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Avoid leaking internal types in GadtConstraint.approximation #15558

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Jun 30, 2022
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Avoid leaking internal types in GadtConstraint.approximation #15558

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10 changes: 9 additions & 1 deletion compiler/src/dotty/tools/dotc/core/GadtConstraint.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -209,7 +209,15 @@ final class ProperGadtConstraint private(
def isNarrowing: Boolean = wasConstrained

override def approximation(sym: Symbol, fromBelow: Boolean)(using Context): Type = {
val res = approximation(tvarOrError(sym).origin, fromBelow = fromBelow)
val res =
approximation(tvarOrError(sym).origin, fromBelow = fromBelow) match
case tpr: TypeParamRef =>
// Here we do externalization when the returned type is a TypeParamRef,
// b/c ConstraintHandling.approximation may return internal types when
// the type variable is instantiated. See #15531.
externalize(tpr)
case tp => tp

gadts.println(i"approximating $sym ~> $res")
res
}
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9 changes: 9 additions & 0 deletions tests/pos/i15531.scala
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Original file line number Diff line number Diff line change
@@ -0,0 +1,9 @@
trait Tag { val data: Int }

enum EQ[A, B]:
case Refl[C]() extends EQ[C, C]

def foo[T, B <: Tag](ev: EQ[T, B], x: T) = ev match
case EQ.Refl() =>
val i: Int = x.data