24 Ranges library [ranges]

24.6 Range factories [range.factories]

24.6.2 Single view [range.single]

24.6.2.3 views​::​single [range.single.adaptor]

The name views​::​single denotes a customization point object ([customization.point.object]).
For some subexpression E, the expression views​::​single(E) is expression-equivalent to single_­view{E}.