exception when traversing Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.Test]().test$Test$$FOO_0
exception when traversing Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.Test]().test$Test$$FOO_0()
\

exception when traversing def foo(): test.Test$Foo = Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.\
Test]().test$Test$$FOO_0()
exception when traversing java.lang.Object with scala.ScalaObject with test.Test$X {
  def foo(): test.Test$Foo = Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.Test]().test$Test$$FOO_0\
();
  private   val test$Test$X$$$outer : test.Test = _;
     def test$Test$X$$$outer(): test.Test = Test$X$class.this.test$Test$X$$$out\
er ;
  def /*Test$X$class*/$init$($outer: test.Test): scala.Unit = {
    if ($outer.eq(null))
      throw new java.lang.NullPointerException.this()
    else
      Test$X$class.this.test$Test$X$$$outer  = $outer;
    ()
  }
}
exception when traversing abstract trait Test$X$class extends java.lang.Object with scala.ScalaObject with test.\
Test$X {
  def foo(): test.Test$Foo = Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.Test]().test$Test$$FOO_0\
();
  private   val test$Test$X$$$outer : test.Test = _;
     def test$Test$X$$$outer(): test.Test = Test$X$class.this.test$Test$X$$$out\
er ;
  def /*Test$X$class*/$init$($outer: test.Test): scala.Unit = {
    if ($outer.eq(null))
      throw new java.lang.NullPointerException.this()
    else
      Test$X$class.this.test$Test$X$$$outer  = $outer;
    ()
  }
}
exception when traversing package test {
  abstract trait Test extends java.lang.Object with scala.ScalaObject;
  abstract trait Test$class extends java.lang.Object with scala.ScalaObject with test.Test {
    final  def test$Test$$FOO_0(): object test.Test$FOO_0;
    def /*Test$class*/$init$(): scala.Unit = {
      ()
    }
  };
  private[Test] abstract class Test$Foo extends java.lang.Object with scala.ScalaObject {
    private   val $outer : test.Test = _;
       def $outer(): test.Test = Test$Foo.this.$outer ;
    def this($outer: test.Test): test.Test$Foo = {
      if ($outer.eq(null))
        throw new java.lang.NullPointerException.this()
      else
        Test$Foo.this.$outer  = $outer;
      {
        Test$Foo.super.this();
        ()
      };
      ()
    }
  };
  final class Test$FOO_0 extends test.Test$Foo with scala.ScalaObject {
    private   val $outer : test.Test = _;
       def $outer(): test.Test = Test$FOO_0.this.$outer ;
    def this($outer: test.Test): object test.Test$FOO_0 = {
      if ($outer.eq(null))
        throw new java.lang.NullPointerException.this()
      else
        Test$FOO_0.this.$outer  = $outer;
      {
        Test$FOO_0.super.this($outer);
        ()
      };
      ()
    }
  };
  abstract trait Test$X extends java.lang.Object with scala.ScalaObject {
    private[Test] def foo(): test.Test$Foo;
       def test$Test$X$$$outer(): test.Test
  };
  abstract trait Test$X$class extends java.lang.Object with scala.ScalaObject with test.Test$X {
    def foo(): test.Test$Foo = Test$X$class.this.test$Test$X$$$outer().$asInstanceOf[test.Test]().test$Test$$FOO\
_0();
    private   val test$Test$X$$$outer : test.Test = _;
       def test$Test$X$$$outer(): test.Test = Test$X$class.this.test$Test$X$$$o\
uter ;
    def /*Test$X$class*/$init$($outer: test.Test): scala.Unit = {
      if ($outer.eq(null))
        throw new java.lang.NullPointerException.this()
      else
        Test$X$class.this.test$Test$X$$$outer  = $outer;
      ()
    }
  }
}
Exception in thread "main" java.lang.AssertionError: assertion failed: object test$Test$$FOO_0
	at scala.Predef$.assert(Predef.scala:182)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.scala$tools$nsc$transform$Mixin$MixinTransformer$$postTrans\
