Wrong error message in pattern matching where "not found" would be expected

[Duhemm]

The following snippet is correctly rejected by the compiler, but the error message is confusing:

object Bar {
  trait A
  case class A0(foo: Int) extends A
}

object Foo {
  import Bar.A

  def buzz(a: A) = {
    a match {
      case A0(_) =>
    }
  }
}

-- Error: Foo.scala:11:11 ------------------------------------------------------                   
11 |      case A0(_) =>                                                                            
   |           ^^                                                                                  
   |A0 cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method
one error found

A0 defines an unapply method, so the error message is wrong. The actual problem is that A0 is simply not in scope.

The error message that I would expect is:

-- Error: Foo.scala:11:11 ------------------------------------------------------                   
11 |      case A0(_) =>                                                                            
   |           ^^                                                                                  
   |           not found: A0

[odersky]

Yes, that's been bugging us for a while. The problem comes from the backtracking we have to do to explore various possibilities (e.g. unapply vs unapplySeq, name-based vs option-based). It would be good to fix this, since the error message is really bad.