From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id B33B73952DAC; Tue, 4 May 2021 11:19:16 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org B33B73952DAC MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-436] libstdc++: Do not use deduced return type for std::visit [PR 100384] X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: ad0a3be4df5eecc79075d899fd79179d0f61270e X-Git-Newrev: af5b2b911dd80ae9cc87404b7e7ab807cf6655d4 Message-Id: <20210504111916.B33B73952DAC@sourceware.org> Date: Tue, 4 May 2021 11:19:16 +0000 (GMT) X-BeenThere: libstdc++-cvs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libstdc++-cvs mailing list List-Unsubscribe: , List-Archive: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 04 May 2021 11:19:16 -0000 https://gcc.gnu.org/g:af5b2b911dd80ae9cc87404b7e7ab807cf6655d4 commit r12-436-gaf5b2b911dd80ae9cc87404b7e7ab807cf6655d4 Author: Jonathan Wakely Date: Tue May 4 12:16:46 2021 +0100 libstdc++: Do not use deduced return type for std::visit [PR 100384] This avoids errors outside the immediate context when std::visit is an overload candidate because of ADL, but not actually viable. The solution is to give std::visit a non-deduced return type. New helpers are introduced for that, and existing ones refactored slightly. libstdc++-v3/ChangeLog: PR libstdc++/100384 * include/std/variant (__get_t): New alias template yielding the return type of std::get on a variant. (__visit_result_t): New alias template yielding the result of std::visit. (__same_types): Move into namespace __detail::__variant. (__check_visitor_results): Likewise. Use __invoke_result_t and __get_t. (__check_visitor_result): Remove. (visit): Use __visit_result_t for return type. * testsuite/20_util/variant/100384.cc: New test. Diff: --- libstdc++-v3/include/std/variant | 50 ++++++++++++------------ libstdc++-v3/testsuite/20_util/variant/100384.cc | 9 +++++ 2 files changed, 34 insertions(+), 25 deletions(-) diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant index 1ef9b9725e2..9b2bc440841 100644 --- a/libstdc++-v3/include/std/variant +++ b/libstdc++-v3/include/std/variant @@ -1063,6 +1063,25 @@ namespace __variant std::index_sequence<__indices...>> : _Base_dedup<__indices, __poison_hash>>... { }; + template + using __get_t = decltype(std::get<_Np>(std::declval<_Variant>())); + + // Return type of std::visit. + template + using __visit_result_t + = invoke_result_t<_Visitor, __get_t<0, _Variants>...>; + + template + constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...); + + template + constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>) + { + return __same_types< + invoke_result_t<_Visitor, __get_t<_Idxs, _Variant>>... + >; + } + } // namespace __variant } // namespace __detail @@ -1248,7 +1267,8 @@ namespace __variant #endif template - constexpr decltype(auto) visit(_Visitor&&, _Variants&&...); + constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...> + visit(_Visitor&&, _Variants&&...); template inline enable_if_t<(is_move_constructible_v<_Types> && ...) @@ -1715,41 +1735,21 @@ namespace __variant std::forward<_Variants>(__variants)...); } - template - constexpr inline bool __same_types = (is_same_v<_Tp, _Types> && ...); - - template - decltype(auto) - __check_visitor_result(_Visitor&& __vis, _Variant&& __variant) - { - return std::__invoke(std::forward<_Visitor>(__vis), - std::get<_Idx>(std::forward<_Variant>(__variant))); - } - - template - constexpr bool __check_visitor_results(std::index_sequence<_Idxs...>) - { - return __same_types( - std::declval<_Visitor>(), - std::declval<_Variant>()))...>; - } - - template - constexpr decltype(auto) + constexpr __detail::__variant::__visit_result_t<_Visitor, _Variants...> visit(_Visitor&& __visitor, _Variants&&... __variants) { if ((__variants.valueless_by_exception() || ...)) __throw_bad_variant_access("std::visit: variant is valueless"); - using _Result_type = std::invoke_result_t<_Visitor, - decltype(std::get<0>(std::declval<_Variants>()))...>; + using _Result_type + = __detail::__variant::__visit_result_t<_Visitor, _Variants...>; using _Tag = __detail::__variant::__deduce_visit_result<_Result_type>; if constexpr (sizeof...(_Variants) == 1) { - constexpr bool __visit_rettypes_match = + constexpr bool __visit_rettypes_match = __detail::__variant:: __check_visitor_results<_Visitor, _Variants...>( std::make_index_sequence< std::variant_size...>::value>()); diff --git a/libstdc++-v3/testsuite/20_util/variant/100384.cc b/libstdc++-v3/testsuite/20_util/variant/100384.cc new file mode 100644 index 00000000000..4866aa017ff --- /dev/null +++ b/libstdc++-v3/testsuite/20_util/variant/100384.cc @@ -0,0 +1,9 @@ +// { dg-do compile { target c++17 } } + +#include + +int visit(int*, std::true_type) { return 0; } + +const std::true_type dat; + +int i = visit(nullptr, dat);