Does not crash compiler

NotCrash.swift

public protocol X {
    func _ok(a: Any)
}

public protocol StoreSubscriber: X {
    typealias StoreSubscriberStateType
    typealias AppStateType

    func newState(state: StoreSubscriberStateType)
    func substateSelector(state: AppStateType) -> StoreSubscriberStateType
}

extension StoreSubscriber {
    public func _ok(a: Any) {
        if let a = a as? StoreSubscriberStateType {
            newState(a)
        }
    }
}

struct A {
    var substate: Int
}

class Thing: StoreSubscriber {

    func newState(state: Int) {
        print(state)
    }

    func substateSelector(state: A) -> Int {
        return state.substate
    }

}

let substate = Thing().substateSelector(A(substate: 5))
Thing().newState(substate)