Hey guys! While reviewing PR I've spotted a smelly code.

fun x(anInt: Int) : String {
    when (anInt) {
        0 -> return "zero"
        1 -> return "one"
    }
    return null!!
}

Despite the 'quality` of code itself I'm curious why compiler thinks that

return null!!

is fine (IDE hints that return statement is unreachable which is false)? And similarly why this

val a : Int = null!!

is allowed as well.

Because it is as fine as

throw Exception()

null!!

has type

Nothing

, which is a subclass of every type. You can therefore return it from everything (for the same reason you can throw exceptions). The

return

is unreachable because there is an expression of type

Nothing

before it.

Cool beans, checked with decompiled code and make sense.

The type of

null

is `Nothing?`:

Yes, therefore the type of

null!!

becomes

Nothing

.

I'd move the return out of the when block and use an

else -> error("Invalid anInt=$anInt")