From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugzilla@gcc.gnu.org>
Received: by sourceware.org (Postfix, from userid 48)
 id 1014F3858C83; Wed, 20 Jul 2022 21:31:57 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 1014F3858C83
From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/106150] [DR 2084] union with more than one variant and
 non-trivial constructor is not accepted
Date: Wed, 20 Jul 2022 21:31:56 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 12.1.0
X-Bugzilla-Keywords: rejects-valid
X-Bugzilla-Severity: normal
X-Bugzilla-Who: redi at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Resolution: 
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Flags: 
X-Bugzilla-Changed-Fields: see_also
Message-ID: <bug-106150-4-PSSPMP3qx2@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-106150-4@http.gcc.gnu.org/bugzilla/>
References: <bug-106150-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-BeenThere: gcc-bugs@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-bugs mailing list <gcc-bugs.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-bugs>,
 <mailto:gcc-bugs-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Wed, 20 Jul 2022 21:31:57 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106150

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://gcc.gnu.org/bugzill
                   |                            |a/show_bug.cgi?id=3D98423

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #3)
> Here is an example which is valid (after
> https://cplusplus.github.io/CWG/issues/2084.html):
> struct S1 {
>     S1();
> };
> struct S {
>     S();
> };
> union U {
>     S s{};
>     S1 s1;
> } u;
>=20
> The check in GCC for this seems to be off, if only the variant s is there,
> GCC (and clang) accepts it.
>=20
> So the full check for the defect report was never really done (and it was
> not even mentioned in the defect report commentary either).

Yes, all of gcc, clang, edg and msvc reject cases like this.

It should not matter that S1 does not have a trivial default ctor, because =
the
default member initializer should make this equivalent to:

union U {
  S s;
  S1 s1;
  U() : s() { }
};

Having to write a user-provided constructor is annoying, because to do it
"right" in a generic std::lib template requires:

  constexpr U() noexcept(is_nothrow_default_constructible_v<S>) requires
default_initializable<S> { }

But that should be approximately how the defaulted default ctor is defined
automatically, without all that verbosity.

This is a dup of PR 98423, where Jakub pointed out the code that needs to
change, and what needs to happen.=