Class-only protocols

If a protocol is to apply to types that are assumed to use reference semantics (rather than being passed by value), then we can create a class protocol that restricts its use to classes:

protocol ClassyProtocol: class 
{ 
  func addClassToClassesList() // or whatever 
} 

Any attempt to declare a value type, whether struct, Int or any other, will be met with a compiler error.