re-disallow DESC with ORDER BY ANN

src/java/org/apache/cassandra/cql3/restrictions/StatementRestrictions.java

@@ -682,7 +682,8 @@ else if (indexOrderings.size() == 1)
                 if (orderings.size() > 1)
                     throw new InvalidRequestException("Cannot combine clustering column ordering with non-clustering column ordering");
                 Ordering ordering = indexOrderings.get(0);
-                if (ordering.direction != Ordering.Direction.ASC && ordering.expression.isScored())
+                // TODO remove the instanceof with SelectStatement.ANN_USE_SYNTHETIC_SCORE.
+                if (ordering.direction != Ordering.Direction.ASC && (ordering.expression.isScored() || ordering.expression instanceof Ordering.Ann))
                     throw new InvalidRequestException("Descending ANN ordering is not supported");
                 if (!ENABLE_SAI_GENERAL_ORDER_BY && ordering.expression instanceof Ordering.SingleColumn)
                     throw new InvalidRequestException("SAI based ORDER BY on non-vector column is not supported");

src/java/org/apache/cassandra/cql3/statements/SelectStatement.java

@@ -101,8 +101,11 @@
  */
 public class SelectStatement implements CQLStatement.SingleKeyspaceCqlStatement
 {
-    // TODO remove this when we no longer need to downgrade to replicas that don't know about synthetic columns
-    // (And don't forget to remove the related hacks in Columns.Serializer.encodeBitmap and UnfilteredSerializer.serializeRowBody)
+    // TODO remove this when we no longer need to downgrade to replicas that don't know about synthetic columns,
+    // and the related code in
+    //  - Columns.Serializer.encodeBitmap
+    //  - UnfilteredSerializer.serializeRowBody)
+    //  - StatementRestrictions.addOrderingRestrictions
     public static final boolean ANN_USE_SYNTHETIC_SCORE = Boolean.parseBoolean(System.getProperty("cassandra.sai.ann_use_synthetic_score", "false"));
 
     private static final Logger logger = LoggerFactory.getLogger(SelectStatement.class);