24 Ranges library [ranges]

24.7 Range adaptors [range.adaptors]

24.7.10 Join view [range.join]

24.7.10.5 views​::​join [range.join.adaptor]

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