The sentinel_for concept specifies the relationship between an input_or_output_iterator type and a semiregular type whose values denote a range. The exposition-only concept __WeaklyEqualityComparableWith is described in equality_comparable.
Let s and i be values of type S and I, respectively, such that [i, s) denotes a range. sentinel_for<S, I> is modeled only if:
i is dereferenceable and [++i, s) denotes a range.The domain of == can change over time. Given an iterator i and sentinel s such that [i, s) denotes a range and i != s, [i, s) is not required to continue to denote a range after incrementing any iterator equal to i (and so i == s is no longer required to be well-defined after such an increment).
A sentinel type and its corresponding iterator type are not required to model equality_comparable_with, because the sentinel type may not be comparable with itself, and they are not required to have a common reference type.
It has been permitted to use a sentinel type different from the iterator type in the range-based for loop since C++17.
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior LWG 3453 C++20 semantic requirements forsentinel_for were too loose for ranges::advance strengthened
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