Cast to constrained protocol can not access default method - swift

Question

I'm trying to access a default method implementation defined in a protocol extension that's constrained in implementation to a class. A 'regular' declaration works fine, however when I try to cast to the protocol I can not access the default defined method on the protocol, though the type satisfies the where clause.
Please consider this example:
class Person {
var name: String
init(name: String) {
self.name = name
}
}
class Hero: Person {
var ability: String
init(name: String, ability: String) {
self.ability = ability
super.init(name: name)
}
}
class Pilot: Person {
var callSign: String
init(name: String, callSign: String) {
self.callSign = callSign
super.init(name: name)
}
}
class Programmer: Person {
var favoriteLanguage: String
init(name: String, favoriteLanguage: String) {
self.favoriteLanguage = favoriteLanguage
super.init(name: name)
}
}
// define a protocol
protocol PersonPresenter: class { }
// extend it where the conformer is a type of Person
extension PersonPresenter where Self: Person {
func displayName() {
print(name.uppercased())
}
}
// conform subclasses of Person to PersonPresenter
extension Hero: PersonPresenter { }
extension Pilot: PersonPresenter { }
extension Programmer: PersonPresenter { }
let myHero = Hero(name: "Hiro", ability: "Bend time & space")
myHero.displayName() // prints 'HIRO'
let myPilot = Pilot(name: "Pete", callSign: "Maverick")
myPilot.displayName() // prints 'PETE'
let myProgrammer = Programmer(name: "Chris", favoriteLanguage: "Swift")
myProgrammer.displayName() // prints 'CHRS'
let people: [Person] = [myHero,myPilot,myProgrammer]
if let presenter = people[0] as? PersonPresenter {
presenter.displayName() // Errror, PerseonPresenter is not a subtype of 'Person'
}
I would like to find a way to cast to PersonPresenter while satisfying the where constraint so as not to be forced to try and cast to each specific sub-class to access the default implementation of the protocol extension. Or to not have to conform the super class (which may be used in many other places) to the protocol.

Answer 1

The real problem is that you are extending an empty protocol.
By casting to Hero, the compiler knows it is a subclass of Person and then it satisfies the extension constraint.
But when casting to the protocol PersonPresenter itself, the compiler doesn't know if the constraint (being a Person) can be satisfied.
If you declare the requirement in the protocol, it will work:
protocol PersonPresenter: class {
func displayName()
}