struct JSON {
    let json: [String: Any]
    func value<Value>(_ key: String) throws -> Value {
        guard let value = json[key] as? Value else { throw NSError() }
        return value
    }
}

protocol JSONDeserializable {
    init(json: JSON) throws
}

protocol UserModel: JSONDeserializable {
    var username: String { get set } // Unwanted requirement (UR) #1: property needs "set" so that it can be initialized within protocol
    init() // UR2: needs empty init, because of default implementation of `init(json: JSON)` in `extension UserModel`
}

extension UserModel {
    init(json: JSON) throws {
        self.init() // UR3: Needs to call this otherwise compilation error: `'self' used before chaining to another self.init requirement`
        username = try json.value("username")
    }
}

struct UserStruct: UserModel {
    // UR4: property cannot be `let`, beause of `set` in protocol.
    var username: String = "" // UR5: Property have to have default value because of it being a empty init
    init() {}
}

final class UserClass: NSObject, UserModel {
    // UR6: analogue with UR4
    var username: String = "" // UR7: analogue with UR5
}

let json: JSON = JSON(json: ["username": "Sajjon"])
let u1 = try UserStruct(json: json)
let u2 = try UserClass(json: json)
print(u1.username) // prints "Sajjon"
print(u2.username) // prints "Sajjon"