[range.prim.empty]

The name ranges::empty denotes a customization point object ([customization.point.object]).

Given a subexpression ranges::empty(E) with type T, let t be an lvalue that denotes the reified object for E.

Then:

(2.1)
If T is an array of unknown bound ([basic.compound]), ranges::empty(E) is ill-formed.
(2.2)
Otherwise, if bool(t.empty()) is a valid expression, ranges::empty(E) is expression-equivalent to bool(t.empty()).
(2.3)
Otherwise, if (ranges::size(t) == 0) is a valid expression, ranges::empty(E) is expression-equivalent to (ranges::size(t) == 0).
(2.4)
Otherwise, if bool(ranges::begin(t) == ranges::end(t)) is a valid expression and the type of ranges::begin(t) models forward_iterator, ranges::empty(E) is expression-equivalent to bool(ranges::begin(t) == ranges::end(t)).
(2.5)
Otherwise, ranges::empty(E) is ill-formed.