interface Gen<A : Gen.A, B : Gen.B> {
    fun a(): A
    fun b(): B

    open class A
    open class B
}

class Values

class Scope<A : Gen.A, B : Gen.B>(
    val gen: Gen<A, B>
) {
    operator fun String.invoke(values: context(Values) B.() -> Unit) {
        with(Values()) {
            values(this, gen.b())
        }
    }
}

operator fun <A : Gen.A, B : Gen.B> Gen<A, B>.invoke(
    rules: context(Scope<A, B>) A.() -> Unit
) {
    with(Scope(this)) {
        rules(this, a())
    }
}

object MyGen : Gen<MyGen.A, MyGen.B> {
    override fun a() = A()
    override fun b() = B()

    open class A : Gen.A() {
        val a = "a"
    }

    open class B : Gen.B() {
        val b = "b"
    }
}

fun main() {
    MyGen {
        a {
            // `this` is `Any?`, but should be `MyGen.B`
            println(b)  // Error: Unresolved reference: b
        }
    }
}

this

MyGen.B

a

public final operator fun String.invoke(
    values: @ContextFunctionTypeParams(count = 1)() (P.() -> Unit)
): Unit

operator fun <A : Gen.A, B : Gen.B> Gen<A, B>.invoke(
    rules: context(Scope<A, B>) A.(TypeWrapper<B>) -> Unit
) {
    with(Scope(this)) {
        rules(this, a(), TypeWrapper.IMPL)
    }
}
fun main() {
    MyGen {
        a {
            scopeId()
            // `this` now shows `MyGen.B`
            println(b)  // Error: Unresolved reference: b
        }
    }
}

fun <S: Gen.A, P: Gen.B> Scope<S, P>.scopeId() = this

TypeWrapper