Try to do without leftRoot

Author: odersky

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -1066,7 +1066,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
          */
         def compareCaptured(arg1: TypeBounds, arg2: Type) = tparam match {
           case tparam: Symbol
-          if leftRoot.isStable || ctx.isAfterTyper || ctx.mode.is(Mode.TypevarsMissContext) =>
+          if /*leftRoot.isStable ||*/ ctx.isAfterTyper || ctx.mode.is(Mode.TypevarsMissContext) =>
             val captured = TypeRef(leftRoot, tparam)
             assert(captured.exists, i"$leftRoot has no member $tparam in isSubArgs($args1, $args2, $tp1, $tparams2)")
             isSubArg(captured, arg2)