Kotlin: Constructor of inner class can be called only with receiver of containing class

We can use it this way too

OuterClassName().NestedClassName()

Your code basically means that InnerClassX is an inner class of InnerClassSuper, not OuterClass, so the error means you need to provide a receiver object of InnerClasssSuper upon construction of InnerClassX.

At this point, Kotlin allows having neither an inner sealed class nor a derived class for a sealed class as an inner class of another class.

You can, however, make an abstract class derived from the sealed one and inherit from it inside the OuterClass:

sealed class SealedClassSuper {
    abstract class SealedClassChild(val x: String): SealedClassSuper()
}

class OuterClass {
    inner class InnerClassX(x: String) : SealedClassSuper.SealedClassChild(x)

    fun someFun(): SealedClassSuper {
        return InnerClassX("Hello")
    }
}