模式匹配拒绝识别成员类型（值X不是_2的成员）

Question

In the following situation Scala seems to not acknowledge the type of system :

sealed trait Bar[S] {
  def system: S
}

trait Foo { def bar(): Unit }

trait FooBar extends Bar[Foo]

If I'm not completely confused, this means that I should be able to do the following:

def test(fb: FooBar) { fb.system.bar() }  // ok, this works

But the following fails:

trait Test[S] {
  val bar: Bar[S]

  bar match {
    case fb: FooBar => fb.system.bar() // error: value bar is not a member of _2
    case _ =>
  }
}

Is this a bug in the pattern matcher, or am I missing a crucial bit?

---

EDIT : Note, the following works:

trait Test[S] {
  val bar: Bar[S]

  bar match {
    case fb: FooBar => (fb: FooBar).system.bar()
    case _ =>
  }
}

I guess I should file a bug?

Answer 1

Type erasure is definitely involved here, but I'm not 100% sure that what you are seeing is a bug or expected behavior. Because you declare the bar val as Bar[S] , S gets erased and the pattern matcher does not have the underlying type to work with. You can see the erasure warning by changing your match to:

case fb:Bar[Foo] =>

The odd thing is that if you declared bar as type Any your original match would work. Also, if you hover over fb in your match statement, you can see that it's being treated as:

Bar[S] with FooBar

Again, I believe this all goes back to how bar is being declared. Not sure this is a bug though and I imagine someone with more experience in the inner workings of the pattern matcher might have a more in depth explanation.