fix CCE for RoaringTagIndex.empty (#1312)

Fix ClassCastException for casting the array based on
the generic type.

atlas-core/src/main/scala/com/netflix/atlas/core/index/RoaringTagIndex.scala

@@ -21,14 +21,15 @@ import com.netflix.atlas.core.model.ItemId
 import com.netflix.atlas.core.model.Query
 import com.netflix.atlas.core.model.Tag
 import com.netflix.atlas.core.model.TaggedItem
-import com.netflix.atlas.core.util.ArrayHelper
 import com.netflix.atlas.core.util.IntIntHashMap
 import com.netflix.atlas.core.util.IntRefHashMap
 import com.netflix.atlas.core.util.LongHashSet
 import com.netflix.atlas.core.util.RefIntHashMap
 import org.roaringbitmap.RoaringBitmap
 import org.slf4j.LoggerFactory
 
+import scala.reflect.ClassTag
+
 /**
   * Create a new index based on roaring bitmaps.
   *
@@ -493,8 +494,8 @@ class RoaringTagIndex[T <: TaggedItem](items: Array[T], stats: IndexStats) exten
 object RoaringTagIndex {
   private val logger = LoggerFactory.getLogger(getClass)
 
-  def empty[T <: TaggedItem]: RoaringTagIndex[T] = {
-    new RoaringTagIndex(ArrayHelper.newInstance[T](0), new IndexStats())
+  def empty[T <: TaggedItem: ClassTag]: RoaringTagIndex[T] = {
+    new RoaringTagIndex(new Array[T](0), new IndexStats())
   }
 
   private[index] def hasNonEmptyIntersection(b1: RoaringBitmap, b2: RoaringBitmap): Boolean = {

atlas-core/src/test/scala/com/netflix/atlas/core/index/RoaringTagIndexSuite.scala

@@ -15,6 +15,7 @@
  */
 package com.netflix.atlas.core.index
 
+import com.netflix.atlas.core.model.Datapoint
 import com.netflix.atlas.core.model.TimeSeries
 import org.roaringbitmap.RoaringBitmap
 
@@ -24,6 +25,11 @@ class RoaringTagIndexSuite extends TagIndexSuite {
     new RoaringTagIndex(TagIndexSuite.dataset.toArray, new IndexStats())
   }
 
+  test("empty") {
+    val idx = RoaringTagIndex.empty[Datapoint]
+    assert(idx.size == 0)
+  }
+
   test("hasNonEmptyIntersection: empty, empty") {
     val b1 = new RoaringBitmap()
     val b2 = new RoaringBitmap()