↓Kotlinでやってみた。

// Kotlin open class Animal() class Cow : Animal() class Sheep : Animal() class Fence<out T> { var animal: T? = null } class Farm<out T> { val fence: Fence<T> = Fence<T>() } fun buySheep(farm: Farm<Animal>) { farm.fence.animal = Sheep() } fun main(args: Array<String>) { val farm = Farm<Cow>() buySheep(farm) }

予想通り Fence の animal でコンパイルエラー。

$ kotlinc NestedGenerics.kt NestedGenerics.kt:6:17: error: type parameter T is declared as 'out' but occurs in 'invariant' position in type T? var animal: T? = null ^