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From the paper's abstract: Change the iterator requirements for non-Ranges algorithms. For forward iterators and above that are constant iterators, instead of requiring that iterators meet certain Cpp17...Iterator requirements, require that the iterators model certain iterator concepts. This makes iterators from several standard views usable with non-Ranges algorithms that require forward iterators or above, such as the parallel overloads of most algorithms. This patch narrowly implements P2408R5 in C++23 mode and C++20 mode (as an extension). "Narrowly" because just as in the paper, we don't attempt to relax the requirements of mutable iterators even though it's possible in theory. Note that the PSTL algorithm requirements have already been relaxed in r15-3650. And we don't bother touching the deprecated parallel mode algorithms under ./include/parallel. The main workhorse of this paper is a new helper __iterator_concept_or_category that replaces eligible uses of __iterator_category and iterator_traits::iterator_category. This new helper considers both the iterator_concept and iterator_category of the given iterator and returns the former if it's at least as strong as the latter. It's implemented in terms of the __promotable_iterator concept added in r16-2588 that made std::advance etc aware of C++20 iterators. Note that this helper doesn't check the actual C++20 iterator concepts (which check syntactic requirements along with iterator_concept if it's defined) and instead just checks for, and fully trusts, the iterator_concept defined by the iterator type. This is a slight deviation from the paper but IMHO it's consistent with the existing trusting of iterator_category and should be good enough in practice, though it means C++20 iterators that don't define iterator_concept will not be recognized as such by this helper even if they otherwise model the std::foo_iterator concept. (An undefined iterator_concept effectively defaults to random_access_iterator_tag.) Most of the changes made here are effectively optimizations that don't have a semantic impact, e.g. for std::reduce. I added tests for a couple of algorithms where these changes are observable. The new __iterator_concept_or_category helper can probably also be used to fix PR100070 "Standard library container iterator-pair constructors should check C++20 iterator concepts". As a follow-up to this patch we should either remove the Boost-style concept checks, or relax them accordingly. It seems we're leaning towards removing them outright; see this thread: https://gcc.gnu.org/pipermail/libstdc++/2025-May/061568.html As suggested by Tomasz, this patch also introduces a _GLIBCXX_ITER_MOVE wrapper around ranges::iter_move that also converts to the iterator's value type (and is usable before C++20 as well). PR libstdc++/113299 libstdc++-v3/ChangeLog: * include/bits/deque.tcc (__copy_move_a1): Constrain with __is_any_random_access_iter instead of __is_random_access_iter. (__copy_move_backward_a1): Likewise. (__equal_aux1): Likewise. * include/bits/stl_algo.h (__search_n): Use __iter_concept_or_category instead of __iterator_category or iterator_traits::iterator_category. (find_end): Likewise. (__is_permutation): Likewise. (for_each_n): Likewise. (unique_copy): Likewise, for constant iterators. (sample): Likewise, for constant iterators. * include/bits/stl_algobase.h (__copy_move_a1): Adjust deque-based forward declaration accordingly. (__copy_move_backward_a1): Likewise. (__equal_aux1): Likewise. (__lexicographical_compare_impl): Use __iter_concept_or_category instead of __iterator_category or iterator_traits::iterator_category. (__equal4): Likewise. * include/bits/stl_iterator_base_funcs.h (__iter_concept_or_category): New. (__is_any_random_access_iter): New. (_GLIBCXX_ITER_MOVE): New. * include/bits/stl_uninitialized.h (uninitialized_copy_n): Use __iterator_concept_or_category instead of __iterator_category for the constant iterator __first. (__uninitialized_copy_n_pair): Likewise. * include/bits/version.def (algorithm_iterator_requirements): Define. * include/bits/version.h: Regenerate. * include/std/algorithm: Provide the FTM __cpp_lib_algorithm_iterator_requirements. * include/std/memory: Likewise. * include/std/numeric: Likewise. Include <bits/stl_iterator_base_funcs.h>. (reduce): Use __is_any_random_access_iter instead of __is_random_access_iter. (transform_reduce): Likewise. (inclusive_scan): Use _GLIBCXX_ITER_MOVE instead of std::move. * testsuite/25_algorithms/find_end/c++20_iter.cc: New test. * testsuite/25_algorithms/sample/c++20_iter.cc: New test. * testsuite/25_algorithms/search_n/c++20_iter.cc: New test. * testsuite/25_algorithms/unique_copy/c++20_iter.cc: New test. * testsuite/26_numerics/inclusive_scan/c++20_iter.cc: New test. Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com> Reviewed-by: Jonathan Wakely <jwakely@redhat.com>