Kotlin

data class CustomerData<out T : String?>(
    val id: T,
    val name: String,
)

typealias AnyCustomer = CustomerData<*>
val AnyCustomer.isSaved get() = id != null

typealias SavedCustomer = CustomerData<String>
typealias UnsavedCustomer = CustomerData<Nothing?>

@Suppress("UNCHECKED_CAST")
fun UnsavedCustomer.saved(id: String) = (this as SavedCustomer).copy(id)

// Ok
val saved = SavedCustomer("123", "Name")
val unsaved = UnsavedCustomer(null, "Fred")

// Type mismatch
val err1 = SavedCustomer(null, "name")    // Null can not be a value of a non-null type String
val err2 = UnsavedCustomer("123", "Name") // inferred type is String but Nothing? was expected
val err3: String = unsaved.id     // inferred type is Nothing? but String was expected
val err4: SavedCustomer = unsaved // inferred type is UnsavedCustomer but SavedCustomer was expected
val err5: UnsavedCustomer = saved // inferred type is SavedCustomer but UnsavedCustomer was expected