protocol CoolProtocol {
    var coolness: Int { get set }
}

protocol CoolClassProtocol: class, CoolProtocol {
   // if you re-declare coolness here all works as expected
   // var coolness: Int { get set }
}

class Cool {
    var coolness = 1
}

func incrementCool(_ cool: CoolClassProtocol) {
   // here you get compiler error saying cool should be var.
   // if you do it as in: 
   // var cool = cool
   // you'll get a warning telling you that cool is never mutated and should be made a let constant.
    cool.coolness += 1
}