From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id 8943C3858D32 for ; Tue, 18 Oct 2022 17:38:37 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 8943C3858D32 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666114717; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=EmDWNgx0eGVDoGDQWfHsk+JsEYef5GmnZIAzQ2J8sMU=; b=IgYXC/GBtgoGqKAeqgZcxgQLpWHtC21Zdt80bE4vZrs1AbXtHt3n7e6DlfMx2hgBfJl6fq ng3wYI026efTnUrGTaJZFaFWu73EqG0XUX+AUHD85lq89X5jemMcBB4cQZslB2V9fQSMMk q7jizvjum1XEtK4bch/x6v5OmHJzF9Q= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-185-1M_OMvSqNr2lMi9Qb1XHUQ-1; Tue, 18 Oct 2022 13:38:34 -0400 X-MC-Unique: 1M_OMvSqNr2lMi9Qb1XHUQ-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id A1E2A381A728 for ; Tue, 18 Oct 2022 17:38:34 +0000 (UTC) Received: from pdp-11.redhat.com (unknown [10.22.17.93]) by smtp.corp.redhat.com (Postfix) with ESMTP id 846EA2022EBA; Tue, 18 Oct 2022 17:38:34 +0000 (UTC) From: Marek Polacek To: GCC Patches , Jason Merrill Subject: [PATCH] c++: Mitigate -Wuseless-cast with classes [PR85043] Date: Tue, 18 Oct 2022 13:38:29 -0400 Message-Id: <20221018173829.392773-1-polacek@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.4 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: 8bit Content-Type: text/plain; charset="US-ASCII"; x-default=true X-Spam-Status: No, score=-12.8 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: -Wuseless-cast (not part of -Wall/-Wextra) warns here: struct S { }; void g (S&&); void f (S&& arg) { g (S(arg)); // warning: useless cast to type 'struct S' } which is wrong: the code will not compile without the cast because "arg" is an lvalue which cannot bind to S&&. I'd like to disable the warning when a class object is cast to a non-reference type, which seems like a minimal patch that fixes the problems reported in our Bugzilla. Of course, the cast in "(int)i" may not be useless, either, but I'm not changing that here. Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? PR c++/85043 gcc/cp/ChangeLog: * typeck.cc (maybe_warn_about_useless_cast): Don't warn when a class object is cast to a non-reference type. gcc/ChangeLog: * doc/invoke.texi: Update documentation of -Wuseless-cast. gcc/testsuite/ChangeLog: * g++.dg/warn/Wuseless-cast.C: Remove dg-warning. * g++.dg/warn/Wuseless-cast3.C: New test. --- gcc/cp/typeck.cc | 5 +++++ gcc/doc/invoke.texi | 13 ++++++++++- gcc/testsuite/g++.dg/warn/Wuseless-cast.C | 4 ++-- gcc/testsuite/g++.dg/warn/Wuseless-cast3.C | 25 ++++++++++++++++++++++ 4 files changed, 44 insertions(+), 3 deletions(-) create mode 100644 gcc/testsuite/g++.dg/warn/Wuseless-cast3.C diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc index da0e1427b97..a7587f56720 100644 --- a/gcc/cp/typeck.cc +++ b/gcc/cp/typeck.cc @@ -8104,6 +8104,11 @@ maybe_warn_about_useless_cast (location_t loc, tree type, tree expr, if (warn_useless_cast && complain & tf_warning) { + /* Don't warn when converting a class object to a non-reference type, + because that's a common way to create a temporary. */ + if (!TYPE_REF_P (type) && CLASS_TYPE_P (TREE_TYPE (expr))) + return; + if ((TYPE_REF_P (type) && (TYPE_REF_IS_RVALUE (type) ? xvalue_p (expr) : lvalue_p (expr)) diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index c176e2dc646..cd4d3c1d72c 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -4551,7 +4551,18 @@ pointers after reallocation. @item -Wuseless-cast @r{(C++ and Objective-C++ only)} @opindex Wuseless-cast @opindex Wno-useless-cast -Warn when an expression is casted to its own type. +Warn when an expression is cast to its own type. This warning does not +occur when a class object is converted to a non-reference type as that +is a way to create a temporary: + +@smallexample +struct S @{ @}; +void g (S&&); +void f (S&& arg) +@{ + g (S(arg)); // make arg prvalue so that it can bind to S&& +@} +@end smallexample @item -Wno-conversion-null @r{(C++ and Objective-C++ only)} @opindex Wconversion-null diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C index 2fd6bc45102..6084b4cef49 100644 --- a/gcc/testsuite/g++.dg/warn/Wuseless-cast.C +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast.C @@ -100,8 +100,8 @@ void f() A a; - (A)(a); // { dg-warning "3:useless cast" } - static_cast(a); // { dg-warning "3:useless cast" } + (A)(a); + static_cast(a); (A*)(&a); // { dg-warning "3:useless cast" } const_cast(&a); // { dg-warning "3:useless cast" } diff --git a/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C new file mode 100644 index 00000000000..2db07de731f --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wuseless-cast3.C @@ -0,0 +1,25 @@ +// PR c++/85043 +// { dg-do compile { target c++11 } } +// { dg-options "-Wuseless-cast" } + +struct S { int s; void bump () { s++; } }; + +void +foo () +{ + S s = { 1 }; + s.bump (); + S (s).bump (); // { dg-bogus "useless" } + ((S) s).bump (); // { dg-bogus "useless" } + static_cast (s).bump (); // { dg-bogus "useless" } +} + +struct X { }; +void g(X&&); + +void +f (X&& arg) +{ + g(X(arg)); // { dg-bogus "useless" } + g(static_cast(arg)); +} base-commit: 885b6660c17fb91980b5682514ef54668e544b02 -- 2.37.3