so the function body is never instantiated and adding a static
assertion there doesn't help.