unpack RDataFrame in CallRFFIHelper.VECTOR_ELT

com.oracle.truffle.r.runtime.ffi/src/com/oracle/truffle/r/runtime/ffi/jnr/CallRFFIHelper.java

@@ -36,6 +36,7 @@ import com.oracle.truffle.r.runtime.data.RAttributes;
 import com.oracle.truffle.r.runtime.data.RComplex;
 import com.oracle.truffle.r.runtime.data.RComplexVector;
 import com.oracle.truffle.r.runtime.data.RDataFactory;
+import com.oracle.truffle.r.runtime.data.RDataFrame;
 import com.oracle.truffle.r.runtime.data.RDoubleSequence;
 import com.oracle.truffle.r.runtime.data.RDoubleVector;
 import com.oracle.truffle.r.runtime.data.RExpression;
@@ -485,7 +486,11 @@ public class CallRFFIHelper {
     }
 
     public static Object VECTOR_ELT(Object x, int i) {
-        RAbstractListVector list = guaranteeInstanceOf(RRuntime.asAbstractVector(x), RAbstractListVector.class);
+        Object vec = x;
+        if (vec instanceof RDataFrame) {
+            vec = ((RDataFrame) vec).getVector();
+        }
+        RAbstractListVector list = guaranteeInstanceOf(RRuntime.asAbstractVector(vec), RAbstractListVector.class);
         return list.getDataAt(i);
     }