Swift Class Extension Only When Conforming to Protocol

Question

Is it possible to provide an extension that only adds functionality to classes conforming to a protocol? The functionality I'm trying to achieve is something like this:

protocol Identifiable {
  var id: String { get }
}

class Model {
  func report(data: String) {
    ...
  }
}

class Thing: Model, Identifiable {
  var id: String
  ...
}

class Place: Model, Identifiable {
  var id: String
  ...
}

extension (Model + Identifiable) {

  func identifiy() {
    report("\(self.id)")
  }

}

// Invalid: Model().identify()
Place().identify() // OK
Thing().identify() // OK

Extending the protocol itself isn't possible because the extension requires access to methods defined on model. Extending model fails because the id is only defined on the child objects. Extending Model: Identifiable fails because Model does not conform to protocol Identifiable.

Answer 1

You could extend Identifiable, you just need to specify that Self is a Model, or subclass of Model:

extension Identifiable where Self: Model {
    func identifiy() {
        report(id) 
        // `id` is already a `String` so there's 
        // no need to use String Interpolation.
    }
}