Pattern matching with an `AnyVal` scrutinee fails with unreachable case error

[milessabin]

Compiling this,

object Test {
  def foo(t: AnyVal): Unit = t match {
    case _: Int =>
  }
}

gives me the following unreachable case error rather than the non-exhaustivity warning I was hoping for,

-- Error: local/nulls.scala:3:9 ------------------------------------------------
3 |    case _: Int =>
  |         ^
  |this case is unreachable since class AnyVal and class Integer are unrelated
1 error found

Assuming this is a bug and that this should instead give us a warning about a non-exhaustive match, then does this make the introduction of Null as a subtype of AnyVal with -Yexplict-nulls in #7546 problematic? Presumably the extra case would make previously exhaustive matches over existing AnyVals non-exhaustive?

[odersky]

This was indeed a bug. But you still won't get exhaustivity warnings since AnyVal is not sealed (it can have user-defined value classes besides the primitive value classes as children).

[milessabin]

But you still won't get exhaustivity warnings since AnyVal is not sealed (it can have user-defined value classes besides the primitive value classes as children).

Nevertheless, should we be concerned that code which previously matched AnyVal scrutinees now has to be concerned about null?