Creating Java witnesses for nullable types

How do we implement a Java interface like this in Kotlin with a nullable (!) type for T?

interface Foo<T> {
    Class<T> typeWitness();

Real use cases include jOOQ’s Converter, and I guess every library that relies on Java reflection will have similar cases.

Now, in Kotlin we observe:

import kotlin.reflect.full.*
import kotlin.reflect.jvm.javaType

class Bar: Foo<String?> {
    override fun typeWitness(): Class<String?> {
        var result: Class<String?>

        // Okay but unchecked cast:
        result = as Class<String?>

        // Nope, none of those compile:
        result = String?::javaClass
        result = String?
        result =

        // A more complicated route, but _still_ requires the unsafe cast
         result = String::class.createType(nullable = true).javaType
            as Class<String?>
        // (!) BUT: 
        //     An operation is not implemented: 
        //     Java type is not yet supported for types created 
        //     with createType

        return result

So the only variant that works uses an unsafe cast (which we know to not cause a runtime error ever, but still). Of course, we can hide the unpleasantness away:

fun <T> Class<T>.nullable() : Class<T?> {
        return this as Class<T?>

But that still leaves a taste of defeat. Should there not be a statically type safe way to construct a Class instances for nullable types? String? strikes me as obvious candidate.

Side Note: If we implement the interface in Java with all the annotations:

public class JBar implements Foo<@Nullable String> {
    @Override @NotNull
    public Class<@Nullable String> typeWitness() {
        return String.class;

We might expect that, after mapping, JBar : Foo<String?>. But:

val f: Foo<String> = JBar()

compiles. Bug or Feature?