Github user zsxwing commented on the pull request:
https://github.com/apache/spark/pull/3378#issuecomment-63775682
Cogroup uses `CompactBuffer`. However, it cannot add ClassTag due to its
signature:
```Scala
class CoGroupedRDD[K](@transient var rdds: Seq[RDD[_ <: Product2[K, _]]],
part: Partitioner)
extends RDD[(K, Array[Iterable[_]])](rdds.head.context, Nil)
```
Here `rdds` is `Seq[RDD[_ <: Product2[K, _]]]` without the real template
type of `RDD`s
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