Why 'is' keyword only work with open classes in kotlin? - kotlin

Question

My question might be noob but please help me. I don't understand what is purpose of not allowing non-open classes with "is" keyword in kotlin.
Sample code 1
fun main(){
val randomclassobject = RandomClass()
println(randomclassobject is someRandomInterface)
}
open class RandomClass{
}
interface someRandomInterface{
fun mustImplementThis()
}
The above code works perfectly fine
Now
Sample code 2
fun main(){
val randomclassobject = RandomClass()
println(randomclassobject is someRandomInterface)
}
class RandomClass{
}
interface someRandomInterface{
fun mustImplementThis()
}
without open keyword it shows the error "Error:(3, 34) Kotlin: Incompatible types: someRandomInterface and RandomClass"
Why open keyword really matters?

Answer 1

When you write it like this
class RandomClass {
}
interface SomeRandomInterface {
fun mustImplementThis()
}
It is not possible for any object to be an instance of both RandomClass and SomeRandomInterface because RandomClass itself does not implement SomeRandomInterface and it cannot have any subclasses that implement it either because it is not open (Kotlin classes by default cannot be extended unless you add open).
Since the compiler knows that this check cannot return true, it marks it as an error. Most other languages would probably just warn you that the check is useless, but Kotlin makes it illegal entirely.
On the other hand, when you write
open class RandomClass {
}
interface SomeRandomInterface {
fun mustImplementThis()
}
even though the class itself does not implement the interface, it could have a subclass that implements it, for example
open class RandomClass {
}
interface SomeRandomInterface {
fun mustImplementThis()
}
class RandomSubClass : RandomClass(), SomeRandomInterface {
fun mustImplementThis() {}
}
which means that the check can return true, so the compiler allows it in that case.