🌐 [range.access.end] - timsong-cpp.github.io

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 ([basic.compound]) 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 decay-copy(t.end()) is a valid expression whose type models sentinel_for<iterator_t<T>> then ranges::end(E) is expression-equivalent to decay-copy(t.end()).
(2.6)
Otherwise, if T is a class or enumeration type and decay-copy(end(t)) is a valid expression whose type models sentinel_for<iterator_t<T>> with overload resolution performed in a context in which unqualified lookup for end finds only the declarations
```
void end(auto&) = delete;
void end(const auto&) = delete;
```
then ranges::end(E) is expression-equivalent to decay-copy(end(t)) with overload resolution performed in the above context.
(2.7)
Otherwise, ranges::end(E) is ill-formed.