How to make sealed classes generic in kotlin?

Via the Google Guidelines: https://developer.android.com/jetpack/docs/guide

sealed class Resource<T>(
        val data: T? = null,
        val message: String? = null
) {
    class Success<T>(data: T) : Resource<T>(data)
    class Loading<T>(data: T? = null, var refreshing: Boolean = false) : Resource<T>(data)
    class Error<T>(data: T? = null, message: String) : Resource<T>(data, message)
}

your Object can't have a generic type in Kotlin but this could be solved simply by following the example below:

sealed class ResponseState<out T> {
    object Loading : ResponseState<Nothing>()
    data class Error(val throwable: Throwable) : ResponseState<Nothing>()
    data class Success<T>(val item: T) : ResponseState<T>()
}

writing:

val _state = MutableLiveData<ResponseState<MessageModle>>() 


_state.postValue(ResponseState.Loading)

myNetworkCall { response, e
  if (e != null) _state.postValue(ResponseState.Error(e))
  else _state.postValue(ResponseState.Success(response))
}

reading:

state.observe(..., {state -> 
  when(state) {
    Loading -> showLoading()
    is Error -> showError(state.throwable)
    is Success -> onSuccess(state.item)
  }
}

It's not possible in Kotlin. Every type you use must have an explicitly declared class somewhere. Classes are not created implicitly by the compiler even in the case when nested classes are declared in the superclass.

For your problem, I recommend you rewrite the code from combining two inheritance-based hierarchies to one of the two combining inheritance and composition, or just restructure the hierarchy in some way, for example (I suppose the exact instance of a result would be irrelevant to you in case when it's not Loaded):

sealed class AsyncResult {
    object InFlight : AsyncResult()
    data class Error(val errorMessage: String) : AsyncResult()
    sealed class Loaded<out T>(val result: T) : AsyncResult() {
        sealed class UserDataAppResult(users: List<User>) : Loaded<List<User>>(users)
        sealed class CreateUserResult(user: User) : Loaded<User>(user)
    }
}