How to create and GADT type with Kotlin

Question

What I would like to declare in Kotlin is an operation with different possible results with what is called GADT (Generalized algebraic data type) using a declaration like this:

sealed interface Response<T> {
    data object Pending: Response<T>
    data class Ok(val result: T): Response<T>
    data class Error(val reason: String): Response<T>
}

But sadly the declared type T is not propagated to the internal declarations.

This is something rather normal in other functional programming languages like Scala, Haskell or Elm but it looks that for Kotlin this requires a different syntax or maybe is even impossible in Kotlin.

Someone knows a possible answer?