typealias が protocol に復活したときに typealias Element = Iterator.Element いけるじゃん！みたいなので騒いでいたのは事実で https://github.com/apple/swift/pull/1610

[Parse/AST/Sema] Split parsing for typealias & associatedtype, allow typealias in protocols and generic constraints by gregomni · Pull Request #1610 · apple/swift

Split up parsing of typealias and associatedtype, including dropping a now unneeded ParseDeclOptions flag. Then made typealias in a protocol actually valid, which works great except that you can't ...