infinite loop typechecking nominal subtyping hierarchies

[namin]

I wanted to test out in DOT a way to create nominal subtyping hierarchies, that can be modular, and both sealed or open. I get a StackOverflow for the newSubCat def.

package hello

object world extends App {
  println("hello dotty!")

  trait AnimalPackage {
    type Animal <: AnimalU
    type AnimalU = { val age: Int }
    def newAnimal(a: AnimalU): Animal
    def newSubAnimal[T](a: AnimalU & T): Animal & T
  }
  val p: AnimalPackage = new AnimalPackage { p =>
    type Animal = AnimalU
    override def newAnimal(a: AnimalU): Animal = a
    override def newSubAnimal[T](a: AnimalU & T): Animal & T = a
  }
  val lambda: p.Animal = p.newAnimal(new { val age = 1 })
  trait CatPackage { pc =>
    type Cat <: p.Animal & pc.CatDelta
    type CatDelta = { val meow: Int }
    type CatU = p.AnimalU & pc.CatDelta
    def newCat(c: CatU): Cat
    def newSubCat[T](c: CatU & T): Cat & T
  }
  val pc: CatPackage = new CatPackage { pc =>
    type Cat = p.Animal & pc.CatDelta
    def newCat(c: CatU): Cat = p.newSubAnimal[pc.CatDelta](c)
    def newSubCat[T](c: CatU & T): Cat & T = p.newSubAnimal[pc.CatDelta & T](c)
  }
  val felix: pc.Cat = pc.newCat(new { val age = 1; val meow = 2 })
}

The error is

[info] Running dotty.tools.dotc.Main examples/hello.scala
[error] Exception in thread "main" java.lang.StackOverflowError
[error]     at dotty.tools.dotc.core.Scopes$Scope.denotsNamed(Scopes.scala:117)
[error]     at dotty.tools.dotc.core.SymDenotations$ClassDenotation.membersNamed(SymDenotations.scala:1498)
[error]     at dotty.tools.dotc.core.SymDenotations$ClassDenotation.findMember(SymDenotations.scala:1551)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:432)
[error]     at dotty.tools.dotc.core.Types$Type.goRefined$1(Types.scala:457)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:428)
[error]     at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:522)
[error]     at dotty.tools.dotc.core.Denotations$Denotation.findMember(Denotations.scala:177)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:432)
[error]     at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:522)
[error]     at dotty.tools.dotc.core.Denotations$Denotation.findMember(Denotations.scala:177)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:432)
[error]     at dotty.tools.dotc.core.Types$Type.goAnd$1(Types.scala:512)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:446)
[error]     at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:522)
[error]     at dotty.tools.dotc.core.Denotations$Denotation.findMember(Denotations.scala:177)
[error]     at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:432)
[error]     at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:522)
[error]     at dotty.tools.dotc.core.Types$Type.memberExcluding(Types.scala:418)
[error]     at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:409)
[error]     at dotty.tools.dotc.core.Types$Type$$anonfun$member$1.apply(Types.scala:409)
[error]     at dotty.tools.dotc.util.Stats$.track(Stats.scala:36)
[error]     at dotty.tools.dotc.core.Types$Type.member(Types.scala:408)
[error]     at dotty.tools.dotc.core.Types$abstractTermNameFilter$.apply(Types.scala:3354)
[error]     at dotty.tools.dotc.core.Types$Type.memberNames(Types.scala:547)
[error]     at dotty.tools.dotc.core.Types$Type.memberNames(Types.scala:551)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1$$anonfun$apply$1.apply(TypeOps.scala:356)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1$$anonfun$apply$1.apply(TypeOps.scala:353)
[error]     at scala.collection.LinearSeqOptimized$class.exists(LinearSeqOptimized.scala:93)
[error]     at scala.collection.immutable.List.exists(List.scala:84)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:353)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:348)
[error]     at scala.collection.LinearSeqOptimized$class.forall(LinearSeqOptimized.scala:83)
[error]     at scala.collection.immutable.List.forall(List.scala:84)
[error]     at dotty.tools.dotc.core.TypeOps$class.isVolatile(TypeOps.scala:348)
[error]     at dotty.tools.dotc.core.Contexts$Context.isVolatile(Contexts.scala:53)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:359)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:348)
[error]     at scala.collection.LinearSeqOptimized$class.forall(LinearSeqOptimized.scala:83)
[error]     at scala.collection.immutable.List.forall(List.scala:84)
[error]     at dotty.tools.dotc.core.TypeOps$class.isVolatile(TypeOps.scala:348)
[error]     at dotty.tools.dotc.core.Contexts$Context.isVolatile(Contexts.scala:53)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:359)
[error]     at dotty.tools.dotc.core.TypeOps$$anonfun$isVolatile$1.apply(TypeOps.scala:348)
[error]     at scala.collection.LinearSeqOptimized$class.forall(LinearSeqOptimized.scala:83)
...