Null checks with pattern matching are not reflected to the return type with `-Yexplicit-nulls`

[lolgab]

Compiler version

Scala 3.0.0-RC2 with -Yexplicit-nulls

Minimized code

// This works correctly
def f1(s: String | Null): String = if(s == null) "other" else s

// This fails to compile
def f(s: String | Null): String = s match {
  case null => "other"
  case s => s
}

Output

  case s => s
            ^
            Found:    (s : String | Null)
            Required: String

Expectation

My expectation is that what happens with def f(s: String | Null): String = if(s == null) "other" else s should apply to pattern matching too.

I couldn't find any open issue treating this, so I opened one.

Thank you.

[noti0na1]

Currently, the flow typing of explicit nulls only detects the NotNull cases in pattern matching (see Nullables.matchesNotNull).

For example:

val s: String | Null = ???
s match {
  case _: String => println(s.length)
  case _ => println(0)
}

I think it would be better to detect the null case as well.

[noti0na1]

Sorry, I made a mistake. The null case is indeed detected by flow typing.
In your example, the s at the case position is new variable, it will still have the original type.

If you write it in this way, then it can be compiled.

def f(s: String | Null): String = s match {
  case null => "other"
  case _ => s
}

[lolgab]

In your example, the s at the case position is new variable, it will still have the original type.

It is a new variable yes, but it's defined after the null is already matched, so in theory it's type should be String, not String | Null, right?

[noti0na1]

The variables binded by the patterns are not handled by flow typing now. I agree we should consider them as well.