[Bug c++/106838] Built-in traits have wrong preconditions

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.