24 Ranges library [ranges]

24.3 Range access [range.access]

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

The name size denotes a customization point object.
The expression ranges::size(E) for some subexpression E with type T is expression-equivalent to:
[Note
:
Whenever ranges::size(E) is a valid expression, its type models Integral.
end note
]