Associated types can have defaults in Swift - which is super useful for types that are not easily inferred (for example when they're not used for a specific instance method or property).

protocol Identifiable {
    associatedtype RawIdentifier: Codable = String

    var id: Identifier<Self> { get }
}

struct User: Identifiable {
    let id: Identifier<User>
    let name: String
}

struct Group: Identifiable {
    typealias RawIdentifier = Int

    let id: Identifier<Group>
    let name: String
}