Avoid using ScanOp.get_output_pos()

theano/scan_module/scan_op.py

@@ -1937,10 +1937,15 @@ class Scan(PureOp):
 
                 for pos, inp in enumerate(states):
                     if inp in theano.gof.graph.inputs([Xt]):
-                        oidx = self.get_output_pos(pos)
-                        if not isinstance(dC_douts[oidx].type,
+                        # Get the index of the outer output that to which
+                        # the state variable 'inp' corresponds.
+                        outer_iidx = self.get_outer_iidx_from_inner_iidx_seq()[self.n_seqs +
+                                                                               pos]
+                        outer_oidx = self.get_outer_iidx_from_outer_oidx_seq().index(outer_iidx)
+
+                        if not isinstance(dC_douts[outer_oidx].type,
                                           DisconnectedType):
-                            dtypes.append(dC_douts[oidx].dtype)
+                            dtypes.append(dC_douts[outer_oidx].dtype)
                 if dtypes:
                     new_dtype = theano.scalar.upcast(*dtypes)
                 else:
@@ -1984,14 +1989,25 @@ class Scan(PureOp):
         # construct dX_dtm1
         dC_dXtm1s = []
         for pos, x in enumerate(dC_dinps_t[self.n_seqs:]):
-            opos = self.get_output_pos(pos)
-            if opos >= 0:
+
+            # Get the index of the first outer input corresponding to the
+            # pos-ieth inner input state
+            idxs = self.get_inner_oidx_from_inner_iidx_seq()[self.n_seqs +
+                                                             pos]
+
+            # Check if the pos-th input is associated with one of the
+            # recurrent states
+            x_is_state = pos < sum([len(t) for t in self.tap_array])
+
+            if x_is_state and len(idxs) > 0:
+                opos = idxs[0]
                 dC_dXtm1s.append(safe_new(dC_dXts[opos]))
                 if hasattr(x, 'dtype') and x.dtype != dC_dXts[opos].dtype:
                     dC_dinps_t[pos + self.n_seqs] = \
                             x.astype(dC_dXts[opos].dtype)
             else:
                 dC_dXtm1s.append(safe_new(x))
+
         for dx, dC_dXtm1 in enumerate(dC_dXtm1s):
             if isinstance(dC_dinps_t[dx + self.n_seqs].type, NullType):
                 # The accumulated gradient is undefined