crash on compilable code: cyclic reference involving class Visitor

[helloqirun]

scalac can compile but dotc crashes.

$ cat abc.scala

trait Lang1 {
  trait Exp
  trait Visitor { def f(left: Exp): Unit }
  class Eval1 extends Visitor { self =>
    def f(left: Exp) = ()
  }
}
trait Lang2 extends Lang1 {
  class Visitor extends Eval1 { Visitor =>
  }
}

$ dotc abc.scala

exception while transforming () extends Lang2.this.Eval1() {} of class class dotty.tools.dotc.ast.Trees$Template # 174
exception while transforming class Visitor() extends Lang2.this.Eval1() {} of class class dotty.tools.dotc.ast.Trees$TypeDef # 175
exception while transforming () extends Object with Lang1 {
  class Visitor() extends Lang2.this.Eval1() {}
} of class class dotty.tools.dotc.ast.Trees$Template # 176
exception while transforming @scala.annotation.internal.SourceFile("abc.scala") <trait> trait Lang2() extends

Object with Lang1 {
  class Visitor() extends Lang2.this.Eval1() {}
} of class class dotty.tools.dotc.ast.Trees$TypeDef # 177
exception while transforming package <empty> {
  @scala.annotation.internal.SourceFile("abc.scala") <trait> trait Lang1()
     extends
   Object {
    <trait> interface trait Exp() extends Object {}
    <trait> interface trait Visitor() extends Object {
      def f(left: Lang1.this.Exp): Unit
    }
    class Eval1() extends Object() with Lang1.this.Visitor {
      def f(left: Lang1.this.Exp): Unit = ()
    }
  }
  @scala.annotation.internal.SourceFile("abc.scala") <trait> trait Lang2()
     extends
   Object with Lang1 {
    class Visitor() extends Lang2.this.Eval1() {}
  }
} of class class dotty.tools.dotc.ast.Trees$PackageDef # 178

exception occurred while compiling abc.scala
Exception in thread "main" dotty.tools.dotc.core.Types$CyclicReference: cyclic reference involving class Visitor
	at dotty.tools.dotc.core.Types$CyclicReference$.apply(Types.scala:3838)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation$$anonfun$baseTypeRefOf$2.apply(SymDenotations.scala:1720)
	at dotty.tools.dotc.core.SymDenotations$ClassDenotation$$anonfun$baseTypeRefOf$2.apply(SymDenotations.scala:1709)
	at dotty.tools.dotc.reporting.Reporting$class.conditionalTraceIndented(Reporter.scala:132)
	at dotty.tools.dotc.core.Contexts$Context.conditionalTraceIndented(Contexts.scala:57)
	at dotty.tools.dotc.reporting.Reporting$class.debugTraceIndented(Reporter.scala:127)
	at dotty.tools.dotc.core.Contexts$Context.debugTraceIndented(Contexts.scala:57)
<snipped>

[liufengyun]

The exception happens in the phase RefChecks.

[liufengyun]

I've been debugging this a little bit last night, this test case is very tricky, it doesn't appear if either:

1. we change def f(left: Exp): Unit to def f(x: Int): Unit, or
2. we rename Visitor to a different name in lang2.

The combination of def f(left: Exp) and duplicate class name will trigger a very deep asSeenFrom in ref-checking overriding of def f, which results in a cyclic loop in baseTypeRefOf. This is because baseTypeRefOf(lang2.this.Visitor) will cause the parent call baseTypeRefOf(Lang2.this.Eval1), which in turn triggers the parent call baseTypeRefOf (Lang2.this.Visitor) again (NOT Lang1, because asSeenFrom), thus a cyclic reference is detected.

There is also a specification different from Scala here: Scalac allows overriding of Visitor, while Dotty disallows overriding of classes.