open class Test {
  open fun SomeClass.test(){}
}

class Test2 : Test() {
  override fun SomeClass.test() {
    super.test() // is compilation error
  }
}

abstract class Base {
    abstract fun String.foo()

    open fun applyFoo(s: String) {
        s.foo()
    }
}

open class Derived : Base() {
    override fun String.foo() {
        // do something
    }
}

class Impl : Derived() {
    override fun applyFoo(s: String) {
        super.applyFoo(s) // calls `foo` from Derived
        // do something else
    }
}

fun use(base: Base, s: String) {
    base.applyFoo(s)
}