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Fri, 05 Dec 2025 06:56:00 -0800 (PST) X-Google-Smtp-Source: AGHT+IEUUjsLND1Q6tzh7xufFjIaihfkAnyf+hgQ4GeInMftkzlI4lOFCW4j+ArbigV0iMvI//howA== X-Received: by 2002:ac8:5793:0:b0:4ed:a574:64cb with SMTP id d75a77b69052e-4f01b0c0edfmr99448361cf.3.1764946559993; Fri, 05 Dec 2025 06:55:59 -0800 (PST) Received: from idea ([2600:4040:aa68:6000:9e8e:99ff:fed1:71f]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4f0276b0351sm27429671cf.8.2025.12.05.06.55.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 05 Dec 2025 06:55:59 -0800 (PST) From: Patrick Palka To: gcc-patches@gcc.gnu.org Cc: libstdc++@gcc.gnu.org, Patrick Palka Subject: [PATCH] libstdc++: Use deducing this in std::bind when available [PR80564] Date: Fri, 5 Dec 2025 09:55:58 -0500 Message-ID: <20251205145558.1952982-1-ppalka@redhat.com> X-Mailer: git-send-email 2.52.0.154.gf0ef5b6d9b MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: ZKZtZ7q4oKNsEE2T4YtujZ6Vgdu212fdYEyyQXnasDU_1764946561 X-Mimecast-Originator: redhat.com content-type: text/plain; charset="US-ASCII"; x-default=true X-Spam-Status: No, score=-13.9 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, RCVD_IN_DNSWL_BLOCKED, RCVD_IN_MSPIKE_H2, RCVD_IN_VALIDITY_RPBL_BLOCKED, RCVD_IN_VALIDITY_SAFE_BLOCKED, SPF_HELO_PASS, SPF_NONE, TXREP autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on 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~patchwork=sourceware.org@gcc.gnu.org Tested on x86_64-pc-linu-xgnu, does this look OK for trunk? -- >8 -- Implement the cv-qual forwarding required by std::bind using deducing this when available, instead of needing 4 operator() overloads. Using deducing this here is more complicated here than in other call wrappers because std::bind is not really "perfect forwarding": it doesn't forward value category, and along with const-ness it also forwards volatile-ness (before C++20). The old implementation suffers from the same problem that other SFINAE-friendly call wrappers have which is solved by using deducing this (see p5.5 of the deducing this paper P0847R7). PR libstdc++/80564 libstdc++-v3/ChangeLog: * include/std/functional (__cv_like): New. (_Bind::_Res_type): Don't define when not needed. (_Bind::__dependent): Likewise. (_Bind::_Res_type_cv): Likewise. (_Bind::operator()) [_GLIBCXX_EXPLICIT_THIS_PARAMETER]: Define as two instead of four overloads using deducing this. * testsuite/20_util/bind/cv_quals_2.cc: Ignore SFINAE diagnostics inside headers. * testsuite/20_util/bind/ref_neg.cc: Likewise. * testsuite/20_util/bind/cv_quals_4.cc: New test. --- libstdc++-v3/include/std/functional | 81 +++++++++++++++++++ .../testsuite/20_util/bind/cv_quals_2.cc | 3 + .../testsuite/20_util/bind/cv_quals_4.cc | 39 +++++++++ .../testsuite/20_util/bind/ref_neg.cc | 1 + 4 files changed, 124 insertions(+) create mode 100644 libstdc++-v3/testsuite/20_util/bind/cv_quals_4.cc diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional index 1928a27d3fd6..9348356f03ce 100644 --- a/libstdc++-v3/include/std/functional +++ b/libstdc++-v3/include/std/functional @@ -495,6 +495,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION # define _GLIBCXX_DEPR_BIND #endif +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER + // Return a _Up that has the same cv-quals as _Tp. + template + struct __cv_like + { using type = _Up; }; + + template + struct __cv_like + { using type = const _Up; }; + + template + struct __cv_like + { using type = volatile _Up; }; + + template + struct __cv_like + { using type = const volatile _Up; }; + + template + using __cv_like_t = typename __cv_like<_Tp, _Up>::type; +#endif + /// Type of the function object returned from bind(). template class _Bind; @@ -564,6 +586,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION using _Res_type_impl = __invoke_result_t<_Fn&, _Mu_type<_BArgs, _CallArgs>&&...