template<class S, class I>conceptsentinel_for=semiregular<S>&&input_or_output_iterator<I>&&weakly-equality-comparable-with<S, I>; // See [concept.equalitycomparable]
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.