https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87431
--- Comment #23 from Jonathan Wakely <redi at gcc dot gnu.org> --- Author: redi Date: Tue Apr 23 09:55:33 2019 New Revision: 270502 URL: https://gcc.gnu.org/viewcvs?rev=270502&root=gcc&view=rev Log: Fix std::variant regression caused by never-valueless optimization A regression was introduced by the recent changes to provide the strong exception safety guarantee for "never valueless" types that have O(1), non-throwing move assignment. The problematic code is: else if constexpr (__detail::__variant::_Never_valueless_alt<type>()) { // This construction might throw: variant __tmp(in_place_index<_Np>, __il, std::forward<_Args>(__args)...); // But _Never_valueless_alt<type> means this won't: *this = std::move(__tmp); } When the variant is not assignable, the assignment is ill-formed, so should not be attempted. When the variant has a copy assignment operator but not a move assignment operator, the assignment performs a copy assignment and that could throw, so should not be attempted. The solution is to only take that branch when the variant has a move assignment operator, which is determined by the _Traits::_S_move_assign constant. When that is false the strong exception safety guarantee is not possible, and so the __never_valueless function should also depend on _S_move_assign. While testing the fixes for this I noticed that the partial specialization _Never_valueless_alt<basic_string<C,T,A>> incorrectly assumed that is_nothrow_move_constructible<basic_string<C,T,A>> is always true, but that's wrong for fully-dynamic COW strings. Fix the partial specialization, and improve the comment describing _Never_valueless_alt to be clear it depends on move construction as well as move assignment. Finally, I also observed that _Variant_storage<false, T...>::_M_valid() was not taking advantage of the __never_valueless<T...>() function to avoid a runtime check. Only the _Variant_storage<true, T...>::_M_valid() function was using __never_valueless. That is also fixed. PR libstdc++/87431 * include/bits/basic_string.h (_Never_valueless_alt): Make partial specialization also depend on is_nothrow_move_constructible. * include/std/variant (__detail::__variant::__never_valueless()): Only true if the variant would have a move assignment operator. (__detail::__variant::_Variant_storage<false, T...>::_M_valid()): Check __never_valueless<T...>(). (variant::emplace): Only perform non-throwing move assignments for never-valueless alternatives if the variant has a move assignment operator. * testsuite/20_util/variant/compile.cc: Check that never-valueless types can be emplaced into non-assignable variants. * testsuite/20_util/variant/run.cc: Check that never-valueless types don't get copied when emplaced into non-assignable variants. Modified: trunk/libstdc++-v3/ChangeLog trunk/libstdc++-v3/include/bits/basic_string.h trunk/libstdc++-v3/include/std/variant trunk/libstdc++-v3/testsuite/20_util/variant/compile.cc trunk/libstdc++-v3/testsuite/20_util/variant/run.cc
