StackOverflow when wrapping F-Bound type

[Katrix]

Minimized code

trait Foo[A <: Foo[A]]
trait FooCreator[A <: Foo[A]] {
  def createFoo(): A
}

trait FooWrapper {
  type A <: Foo[A]
  def foo: A
}
object FooWrapper {
  def apply[A0 <: Foo[A0]](toWrap: A0): FooWrapper { type A = A0 } = new FooWrapper {
    type A = A0
    def foo: A0 = toWrap
  }
}

trait FooCreatorWrapper {
  type A <: Foo[A]
  def fooCreator: FooCreator[A]
}

sealed trait Bar
object Bar {
  case class Baz(wrapper: FooCreatorWrapper) extends Bar
}

def process(bar: Bar): FooWrapper = bar match {
  case Bar.Baz(wrapper) => FooWrapper(wrapper.fooCreator.createFoo())
}

Stuff which does not produce the error. No match, no creator, breaking out the expression into two lines with a val inbetween.

Output (click arrow to expand)

[error] java.lang.StackOverflowError
[error] dotty.tools.dotc.core.Types$NamedType.denot(Types.scala:2085)
[error] dotty.tools.dotc.core.TypeApplications$.safeDealias$extension(TypeApplications.scala:271)
[error] dotty.tools.dotc.core.TypeApplications$.appliedTo$extension(TypeApplications.scala:311)
[error] dotty.tools.dotc.core.Types$AppliedType.derivedAppliedType(Types.scala:4074)
[error] dotty.tools.dotc.core.Types$ApproximatingTypeMap.derivedAppliedType(Types.scala:5439)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5120)
[error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:442)
[error] dotty.tools.dotc.core.Types$ApproximatingTypeMap.reapply(Types.scala:5492)
[error] dotty.tools.dotc.core.Types$ApproximatingTypeMap.expandBounds(Types.scala:5293)
[error] dotty.tools.dotc.core.Types$ApproximatingTypeMap.tryWiden(Types.scala:5309)
[error] dotty.tools.dotc.core.TypeOps$$anon$1.derivedSelect(TypeOps.scala:457)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5117)
[error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:442)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver$$anonfun$1(Types.scala:5167)
[error] dotty.tools.dotc.core.Types$LazyRef$.apply$$anonfun$1(Types.scala:2735)
[error] dotty.tools.dotc.core.Types$LazyRef.ref(Types.scala:2748)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver$$anonfun$1(Types.scala:5166)
[error] dotty.tools.dotc.core.Types$LazyRef$.apply$$anonfun$1(Types.scala:2735)
[error] dotty.tools.dotc.core.Types$LazyRef.ref(Types.scala:2748)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver$$anonfun$1(Types.scala:5166)
[error] dotty.tools.dotc.core.Types$LazyRef$.apply$$anonfun$1(Types.scala:2735)
[error] dotty.tools.dotc.core.Types$LazyRef.ref(Types.scala:2748)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5605)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5661)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5639)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy19$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5539)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5653)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5639)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5605)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5661)
[error] dotty.tools.dotc.core.Types$NamedPartsAccumulator.apply(Types.scala:5639)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy19$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5539)
...

[Katrix]

Ran into this as well, using some of the same code. The stack trace is a bit different, so can make a new issue for it if it's not actually the same crash. The cause of the error is the type signature of doThing, if it's changed to FooWrapper, everything compiles.

trait Foo[A <: Foo[A]]
trait FooWrapper:
  self =>
  type A <: Foo[A]
  def doThing(foo: FooWrapper): FooWrapper { type A = self.A } = ???
end FooWrapper

val foos: scala.Seq[FooWrapper] = ???
val newFoo = foos.foldLeft(??? : FooWrapper)((topFoo, foo) => topFoo.doThing(foo))

Error:

[error] java.lang.StackOverflowError
[error] dotty.tools.dotc.core.Types$NamedType.denot(Types.scala:2085)
[error] dotty.tools.dotc.core.Types$NamedType.info(Types.scala:2077)
[error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:417)
[error] dotty.tools.dotc.core.Types$TypeMap.op$proxy14$1(Types.scala:5110)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:5110)
[error] dotty.tools.dotc.core.TypeOps$$anon$1.apply(TypeOps.scala:442)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver$$anonfun$1(Types.scala:5167)
[error] dotty.tools.dotc.core.Types$LazyRef$.apply$$anonfun$1(Types.scala:2735)
[error] dotty.tools.dotc.core.Types$LazyRef.ref(Types.scala:2748)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver$$anonfun$1(Types.scala:5166)
[error] dotty.tools.dotc.core.Types$LazyRef$.apply$$anonfun$1(Types.scala:2735)
[error] dotty.tools.dotc.core.Types$LazyRef.ref(Types.scala:2748)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5605)
[error] dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:5622)
[error] dotty.tools.dotc.sbt.ExtractDependenciesCollector$TypeDependencyTraverser.traverse(ExtractDependencies.scala:468)
[error] dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:5621)
[error] dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:5621)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.op$proxy19$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldArgs$1(Types.scala:5535)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5539)
[error] dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:5622)
[error] dotty.tools.dotc.sbt.ExtractDependenciesCollector$TypeDependencyTraverser.traverse(ExtractDependencies.scala:468)
[error] dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:5621)
[error] dotty.tools.dotc.core.Types$TypeTraverser.apply(Types.scala:5621)
[error] dotty.tools.dotc.core.Types$TypeAccumulator.foldOver(Types.scala:5605)
[error] dotty.tools.dotc.core.Types$TypeTraverser.traverseChildren(Types.scala:5622)
[error] dotty.tools.dotc.sbt.ExtractDependenciesCollector$TypeDependencyTraverser.traverse(ExtractDependencies.scala:468)

[Katrix]

And another one. Don't think this is a crash, it just throws a really, REALLY large type back. Again, if I should make another issue, just say so. Too large for me to post here. Uses Foo and FooWrapper as defined earlier

def error(fooWrapper: FooWrapper, processor: [A <: Foo[A]] => A => A): FooWrapper =
  FooWrapper(processor(fooWrapper.foo))

[liufengyun]

@Katrix Thanks for your report. We still need to fix the problem related to the following piece of code:

def error(fooWrapper: FooWrapper, processor: [A <: Foo[A]] => A => A): FooWrapper =
  FooWrapper(processor(fooWrapper.foo))

A workaround is to supply the type argument explicitly:

def error(fooWrapper: FooWrapper, processor: [A <: Foo[A]] => A => A): FooWrapper =
-  FooWrapper(processor(fooWrapper.foo))
+  FooWrapper[fooWrapper.A](processor(fooWrapper.foo))