Relax comparison between Null and reference types in explicit nulls

[SuperCl4sh]

An up-to-date version of #19258 with merge conflicts resolved.

[SuperCl4sh]

cc @olhotak @HarrisL2

[olhotak]

Before this PR, all three errors in flow-match come from the typer phase:

-- [E007] Type Mismatch Error: tests/explicit-nulls/neg/flow-match.scala:5:15 --
5 |    case s2 => s2 // error
  |               ^^
  |               Found:    (s2 : String | Null)
  |               Required: String
  |
  | longer explanation available when compiling with `-explain`
-- [E172] Type Error: tests/explicit-nulls/neg/flow-match.scala:6:9 ------------
6 |    case null => "other" // error
  |         ^^^^
  |         Values of types Null and String cannot be compared with == or !=
-- [E172] Type Error: tests/explicit-nulls/neg/flow-match.scala:12:9 -----------
12 |    case null => "other" // error
   |         ^^^^
   |        Values of types Null and String cannot be compared with == or !=
3 errors found

After this PR, the two errors on lines 6 and 12 understandably go away.

The error on line 5 prevents the compiler from getting past typer, so match reachability checking never happens.

The test was not intended for match reachability checking but to test for the flow-sensitive analysis in typer.

We should split the test into two, one that errors only in typer, and one that tests only match reachability checking.

[olhotak]

Here are the proposed two tests. Note that the second one needs to go in warn, not neg.

// Test flow-typing when NotNullInfos are from cases

object MatchTest {
  def f6(s: String | Null): String = s match {
    case s2 => s2 // error
    case s3 => s3 // OK since not null
  }

  def f7(s: String | Null): String = s match {
    case null => "other"
    case s3 => s3 // OK since not null
  }
}

// Test unreachable matches in presence of nulls

object MatchTest2 {
  def f6(s: String | Null): String = s match {
    case s2 => s2.nn
    case null => "other" // warn
    case s3 => s3 // warn
  }

  def f7(s: String | Null): String = s match {
    case null => "other"
    case null => "other" // warn
    case s3: String => s3
    case s4 => s4.nn // warn
  }
}

[olhotak]

This should go in warn instead of enabling fatal warnings.

[SuperCl4sh]

OK I think all the comments have been addressed, apologies for the delay. Please let me know if I missed something.

[olhotak]

LGTM. @noti0na1 can you also have a look?