[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.

Then:

(2.1)
If E is an rvalue and enable_borrowed_range<remove_cv_t<T>> is false, ranges::begin(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::begin(E) is ill-formed with no diagnostic required.
(2.3)
Otherwise, if T is an array type, ranges::begin(E) is expression-equivalent to t + 0.
(2.4)
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()).
(2.5)
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.
(2.6)
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]

26 Ranges library [ranges]