Fix but in RMissing filter of CachedVectorNodes. Improved comment about its rationale.

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/access/vector/CachedVectorNode.java

@@ -77,10 +77,17 @@ abstract class CachedVectorNode extends RBaseNode {
         }
     }
 
-    @ExplodeLoop
     protected Object[] filterPositions(Object[] positions) {
-        // we assume that the positions count cannot change as the isRemovePosition check
-        // is just based on types and therefore does not change per position instance.
+        /*
+         * we assume that the positions count cannot change as the isRemovePosition check is just
+         * based on types and therefore does not change per position instance.
+         * 
+         * normally empty positions are just empty but with S3 function dispatch it may happen that
+         * positions are also of type RMissing. These values should not contribute to the access
+         * dimensions.
+         * 
+         * Example test case: dimensions.x<-data.frame(c(1,2), c(3,4)); x[1]
+         */
         assert initializeFilteredPositionsCount(positions) == filteredPositionsLength;
         if (filteredPositionsLength != -1) {
             Object[] newPositions = new Object[filteredPositionsLength];