Why no Type Erasure Warning

Question

After reading this excellent question/answer on type erasure in Scala, I tried this code. The Scala compiler did not output a type erasure warning.

scala> val x: List[Int] = List(1,2,3)
x: List[Int] = List(1, 2, 3)

scala> x match {
     |   case List(x: Int) => println("a")
     |   case _ => println("false")
     |  }
false

Why doesn't the above code output the same warning as this code:

scala> List(1,2,3) match {
     |  case l: List[String] => println("list of strings")
     |  case _ => println("ok")
     | }
<console>:9: warning: fruitless type test: a value of type List[Int] cannot 
also be a List[String] (but still might match its erasure)
                   case l: List[String] => println("list of strings")
                           ^
    list of strings

Answer 1

第一种情况不仅仅是测试类型 - 它通过模式匹配进行测试，列表只有一个整数元素。