>; +#if !_GLIBCXX_EXPLICIT_THIS_PARAMETER template using _Res_type = _Res_type_impl<_Functor, _CallArgs, _Bound_args...>; @@ -576,6 +599,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION typename __cv_quals<__dependent<_CallArgs>>::type, _CallArgs, typename __cv_quals<_Bound_args>::type...>; +#endif public: template @@ -593,6 +617,62 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION _Bind(const _Bind&) = default; _Bind(_Bind&&) = default; +#if _GLIBCXX_EXPLICIT_THIS_PARAMETER +# pragma GCC diagnostic push +# pragma GCC diagnostic ignored "-Wc++17-extensions" // if constexpr +# pragma GCC diagnostic ignored "-Wc++23-extensions" // deducing this + // Call unqualified + template::type, + __enable_if_t::value, int> = 0, + typename _Result + = _Res_type_impl<__cv_like_t<_Self_nonref, _Functor>, + tuple<_Args...>, + __cv_like_t<_Self_nonref, _Bound_args>...>> + _GLIBCXX20_CONSTEXPR + _Result + operator()(this _Self&& __self, _Args&&... __args) + { + if constexpr (is_const<_Self_nonref>::value) + return static_cast(__self) + .template __call_c<_Result>(std::forward_as_tuple + (std::forward<_Args>(__args)...), + _Bound_indexes()); + else + return static_cast<_Bind&>(__self) + .template __call<_Result>(std::forward_as_tuple + (std::forward<_Args>(__args)...), + _Bound_indexes()); + } + +# if defined(_GLIBCXX_VOLATILE_BIND) + template::type, + __enable_if_t::value, int> = 0, + typename _Result + = _Res_type_impl<__cv_like_t<_Self_nonref, _Functor>, + tuple<_Args...>, + __cv_like_t<_Self_nonref, _Bound_args>...>> + _GLIBCXX_DEPR_BIND + _Result + operator()(this _Self&& __self, _Args&&... __args) + { + if constexpr (is_const<_Self_nonref>::value) + return static_cast(__self) + .template __call_c_v<_Result>(std::forward_as_tuple + (std::forward<_Args>(__args)...), + _Bound_indexes()); + else + return static_cast(__self) + .template __call_v<_Result>(std::forward_as_tuple + (std::forward<_Args>(__args)...), + _Bound_indexes()); + } +# endif +# pragma GCC diagnostic pop +#else // Call unqualified template>> @@ -642,6 +722,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION _Bound_indexes()); } #endif // volatile +#endif }; /// Type of the function object returned from bind(). diff --git a/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc b/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc index e4c348f7a3ca..6d37cc43fd3a 100644 --- a/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc +++ b/libstdc++-v3/testsuite/20_util/bind/cv_quals_2.cc @@ -44,6 +44,9 @@ void test01() // { dg-error "no match" "" { target c++20 } 43 } } +// Ignore the reasons for deduction/substitution failure in the headers. +// { dg-prune-output "no type named 'type' in 'struct std::enable_if + +struct A +{ + template + auto operator()(T&) + { } + + template + auto operator()(T&) const + { T::fail; } +}; + +void +test01() +{ + A a; + std::bind(a, 0)(); // doesn't consider the const overload + std::bind(a, 0)(); +} + +void +test02() +{ + auto f = [] (auto& x) { x = 1; }; + int i; + std::bind(f, i)(); // doesn't try const-invoking the lambda + std::bind(f, i)(); +} + +int +main() +{ + test01(); + test02(); +} diff --git a/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc b/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc index dd47c437d426..46cc4bb330e2 100644 --- a/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc +++ b/libstdc++-v3/testsuite/20_util/bind/ref_neg.cc @@ -51,6 +51,7 @@ void test02() // Ignore the reasons for deduction/substitution failure in the headers. // Arrange for the match to work on installed trees as well as build trees. // { dg-prune-output "no type named 'type' in 'struct std::__invoke_result" } +// { dg-prune-output "no type named 'type' in 'struct std::enable_if