From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id 836C53984012; Thu, 22 Oct 2020 11:34:27 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 836C53984012 From: "cvs-commit at gcc dot gnu.org" To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/96575] std::ranges::sort is not usable as a 'constexpr' function when saving its return value in lambda function Date: Thu, 22 Oct 2020 11:34:27 +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: 11.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: cvs-commit 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, 22 Oct 2020 11:34:27 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D96575 --- Comment #3 from CVS Commits --- The master branch has been updated by Patrick Palka : https://gcc.gnu.org/g:46fdced6a9f936ae4d5b42347d7d87f69875683a commit r11-4230-g46fdced6a9f936ae4d5b42347d7d87f69875683a Author: Patrick Palka Date: Thu Oct 22 07:33:58 2020 -0400 c++: constexpr evaluation and bare EMPTY_CLASS_EXPR [PR96575] In the testcase below, folding of the initializer for 'ret' inside the instantiated f::lambda ends up yielding an initializer for which potential_constant_expression returns false. This causes finish_functi= on to mark the lambda as non-constexpr, which ultimately causes us to reje= ct 'f(g)' as a call to a non-constexpr function. The initializer for 'ret' inside f::lambda, prior to folding, is the CALL_EXPR ::operator() (&cb, ({}, <<< Unknown tree: empty_class_expr >>>;)) where the second argument is a COMPOUND_EXPR whose second operand is an EMPTY_CLASS_EXPR that was formed by build_class_a. cp_fully_fold_init is able to only partially fold this initializer: it gets rid of the side-effectless COMPOUND_EXPR to obtain ::operator() (&cb, <<< Unknown tree: empty_class_expr >>>) as the final initializer for 'ret'. This initializer no longer satifies potential_constant_expression due to the bare EMPTY_CLASS_EXPR which is not wrapped in a COMPOUND_EXPR. (cp_fully_fold_init first tries maybe_constant_value on the original CALL_EXPR, but constexpr evaluation punts upon seeing __builtin_is_constant_evaluated, since manifestly_const_eval is false.) To fix this, it seems we could either make cp_fold preserve the COMPOUND_EXPR trees produced by build_call_a, or we could improve the constexpr machinery to treat EMPTY_CLASS_EXPR trees as first-class citizens. Assuming it's safe to continue folding away these COMPOUND_EXPRs, the second approach seems cleaner, so this patch implements the second approach. gcc/cp/ChangeLog: PR c++/96575 * constexpr.c (cxx_eval_constant_expression) : Lower it to a CONSTRUCTOR. (potential_constant_expression_1) : Remove now-redundant handling of COMPOUND_EXPR with EMPTY_CLASS_EXPR second operand. : Return true instead of false. gcc/testsuite/ChangeLog: PR c++/96575 * g++.dg/cpp1z/constexpr-96575.C: New test.=