Why does pattern match on case object fail when using -Xfuture

Question

The following code compiles fine without -Xfuture:

package sandbox

trait M

case object Obj extends M {
  def run: Unit = println("RUNNING")
}

object Sandbox extends App {
  (Obj: M) match {
    case x @ Obj => x.run
  }
}

But fails with it:

error: value run is not a member of sandbox.M
  case x @ Obj => x.run

Why is it assuming x to be of type M and not Obj.type?

By the way, it appears to compile fine with a case class.

Answer 1

它与您在模式匹配中期望的相等性有关。

https://issues.scala-lang.org/browse/SI-1503

scala> trait M
defined trait M

scala> trait X { def x: Int = 42 }
defined trait X

scala> case object O extends X with M
defined object O

scala> (O: M) match { case x @ O => x.x }
res0: Int = 42

scala> :replay -Xfuture
Replaying: trait M
defined trait M

Replaying: trait X { def x: Int = 42 }
defined trait X

Replaying: case object O extends X with M
defined object O

Replaying: (O: M) match { case x @ O => x.x }
<console>:14: error: value x is not a member of M
       (O: M) match { case x @ O => x.x }
                                      ^


scala> (O: M) match { case x: O.type => x.x }
res1: Int = 42

发出-Xlint警告：

scala> (O: M) match { case x @ O => x.x }
<console>:14: warning: The value matched by O is bound to x, which may be used under the
unsound assumption that it has type O.type, whereas we can only safely
count on it having type M, as the pattern is matched using `==` (see SI-1503).
       (O: M) match { case x @ O => x.x }
                               ^
res0: Int = 42