24 Ranges library [ranges]

24.3 Range access [range.access]

24.3.10 ranges​::​empty [range.prim.empty]

The name empty denotes a customization point object.
The expression ranges::empty(E) for some subexpression E is expression-equivalent to:
[Note
:
Whenever ranges::empty(E) is a valid expression, it has type bool.
end note
]