fun main() {
    val b = B()
    val c = C()
    val dcWithParameter = DC<B>(b)
    val dcWithoutParameter = DC(b)
}

interface A {
    //...
}

data class DC<T : A>(
    val a: A
    //...
)

class B : A {
    //...
}

class C : A {
    //...
}