CurrentState

DesiredState

val x = when (CurrentState to DesiredState) { ... }

else

sealed class A
class B : A()
class C : A()

sealed class X
class Y : X()
class Z : X()

fun test(a: A, x: X) {
    val result = when (a) {
        is B -> when (x) {
            is Y -> 1
            is Z -> 2
        }
        is C -> when (x) {
            is Y -> 3
            is Z -> 4
        }
    }
}

when

sealed class DesiredState: Parcelable {
    @Parcelize
    object MainPage: DesiredState()
    @Parcelize
    object AcceptCall: DesiredState()
}

sealed class DesiredState: Parcelable {
    @Parcelize
    object MainPage: DesiredState()
    @Parcelize
    object AcceptCall: DesiredState()
}

fun test(state: DesiredState) = when (state) {
    is DesiredState.MainPage -> "OK"
    is DesiredState.AcceptCall -> "Error"
}