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

[noti0na1]

I'm not sure about this test. If we matched null value, then it is clear the following null cases are unreachable?

[olhotak]

I wonder if this is caused by the change here:
https://github.com/scala/scala3/pull/23308/files#diff-9678fa10ff9f7a2cdf71755535b6ef49afd31880e9d159ed4c40a39921d43454R972

That change was in the original PR #19258 . I wonder if it has been subsumed by other changes since then and might no longer be necessary. Without that change, what would break?

[SuperCl4sh]

Running scalac tests/explicit-nulls/neg/flow-match.scala -Yexplicit-nulls with checkReachability(m) produces the same compiler output as withoutMode(Mode.safeNulls) { checkReachability(m) }:

-- [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`
1 error found

Deleting f6 and re-running causes both changes to warn about unreachability in f7:

-- [E030] Match case Unreachable Warning: tests\explicit-nulls\neg\flow-match.scala:7:9 
7 |    case null => "other"
  |         ^^^^
  |         Unreachable case
1 warning found

[HarrisL2]

This is not an error, just a warning. We can change the test case to remove f7 entirely.

[olhotak]

Yes, or keep it in as a positive part of the test. (No need to make it a separate pos test file.)
#23308 (comment)

[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]

We discussed with @noti0na1 and agreed that this change should be removed. It is outdated and should no longer be necessary.

[noti0na1]

The PRs may be related to this: #21850 #21934

[olhotak]

This should go in warn instead of enabling fatal warnings.