Unreachable case warning could point to previous matching case

Given:

trait Foo
class One extends Foo
class Two extends Foo
class Bla extends One

object Test {
  def test(f: Foo) = f match {
    case f: One =>
    case f: Two =>
    case f: Bla =>
  }
}

We get the warning:

-- [E120] Only null matched Warning: try/caseun.scala:10:9 ---------------------
10 |    case f: Bla =>
   |         ^^^^^^
   |         Only null is matched. Consider using `case null =>` instead.

This is confusing since it doesn’t explain why only null is matched. Here it’s easy to see that this is because we matched on One previously and that Bla is a subtype of One, but in a long series of case this might be very hard to understand. A better warning message would explain all of this.

[Blaisorblade]

Created #4953 for -Ylog, and closing with stat:revisit.

[abeln]

I see. That feels like a change with larger implications, though. I’d agree with Fengyun that we either put on hold or close this, since we now have the correct error message that makes more sense. We could file a separate bug for “-Ylog”.