24 Ranges library [ranges]

24.3 Range access [range.access]

24.3.2 ranges​::​end [range.access.end]

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