nullability flow typing ignores Predef.eq/ne

[olhotak]

Compiler version

Scala compiler version 3.3.2-RC1-bin-SNAPSHOT-git-ce1ce99 -- Copyright 2002-2023, LAMP/EPFL

Minimized code

def f(s: String|Null): String = {
  if(s eq null) "foo" else s
}

def f2(s: String|Null): String = {
  if(s ne null) s else "foo"
}

Compile with scalac -Yexplicit-nulls.

Output

-- [E007] Type Mismatch Error: tests/explicit-nulls/pos/flow-predef-eq.scala:2:27 ---------------------------------------------------------
2 |  if(s eq null) "foo" else s
  |                           ^
  |                           Found:    (s : String | Null)
  |                           Required: String
  |
  | longer explanation available when compiling with `-explain`
-- [E007] Type Mismatch Error: tests/explicit-nulls/pos/flow-predef-eq.scala:6:16 ---------------------------------------------------------
6 |  if(s ne null) s else "foo"
  |                ^
  |                Found:    (s : String | Null)
  |                Required: String
  |
  | longer explanation available when compiling with `-explain`
4 errors found

Expectation

Compiles.

The problem is that since s is of type AnyRef | Null, the eq/ne are actually extension methods in Predef. The flow typing code looks for the pattern l.eq(r) and l.==(r) but not the more complicated (Predef.eq(l))(r).

[olhotak]

I tried adding a search for this pattern to the CompareNull extractor, but got stuck. To determine whether the operation is eq or ne, we need to resolve the symbol, but that results in an overload resolution ambiguity, presumably because of not enough context.