https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70528
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> --- I don't know how to implement is_default_constructible without using that decltype, or something similar that will cause the same problem. If we had a __is_constructible(_Tp, _Args...) builtin we could use that (for is_constructible, and then define is_default_constructible in terms of that).
