AbstractMethodError with separate compilation of subclass of trait containing inner class

This bug only occurs when separately compiling the two files:

File 1:

trait TypeHints {

  def + (hints: TypeHints): TypeHints = CompositeTypeHints()

  private[TypeHints] case class CompositeTypeHints() extends TypeHints

}

File 2:

class CustomTypeHints extends TypeHints {
}

object Test extends App {
  new CustomTypeHints() + new TypeHints{}
}

Expected output: (no output)

Actual output:

java.lang.AbstractMethodError: CustomTypeHints.TypeHints$$CompositeTypeHints()LTypeHints$CompositeTypeHints$;
        at TypeHints$class.$plus(Bug.scala:3)
        at CustomTypeHints.$plus(Bug2.scala:1)
        at Test$.delayedEndpoint$Test$1(Bug2.scala:5)
        at Test$delayedInit$body.apply(Bug2.scala:4)
        at scala.Function0$class.apply$mcV$sp(Function0.scala:34)
        at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
        at scala.App$$anonfun$main$1.apply(App.scala:76)
        at scala.App$$anonfun$main$1.apply(App.scala:76)
        at scala.collection.immutable.List.foreach(List.scala:381)
        at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
        at scala.App$class.main(App.scala:76)
        at Test$.main(Bug2.scala:4)
        at Test.main(Bug2.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at scala.reflect.internal.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:70)
        at scala.reflect.internal.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:31)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:101)
        at scala.reflect.internal.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:70)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:101)
        at scala.tools.nsc.CommonRunner$class.run(ObjectRunner.scala:22)
        at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:39)
        at scala.tools.nsc.CommonRunner$class.runAndCatch(ObjectRunner.scala:29)
        at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:39)
        at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:65)
        at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:87)
        at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:98)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:103)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

scala / bug

AbstractMethodError with separate compilation of subclass of trait containing inner class #9948