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[libc++] Disallow std::prev(non_cpp17_bidi_iterator) (#112102)
The std::prev function appeared to work on non Cpp17BidirectionalIterators, however it behaved strangely by being the identity function. That was extremely misleading and potentially dangerous, since several recent iterators that are C++20 bidirectional_iterators don't satisfy the Cpp17BidirectionalIterator named requirement. For example: auto zv = std::views::zip(vec1, vec2); auto it = zv.begin() + 5; auto it2 = std::prev(it); // "it2" will be the same as "it", instead of the previous one Here, zip_view::iterator is a c++20 random_access_iterator, but it only satisfies the Cpp17InputIterator named requirement because its reference type is a proxy type. Hence `std::prev` would silently accept that iterator but it would do a no-op instead of going to the previous position. This patch changes `std::prev(it)` to produce an error at compile-time when instantiated with a type that is not a Cpp17BidirectionalIterator. Fixes #109456
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