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Fix captureWildcards with FlexibleType #24070

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noti0na1 merged 1 commit into from
Sep 26, 2025
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Fix captureWildcards with FlexibleType #24070

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2 changes: 2 additions & 0 deletions compiler/src/dotty/tools/dotc/typer/Inferencing.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -626,11 +626,13 @@ object Inferencing {
case tp: RecType => tp.derivedRecType(captureWildcards(tp.parent))
case tp: LazyRef => captureWildcards(tp.ref)
case tp: AnnotatedType => tp.derivedAnnotatedType(captureWildcards(tp.parent), tp.annot)
case tp: FlexibleType => tp.derivedFlexibleType(captureWildcards(tp.hi))
case _ => tp
}

def hasCaptureConversionArg(tp: Type)(using Context): Boolean = tp match
case tp: AppliedType => tp.args.exists(_.typeSymbol == defn.TypeBox_CAP)
case tp: FlexibleType => hasCaptureConversionArg(tp.hi)
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@olhotak olhotak Sep 26, 2025

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For this one, I don't understand why the other cases are not needed (i.e. what makes FlexibleType special). For example, what if you get the right AppliedType under an AnnontatedType? In that case, this method won't find it either.

But the case for FlexibleType looks right to me.

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@noti0na1 noti0na1 Sep 26, 2025 •
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The real fix is in captureWildcards. I guess hasCaptureConversionArg is only used to check top-level AppliedType of a tree for error messages. Maybe adding a stripped after tp can handle more cases, idk. Given the places where FlexibleType is expected to appear, I just added this case.

case _ => false
}

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6 changes: 6 additions & 0 deletions tests/explicit-nulls/pos/java-collectors.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
import java.util.stream._

def test =
val seqStream: Stream[String] = ???
val shouldNotNPE = seqStream.collect(Collectors.toList())
val shouldNotNPE2 = seqStream.collect(Collectors.toList[String]())
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