智能转换为 'Type' 是不可能的，因为 'variable' 是一个可变属性，此时可能已经更改

问题：

And the Kotlin newbie asks, "why won't the following code compile?": Kotlin 新手会问，“为什么下面的代码不能编译？”：

var left: Node? = null
    
fun show() {
    if (left != null) {
        queue.add(left) // ERROR HERE
    }
}

Smart cast to 'Node' is impossible, because 'left' is a mutable property that could have been changed by this time智能转换到“节点”是不可能的，因为“左”是一个可变属性，此时可能已经改变

I get that left is mutable variable, but I'm explicitly checking left != null and left is of type Node so why can't it be smart-casted to that type?我知道left是可变变量，但我明确检查left != null并且left是Node类型，所以为什么不能将它智能转换为该类型？

How can I fix this elegantly?我该如何优雅地解决这个问题？

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解决方案：

参考一： https://en.stackoom.com/question/317Kj
参考二： https://stackoom.com/question/317Kj