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.
The expression ranges::rend(E) for some subexpression E is expression-equivalent to:
[Note
:
Whenever ranges::rend(E) is a valid expression, the types S and I of ranges::rend(E) and ranges::rbegin(E) model Sentinel<S, I>.
end note
]