Compiler generates wrong code from some code using extractor

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I wrote following code and compiled it(scala 2.7.3.final).

case class Num(n : scala.Int) extends Term
case class Add(x : Term,y : Term) extends Term
object Value {
  def unapply(term : Term) : Boolean =
    term match {
      case Num(_) => true
      case Add(_,_) => false
    }
}

object App {
  def main(args: Array[String]) {
    val term = Add(Num(1), Add(Num(2),Num(3)))
    term match {
      case Add(Num(x),Num(y)) =>
        println("Add(Num, Num)")
      case Add(Value(),y) => // <- not match!
        println("Add(Value, ?)")
      case _ =>
        println("?")
    }
  }
}

I expects that "Add(Value, ?)" is displayed , but "?" is displayed.
Diassembling the compiled code, It seems that the compiler generates the code that skips pattern "Add(Value(),y)" when pattern "Num(y)" fails.

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Imported From: https://issues.scala-lang.org/browse/SI-1697?orig=1
Reporter: @kmizu

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Justin Wick (justinwick) said:
This issue appears to be solved in scalac as of scala-2.8.0.r18746-b20090923021937. fsc refuses to compile the code, however. Is that a bug?

Also, I motion that someone create a script to make simple combinations of standalone code and expected output into a ParTest test.

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@paulp said:
Replying to [comment:6 JustinWick]:

This issue appears to be solved in scalac as of scala-2.8.0.r18746-b20090923021937. fsc refuses to compile the code, however. Is that a bug?

I don't know what refusal it is giving you but fsc works for me. The exact case given above I fixed in r18184 but the general issue is not fixed so I left this open.

Also, I motion that someone create a script to make simple combinations of standalone code and expected output into a ParTest test.

I second the motion! All those in favor of assert(Someone == JustinWick)? The motion is carried unanimously!

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@ingoem said:
Some of our students trapped into this issue. I am not sure it is the very same underlying issue, so here is another test case:

abstract class Term
case object Var extends Term
case object Abs extends Term

object Ex {
  def unapply(t: Term): Option[Term] = {
    println("unapply " + t)
    t match {
      case Abs => Some(t)
      case _ => None
    }
  }
}

object Test extends Application {
  (Abs, Var) match {
    case (Abs, Ex(v)) => println("Hm") // Comment this line out to get "Success"
    case (Ex(v), t) => println("Success")
    case (a, b) if Value.unapply(a) != None => println("Fail")
  }
}

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@sschaef said:
I got also problems with extractors - but I used unapplySeq instead of unapply. Comment out the first case to get the expected output.

object MyListTest extends App {
  val xs = MyList(1 to 5: _*)
  xs match {
//    case MyList(head) => println("no") // works fine
    case head :: MyNil => println("no") // produces a MatchError
    case MyList(head, tail @ _*) => println("yay")
  }
}

object MyList {
  def apply[A](a: A*): MyList[A] =
    (a :\ empty[A]) { _ :: _ }
    
  def empty[A]: MyList[A] = MyNil
  
  def unapplySeq[A](a: MyList[A]): Option[Seq[A]] = {
    var xs: List[A] = Nil
    var ys = a
    while (!ys.isEmpty) {
      xs = ys.head :: xs
      ys = ys.tail
    }
    Some(xs.reverse)
  }
}

abstract class MyList[+A] {
  def head: A
  def tail: MyList[A]
  def isEmpty: Boolean
  
  def :: [B >: A](b: B): MyList[B] =
    new ::(b, this)
}

case class :: [A](head: A, tail: MyList[A]) extends MyList[A] {
  def isEmpty = false
}

case object MyNil extends MyList[Nothing] {
  def head = throw new UnsupportedOperationException("nil head")
  def tail = throw new UnsupportedOperationException("nil tail")
  def isEmpty = true
}

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Commit Message Bot (anonymous) said:
(moors in r25966) smarter bridges to unapplies

wraps the call to a bridged synthetic unapply(Seq) in a defensive if-test:

if (x.isInstanceOf[expectedType]) real.unapply(x.asInstanceOf[expectedType]) else None

NOTE: the test is WRONG, but it has to be due to #1697/#2337 -- once those are fixed, this one should generate the expected output

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@paulp said:
virtpatmat saves day, test in 8bc8b83f0b .