Opaque type with cyclic bound: assertion error

[Jasper-M]

minimized code

trait Foo[F <: Foo[F]]
class Bar extends Foo[Bar]

object Q {
  opaque type X <: Foo[X] = Bar
}

Crash output (click arrow to expand)

-- Error: test.scala:5:19 ------------------------------------------------------
5 |  opaque type X <: Foo[X] = Bar
  |                   ^^^^^^^^^^^^
  |                   type Bar outside bounds  <: Foo[Q.X]
exception occurred while typechecking test.scala
exception occurred while compiling test.scala
java.lang.AssertionError: assertion failed while compiling test.scala
Exception in thread "main" java.lang.AssertionError: assertion failed
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:16)
	at dotty.tools.dotc.core.Types$LazyRef.update(Types.scala:2598)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.recur$3(SymDenotations.scala:445)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.setAlias$1(SymDenotations.scala:448)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.opaqueToBounds(SymDenotations.scala:462)
	at dotty.tools.dotc.typer.Namer$TypeDefCompleter.typeSig(Namer.scala:1006)
	at dotty.tools.dotc.typer.Namer$Completer.completeInCreationContext(Namer.scala:932)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:840)
	at dotty.tools.dotc.core.SymDenotations$SymDenotation.completeFrom(SymDenotations.scala:260)
	at dotty.tools.dotc.core.Denotations$Denotation.completeInfo$1(Denotations.scala:185)
	at dotty.tools.dotc.core.Denotations$Denotation.info(Denotations.scala:187)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.computeAsSeenFrom(Denotations.scala:1169)
	at dotty.tools.dotc.core.Denotations$SingleDenotation.computeAsSeenFrom(Denotations.scala:1141)
	at dotty.tools.dotc.core.Denotations$PreDenotation.asSeenFrom(Denotations.scala:132)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation.findMember(SymDenotations.scala:1851)
	at dotty.tools.dotc.core.Types$Type.go$1(Types.scala:580)
	at dotty.tools.dotc.core.Types$Type.findMember(Types.scala:741)
	at dotty.tools.dotc.core.Types$Type.memberBasedOnFlags(Types.scala:563)
	at dotty.tools.dotc.core.Types$Type.member(Types.scala:547)
	at dotty.tools.dotc.typer.Typer.memberInSelfButNotThis$2(Typer.scala:1794)
	at dotty.tools.dotc.typer.Typer.classExistsOnSelf$4$$anonfun$3(Typer.scala:1796)
	at dotty.tools.dotc.typer.Typer.classExistsOnSelf$5$$anonfun$adapted$1(Typer.scala:1801)
	at scala.collection.IterableOnceOps.foldLeft(IterableOnce.scala:636)
	at scala.collection.IterableOnceOps.foldLeft$(IterableOnce.scala:632)
	at scala.collection.AbstractIterator.foldLeft(Iterator.scala:1274)
	at dotty.tools.dotc.typer.Typer.classExistsOnSelf$1(Typer.scala:1801)
	at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1812)
	at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:2141)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2208)
	at dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2247)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2256)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2259)
	at dotty.tools.dotc.typer.Typer.traverse$1(Typer.scala:2281)
	at dotty.tools.dotc.typer.Typer.typedStats(Typer.scala:2325)
	at dotty.tools.dotc.typer.Typer.typedPackageDef(Typer.scala:1944)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:2182)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:2209)
	at dotty.tools.dotc.typer.Typer.op$1(Typer.scala:2247)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2256)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:2259)
	at dotty.tools.dotc.typer.Typer.typedExpr(Typer.scala:2367)
	at dotty.tools.dotc.typer.FrontEnd.liftedTree1$2(FrontEnd.scala:78)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck$$anonfun$1(FrontEnd.scala:83)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.typer.FrontEnd.monitor(FrontEnd.scala:42)
	at dotty.tools.dotc.typer.FrontEnd.typeCheck(FrontEnd.scala:84)
	at dotty.tools.dotc.typer.FrontEnd.runOn$$anonfun$3(FrontEnd.scala:114)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.immutable.List.foreach(List.scala:305)
	at dotty.tools.dotc.typer.FrontEnd.runOn(FrontEnd.scala:114)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:167)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:177)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:185)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:64)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:192)
	at dotty.tools.dotc.Run.compileSources(Run.scala:129)
	at dotty.tools.dotc.Run.compile(Run.scala:112)
	at dotty.tools.dotc.Driver.doCompile(Driver.scala:36)
	at dotty.tools.dotc.Driver.process(Driver.scala:189)
	at dotty.tools.dotc.Driver.process(Driver.scala:158)
	at dotty.tools.dotc.Driver.process(Driver.scala:170)
	at dotty.tools.dotc.Driver.main(Driver.scala:197)
	at dotty.tools.dotc.Main.main(Main.scala)

---

Besides the crash, I think it should be possible to define this type without errors, given that X =:= Bar and Bar <: Foo[Bar].

Also consider the following which already (or still?) works:

scala> trait Foo[F <: Foo[F]] { this: F => def self: F = this }
// defined trait Foo

scala> class Bar extends Foo[Bar]
// defined class Bar

scala> trait Q { type X <: Foo[X]; def mkX(b: Bar): X }
// defined trait Q

scala> val XMaker: Q = new Q { type X  = Bar; def mkX(b: Bar) = b }
val XMaker: Q = anon$1@67671db1

scala> val x = XMaker.mkX(new Bar)
val x: XMaker.X = Bar@8e77c5b

scala> x.self                                                                   
val res2: LazyRef(XMaker.X) = Bar@8e77c5b

And also notice the strange LazyRef(XMaker.X) type.