24 Ranges library [ranges]

24.3 Range access [range.access]

24.3.9 ranges​::​size [range.prim.size]

The name ranges​::​size 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:
[Note:
Diagnosable ill-formed cases above result in substitution failure when ranges​::​size(E) appears in the immediate context of a template instantiation.
— end note]
[Note:
Whenever ranges​::​size(E) is a valid expression, its type is integer-like.
— end note]