"unreachable code" warning on reachable code

[scabug]

Consider the following code which I've put in a file "unreachable.scala":

trait T1 {}

sealed abstract class Base

case class A() extends Base
case class B() extends Base

object O1 {
  def fn[V](x: Base with T1) = {
    x match {
      case a: A => "A"
      case b: B => "B"
    }
  }
}

object Main {
  def main(args: Array[String]) = {
    import O1.fn

    println(s"fn(fn(new A() with T1)) => ${fn(new A() with T1)}")
    println(s"fn(fn(new B() with T1)) => ${fn(new B() with T1)}")
  }
}

I get a "warning: unreachable code" for line 14 (the "case b: B" statement above) when I compile this.

However when I run p1.Main that line is clearly reachable, because "B" is output.

[jluxenberg-workstation ~]$ ~/Downloads/scala-2.11.7/bin/scalac unreachable.scala
unreachable.scala:14: warning: unreachable code
      case b: B => "B"
                   ^
one warning found
[jluxenberg-workstation ~]$ ~/Downloads/scala-2.11.7/bin/scala -classpath . p1.Main
fn(fn(new A() with T1)) => A
fn(fn(new B() with T1)) => B

Might be related to #7369

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9505?orig=1
Reporter: Jared Luxenberg (jluxenberg)
Affected Versions: 2.11.7
See #7369
Attachments:

• unreachable.scala (created on Oct 6, 2015 1:16:04 AM UTC, 434 bytes)

[scabug]

@JustinPihony said:
Just as a note that the below code seems to be similar and have the same problem, however it can be remedied by marking the abstract classes as sealed:

sealed trait Result

abstract class FullResult(values: Int) extends Result

abstract class PartialResult(values: Int) extends Result

case class GetResult(events: Int) extends FullResult(events) with Result

case class PGetResult(events: Int, isLast: Boolean) extends PartialResult(events) with Result

object Test {
  def handle(r: Result) = r match {
    case res: GetResult => res.events
    case res: PGetResult => res.events
    case res => 42
  }
}

[pabloazul]

Still occurs in 2.13.0

[SethTisue]

This seems pretty interesting and surprising, somebody want to look into it?

[Jasper-M]

There's also #11457 which looks like it could be a duplicate, but that one works correctly in 2.12 and 2.11. I think I've looked into that one for a bit, but didn't find anything. All I remember is that the code in this area is very scary.
There's a bunch of other seemingly related tickets if you search for "unreachable".