Swift constrained extension where fixed type is generic

example.swift

public protocol Route: Equatable {
    
    var previous: Self? { get }
    
}

internal enum InternalAction<RouteType: Route, MessageType> {

    case navigateToPreviousRouteAfterPop
    case action(Action<RouteType, MessageType>)
    
}

public final class Mailbox<MessageType> {
    
    fileprivate var subscribers: [(MessageType) -> ()] = []
    
    public func subscribe(subscriber: @escaping (MessageType) -> ()) {
        subscribers.append(subscriber)
    }
    
}

// This does not compile because Any: Route is not valid syntax.
extension Mailbox where MessageType == InternalAction<Any: Route, Any> {

}