[Bug c++/106838] New: 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

            Bug ID: 106838
           Summary: Built-in traits have wrong preconditions
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

According to https://gcc.gnu.org/onlinedocs/gcc/Type-Traits.html the
precondition for __has_virtual_destructor is:

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

This seems to be what the implementation actually requires:

union Inc;
static_assert( !__has_virtual_destructor(Inc));
struct Inc2;
static_assert( !__has_virtual_destructor(Inc2[1]) );


inc.C:2:45: error: invalid use of incomplete type 'union Inc'
    2 | static_assert( !__has_virtual_destructor(Inc));
      |                                             ^
inc.C:1:7: note: forward declaration of 'union Inc'
    1 | union Inc;
      |       ^~~
inc.C:4:49: error: invalid use of incomplete type 'struct Inc2'
    4 | static_assert( !__has_virtual_destructor(Inc2[1]) );
      |                                                 ^
inc.C:3:8: note: forward declaration of 'struct Inc2'
    3 | struct Inc2;
      |        ^~~~


However, the precondition for the std::has_virtual_destructor trait is
different:

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

So std::has_virtual_destructor_v<Inc> and
std::has_virtual_destructor_v<Inc2[1]> should both be false. But because the
library trait just uses __has_virtual_destructor, those are ill-formed.

Should the built-in (and its documentation) be adjusted to match the library's
requirements?

Clang gives the same errors as GCC. EDG rejects the incomplete union, but
allows the array of incomplete type. This suggests that the library trait needs
to avoid using the built-in for those types, even if GCC's built-in is changed
to support it.