fun main() { // 找到一家肯德基 // ↓ val kfc = Restaurant<KFC>() // 需要普通饭店，传入了肯德基，编译器报错 orderFood(kfc) val success:Result.Success<Cat> = Result.Success<Cat>(Cat()) println(success.data) var s:Result.Success<Animal> = success s.data = Dog() println(success.data as Cat) }

sealed class Result<out R> { // 协变 ① // ↓ ↓ data class Success<out T:Animal>(var data: @UnsafeVariance T, val message: String = "") : Result<T>()

data class Error(val exception: Exception) : Result<Nothing>()

data class Loading(val time: Long = System.currentTimeMillis()) : Result<Nothing>()

}