Explicitly cast iterator in branches for type clarity

core/src/main/scala/org/apache/spark/shuffle/hash/HashShuffleReader.scala

@@ -65,13 +65,21 @@ private[spark] class HashShuffleReader[K, C](
        readMetrics.incRecordsRead(1)
        delegate.next()
      }
-    }.asInstanceOf[Iterator[Nothing]]
+    }
 
     val aggregatedIter: Iterator[Product2[K, C]] = if (dep.aggregator.isDefined) {
       if (dep.mapSideCombine) {
-        new InterruptibleIterator(context, dep.aggregator.get.combineCombinersByKey(iter, context))
+        // We are reading values that are already combined
+        val combinedKeyValuesIterator = iter.asInstanceOf[Iterator[(K,C)]]
+        new InterruptibleIterator(context,
+          dep.aggregator.get.combineCombinersByKey(combinedKeyValuesIterator, context))
       } else {
-        new InterruptibleIterator(context, dep.aggregator.get.combineValuesByKey(iter, context))
+        // We don't know the value type, but also don't care -- the dependency *should*
+        // have made sure its compatible w/ this aggregator, which will convert the value
+        // type to the combined type C
+        val keyValuesIterator = iter.asInstanceOf[Iterator[(K,Nothing)]]
+        new InterruptibleIterator(context,
+          dep.aggregator.get.combineValuesByKey(keyValuesIterator, context))
       }
     } else {
       require(!dep.mapSideCombine, "Map-side combine without Aggregator specified!")