cpp/ranges/join with view

@1@ A range adaptor that represents consisting of the sequence obtained from flattening a view of ranges, with every element of the delimiter inserted in between elements of the view. The delimiter can be a single element or a view of elements.

@2@ . The expression is expression-equivalent to  for any suitable subexpressions  and.

and have compatible elements if all of the following concepts are modeled:

models.

models when:
 * is a reference, and
 * and each model.

models when:
 * is a reference,
 * ,, and each models , and
 * and each model.

models when:
 * is a reference, and
 * and each model  and.