暗黙の型変換出ない本物のサブタイピングである Kotlin なら何の問題もありませんね。 // Kotlin open class Animal class Cat: Animal() // Rule A // Cat is subtype of Animal run { val cat: Cat = Cat() val animal: Animal = cat } // Rule B // Int is subtype of Int? run { val int: Int = 0 val intOptional: Int? = int // 0 } // Rule C // Optional has covariance run { val catOptional: Cat? = null val animalOptional: Animal? = catOptional // null } // Question // What happen replace `Cat : Animal` to `Int : Int?` run { val intOptional: Int? = null val intOptionalOptional: Int?? = intOptional // null (Int?? == Int?) }
