protocol P {
 init()
 init(str: String)
 }

 extension P {
 init() {
 self.init(str: "some default")
   }
  }

  struct S : P {
  var str: String
 }

let s = S()
 print(s.str) // prints - some default

class S : P {
var str: String {
init() {
self.init(str: String)
}