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kotlinlang

Can anyone explain why the compiler can't smart cast `b` in `b.isEmpty()` but it does for `a` in `a.isEmpty()`?

```
val a: String? = null
val b: String? = null
when {
  a == null &amp;&amp; b == null -&gt; true
  a == null -&gt; b.isEmpty()
  b == null -&gt; a.isEmpty()
}
```

Someone should correct me if I'm wrong but in this case it can smart cast `a` because you null check it prior so it cannot be null by the time it gets to executing `isEmpty()` while `b` can be null and the compiler can't 'smart cast'. You can fix this by null checking `b` prior and handing the null case or using `b?.isEmpty() ?: true`.

To clarify the 'smart cast' in this scenario is the cast from nullable to nonnull types. E.g `Boolean?` -&gt; `Boolean`

<@UBGKY6L2F> 
Assuming the first clause of the where is false and the second is true:

1. `a` and `b` cannot both be null
2. `a` is null
3. Since `a` is null and both `a` and `b` cannot be null, `b` must be not null