[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 #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #4)
> All traits of kind C currently reject T[] and T[1], but should accept them.

Gah, they do in gcc-12, but not trunk, sorry.

But they do reject incomplete unions, which should be accepted.

And traits of kind A incorrectly allow Incomplete[1].

And traits of kind B incorrectly allow Incomplete[].


Here's a testcase:

struct I;

// A.
// "T shall be a complete type, (possibly cv-qualified) void,"
// "or an array of unknown bound."

void kindA()
{
  // No incomplete object types.
  (void) __is_constructible(I); // { dg-error "incomplete type" "FAILS" }
  (void) __is_constructible(I[1]); // { dg-error "incomplete type" "FAILS" }
  // Except arrays of unknown bound.
  (void) __is_constructible(I[]);
}

// B.
// "remove_all_extents_t<T> shall be a complete type or cv void."

void kindB()
{
  // No incomplete types, including arrays.
  (void) __is_trivial(I); // { dg-error "incomplete type" }
  (void) __is_trivial(I[1]); // { dg-error "incomplete type" }
  (void) __is_trivial(I[]); // { dg-error "incomplete type" "FAILS" }
}

union U;

// C.
// "If T is a non-union class type, T shall be a complete type."

void kindC()
{
  // No incomplete non-union class types.
  (void) __is_empty(I); // { dg-error "incomplete type" }
  // But arrays of incomplete type are OK.
  (void) __is_empty(I[1]); // { dg-bogus "incomplete type" "FAILS" }
  (void) __is_empty(I[]);
  // And incomplete unions are OK.
  (void) __is_empty(U); // { dg-bogus "incomplete type" "FAILS" }
  (void) __is_empty(U[1]); // { dg-bogus "incomplete type" "FAILS" }
  (void) __is_empty(U[]);
}

The dg-error lines marked FAIL are incorrectly accepted. The dg-bogus lines are
incorrectly rejected.