[Bug c++/92067] __is_constructible(incomplete_type) should make the program ill-formed

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92067

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We need to be careful about this in SFINAE contexts. It can't be a hard error,
because it's extremely common for constructors to be constrained with
std::is_constructible, and for overload resolution to consider constructors for
incomplete types.