From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 5E93F385841D; Fri, 14 Jan 2022 11:10:21 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 5E93F385841D From: "cvs-commit at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/103991] [12 Regression] Bogus -Wreturn-type with constexpr if and local var with destructor Date: Fri, 14 Jan 2022 11:10:21 +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: 12.0 X-Bugzilla-Keywords: diagnostic X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit at gcc dot gnu.org X-Bugzilla-Status: NEW X-Bugzilla-Resolution: X-Bugzilla-Priority: P1 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: 12.0 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: Fri, 14 Jan 2022 11:10:21 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103991 --- Comment #9 from CVS Commits --- The master branch has been updated by Jakub Jelinek : https://gcc.gnu.org/g:cbf06187d5f246634272e3d2892501563bff3d99 commit r12-6579-gcbf06187d5f246634272e3d2892501563bff3d99 Author: Jakub Jelinek Date: Fri Jan 14 12:09:19 2022 +0100 c++: Avoid some -Wreturn-type false positives with const{expr,eval} if [PR103991] The changes done to genericize_if_stmt in order to improve -Wunreachable-code* warning (which Richi didn't actually commit for GCC 12) are I think fine for normal ifs, but for constexpr if and consteval if we have two competing warnings. The problem is that we replace the non-taken clause (then or else) with void_node and keep the if (cond) { something } else {} or if (cond) {} else { something }; in the IL. This helps -Wunreachable-code*, if something can't fallthru but the non-taken clause can, we don't warn about code after it because it is still (in theory) reachable. But if the non-taken branch can't fallthru, we can get false positive -Wreturn-type warnings (which are enabled by default) if there is nothing after the if and the taken branch can't fallthru either. One possibility to fix this is revert at least temporarily to the previous behavior for constexpr and consteval if, yes, we can get false positive -Wunreachable-code* warnings but the warning isn't present in GCC 12. The patch below implements that for constexpr if which throws its clauses very early (either during parsing or during instantiation), and for consteval if it decides based on block_may_fallthru on the non-taken (for constant evaluation only) clause - if the non-taken branch may fallthru, it does what you did in genericize_if_stmt for consteval if, if it can't fallthru, it uses the older way of pretending there wasn't an if and just replacing it with the taken clause. There are some false positive risks with this though, block_may_fallthru is optimistic and doesn't handle some statements at all (like FOR_STMT, WHILE_STMT, DO_STMT - of course handling those is quite hard). For constexpr if (but perhaps for GCC 13?) we could try to block_may_fallthru before we throw it away and remember it in some flag on the IF_STMT, but am not sure how dangerous would it be to call it on the discarded stmts. Or if it is too dangerous e.g. just remember whether the discarded block of consteval if wasn't present or was empty, in that case assume fallthru, and otherwise assume it can't fallthru (-Wunreachable-code possible false positives). 2022-01-14 Jakub Jelinek PR c++/103991 * cp-objcp-common.c (cxx_block_may_fallthru) : For IF_STMT_CONSTEXPR_P with constant false or true condition only check if the taken clause may fall through. * cp-gimplify.c (genericize_if_stmt): For consteval if, revert to r12-5638^ behavior if then_ block can't fall through. For constexpr if, revert to r12-5638^ behavior. * g++.dg/warn/Wreturn-type-13.C: New test.=