Pattern match on this fails to compile when type parameter varies by subclass

[scabug]

sealed trait Base
case class Up() extends Base
case class Down() extends Base

sealed trait Tracker[B <: Base] {

  def bar = this match {
    case UpTracker() => ???
    case DownTracker() => ???
  }

}
case class UpTracker() extends Tracker[Up]
case class DownTracker() extends Tracker[Down]

produces

PatternMatch.scala:8: error: constructor cannot be instantiated to expected type;
 found   : UpTracker
 required: Tracker[B]
    case UpTracker() => ???
         ^
PatternMatch.scala:9: error: constructor cannot be instantiated to expected type;
 found   : DownTracker
 required: Tracker[B]
    case DownTracker() => ???
         ^
two errors found

This does not occur if we move the method into a separate object:

sealed trait Base
case class Up() extends Base
case class Down() extends Base

sealed trait Tracker[B <: Base]
case class UpTracker() extends Tracker[Up]
case class DownTracker() extends Tracker[Down]

object Foo {
  def bar[B <: Base](t: Tracker[B]) = t match {
    case UpTracker() => ???
    case DownTracker() => ???
  }
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9394?orig=1
Reporter: Robin Green (ST_robin_green)
Affected Versions: 2.11.7

[scabug]

@retronym said:
Another workaround is to modify the original example with:

  def bar = (this: Tracker[_ <: B]) match {
    case UpTracker() => ???
    case DownTracker() => ???
  }

[scabug]

@retronym said:
Potential fix: scala/scala#4625