Fix broadcastable pattern of flatten output

theano/tensor/basic.py

@@ -4190,8 +4190,18 @@ class Flatten(Op):
         if self.outdim < 1 or (x.ndim and self.outdim > x.ndim):
             raise ValueError('invalid output ndimensions (%i) for tensor of '
                              'rank %i' % (self.outdim, t_x.ndim))
+
+        # Infer the broadcastable pattern of the output. For every dimension
+        # unaffected by the flatten, the broadcast flag should be unchanged.
+        # For the dimension resulting from the collapse of other dimensions,
+        # it should be broadcastable iff all the collapsed dimensions were
+        # broadcastable.
+        bcast_kept_dims = x.broadcastable[:self.outdim - 1]
+        bcast_new_dim = python_all(x.broadcastable[self.outdim - 1:])
+        broadcastable = bcast_kept_dims + (bcast_new_dim,)
+
         return gof.Apply(self, [t_x], [tensor(x.type.dtype,
-                                              (False,) * self.outdim)])
+                                              broadcastable)])
 
     def perform(self, node, inp, out_):
         x, = inp