26 Ranges library [ranges]

26.3 Range access [range.access]

26.3.2 ranges​::​begin [range.access.begin]

The name ranges​::​begin 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.
  • If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges​::​begin(E) is ill-formed.
  • Otherwise, if T is an array type ([dcl.array]) and remove_all_extents_t<T> is an incomplete type, ranges​::​begin(E) is ill-formed with no diagnostic required.
  • Otherwise, if T is an array type, ranges​::​begin(E) is expression-equivalent to t + 0.
  • Otherwise, if auto(t.begin()) is a valid expression whose type models input_or_output_iterator, ranges​::​begin(E) is expression-equivalent to auto(t.begin()).
  • Otherwise, if T is a class or enumeration type and auto(begin(t)) is a valid expression whose type models input_or_output_iterator where the meaning of begin is established as-if by performing argument-dependent lookup only ([basic.lookup.argdep]), then ranges​::​begin(E) is expression-equivalent to that expression.
  • Otherwise, ranges​::​begin(E) is ill-formed.
[Note 1: 
Diagnosable ill-formed cases above result in substitution failure when ranges​::​begin(E) appears in the immediate context of a template instantiation.
— end note]
[Note 2: 
Whenever ranges​::​begin(E) is a valid expression, its type models input_or_output_iterator.
— end note]