deque is inconsistent about using checked vs. unchecked iterators for internal operations where checking is not needed. emplace, for example:

passes checked iterators to rotate which could as well be unchecked - we know that the ranges we're forming are valid. We need to audit the file for such occurrences and change them to use unchecked iterators (returned from _Unchecked_begin and _Unchecked_end) where possible.