Remember to think about filter caching when upgrading to Lucene 4.5.

src/main/java/org/elasticsearch/common/lucene/docset/DocIdSets.java

@@ -25,14 +25,22 @@
 import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.FixedBitSet;
 import org.apache.lucene.util.OpenBitSetIterator;
+import org.apache.lucene.util.Version;
 import org.elasticsearch.common.Nullable;
+import org.elasticsearch.common.lucene.Lucene;
 
 import java.io.IOException;
 
 /**
  */
 public class DocIdSets {
 
+    static {
+        // TODO when upgrading to Lucene 4.5. Lucene's CachingWrapperFilter now doesn't use FixedBitSet all the time in favor of
+        // more memory-efficient implementations. Maybe we should do the same in toCacheable?
+        assert Version.LUCENE_44.onOrAfter(Lucene.VERSION);
+    }
+
     public static long sizeInBytes(DocIdSet docIdSet) {
         if (docIdSet instanceof FixedBitSet) {
             return ((FixedBitSet) docIdSet).getBits().length * 8 + 16;