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When the C++98 std::distance and std::advance functions (and C++11 std::next and std::prev) are used with C++20 iterators there can be unexpected results, ranging from compilation failure to decreased performance to undefined behaviour. An iterator which satisfies std::input_iterator but does not meet the Cpp17InputIterator requirements might have std::output_iterator_tag for its std::iterator_traits<I>::iterator_category, which means it currently cannot be used with std::advance at all. However, the implementation of std::advance for a Cpp17InputIterator doesn't do anything that isn't valid for iterator types satsifying C++20 std::input_iterator. Similarly, a type satisfying C++20 std::bidirectional_iterator might be usable with std::prev, if it weren't for the fact that its C++17 iterator_category is std::input_iterator_tag. Finally, a type satisfying C++20 std::random_access_iterator might use a slower implementation for std::distance or std::advance if its C++17 iterator_category is not std::random_access_iterator_tag. This commit adds a __promotable_iterator concept to detect C++20 iterators which explicitly define an iterator_concept member, and which either have no iterator_category, or their iterator_category is weaker than their iterator_concept. This is used by std::distance and std::advance to detect iterators which should dispatch based on their iterator_concept instead of their iterator_category. This means that those functions just work and do the right thing for C++20 iterators which would otherwise fail to compile or have suboptimal performance. This is related to LWG 3197, which considers making it undefined to use std::prev with types which do not meet the Cpp17BidirectionalIterator requirements. I think making it work, as in this commit, is a better solution than banning it (or rejecting it at compile-time as libc++ does). PR libstdc++/102181 libstdc++-v3/ChangeLog: * include/bits/stl_iterator_base_funcs.h (distance, advance): Check C++20 iterator concepts and handle appropriately. (__detail::__iter_category_converts_to_concept): New concept. (__detail::__promotable_iterator): New concept. * testsuite/24_iterators/operations/cxx20_iterators.cc: New test. Reviewed-by: Patrick Palka <ppalka@redhat.com> Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>