Recursion limit exceeded (F-bounded type and sealed pattern match)

[Sciss]

Minimized code

git clone https://github.com/Sciss/DottyLucre.git
cd DottyLucre
git checkout dotty-issue-9626
sbt compile

Output (click arrow to expand)

crashes in either SkipList or HASkipList of the data module (crash doesn't indicate file or line number)

[info] exception occurred while compiling /home/hhrutz/Documents/devel/DottyLucre/data/src/main/scala/de/sciss/lucre/data/HASkipList.scala, /home/hhrutz/Documents/devel/DottyLucre/data/src/main/scala/de/sciss/lucre/data/Ordered.scala, /home/hhrutz/Documents/devel/DottyLucre/data/src/main/scala/de/sciss/lucre/data/Ordering.scala, /home/hhrutz/Documents/devel/DottyLucre/data/src/main/scala/de/sciss/lucre/data/SkipList.scala
Recursion limit exceeded.
Maybe there is an illegal cyclic reference?
If that's not the case, you could also try to increase the stacksize using the -Xss JVM option.
A recurring operation is (inner to outer):

  subtype LazyRef(ile / compileIncremental 1s
  de.sciss.lucre.stm.Txn[
    LazyRef(
      de.sciss.lucre.stm.Txn[
        LazyRef(
          de.sciss.lucre.stm.Txn[

Compiles fine under ++2.13.3.

[Sciss]

full log: https://gist.github.com/Sciss/254f302679b21ba2c6b4f00b414624f4

[smarter]

Reproduce on the latest nightly. Interestingly the crash happens in the exhaustivity checker and not in the typechecker as I thought (/cc @liufengyun ). The interesting part of the stacktrace is:

