assertion error computing closure

[scabug]

An !AssertionError occurs when compiling the following code. Versions 2.7.0-final, 2.6.1-final, and the April 16 nightly build all produce the error.

class Top[A] {
	type AType = A
}

trait Node extends NotNull { outer =>
	type T <: Node
	def prepend = new Node { type T = outer.type }
}

class Main[NextType <: Node](value: Node { type T = NextType })
	extends Top[Node { type T = NextType }] {
	
	new Main[AType]( (value: AType).prepend )
}

I meant to put

type T <: NextType instead of type T = NextType

in the Main section and with this change, it compiles without an error.

The stack trace using the April 16th nightly build:

Exception in thread "main" java.lang.AssertionError: assertion failed: computing closure of Main.this.AType with Singleton:true
        at scala.Predef$$.assert(Predef.scala:92)
        at scala.tools.nsc.symtab.Types$$CompoundType.computeClosure$$1(Types.scala:1038)
        at scala.tools.nsc.symtab.Types$$CompoundType$$$$anonfun$$closure$$1.apply(Types.scala:1062)
        at scala.tools.nsc.symtab.Types$$CompoundType$$$$anonfun$$closure$$1.apply(Types.scala:1062)
        at scala.tools.nsc.symtab.Types$$CompoundType.memo(Types.scala:1120)
        at scala.tools.nsc.symtab.Types$$CompoundType.closure(Types.scala:1062)
        at scala.tools.nsc.symtab.Types$$Type.closurePos(Types.scala:556)
        at scala.tools.nsc.symtab.Symbols$$Symbol.isNonBottomSubClass(Symbols.scala:728)
        at scala.tools.nsc.symtab.Symbols$$Symbol.isSubClass(Symbols.scala:731)
        at scala.tools.nsc.symtab.Symbols$$Symbol.existentialToString(Symbols.scala:1264)
        at scala.tools.nsc.typechecker.Infer$$Inferencer$$$$anonfun$$scala$$tools$$nsc$$typechecker$$Infer$$Inferencer$$$$existentialContext$$1.apply(Infer.scala:240)
        at scala.tools.nsc.typechecker.Infer$$Inferencer$$$$anonfun$$scala$$tools$$nsc$$typechecker$$Infer$$Inferencer$$$$existentialContext$$1.apply(Infer.scala:240)
        at scala.List.map(List.scala:759)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.scala$$tools$$nsc$$typechecker$$Infer$$Inferencer$$$$existentialContext(Infer.scala:240)
        at scala.tools.nsc.typechecker.Infer$$Inferencer$$$$anonfun$$foundReqMsg$$1.apply(Infer.scala:245)
        at scala.tools.nsc.typechecker.Infer$$Inferencer$$$$anonfun$$foundReqMsg$$1.apply(Infer.scala:246)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.withDisambiguation(Infer.scala:303)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.foundReqMsg(Infer.scala:244)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.typeErrorMsg(Infer.scala:250)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.typeError(Infer.scala:266)
        at scala.tools.nsc.typechecker.Infer$$Inferencer.typeErrorTree(Infer.scala:272)
        at scala.tools.nsc.typechecker.Typers$$Typer.adapt(Typers.scala:813)
        at scala.tools.nsc.typechecker.Typers$$Typer.adapt(Typers.scala:618)
        at scala.tools.nsc.typechecker.Typers$$Typer.adapt(Typers.scala:627)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3248)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedArg(Typers.scala:1591)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$typedArgs$$2.apply(Typers.scala:1607)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$typedArgs$$2.apply(Typers.scala:1607)
        at scala.List$$.map2(List.scala:287)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedArgs(Typers.scala:1607)
        at scala.tools.nsc.typechecker.Typers$$Typer.doTypedApply(Typers.scala:1691)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedApply$$1(Typers.scala:2553)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:3140)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3243)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3280)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedStat$$1(Typers.scala:1540)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$19.apply(Typers.scala:1585)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$19.apply(Typers.scala:1585)
        at scala.List$$.loop$$1(List.scala:254)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.List$$.loop$$1(List.scala:258)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.List$$.loop$$1(List.scala:258)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedStats(Typers.scala:1585)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedTemplate(Typers.scala:1165)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedClassDef(Typers.scala:1039)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:2972)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3243)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3280)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedStat$$1(Typers.scala:1540)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$19.apply(Typers.scala:1585)
        at scala.tools.nsc.typechecker.Typers$$Typer$$$$anonfun$$19.apply(Typers.scala:1585)
        at scala.List$$.loop$$1(List.scala:254)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.List$$.loop$$1(List.scala:258)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.List$$.loop$$1(List.scala:258)
        at scala.List$$.mapConserve(List.scala:271)
        at scala.tools.nsc.typechecker.Typers$$Typer.typedStats(Typers.scala:1585)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed1(Typers.scala:2968)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3243)
        at scala.tools.nsc.typechecker.Typers$$Typer.typed(Typers.scala:3280)
        at scala.tools.nsc.typechecker.Analyzer$$typerFactory$$$$anon$$2.apply(Analyzer.scala:40)
        at scala.tools.nsc.Global$$GlobalPhase.applyPhase(Global.scala:257)
        at scala.tools.nsc.Global$$GlobalPhase$$$$anonfun$$run$$1.apply(Global.scala:243)
        at scala.tools.nsc.Global$$GlobalPhase$$$$anonfun$$run$$1.apply(Global.scala:243)
        at scala.Iterator$$class.foreach(Iterator.scala:395)
        at scala.collection.mutable.ListBuffer$$$$anon$$1.foreach(ListBuffer.scala:266)
        at scala.tools.nsc.Global$$GlobalPhase.run(Global.scala:243)
        at scala.tools.nsc.Global$$Run.compileSources(Global.scala:561)
        at scala.tools.nsc.Global$$Run.compile(Global.scala:646)
        at scala.tools.nsc.Main$$.process(Main.scala:84)
        at scala.tools.nsc.Main$$.main(Main.scala:98)
        at scala.tools.nsc.Main.main(Main.scala

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-764?orig=1
Reporter: @harrah

[scabug]

@odersky said:
fixed in r15527

[scabug]

@odersky said:
Milestone next_bugfix deleted