new test checking the borrow flag

theano/scan_module/tests/test_scan.py

@@ -3206,6 +3206,27 @@ class T_Scan(unittest.TestCase):
         f = theano.function([seq], results[1])
         assert numpy.all(exp_out == f(inp))
 
+    def test_borrow_bug_jeremiah(self):
+        # This test fails if scan uses wrongly the borrow flag
+        inp = numpy.arange(10).reshape(-1,1).astype(theano.config.floatX)
+        exp_out = numpy.zeros((10,1)).astype(theano.config.floatX)
+        exp_out[4:] = inp[:-4]
+
+        def onestep(x, x_tm4):
+            return x, x_tm4
+
+        seq = tensor.matrix()
+        initial_value = theano.shared(numpy.zeros((4,1),
+                                                  dtype=theano.config.floatX))
+        outputs_info = [{'initial' : initial_value, 'taps' : [-4]}, None]
+        results, _ = theano.scan(fn=onestep,
+                                       sequences=seq,
+                                       outputs_info=outputs_info)
+        sharedvar = theano.shared(numpy.zeros((1,1)))
+        updates = {sharedvar : results[0][-1:]}
+
+        f = theano.function([seq], results[1], updates=updates)
+        assert numpy.all(exp_out == f(inp))
 
 def test_speed():
     #