Pattern Matching Bug When matching against case classes defined in trait

[scabug]

When pattern matching, compiler :

• doesn't emit the "match is not exhaustive" warning (it should)
• emits "unreachable code" (it should not)

trait Hierarchy {
  sealed trait Expr
}
trait If {
  this: Hierarchy =>
  case class If(cond: Expr, yes: Expr, no: Expr) extends Expr
}
trait Word {
  this: Hierarchy =>
  case class Word(name: String) extends Expr
}
trait IntExpr {
  this: Hierarchy =>
  case class IntExpr(value : Int) extends Expr
}

object SimpleExpr extends Hierarchy with If with Word with IntExpr
//object OtherExpr extends Hierarchy with If with IntExpr

object Demo extends App {
  import SimpleExpr._
  def func(expr: Expr) = expr match {
    case If(cond, yes, no) => cond
    case Word(name) => name
    // compiler should emit warning "missing case statement"
    // emits the wrong warning "unreachable code"
  }
}

see also here : http://stackoverflow.com/questions/35259337/scala-compiler-emits-no-warning-when-pattern-matching-against-a-sealed-trait

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9672?orig=1
Reporter: Julien Harbulot (JulienH)
Affected Versions: 2.11.7

[hrhino]

potentially related, and simpler if so:

  trait M
  sealed trait A
  class B extends A
  class C extends A

  val k: A with M = new B with M {}

  k match {
    case _: B => ???
    case _: C => ??? // unreachable code warning here
  }

(reported on gitter by @eugengarkusha)

[martijnhoekstra]

Another variation reported on gitter by @disblader

sealed trait Test

class TestOne(val id: Int) extends Test
class TestTwo(val id: String) extends Test

trait Testorino { this: Test =>
  val reachable = this match {
    case _: TestOne => false
    case _: TestTwo => true
  }
}

object Leaf extends TestTwo("two") with Testorino

Leaf.reachable //true