From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 26603 invoked by alias); 9 Apr 2012 21:18:28 -0000 Received: (qmail 26591 invoked by uid 22791); 9 Apr 2012 21:18:26 -0000 X-SWARE-Spam-Status: No, hits=-4.3 required=5.0 tests=ALL_TRUSTED,AWL,BAYES_00,KHOP_THREADED X-Spam-Check-By: sourceware.org Received: from localhost (HELO gcc.gnu.org) (127.0.0.1) by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Mon, 09 Apr 2012 21:18:13 +0000 From: "redi at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug libstdc++/52917] [DR 2048] explicitly stated return type in std::mem_fn cannot be compiled Date: Mon, 09 Apr 2012 21:18:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: libstdc++ X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: redi at gcc dot gnu.org X-Bugzilla-Status: SUSPENDED X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Changed-Fields: Status Last reconfirmed Summary Ever Confirmed Message-ID: In-Reply-To: References: X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated Content-Type: text/plain; charset="UTF-8" MIME-Version: 1.0 Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm Precedence: bulk List-Id: List-Archive: List-Post: List-Help: Sender: gcc-bugs-owner@gcc.gnu.org X-SW-Source: 2012-04/txt/msg00571.txt.bz2 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52917 Jonathan Wakely changed: What |Removed |Added ---------------------------------------------------------------------------- Status|UNCONFIRMED |SUSPENDED Last reconfirmed| |2012-04-09 Summary|explicitly stated return |[DR 2048] explicitly stated |type in std::mem_fn cannot |return type in std::mem_fn |be compiled |cannot be compiled Ever Confirmed|0 |1 --- Comment #1 from Jonathan Wakely 2012-04-09 21:18:11 UTC --- (In reply to comment #0) > (2) needs the return type to diffrentiate between the const and the non-const > getter. Or you can do auto y = std::mem_fn((int& (X::*)())&X::get ); Which also works in this case where specifying the return type doesn't help: struct X { int get() { return 0; } int get() const { return 1; } }; > The specification for mem_fn is > > template< class R, class T > /*unspecified*/ mem_fn(R T::* pm); > template< class R, class T, class... Args > /*unspecified*/ mem_fn(R (T::* > pm)(Args...)); > [ ... some more overloads for const, volatile and references] Maybe not for long: http://cplusplus.github.com/LWG/lwg-active.html#2048 The libstdc++ implementation exactly matches that issue's proposed resolution, which has been voted Tentatively Ready. > (2) should result in an instantiation of the second version with > sizeof...(Args)==0, because argument to the first version is a "pointer to > member data", not "pointer to member function". No, it's a pointer to member, which can match either a pointer to member data or pointer to member function. If the second overload exists (as it presumably does in libc++) then it's a better match than the first overload, but the first overload is still viable. > Both g++-4.7 and clang++-3.1 (when compiling with libstdc++) try to instantiate > the first version by mistake. Not by mistake, by design, that overload can match any pointer to member. I'm suspending this because I believe the resolution to DR 2048 would make libstdc++ correct.