23 Ranges library [ranges]

23.8 Range adaptors [range.adaptors]

23.8.7 Join view [range.join]

23.8.7.5 view​::​join [range.join.adaptor]

The name view::join denotes a range adaptor object.
For some subexpression E, the expression view::join(E) is expression-equivalent to join_­view{E}.