I’m assuming that you’ve got a typo and meant if (feed.second != null).
feed.second is a property. Accessing a property causes some code to run. This code usually returns the same value, but that’s not guaranteed. Compiler can’t rely on that behaviour. Getter could do anything including formatting your hard disk driver or launching nuclear attack. Therefore compiler can’t assume that second invocation of feed.second would return the same value. So even if you’ve checked that value to be not null, second invocation might return null and cause NullPointerExcetion and Kotlin tries to avoid that at all costs.
You can extract that value into another variable:
val feedSecond = feed.second
and then use it. Now compiler can be sure that this value won’t change between calls and smart cast will work.
Another option is just to use when:
when (feed.second) {
HttpURLConnection.HTTP_NOT_FOUND -> call.respond(HttpStatusCode.NotFound)
null -> call.respond(HttpStatusCode.OK, feed.first!!)
}