[error] dotty.tools.dotc.core.TypeComparer.compareAppliedType2$1(TypeComparer.scala:1088)
[error] dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:504)
[error] dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:454)
[error] dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:351)
[error] dotty.tools.dotc.core.TypeComparer.op$proxy4$1(TypeComparer.scala:1204)
[error] dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1219)
[error] dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:408)
[error] dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:351)
[error] dotty.tools.dotc.core.TypeComparer.op$proxy4$1(TypeComparer.scala:1204)
[error] dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1219)
[error] dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:296)
[error] dotty.tools.dotc.core.TypeComparer.op$proxy4$1(TypeComparer.scala:1204)
[error] dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1219)
[error] dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:164)
[error] dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:174)
[error] dotty.tools.dotc.core.TypeComparer.isSubArg$1(TypeComparer.scala:1415)
[error] dotty.tools.dotc.core.TypeComparer.recurArgs$1(TypeComparer.scala:1420)
[error] dotty.tools.dotc.core.TypeComparer.isSubArgs(TypeComparer.scala:1423)
[error] dotty.tools.dotc.core.TypeComparer.checkSubArgs$1(TypeComparer.scala:1012)
[error] dotty.tools.dotc.core.TypeComparer.loop$3(TypeComparer.scala:1014)
[error] dotty.tools.dotc.core.TypeComparer.isMatchingApply$1(TypeComparer.scala:1029)
[error] dotty.tools.dotc.core.TypeComparer.compareAppliedType2$1(TypeComparer.scala:1088)
[error] dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:504)
[error] dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:454)
[error] dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:351)
[error] dotty.tools.dotc.core.TypeComparer.op$proxy4$1(TypeComparer.scala:1204)
[error] dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1219)
[error] dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:322)
[error] dotty.tools.dotc.core.TypeComparer.op$proxy4$1(TypeComparer.scala:1204)
[error] dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1219)
[error] dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:164)
[error] dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:174)
[error] dotty.tools.dotc.core.TypeComparer.isSub(TypeComparer.scala:176)
[error] dotty.tools.dotc.core.ConstraintHandling.op$proxy1$1(ConstraintHandling.scala:233)
[error] dotty.tools.dotc.core.ConstraintHandling.isSubTypeWhenFrozen(ConstraintHandling.scala:233)
[error] dotty.tools.dotc.core.ConstraintHandling.isSubTypeWhenFrozen$(ConstraintHandling.scala:25)
[error] dotty.tools.dotc.core.TypeComparer.isSubTypeWhenFrozen(TypeComparer.scala:30)
[error] dotty.tools.dotc.core.TypeComparer$.isSubTypeWhenFrozen(TypeComparer.scala:2538)
[error] dotty.tools.dotc.typer.Inferencing$.dotty$tools$dotc$typer$Inferencing$$$instDirection(Inferencing.scala:328)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:130)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:123)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy15$1(Types.scala:5392)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$1(Types.scala:5392)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5396)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:146)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:123)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5427)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.apply(Inferencing.scala:146)
[error] dotty.tools.dotc.typer.Inferencing$IsFullyDefinedAccumulator.process(Inferencing.scala:157)
[error] dotty.tools.dotc.typer.Inferencing$.isFullyDefined(Inferencing.scala:36)
[error] dotty.tools.dotc.core.TypeOps$.instantiateToSubType(TypeOps.scala:739)
[error] dotty.tools.dotc.core.TypeOps$.refineUsingParent(TypeOps.scala:639)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.$anonfun$4(Space.scala:611)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.decompose(Space.scala:623)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.tryDecompose2$2(Space.scala:193)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.op$proxy4$1(Space.scala:203)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect(Space.scala:228)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect$(Space.scala:77)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.intersect(Space.scala:316)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.op$proxy4$4$$anonfun$3(Space.scala:198)
[error] scala.collection.immutable.List.map(List.scala:250)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.op$proxy4$1(Space.scala:198)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect(Space.scala:228)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect$(Space.scala:77)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.intersect(Space.scala:316)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.op$proxy4$2$$anonfun$1(Space.scala:197)
[error] scala.collection.immutable.List.map(List.scala:246)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.op$proxy4$1(Space.scala:197)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect(Space.scala:228)
[error] dotty.tools.dotc.transform.patmat.SpaceLogic.intersect$(Space.scala:77)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.intersect(Space.scala:316)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.checkRedundancy$$anonfun$1(Space.scala:881)
[error] dotty.runtime.function.JFunction1$mcVI$sp.apply(JFunction1$mcVI$sp.java:12)
[error] scala.collection.immutable.Range.foreach(Range.scala:190)
[error] dotty.tools.dotc.transform.patmat.SpaceEngine.checkRedundancy(Space.scala:902)
[error] dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:46)
[error] dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:778)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:368)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] dotty.tools.dotc.transform.MegaPhase.mapDefDef$1(MegaPhase.scala:248)
[error] dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426)
[error] dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:361)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:255)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:426)
[error] dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:436)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:441)
[error] dotty.tools.dotc.transform.MegaPhase.recur$1(MegaPhase.scala:1060)
[error] dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:441)
[error] dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:381)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:384)
[error] dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:428)
[error] dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:447)
[error] dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:459)

(make sure -XX:MaxJavaStackTraceDepth=100000 to not get a truncated stacktrace)

[Sciss]

Well, there is only one pattern match that could cause it, and indeed removing that avoids the crash. It's comparing SkipList[T, A, _] to Impl[T, A, _] in line 85, which is a debugging function, so for now I can just remove that.

[Sciss]

Minimised:

trait Txn[T <: Txn[T]]

object SkipList {
  trait Set[T <: Txn[T], A] extends SkipList[T, A, A]
}
sealed trait SkipList[T <: Txn[T], A, E]

object HASkipList {
  def debug[T <: Txn[T], A](in: SkipList[T, A, _], key: A)(implicit tx: T): Int = in match {
    case impl: Impl[T, A, _] => impl.foo(key)
    case _ => -1
  }

  private trait Impl[T <: Txn[T], A, E] {
    self: SkipList[T, A, E] =>

    def foo(key: A)(implicit tx: T): Int
  }
}