Guarded pattern with existential causes bad matching

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The following code prints "B" instead of the expected result "C":

object MatchTest {
  class A[T] { val op = null }
  class B extends A[Any]
  class C extends B

  def f(o: AnyRef) = o match {
    case a: A[_] if(a.op != null) => "with op"
    case c: C => "C"
    case b: B => "B"
  }

  def main(args: Array[String]) = println(f(new C))
}

Case "a" is never matched because the if() guard always fails but using the existential type A[_] in case a also causes the match for case c to fail. Removing the type parameter on A entirely or matching against A[Any] (which gets erased anyway) in case a results in the correct output "C".

This is with Scala 2.7.2RC3.

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Imported From: https://issues.scala-lang.org/browse/SI-1434?orig=1
Reporter: @szeiger
Assignee: @DRMacIver

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@szeiger said:
I just ran into the same problem again with some new code where it occurs without an if() guard but I wasn't able to reproduce it in a test case which is simple enough to post.

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@paulp said:
Fixed in r18212.