swift VAR restriction of types

Question

how I can add restriction of types to var like this

var contentController: <UIViewController, MyProtocol> // wrong
var contentController: UIViewController where Self: MyProtocol // wrong

but it is wrong declarations for swift. For example what I want - true declarations for func

func setContentController<Controller:UIViewController>(_ controller: Controller) where Controller: MyProtocol

its fine work to func, but I do not add restriction for var or typealias