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Fri, 19 Jan 2024 21:18:30 -0500 X-MC-Unique: EeKpNeCoPbe4geplVUQcgA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id B08303C0C4A3; Sat, 20 Jan 2024 02:18:29 +0000 (UTC) Received: from pdp-11.redhat.com (unknown [10.22.9.190]) by smtp.corp.redhat.com (Postfix) with ESMTP id 874D451D5; Sat, 20 Jan 2024 02:18:29 +0000 (UTC) From: Marek Polacek To: GCC Patches , libstdc++ , Jason Merrill Subject: [PATCH] c++: -Wdangling-reference and lambda false warning [PR109640] Date: Fri, 19 Jan 2024 21:18:23 -0500 Message-ID: <20240120021823.81839-1-polacek@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com X-Spam-Status: No, score=-13.6 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_H3, RCVD_IN_MSPIKE_WL, SPF_HELO_NONE, SPF_NONE, TXREP, T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: gcc-patches-bounces+incoming=patchwork.ozlabs.org@gcc.gnu.org Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- -Wdangling-reference checks if a function receives a temporary as its argument, and only warns if any of the arguments was a temporary. But we should not warn when the temporary represents a lambda or we generate false positives as in the attached testcases. PR c++/113256 PR c++/111607 PR c++/109640 gcc/cp/ChangeLog: * call.cc (do_warn_dangling_reference): Don't warn if the temporary is of lambda type. gcc/testsuite/ChangeLog: * g++.dg/warn/Wdangling-reference14.C: New test. * g++.dg/warn/Wdangling-reference15.C: New test. * g++.dg/warn/Wdangling-reference16.C: New test. --- gcc/cp/call.cc | 9 ++++-- .../g++.dg/warn/Wdangling-reference14.C | 22 +++++++++++++ .../g++.dg/warn/Wdangling-reference15.C | 31 +++++++++++++++++++ .../g++.dg/warn/Wdangling-reference16.C | 13 ++++++++ 4 files changed, 72 insertions(+), 3 deletions(-) create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference14.C create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference15.C create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference16.C base-commit: 615e25c82de97acc17ab438f88d6788cf7ffe1d6 diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index 1f5ff417c81..77f51bacce3 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -14123,7 +14123,10 @@ do_warn_dangling_reference (tree expr, bool arg_p) tree e = expr; while (handled_component_p (e)) e = TREE_OPERAND (e, 0); - if (!reference_like_class_p (TREE_TYPE (e))) + tree type = TREE_TYPE (e); + /* If the temporary represents a lambda, we don't really know + what's going on here. */ + if (!reference_like_class_p (type) && !LAMBDA_TYPE_P (type)) return expr; } @@ -14180,10 +14183,10 @@ do_warn_dangling_reference (tree expr, bool arg_p) initializing this reference parameter. */ if (do_warn_dangling_reference (arg, /*arg_p=*/true)) return expr; - /* Don't warn about member function like: + /* Don't warn about member functions like: std::any a(...); S& s = a.emplace({0}, 0); - which constructs a new object and returns a reference to it, but + which construct a new object and return a reference to it, but we still want to detect: struct S { const S& self () { return *this; } }; const S& s = S().self(); diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C new file mode 100644 index 00000000000..92b38a965e0 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference14.C @@ -0,0 +1,22 @@ +// PR c++/113256 +// { dg-do compile { target c++14 } } +// { dg-options "-Wdangling-reference" } + +#include +#include + +template auto bind(M T::* pm, A) +{ + return [=]( auto&& x ) -> M const& { return x.*pm; }; +} + +template struct arg {}; + +arg<1> _1; + +int main() +{ + std::pair pair; + int const& x = bind( &std::pair::first, _1 )( pair ); // { dg-bogus "dangling reference" } + assert( &x == &pair.first ); +} diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C new file mode 100644 index 00000000000..c39577db64a --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference15.C @@ -0,0 +1,31 @@ +// PR c++/111607 +// { dg-do compile { target c++20 } } +// { dg-options "-Wdangling-reference" } + +#include + +struct S { + constexpr S(int i_) : i(i_) {} + S(S const &) = delete; + S & operator=(S const &) = delete; + S(S &&) = delete; + S & operator=(S &&) = delete; + int i; +}; + +struct A { + S s{0}; +}; + +using V = std::variant; + +consteval auto f(V const & v) { + auto const & s = std::visit([](auto const & v) -> S const & { return v.s; }, v); // { dg-bogus "dangling reference" } + return s.i; +} + +int main() { + constexpr V a{std::in_place_type}; + constexpr auto i = f(a); + return i; +} diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C new file mode 100644 index 00000000000..91996922291 --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference16.C @@ -0,0 +1,13 @@ +// PR c++/109640 +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +bool +fn0 () +{ + int a; + int&& i = [](int& r) -> int&& { return static_cast(r); }(a); // { dg-bogus "dangling reference" } + auto const l = [](int& r) -> int&& { return static_cast(r); }; + int&& j = l(a); + return &i == &j; +}