https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100667
--- Comment #9 from Jonathan Wakely <redi at gcc dot gnu.org> --- The changes in https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1285r0.pdf mean that it's only undefined if the result of is_constructible_v<T&&, T> would change were T completed. So there's no benefit to enforcing the completeness requirement here, it's not UB anyway.
