I'm not finding "Lightweight same-type requirements for primary associated types" in Language Docs

In the current Language Guide, and Language Reference, this 5.7 accepted evolution is not documented. The reference manual uses the grammar production rule: protocol-declaration → attributes? access-level-modifier? protocol protocol-name type-inheritance-clause? generic-where-clause? protocol-body which does not include productions for this evolution. Using for example something like: enum AnEnum<T,C:Collection<T>> { ...} is a fairly common syntax usage. Given this, should this usage not ...