ParTrieMap iteration fails with ClassCastException

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Performing a foreach on a ParTrieMap instance sometimes causes the following ClassCastException to be thrown:

java.lang.ClassCastException: scala.collection.LinearSeqLike$$anon$1 cannot be cast to scala.collection.parallel.IterableSplitter
        at scala.collection.parallel.IterableSplitter$$anonfun$splitWithSignalling$1.apply(RemainsIterator.scala:398)
        at scala.collection.immutable.List.foreach(List.scala:309)
        at scala.collection.parallel.IterableSplitter$class.splitWithSignalling(RemainsIterator.scala:398)
        at scala.collection.parallel.mutable.ParTrieMapSplitter.splitWithSignalling(ParTrieMap.scala:123)
        at scala.collection.parallel.ParIterableLike$Accessor$class.split(ParIterableLike.scala:905)
        at scala.collection.parallel.ParIterableLike$Foreach.split(ParIterableLike.scala:972)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.split(Tasks.scala:516)
        at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.spawnSubtasks(Tasks.scala:197)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.spawnSubtasks(Tasks.scala:514)
        at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.internal(Tasks.scala:170)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.internal(Tasks.scala:514)
        at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.compute(Tasks.scala:162)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.compute(Tasks.scala:514)
        at scala.concurrent.forkjoin.RecursiveAction.exec(RecursiveAction.java:160)
        at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:262)
        at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runSubtask(ForkJoinPool.java:994)
        at scala.concurrent.forkjoin.ForkJoinPool.tryHelpStealer(ForkJoinPool.java:1702)
        at scala.concurrent.forkjoin.ForkJoinPool.awaitJoin(ForkJoinPool.java:1830)
        at scala.concurrent.forkjoin.ForkJoinTask.doJoin(ForkJoinTask.java:344)
        at scala.concurrent.forkjoin.ForkJoinTask.join(ForkJoinTask.java:648)
        at scala.collection.parallel.ForkJoinTasks$WrappedTask$class.sync(Tasks.scala:444)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.sync(Tasks.scala:514)
        at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.internal(Tasks.scala:187)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.internal(Tasks.scala:514)
        at scala.collection.parallel.AdaptiveWorkStealingTasks$WrappedTask$class.compute(Tasks.scala:162)
        at scala.collection.parallel.AdaptiveWorkStealingForkJoinTasks$WrappedTask.compute(Tasks.scala:514)
        at scala.concurrent.forkjoin.RecursiveAction.exec(RecursiveAction.java:160)
        at scala.concurrent.forkjoin.ForkJoinTask.doExec(ForkJoinTask.java:262)
        at scala.concurrent.forkjoin.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:975)
        at scala.concurrent.forkjoin.ForkJoinPool.runWorker(ForkJoinPool.java:1478)
        at scala.concurrent.forkjoin.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:104)

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Imported From: https://issues.scala-lang.org/browse/SI-7498?orig=1
Reporter: William Robertson (nullptr)
Affected Versions: 2.10.1
Other Milestones: 2.11.0-M4

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William Robertson (nullptr) said:
I did some instrumentation of the code for ParTrieMap, and it looks like subiter can refer to a TrieMap iterator rather than a ParTrieMap iterator (see around TrieMap.scala:1018 in 2.10.1). A possible fix would be to make sure that subiter is always assigned the result of newIterator which is overloaded in ParTrieMap, although I'm not familiar enough with the codebase to know whether that's the right solution.

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@retronym said:
Alex: could you please take a look at this one?

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@axel22 said:
Sure. William's analysis is correct - this is due to a cast in the ParTrieMap of the return value of subdivide from TrieMap, that internally does not call newIterator when it deals with LNodes. I will fix this.

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@axel22 said:
Should do the trick:

scala/scala#2610

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@JamesIry said:
Pull request hasn't been merged. We'll close when it has.

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@retronym said:
Reopening for the backport for 2.10.3

scala/scala#2644