form(Mixin.scala:416)
	at scala.tools.nsc.transform.Mixin$MixinTransformer$$anonfun$31.apply(Mixin.scala:438)
	at scala.tools.nsc.transform.Mixin$MixinTransformer$$anonfun$31.apply(Mixin.scala:438)
	at scala.tools.nsc.symtab.SymbolTable.atPhase(SymbolTable.scala:71)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:438)
	at scala.tools.nsc.ast.Trees$Transformer.transform(Trees.scala:1018)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:437)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$16.apply(Trees.scala:963)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$16.apply(Trees.scala:962)
	at scala.tools.nsc.ast.Trees$Transformer.atOwner(Trees.scala:1064)
	at scala.tools.nsc.ast.Trees$Transformer.transform(Trees.scala:961)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:437)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$25.apply(Trees.scala:1058)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$25.apply(Trees.scala:1056)
	at scala.List$.loop$0(List.scala:232)
	at scala.List$.mapConserve(List.scala:249)
	at scala.tools.nsc.ast.Trees$Transformer.transformStats(Trees.scala:1056)
	at scala.tools.nsc.ast.Trees$Transformer.transform(Trees.scala:982)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:437)
	at scala.tools.nsc.ast.Trees$Transformer.transformTemplate(Trees.scala:1044)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$13.apply(Trees.scala:950)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$13.apply(Trees.scala:949)
	at scala.tools.nsc.ast.Trees$Transformer.atOwner(Trees.scala:1064)
	at scala.tools.nsc.ast.Trees$Transformer.transform(Trees.scala:948)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:437)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$25.apply(Trees.scala:1058)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$25.apply(Trees.scala:1056)
	at scala.List$.loop$0(List.scala:232)
	at scala.List$.mapConserve(List.scala:249)
	at scala.List$.loop$0(List.scala:236)
	at scala.List$.mapConserve(List.scala:249)
	at scala.List$.loop$0(List.scala:236)
	at scala.List$.mapConserve(List.scala:249)
	at scala.List$.loop$0(List.scala:236)
	at scala.List$.mapConserve(List.scala:249)
	at scala.List$.loop$0(List.scala:236)
	at scala.List$.mapConserve(List.scala:249)
	at scala.List$.loop$0(List.scala:236)
	at scala.List$.mapConserve(List.scala:249)
	at scala.tools.nsc.ast.Trees$Transformer.transformStats(Trees.scala:1056)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$12.apply(Trees.scala:945)
	at scala.tools.nsc.ast.Trees$Transformer$$anonfun$12.apply(Trees.scala:945)
	at scala.tools.nsc.ast.Trees$Transformer.atOwner(Trees.scala:1064)
	at scala.tools.nsc.ast.Trees$Transformer.transform(Trees.scala:944)
	at scala.tools.nsc.transform.Mixin$MixinTransformer.transform(Mixin.scala:437)
	at scala.tools.nsc.ast.Trees$Transformer.transformUnit(Trees.scala:1059)
	at scala.tools.nsc.transform.Transform$Phase.apply(Transform.scala:22)
	at scala.tools.nsc.Global$GlobalPhase.applyPhase(Global.scala:216)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$1.apply(Global.scala:205)
	at scala.tools.nsc.Global$GlobalPhase$$anonfun$1.apply(Global.scala:205)
	at scala.Iterator$class.foreach(Iterator.scala:304)
	at scala.collection.mutable.ListBuffer$$anon$0.foreach(ListBuffer.scala:216)
	at scala.tools.nsc.Global$GlobalPhase.run(Global.scala:205)
	at scala.tools.nsc.Global$Run.compileSources(Global.scala:467)
	at scala.tools.nsc.Global$Run.compile(Global.scala:531)
	at scala.tools.nsc.Main$.process(Main.scala:76)
	at scala.tools.nsc.Main$.main(Main.scala:98)
	at scala.tools.nsc.Main.main(Main.scala)


Note: changing FOO_0 from private to private[Test] avoids the crash and compiles correctly.