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

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[eygraber]

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 && b == null -> true
  a == null -> b.isEmpty()
  b == null -> a.isEmpty()
}

[karn]

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.

[karn]

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

[eygraber]

@karn
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