template< bool Const >
V is not a simple view,
Const is true for sentinels returned from the const overloads, and false otherwise. If
V is a simple view,
Const is true.
The name of this class template (shown here as /*sentinel*/) is unspecified.
Typical implementation holds only one data member: a sentinel
end_ obtained from (possibly const-qualified)
 Member types
|| const ranges::join_view<V> if Const is |
|| const V if Const is |
 Member functions
| constructs a sentinel |
(public member function)
 Non-member functions
| compares a sentinel with an iterator returned from join_view::begin |