一个合理的解决方案是具有关联类型的枚举

One reasonable solution is an enum with associated type(s)

struct User : Decodable {
    let userId: Int
    let userName: String
}

enum Result : Decodable {
    case success(User), failure

    enum CodingKeys: String, CodingKey { case status, data }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        let status = try container.decode(String.self, forKey: .status)
        if status == "success" {
            let userData = try container.decode(User.self, forKey: .data)
            self = .success(userData)
        } else {
            self = .failure
        }
    }
}

并使用它

do {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase
    let result = try decoder.decode(Result.self, from: data)
    switch result {
      case .success(let user): print(user)
      case .failure: print("An error occurred")
    }
} catch { print(error) }