[Bug c++/104264] New: __is_trivially_copyable fails to report non-recursively trivially copyable classes

[Bug c++/104264] New: __is_trivially_copyable fails to report non-recursively trivially copyable classes

[de34 at live dot cn]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104264

            Bug ID: 104264
           Summary: __is_trivially_copyable fails to report
                    non-recursively trivially copyable classes
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: de34 at live dot cn
  Target Milestone: ---

The current definition of trivially copyable class ([class.prop]
/2) is not recursive. I.e. a trivially copyable class may have a base class or
data member of a non-trivially copyable class type.

The following example is accepted by clang, msvc, and icc, but rejected by gcc
(all of which use __is_trivially_copyable). Perhaps gcc considers Supressor as
non-trivially copyable because it has a non-trivially copyable data member.

struct Weird {
    Weird() = default;
    Weird(const Weird&) = default;
    Weird &operator=(const Weird&) = default;
    // non-trivial
    constexpr Weird(Weird&&) noexcept {}
    constexpr Weird &operator=(Weird&&) noexcept { return *this; }
};

struct Supressor {
    Weird w;

    Supressor() = default;
    Supressor(const Supressor&) = default;
    Supressor &operator=(const Supressor&) = default;
    // move functions are supressed
    // non-trivial move functions are not called
};

int main()
{
    static_assert(!__is_trivially_copyable(Weird), "");
    static_assert(__is_trivially_copyable(Supressor), "");
}

Compiler Explorer link: https://gcc.godbolt.org/z/4o1vPdz86

The issue might be related to CWG 2463 (https://wg21.link/cwg2463). CWG DR 1734
seems unrelated,

[Bug c++/104264] __is_trivially_copyable fails to report non-recursively trivially copyable classes

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104264

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |jason at gcc dot gnu.org
                 CC|                            |jason at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2023-09-23