class View {}
protocol Delegate: class {}
class ViewAndDelegate: View, Delegate {}

class Subview<T>: View where T: View, T: Delegate {
    func something(argument: View) {
        guard let subtype = argument as? Subview<T> else { return }
        // do something with `subtype`
        let _ = subtype
        print("does not fail")
    }
}

let subview = Subview<ViewAndDelegate>()
subview.something(argument: subview)