crash.swift

class C {}

protocol P where Self: C {
  var x: Int { get }
  var y: Int { get }
}

extension P {
  var x: Int { return 42 }
}

class D: C, P {
  var y: Int { return x + 1 }
}

func f(vc: C) {
  if let vc = vc as? P {
    print(vc.x)
    print(vc.y)
  }
}

f(vc: D())