24 Ranges library [ranges]

24.3 Range access [range.access]

24.3.6 ranges​::​rend [range.access.rend]

The name ranges​::​rend denotes a customization point object ([customization.point.object]).
Given a subexpression E with type T, let t be an lvalue that denotes the reified object for E.
Then:
[Note:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​rend(E) appears in the immediate context of a template instantiation.
— end note]
[Note:
Whenever ranges​::​rend(E) is a valid expression, the types S and I of ranges​::​rend(E) and ranges​::​rbegin(E) model sentinel_­for<S, I>.
— end note]