https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71579
--- Comment #21 from Jonathan Wakely <redi at gcc dot gnu.org> --- The precondition for has_virtual_destructor is: "If T is a non-union class type, T shall be a complete type." Our type trait and the __has_virtual_destructor built-in both seem to get this wrong, rejecting Incomplete[2], which is not a class type, and so doesn't need to be complete (or maybe the precondition is wrong and there's a library defect?)
