在swift中使用上限扩展泛型类型

Question

以下是代码：

// won't compile!
extension Array where Element: JsonSerializable : JsonSerializable {
    var JsonRepresentation: AnyObject {
        var representation = [AnyObject]()
        for obj in self {
            representation.append(obj.JsonRepresentation)
        }
        return representation as AnyObject
    }
}

protocol JsonRepresentable {
    var JsonRepresentation: AnyObject {get}
}

protocol JsonSerializable: JsonRepresentable {
}

extension JsonSerializable {
    var JsonRepresentation: AnyObject {
        var representation = [String: AnyObject]()
        for case let (label?, value) in Mirror(reflecting: self).children {
            switch value {
            case let value as JsonRepresentable:
                representation[label] = value.JsonRepresentation
            case let value as AnyObject:
                representation[label] = value as AnyObject
            default:
                break

            }
        }
        return representation as AnyObject
    }

    func toJson() -> String? {
        let representation = JsonRepresentation
//        guard JSONSerialization.isValidJSONObject(representation) else {
//            return nil
//        }
        do {
            let data = try JSONSerialization.data(withJSONObject: representation, options: [.prettyPrinted])
            return String(data: data, encoding: String.Encoding.utf8)
        } catch {
            return nil
        }
    }
}

要点是我想要一个JsonSerializables数组也是JsonSerializable。我怎样才能做到这一点？