https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106838
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> --- We have three main kinds of trait preconditions. A. "T shall be a complete type, (possibly cv-qualified) void, or an array of unknown bound." This only allows T[1] is T is complete, but always allows T[], even if incomplete. The rationale here is that we can always tell if T[] is constructible, destructible, assignable etc (the answer is always no), but to tell whether T[1] is constructible we need to know about T. B. "remove_all_extents_t<T> shall be a complete type or cv void." This only allows T[] and T[1] when T is complete. The rationale is that we need to know properties of the array element, e.g. to tell whether T[1] is an aggregate, we need to know if T is an aggregate. C. "If T is a non-union class type, T shall be a complete type." This always allows T[] and T[1], even if T is incomplete, and allows incomplete T is T is a union type. The rationale is that these traits are always false for array types, e.g. T[] is not a polymorphic type, even if T is polymorphic. Currently it seems that check_trait_type only supports preconditions of kind A, and since r13-25 it allows all arrays, when it should only allow unbounded arrays. So e.g. it doesn't reject __is_constructible(Incomplete[1]) even though it can't know the answer. I think check_trait_type could be adjusted to handle kinds A and B, by adding a bool parameter, but maybe kind C should be checked by a separate function, as it is quite different.
