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From: "de34 at live dot cn" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104264] New: __is_trivially_copyable fails to report
 non-recursively trivially copyable classes
Date: Fri, 28 Jan 2022 03:34:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104264

            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() =3D default;
    Weird(const Weird&) =3D default;
    Weird &operator=3D(const Weird&) =3D default;
    // non-trivial
    constexpr Weird(Weird&&) noexcept {}
    constexpr Weird &operator=3D(Weird&&) noexcept { return *this; }
};

struct Supressor {
    Weird w;

    Supressor() =3D default;
    Supressor(const Supressor&) =3D default;
    Supressor &operator=3D(const Supressor&) =3D 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,=