DynamicNodeEncoding Doesn't Work with Optional

[owenzhao]

// Model

class Bar:Codable, DynamicNodeEncoding {
    enum CodingKeys: String, CodingKey {
        case id
    }

    static func nodeEncoding(for key: CodingKey) -> XMLEncoder.NodeEncoding {
        switch key {
        case CodingKeys.id:
            return .attribute
        default:
            return .element
        }
    }

    var id = Int.random(in: 1...1000_000)
}

// code

    override func viewDidLoad() {
        super.viewDidLoad()

        let bar = Bar()
        output(bar)

        print()

        let foo:Bar? = bar
        output(foo)
    }

    private func output<T:Encodable>(_ t:T) {
        let encoder = XMLEncoder()
        encoder.outputFormatting = .prettyPrinted
        let encodedXML = try! encoder.encode(t, withRootKey: "bar")

        let str = String(data: encodedXML, encoding: .utf8)
        print(str!)
    }

// result

<bar id="974440" />

<bar>
    <id>974440</id>
</bar>

As you can see from the results, they were different. We expected they should be the same.