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 16EC53858001; Tue, 26 Jan 2021 18:56:28 +0000 (GMT)
DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 16EC53858001
From: "gcc@nicholas-schwab.de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/98842] optional's spaceship operations generates
 wrong code when operator== is not present
Date: Tue, 26 Jan 2021 18:56:27 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: libstdc++
X-Bugzilla-Version: 11.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: gcc@nicholas-schwab.de
X-Bugzilla-Status: UNCONFIRMED
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: cc
Message-ID: <bug-98842-4-lKcaqhv9nf@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-98842-4@http.gcc.gnu.org/bugzilla/>
References: <bug-98842-4@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: Tue, 26 Jan 2021 18:56:28 -0000

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98842

gcc@nicholas-schwab.de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |gcc@nicholas-schwab.de

--- Comment #1 from gcc@nicholas-schwab.de ---
Three problem lies in the concept three_way_comparable_with. It uses
__detail::__weakly_eq_cmp_with that requires u =3D=3D t. Due to paper P1185
operator=3D=3D will not lookup operator<=3D>. Therefore having only operato=
r<=3D>
defined does not suffice to fulfill __detail::__weakly_eq_cmp. Hence the fi=
rst
of these two templates is SFINAE'D out and the second is taken.=20

   template<typename _Tp, three_way_comparable_with<_Tp> _Up>
    constexpr compare_three_way_result_t<_Tp, _Up>
    operator<=3D>(const optional<_Tp>& __x, const optional<_Up>& __y)

  template<typename _Tp, typename _Up>
    constexpr compare_three_way_result_t<_Tp, _Up>
    operator<=3D>(const optional<_Tp>& __x, const _Up& __v)=20

The second will however compare false whenever the left hand side is empty.=
=20

The standard requires that three_way_comparable_with also requires
weakly_equality_comparable. Hence the implementation of this concept is
correct. However this might be a defect in the standard.=20

Note that the standard also wants three_way_comparable_with<_Tp> for the se=
cond
template (http://eel.is/c++draft/optional#comp.with.t-25). So the second
template above is non-confirming. But it seems to me that a comparison betw=
een
two optionals then is always ambiguous, so this should be a defect.=