From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 776C03858C3A; Thu, 14 Oct 2021 15:22:50 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 776C03858C3A From: "ppalka at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/101263] non-propagating-cache::emplace-deref missing constexpr Date: Thu, 14 Oct 2021 15:22:50 +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: 12.0 X-Bugzilla-Keywords: rejects-valid X-Bugzilla-Severity: normal X-Bugzilla-Who: ppalka at gcc dot gnu.org X-Bugzilla-Status: ASSIGNED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: ppalka at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: 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 List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 14 Oct 2021 15:22:50 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101263 --- Comment #5 from Patrick Palka --- (In reply to Jonathan Wakely from comment #4) > (In reply to Patrick Palka from comment #1) > > We might first need to implement P2231 (for constexpr optional) before = this > > function can be properly constexpr. >=20 > Implemented in r12-4389 Yay, thanks! I thought this would be enough to straightforwardly make emplace-deref constexpr, but I ran into an unrelated wrinkle. During const= expr evaluation we can't use placement new to initialize the contained value directly from *__i and instead have to use the equivalent of construct_at(&value, *__i), but the latter incurs an extra move due to the temporary materialization of *__i, which is contrary to the effects of emplace-deref ([range.nonprop.cache]/1.6): Calls reset(). Then initializes the contained value as if direct-non-list-initializing an object of type T with the argument *i. Am I missing something or is it not possible to perform this direct-init in= the constexpr case?=