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From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r12-1437] libstdc++: Fix common_reference for non-reference results [PR100894] Date: Mon, 14 Jun 2021 20:17:56 +0000 (GMT) [thread overview] Message-ID: <20210614201756.62B1B38515EC@sourceware.org> (raw) https://gcc.gnu.org/g:c37b5ddcc88e0cc0f6a4ad609eda51021df0f6bb commit r12-1437-gc37b5ddcc88e0cc0f6a4ad609eda51021df0f6bb Author: Jonathan Wakely <jwakely@redhat.com> Date: Mon Jun 14 20:31:00 2021 +0100 libstdc++: Fix common_reference for non-reference results [PR100894] The result of COMMON-REF(A&, B&&) where they have no common reference type should not be a reference. The implementation of COMMON-REF fails to check that the result is a reference, so is well-formed when it shouldn't be. This means that common_reference uses that result when it shouldn't. The fix is to reject the result of COMMON-REF(A, B) if it's not a reference, so that common_reference falls through to the next case, which uses COND-RES, which yields a non-reference result. Signed-off-by: Jonathan Wakely <jwakely@redhat.com> libstdc++-v3/ChangeLog: PR libstdc++/100894 * include/std/type_traits (__common_ref_impl<X&, Y&>): Only use the type if it's a reference. * testsuite/20_util/common_reference/100894.cc: New test. Diff: --- libstdc++-v3/include/std/type_traits | 12 +++++++++--- libstdc++-v3/testsuite/20_util/common_reference/100894.cc | 9 +++++++++ 2 files changed, 18 insertions(+), 3 deletions(-) diff --git a/libstdc++-v3/include/std/type_traits b/libstdc++-v3/include/std/type_traits index eaf06fcb036..d9068a06f08 100644 --- a/libstdc++-v3/include/std/type_traits +++ b/libstdc++-v3/include/std/type_traits @@ -3340,11 +3340,17 @@ template <typename _From, typename _To> template<typename _Ap, typename _Bp> using __common_ref = typename __common_ref_impl<_Ap, _Bp>::type; + // COND-RES(COPYCV(X, Y) &, COPYCV(Y, X) &) + template<typename _Xp, typename _Yp> + using __condres_cvref + = __cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>; + // If A and B are both lvalue reference types, ... template<typename _Xp, typename _Yp> - struct __common_ref_impl<_Xp&, _Yp&, - __void_t<__cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>>> - { using type = __cond_res<__copy_cv<_Xp, _Yp>&, __copy_cv<_Yp, _Xp>&>; }; + struct __common_ref_impl<_Xp&, _Yp&, __void_t<__condres_cvref<_Xp, _Yp>>> + : enable_if<is_reference_v<__condres_cvref<_Xp, _Yp>>, + __condres_cvref<_Xp, _Yp>> + { }; // let C be remove_reference_t<COMMON-REF(X&, Y&)>&& template<typename _Xp, typename _Yp> diff --git a/libstdc++-v3/testsuite/20_util/common_reference/100894.cc b/libstdc++-v3/testsuite/20_util/common_reference/100894.cc new file mode 100644 index 00000000000..5e144768002 --- /dev/null +++ b/libstdc++-v3/testsuite/20_util/common_reference/100894.cc @@ -0,0 +1,9 @@ +// { dg-options "-std=gnu++20" } +// { dg-do compile { target c++20 } } +// PR libstdc++/100894 - common_reference implementation seems to be wrong + +#include <type_traits> + +struct A {}; +struct B { B(A); }; +static_assert( std::is_same_v<std::common_reference_t<A&, B&&>, B> );
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