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kotlinlang

I was surprised this compiled:
```interface Uri {
  val fragment: String?
}

class Path : Uri {
  override val fragment: Nothing? = null
}

fun main() {
  val p: Path = Path()
  val x: String? = p.fragment?.substring(0, 1)
  println(x)
}```
I would have hoped the covariant return type of `Nothing?` would have meant the compiler stopped me using `fragment` as a `String?` given `p` is typed as a `Path`.

Actually it works because `substring` is an extension function. `Nothing` can substitute for any type, so you can actually always call _any extension function_ of any type on a value of type `Nothing`. The fact that your `fragment` overrides a property of type `String` doesn’t make a difference one way or the other. This also compiles:
```fun main() {
    val n: Nothing? = null
    println(n?.substring(0, 1))
}```

Ah, thanks! Missed it was an extension function.

Fun fact: IntelliJ deliberately avoids doing autocompletion for extensions on `Nothing`, otherwise it would end up suggesting every extension function in the world