25 Ranges library [ranges]

25.3 Range access [range.access]

25.3.3 ranges​::​end [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:
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​end(E) is ill-formed.
  • 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.
  • Otherwise, if T is an array of unknown bound, ranges​::​end(E) is ill-formed.
  • Otherwise, if T is an array, ranges​::​end(E) is expression-equivalent to t + extent_v<T>.
  • 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()).
  • 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.
  • 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]