23 Ranges library [ranges]

23.7 Range factories [range.factories]

23.7.2 Single view [range.single]

23.7.2.3 view​::​single [range.single.adaptor]

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