[range.access.end]

The name ranges::end 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:

(2.1)
If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::end(E) is ill-formed.
(2.2)
Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges::end(E) is ill-formed with no diagnostic required.
(2.3)
Otherwise, if T is an array of unknown bound, ranges::end(E) is ill-formed.
(2.4)
Otherwise, if T is an array, ranges::end(E) is expression-equivalent to t + extent_v<T>.
(2.5)
Otherwise, if auto(t.end()) is a valid expression whose type models sentinel_for<iterator_t<T>> then ranges::end(E) is expression-equivalent to auto(t.end()).
(2.6)
Otherwise, if T is a class or enumeration type and auto(end(t)) is a valid expression whose type models sentinel_for<iterator_t<T>> where the meaning of end is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges::end(E) is expression-equivalent to that expression.
(2.7)
Otherwise, ranges::end(E) is ill-formed.

[Note 1:

Diagnosable ill-formed cases above result in substitution failure when ranges::end(E) appears in the immediate context of a template instantiation.

— end note]

[Note 2:

Whenever ranges::end(E) is a valid expression, the types S and I of ranges::end(E) and ranges::begin(E) model sentinel_for<S, I>.

— end note]

25 Ranges library [ranges]