Swift Struct Decoder initializer compiler error

Question

我试图更好地理解解码器初始化器和 JSON 解码。此代码运行并产生预期结果：

let json = """
[
    {
        "firstName": "Jon",
        "lastName": "Dough"
   },
    {
        "firstName": "Jane",
        "lastName": "Dough"
    }
]
""".data(using: .utf8)!

struct Character: Codable {
    var firstName: String
    var lastName: String
    
     enum CodingKeys: String, CodingKey {
        case
            firstName,
            lastName
    }
 
    init(from decoder: Decoder) throws {
        
        let data = try decoder.container(keyedBy: CodingKeys.self)

        firstName = try data.decode(String.self, forKey: .firstName)
        lastName = try data.decode(String.self, forKey: .lastName)
    }
}

let decoder = JSONDecoder()
func doDecode() -> [Character] {
    do {
        var cast = try decoder.decode([Character].self, from: json)
        return cast
    } catch {
        print("Error")
    }
    return []
}

let cast = doDecode()

print("# Cs \(cast.count)")

当我尝试添加辅助函数时，

extension Character {
    private func tryDecode<T: Codable>(_ data: KeyedDecodingContainer<CodingKeys>, type: T.Type, forKey: CodingKeys, empty: T) -> T {
        do {
            let result = try data.decode(T.self, forKey: forKey)
            return result
        } catch { return empty }
    }
}

我得到一个编译器错误：

初始化前使用的变量“self.firstName”

当我从初始化程序中调用它时：

   firstName = tryDecode( data, type: String.self, forKey: Character.CodingKeys.firstName, empty: "")

我输入了“forKey:”，有和没有完整路径。同样的结果。

我做错了什么？为什么编译器不知道“forKey:”参数指的是枚举，而不是 self.firstName？

我在 Xcode playground 中运行最新的 Xcode 和 macOS 版本。