[libc++] Fix `reverse_iterator` when underlying is c++20 `bidirectional_iterator` but not `Cpp17BidirectionalIterator` (#112100)

`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`.

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Co-authored-by: Louis Dionne <ldionne.2@gmail.com>

0 files changed, 0 insertions, 0 deletions