22 Iterators library [iterators]

22.3 Iterator requirements [iterator.requirements]

22.3.4 Iterator concepts [iterator.concepts]

22.3.4.7 Concept Sentinel [iterator.concept.sentinel]

The Sentinel concept specifies the relationship between an Iterator type and a Semiregular type whose values denote a range.

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template<class S, class I>
  concept Sentinel =
    Semiregular<S> &&
    Iterator<I> &&
    weakly-equality-comparable-with<S, I>; // See [concept.equalitycomparable]

Let s and i be values of type S and I such that [i, s) denotes a range.

Types S and I model Sentinel<S, I> only if

(2.1)
i == s is well-defined.
(2.2)
If bool(i != s) then i is dereferenceable and [++i, s) denotes a range.

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.