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Quickly fix HKT GADT unsoundness issue #9138

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Quickly fix HKT GADT unsoundness issue #9138

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31 changes: 17 additions & 14 deletions compiler/src/dotty/tools/dotc/typer/Typer.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -3398,7 +3398,7 @@ class Typer extends Namer

// try an implicit conversion
val prevConstraint = ctx.typerState.constraint
def recover(failure: SearchFailureType) = {
def recover(failure: SearchFailureType): Tree = {
def canTryGADTHealing: Boolean = {
val isDummy = tree.hasAttachment(dummyTreeOfType.IsDummyTree)
tryGadtHealing // allow GADT healing only once to avoid a loop
Expand All @@ -3411,20 +3411,23 @@ class Typer extends Namer
else if (canTryGADTHealing) {
// try recovering with a GADT approximation
val nestedCtx = ctx.fresh.setNewTyperState()
val res =
readapt(
tree = tpd.Typed(tree, TypeTree(Inferencing.approximateGADT(wtp))),
shouldTryGadtHealing = false,
)(using nestedCtx)
if (!nestedCtx.reporter.hasErrors) {
// GADT recovery successful
nestedCtx.typerState.commit()
res
} else {
// otherwise fail with the error that would have been reported without the GADT recovery
err.typeMismatch(tree, pt, failure)
val approximated = Inferencing.approximateGADT(wtp)(using nestedCtx)
if (tree.tpe <:< approximated) {
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Shouldn't that check be done in the nestedCtx?

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That's sadly not the part that's making it explode. Not even making the check frozen and doing it in nestedCtx helps. I have no idea why, seems that we will need to take a more scorched-earth type of approach.

val res =
readapt(
tree = tpd.Typed(tree, TypeTree(approximated)),
shouldTryGadtHealing = false,
)(using nestedCtx)

if (!nestedCtx.reporter.hasErrors) {
// GADT recovery successful
nestedCtx.typerState.commit()
return res
}
}
} else err.typeMismatch(tree, pt, failure)
}

err.typeMismatch(tree, pt, failure)
}
if ctx.mode.is(Mode.ImplicitsEnabled) && tree.typeOpt.isValueType then
if pt.isRef(defn.AnyValClass) || pt.isRef(defn.ObjectClass) then
Expand Down
14 changes: 14 additions & 0 deletions tests/neg/i9044.scala
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@@ -0,0 +1,14 @@
sealed trait Test[+F[_], +A] extends Product with Serializable

object Test {

implicit class Syntax[F[_], A](val self: Test[F, A]) extends AnyVal {

def fold[B](completed: F[A] => B): B = self match {
case Completed(fa) => completed(fa) // error
}
}


final case class Completed[F[_], A](fa: F[A]) extends Test[F, A]
}