reified

null is T

fun <T : Any> withNonNull(foo: T)

private infix fun <Key, T, Value, Return : Value> <http://Key.at|Key.at>(map: MutableMap<Key, Value>): ReadWriteProperty<T, Return?> {
    return object : ReadWriteProperty<T, Return?> {
        override fun setValue(thisRef: T, property: KProperty<*>, value: Return?) {
            if (value != null) { // should be only done if Value is non nullable type
                map[this@at] = value
            }
        }

        override fun getValue(thisRef: T, property: KProperty<*>) = map[this@at] as Return?
    }
}

Value

a

a = null