[Bug libstdc++/93923] [10 Regression] std::is_copy_constructible raises compilation error

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93923

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Sorry, that wasn't a correct fix, I missed a ! in the variable template, and
adding that means it gives an error again.

I still think GCC is right to reject the program, because it has an
unresolvable cycle where is_copy_constructible<A> depends on
is_constructible<A, const A&> which is obviously recursive.