Smart cast to ‘Type’ is impossible, because ‘variable’ is a mutable property that could have been changed by this time

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Between execution of left != null and queue.add(left) another thread could have changed the value of left to null.

To work around this you have several options. Here are some:

  1. Use a local variable with smart cast:

     val node = left
 if (node != null) {
     queue.add(node)
 }

  2. Use a safe call such as one of the following:

     left?.let { node -> queue.add(node) }
 left?.let { queue.add(it) }
 left?.let(queue::add)

  3. Use the Elvis operator with return to return early from the enclosing function:

     queue.add(left ?: return)

    Note that break and continue can be used similarly for checks within loops.

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