nullability flow typing ignores Predef.eq/ne #18282
Description
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 foundExpectation
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).