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`reverse_iterator` supports either c++20 `bidirectional_iterator` or `Cpp17BidirectionalIterator ` http://eel.is/c++draft/reverse.iter.requirements The current `reverse_iterator` uses `std::prev` in its `operator->`, which only supports the `Cpp17BidirectionalIterator` properly. If the underlying iterator is c++20 `bidirectional_iterator` but does not satisfy the named requirement `Cpp17BidirectionalIterator`, (examples are `zip_view::iterator`, `flat_map::iterator`), the current `std::prev` silently compiles but does a no-op and returns the same iterator back. So `reverse_iterator::operator->` will silently give a wrong answer. Even if we fix the behaviour of `std::prev`, at best, we could fail to compile the code. But this is not ok, because we need to support this kind of iterators in `reverse_iterator`. The solution is simply to not use `std::prev`. --------- Co-authored-by: Louis Dionne <ldionne.2@gmail.com>