23 Iterators library [iterators]

23.3 Iterator requirements [iterator.requirements]

23.3.4 Iterator concepts [iterator.concepts] Concept sentinel_­for [iterator.concept.sentinel]

The sentinel_­for concept specifies the relationship between an input_­or_­output_­iterator type and a semiregular type whose values denote a range.
Let s and i be values of type S and I such that [i, s) denotes a range.
Types S and I model sentinel_­for<S, I> only if
The domain of == is not static.
Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, i and s are not required to continue to denote a range after incrementing any other iterator equal to i.
Consequently, i == s is no longer required to be well-defined.