[Bug libstdc++/70472] is_copy_constructible>>::value is true

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70472

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, I know how to do it, that doesn't mean we can do so easily in existing
types.