Kotlin 编译器无法确定变量在 do-while 循环中不可为 null

问：

我有以下方法。它的逻辑非常简单，如果设置了 right，则在它有一个值（不是 null）时调用 left。当我以以下方式编写它时，它起作用了。

fun goNext(from: Node): Node? {
    var prev : Node = from
    var next : Node? = from.right
    if (next != null) {
        prev = next
        next = next.left
        while (next != null) {
            prev = next
            next = next.left
        }
    }
    return prev
}

相反，如果我尝试使用 do-while 循环缩短代码，则它不再智能转换为 .它显示以下错误：nextNode

Type mismatch.
Required: Node<T>
Found: Node<T>?

代码如下：

fun goNext(from: Node): Node? {
    var prev : Node = from
    var next : Node? = from.right
    if (next != null) {
        do {
            prev = next // Error is here, even though next can't be null
            next = next.left
        } while (next != null)
    }
    return prev
}