Protocol extension dynamic dispatch

Reproduce.swift

protocol Test {
  func doStuff()
}

extension Test {
  // default protocol implementation
  func doStuff() {
    print("Stuff!")
  }
}

// Using default implementation
class Implementor: Test {
  
}

// subclassing and overriding implementation
class Implementor2: Implementor {
  func doStuff() {
    print("Not stuff!")
  }
}

let y: Test = Implementor2()
y.doStuff() // "Stuff!" is printed?

class Implementor: Test {
  func doStuff() {
    print("Stuff!")
  }
}

class Implementor2: Implementor {
  override func doStuff() {
    print("Not stuff!")
  }
}

let y: Test = Implementor2()
y.doStuff() // "Not stuff!" is printed