Fix #4225: always check redundant cases except for @unchecked

[liufengyun]

Fix #4225: always check redundant cases except for @unchecked

[allanrenucci]

Can you add a test case for objects:

object Bar {
  def unapply(x: String): Some[Int] =
    Some(0)
}

object Test {
  def test(x: String) =
    x match {
      case Bar(a) => a
      case _ => x // this case is reachable, i.e. test(null)
    }
}

[liufengyun]

@allanrenucci : the exhaustivity check both in Dotty and Scalac ignores null. Otherwise, all pattern matchess without a _ will not be exhaustive. Thus in the example above, both Dotty & Scalac will report a warning.

[liufengyun]

Sorry, I was wrong, for reachability check we can handle null thus avoid the warning:

It's implemented in https://github.com/lampepfl/dotty/compare/master...dotty-staging:fix-4225?expand=1#diff-5353f08e8d531fe0ee59335438e6cf7a.

[liufengyun]

Close, it can be reviewed in #4233 .