Fixed crash in a specific concatenate situation

NEWS.txt

@@ -79,6 +79,8 @@ Crash Fix
    (Pascal L., reported by Simon McGregor)
  * Fixed issue with the MaxAndArgmax Op not properly preserving broadcastable
    dimensions, which could typically result in optimization crashes (Olivier D.)
+ * Fixed crash when concatenating some arrays with specific broadcasting
+   patterns (Olivier D.)
 
 =============
 Release Notes

theano/tensor/opt.py

@@ -2078,7 +2078,11 @@ def local_rebroadcast_lift(node):
     input = node.inputs[0]
     inode = input.owner
     if inode and isinstance(inode.op, Elemwise) and len(inode.inputs) == 1:
-        if len(input.clients) == 1:
+        # It may happen that `input` has no client because this optimization
+        # is called from `apply_rebroadcast_opt`, which in particular is used
+        # by the `unbroadcast` function before we are in the actual function
+        # compilation phase.
+        if hasattr(input, 'clients') and len(input.clients) == 1:
             rval = inode.op.make_node(T.Rebroadcast(*op.axis.items())(
                     inode.inputs[0])).outputs
             return rval
@@ -2098,7 +2102,7 @@ def apply_rebroadcast_opt(rval):
     and local_rebroadcast_lift.
 
     :param rval: a Variable
-    :retrun: a Variable. The same if not optimisation can be applied.
+    :return: a Variable (the same if no optimization can be applied)
     """
 
     changed = True

theano/tensor/tests/test_basic.py

@@ -3387,6 +3387,15 @@ class T_Join_and_Split(unittest.TestCase):
         else:
             f(get_mat(3, 4), get_mat(3, 4), get_mat(2, 5))
 
+    def test_rebroadcast(self):
+        """
+        Regression test for a crash that used to happen when rebroadcasting.
+        """
+        x = tensor.TensorType(floatX, [False, False, True])()
+        u = tensor.TensorType(floatX, [False, False, True])()
+        # This line used to crash.
+        z = tensor.concatenate([x, -u], axis=2)
+
 
 class test_comparison(unittest.TestCase):
     def test_gt(self):