关于Kotlin smart cast不生效场景的困惑及验证问询

最近我在读《Kotlin in Action》这本书，里面有一段关于Smart Cast的描述：

The smart cast works only if a variable couldn’t have changed after the is check.

When you’re using a smart cast with a property of a class, as in this example, the property has to be a val and it can’t have a custom accessor. Otherwise, it would not be possible to verify that every access to the property would return the same value.

书中给出的示例代码是这样的：

interface Expr

class Num(val value: Int) : Expr

class Sum(val left: Expr, val right: Expr) : Expr

fun eval(e: Expr): Int {
    if (e is Num) {
        val n = e as Num
        return n.value
    }
    if (e is Sum) {
        return eval(e.right) + eval(e.left)
    }
    throw IllegalArgumentException("Unknown expression")
}

fun main(){
    println(eval(Sum(Sum(Num(1), Num(2)), Num(4))))
}

为了验证这个限制，我尝试了好几种情况：在is检查后修改对象实例和它的属性、用var属性代替val、还有使用自定义访问器，但所有情况下is检查都正常工作了。

我的测试代码如下：

interface face1

class A(): face1{
    var a = 1
    get() = field
    set(value){field = field+value}
}

class B(): face1{
    var b = 1
    get() = field
    set(value){field = field + value}
}

fun Test(x: face1){
    var value = x
    if(value is A){
        value = A()
        println("hello")
        value.a = 123
        println(value.a)
    }
    if(value is B)
        println(value.b)
}

fun main(args: Array<String>){
    val x: face1 = A()
    Test(x)
}

这就让我有点困惑了，书中这段描述到底是什么意思呢？

更新：

我甚至查了Kotlin官方文档，里面也写着：

Note that smart casts work only when the compiler can guarantee that the variable won't change between the check and its usage.
var properties: Never, because the variable can be modified at any time by other code.

但我的测试例子显然和这个描述不符，这是怎么回事？

备注：内容来源于stack exchange，提问作者VanechikSpace