added check that Op.grad and Op.connection_pattern agree

theano/gradient.py

@@ -734,7 +734,38 @@ def _populate_grad_dict(var_to_node_to_idx,
             # we won't be able to post-process out the Nones if it does that
             input_grads = list(input_grads)
 
-            #Do type checking on the result
+            # Do type checking on the result
+
+            #Check that op.connection_pattern matches the connectivity
+            #logic driving the op.grad method
+            for i, packed in \
+                enumerate(zip(inputs, input_grads, inputs_connected)):
+                ipt, ig, connected = packed
+                actually_connected = \
+                    not isinstance(ig.type, DisconnectedType)
+
+                if actually_connected and not connected:
+                    msg = "%s.grad returned %s of type %s for input %d."
+                    msg += " Expected DisconnectedType instance based on "
+                    msg += " the output of the op's connection_pattern "
+                    msg += "method."
+                    msg = msg % (str(node.op), str(ig), str(ig.type), i)
+                    raise TypeError(msg)
+
+                if connected and not actually_connected:
+                    msg = "%s.grad returned DisconnectedType for input"
+                    msg += " %d."
+                    msg = msg % (str(node.op), i)
+                    if hasattr(node.op,'connection_pattern'):
+                        msg += ' Its connection_pattern method does not'
+                        msg += ' allow this.'
+                        raise TypeError(msg)
+                    else:
+                        msg += ' You may want to implement a '
+                        msg += ' connection_pattern method for it.'
+                        warnings.warn(msg)
+
+            # Process out any Nones
             for i, term in enumerate(input_grads):
 
                 if term is None: