Hello Florina Muntenescu and thanks for this post.

In this situation:

sealed class Result<out T : Any> {
    data class Success<out T : Any>(val data: T) : Result<T>()
    sealed class Error(val exception: Exception) : Result<Nothing>() {
        class RecoverableError(exception: Exception) : Error(exception)
        class NonRecoverableError(exception: Exception) : Error(exception)
        …
        class OtherErrors(exception: Exception) : Error(exception)
    }
    object InProgress : Result<Nothing>()
}

Latter when using when I wont be able to do something like:

when (result: Result<Int>) {
    ...
    is Result.RecoverableError,
    is Result.NonRecoverableError -> {
        // it will fail with "Unresolved reference: exception"
        result.exception
    }
    is Result.OtherErrors -> {
        ...
    }
}

How can we solve this?