class Superclass<T: UIView>: UIViewController { ... }

class Subclass: Superclass<UIButton> { ... }

let childViewController: Superclass<UIView>?
    switch type {
    case .typeOne:
      childViewController = Subclass()
    default:
      childViewController = nil
    }

Cannot assign value of type 'Subclass' to type 'Superclass<UIView>?'