https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100639
Bug ID: 100639 Summary: reverse_view<I>::reference erroneously uses iterator_traits<I>::reference instead of iter_reference_t<I> Product: gcc Version: 10.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: barry.revzin at gmail dot com Target Milestone: --- Short example (from https://stackoverflow.com/q/67573305/2069064): #include <ranges> template <typename T> using iota = std::ranges::iota_view<T, T>; template <typename T> using iota_iter = std::ranges::iterator_t<iota<T>>; static_assert(std::same_as< std::reverse_iterator<iota_iter<int64_t>>::reference, int64_t>); This assertion fails when compiling with -std=c++20 (because the reference type is 'void') but passes with -std=gnu++20. The direct reason is that the difference_type of the iota_view iterator is __int128, which is considered a signed_integral with gnu++20 but not c++20. But the reason this matters is because std::reverse_iterator<I>::reference is defined as std::iterator_traits<I>::reference (which checks that I satsifies cpp17-input-iterator which has the signed_integral constraint) instead of being defined as std::iter_reference_t<I> (which has no such check). With the latter implementation, the assertion above would pass on either version. The result is that reversing an iota_view<int64_t, int64_t> isn't a range on -std=c++20.