C.foo D.foo

gistfile1.kt

open class A {
    val c1 = 1
}
class B: A() {
    val c2 = 2
}

open class C
open class D: C()

fun C.foo(f: A.() -> Unit) {}
fun D.foo(f: B.() -> Unit) {}

fun test() {
    D().foo {
        c1
    }
}