MatchError in typer (parser stability)

[sir-wabbit]

trait I3 {
  def foo[i0[_] : i4[_]](a: (List[String]]) = ???
}

Exception in thread "main" scala.MatchError: ContextBounds(TypeBoundsTree(EmptyTree,EmptyTree),List(AppliedTypeTree(AppliedTypeTree(Ident(i4),List(TypeBoundsTree(EmptyTree,EmptyTree))),List(Ident(i0))))) (of class dotty.tools.dotc.ast.untpd$ContextBounds)
	at dotty.tools.dotc.ast.desugar$.apply(Desugar.scala:1109)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1812)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1823)
	at dotty.tools.dotc.typer.Typer.op1$2(Typer.scala:1856)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1852)
	at dotty.tools.dotc.typer.Typer.typed(Typer.scala:1864)
	at dotty.tools.dotc.typer.Namer.typedAheadType$$anonfun$1(Namer.scala:1010)
	at dotty.tools.dotc.typer.Namer.typedAheadImpl(Namer.scala:1003)
	at dotty.tools.dotc.typer.Namer.typedAheadType(Namer.scala:1010)
	at dotty.tools.dotc.typer.Namer.typeDefSig(Namer.scala:1261)
	at dotty.tools.dotc.typer.Namer$TypeDefCompleter.typeSig(Namer.scala:878)
	at dotty.tools.dotc.typer.Namer$Completer.completeInCreationContext(Namer.scala:843)
	at dotty.tools.dotc.typer.Namer$Completer.complete(Namer.scala:821)