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 [170.10.133.124]) by sourceware.org (Postfix) with ESMTPS id D09823858D20 for ; Fri, 11 Nov 2022 20:22:29 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org D09823858D20 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=1668198149; 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=tVwaigk2AUeC5aRetxIgEh6sRV+NMrSslRy9a5CeDBs=; b=Xt2h6QZ4BBIa0gTJg7ASO4BSixjk0ChLG6sXQhyrNF69Xt4NNXTitwhDo+/ehwq+JFt67/ 3g+1QcIfOIDtdi2FgOou0i3ORkjLvBK7K+WtTtaWNKK1s4eHfydXF0JfzZoDqrfv0gv5Ls YQlLmAe3lCEyGWtpvurF5JEmHb4EJTM= 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-88-s29sjlcoPaiyxkx28fRPOw-1; Fri, 11 Nov 2022 15:22:28 -0500 X-MC-Unique: s29sjlcoPaiyxkx28fRPOw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 477C43C021A3 for ; Fri, 11 Nov 2022 20:22:28 +0000 (UTC) Received: from pdp-11.lan (unknown [10.22.33.235]) by smtp.corp.redhat.com (Postfix) with ESMTP id 29BC763A56; Fri, 11 Nov 2022 20:22:28 +0000 (UTC) From: Marek Polacek To: Jason Merrill , GCC Patches Subject: [PATCH] c++: Disable -Wdangling-reference when initing T& Date: Fri, 11 Nov 2022 15:22:26 -0500 Message-Id: <20221111202226.103649-1-polacek@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.5 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=-12.3 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: Non-const lvalue references can't bind to a temporary, so the warning should not be emitted if we're initializing something of that type. I'm not disabling the warning when the function itself returns a non-const lvalue reference, that would regress at least const int &r = std::any_cast(std::any()); in Wdangling-reference2.C where the any_cast returns an int&. Unfortunately, this patch means we'll stop diagnosing int& fn(int&& x) { return static_cast(x); } void test () { int &r = fn(4); } where there's a genuine dangling reference. OTOH, the patch should suppress false positives with iterators, like: auto &candidate = *candidates.begin (); and arguably that's more important than detecting some relatively obscure cases. It's probably not worth it making the warning more complicated by, for instance, not warning when a fn returns 'int&' but takes 'const int&' (because then it can't return its argument). Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? gcc/cp/ChangeLog: * call.cc (maybe_warn_dangling_reference): Don't warn when initializing a non-const lvalue reference. gcc/testsuite/ChangeLog: * g++.dg/cpp23/elision4.C: Remove dg-warning. * g++.dg/warn/Wdangling-reference1.C: Turn dg-warning into dg-bogus. * g++.dg/warn/Wdangling-reference7.C: New test. --- gcc/cp/call.cc | 10 ++++++++-- gcc/testsuite/g++.dg/cpp23/elision4.C | 4 ++-- gcc/testsuite/g++.dg/warn/Wdangling-reference1.C | 4 ++-- gcc/testsuite/g++.dg/warn/Wdangling-reference7.C | 16 ++++++++++++++++ 4 files changed, 28 insertions(+), 6 deletions(-) create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference7.C diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index bd3b64a7e26..ef618d5c485 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -13679,8 +13679,14 @@ maybe_warn_dangling_reference (const_tree decl, tree init) { if (!warn_dangling_reference) return; - if (!(TYPE_REF_OBJ_P (TREE_TYPE (decl)) - || std_pair_ref_ref_p (TREE_TYPE (decl)))) + tree type = TREE_TYPE (decl); + /* Only warn if what we're initializing has type T&& or const T&, or + std::pair. (A non-const lvalue reference can't + bind to a temporary.) */ + if (!((TYPE_REF_OBJ_P (type) + && (TYPE_REF_IS_RVALUE (type) + || CP_TYPE_CONST_P (TREE_TYPE (type)))) + || std_pair_ref_ref_p (type))) return; /* Don't suppress the diagnostic just because the call comes from a system header. If the DECL is not in a system header, or if diff --git a/gcc/testsuite/g++.dg/cpp23/elision4.C b/gcc/testsuite/g++.dg/cpp23/elision4.C index d39053ad741..77dcffcdaad 100644 --- a/gcc/testsuite/g++.dg/cpp23/elision4.C +++ b/gcc/testsuite/g++.dg/cpp23/elision4.C @@ -34,6 +34,6 @@ T& temporary2(T&& x) { return static_cast(x); } void test () { - int& r1 = temporary1 (42); // { dg-warning "dangling reference" } - int& r2 = temporary2 (42); // { dg-warning "dangling reference" } + int& r1 = temporary1 (42); + int& r2 = temporary2 (42); } diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C index 97c81ee716c..1718c28165e 100644 --- a/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference1.C @@ -139,6 +139,6 @@ struct Y { // x1 = Y::operator int&& (&TARGET_EXPR ) int&& x1 = Y(); // { dg-warning "dangling reference" } int&& x2 = Y{}; // { dg-warning "dangling reference" } -int& x3 = Y(); // { dg-warning "dangling reference" } -int& x4 = Y{}; // { dg-warning "dangling reference" } +int& x3 = Y(); // { dg-bogus "dangling reference" } +int& x4 = Y{}; // { dg-bogus "dangling reference" } const int& t1 = Y().foo(10); // { dg-warning "dangling reference" } diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C new file mode 100644 index 00000000000..4b0de2d8670 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference7.C @@ -0,0 +1,16 @@ +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +int& ref(const int&); +int&& rref(const int&); + +void +g () +{ + const int& r1 = ref (1); // { dg-warning "dangling reference" } + int& r2 = ref (2); // { dg-bogus "dangling reference" } + auto& r3 = ref (3); // { dg-bogus "dangling reference" } + int&& r4 = rref (4); // { dg-warning "dangling reference" } + auto&& r5 = rref (5); // { dg-warning "dangling reference" } + const int&& r6 = rref (6); // { dg-warning "dangling reference" } +} base-commit: 0a7b437ca71e2721e9bcf070762fc54ef7991aeb -- 2.38.1