解码 JSON 时 Swift Struct Decoder 初始化程序错误

我正在努力更好地理解解码器初始化器和JSON解码.此代码运行并生成预期结果:

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try 添加助手函数时，

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 }
    }
}

我收到一个编译器错误:

在初始化之前使用的变量‘self.firstName’

当我从初始值设定项调用它时:

init(from decoder: Decoder) throws {
    let data = try decoder.container(keyedBy: CodingKeys.self)
    firstName = tryDecode(data, type: String.self, forKey: Character.CodingKeys.firstName, empty: "")
    lastName = tryDecode(data, type: String.self, forKey: Character.CodingKeys.lastName, empty: "")
}

我输入了带有和不带有完整路径的"Forkey:".结果是一样的.

我做错了什么？为什么编译器不知道"Forkey:"参数引用的是枚举，而不是self.firstName？

我正在Xcode操场上运行最新发布的Xcode&amp；MacOS操作系统.