Overcoming Cyclic Generic Parameters

Using Swift
8

I just had a closer look at the generic types (not the members), I think the reason why the compiler allowed is because it wasn‘t smart enough to catch the complexity of this pattern. I also think this smells like a derivation of CRTP which can be mind bending but still valid. Take this valid Swift 5 for example.

class G<T> {
  class S: G<S>
    var t: T
    init(_ t: T) {
      self.t = t
    }
  }
}

let a = G<Int>.S(42)
let b: G<G<Int>.S> = a // works but mind bending at first glance

I‘m on my iPad so I can‘t test your original example in Swift 5. CRTP was mostly implemented with Swift 5. I said mostly, because there is a potential CRTP pattern in protocols, which is currently rejected by the compiler.