Fix one step scan use wrong seq slice bug (#6598)

theano/scan_module/scan.py

@@ -490,6 +490,12 @@ def scan(fn,
             # go through the indicated slice
             mintap = np.min(seq['taps'])
             maxtap = np.max(seq['taps'])
+            # We cut the sequence such that seq[i] to correspond to
+            # seq[i-k]. For the purposes of cutting the sequences, we
+            # need to pretend tap 0 is used to avoid cutting the sequences
+            # too long if the taps are all lower or all higher than 0.
+            maxtap_proxy = max(maxtap, 0)
+            mintap_proxy = min(mintap, 0)
             for k in seq['taps']:
                 # create one slice of the input
                 # Later on, if we decide not to use scan because we are
@@ -500,9 +506,9 @@ def scan(fn,
 
                 # If not we need to use copies, that will be replaced at
                 # each frame by the corresponding slice
-                actual_slice = seq['input'][k - mintap]
+                actual_slice = seq['input'][k - mintap_proxy]
                 _seq_val = tensor.as_tensor_variable(seq['input'])
-                _seq_val_slice = _seq_val[k - mintap]
+                _seq_val_slice = _seq_val[k - mintap_proxy]
                 nw_slice = _seq_val_slice.type()
 
                 # Try to transfer test_value to the new variable
@@ -529,12 +535,6 @@ def scan(fn,
                         nw_name = seq['input'].name + '[t%d]' % k
                     nw_slice.name = nw_name
 
-                # We cut the sequence such that seq[i] to correspond to
-                # seq[i-k]. For the purposes of cutting the sequences, we
-                # need to pretend tap 0 is used to avoid cutting the sequences
-                # too long if the taps are all lower or all higher than 0.
-                maxtap_proxy = max(maxtap, 0)
-                mintap_proxy = min(mintap, 0)
                 start = (k - mintap_proxy)
                 nw_name = None
                 if k == maxtap_proxy:

theano/scan_module/tests/test_scan.py

@@ -5738,3 +5738,23 @@ def test_condition_hidden_inp():
         outputs=rs)
 
     _sum, total_steps = f(100, 100)
+
+def test_mintap_onestep():
+    seq = theano.tensor.ivector("seq")
+    seq_info = dict(input=seq, taps=[2])
+
+    def accum(seq_t, prev_sum):
+        new_sum = prev_sum + seq_t
+        return new_sum
+
+    rs, updates = theano.scan(fn=accum,
+                              sequences=seq_info,
+                              outputs_info=0,
+                              n_steps=1)
+
+    f = theano.function(inputs=[seq],
+                        outputs=rs)
+    _seq = np.arange(20).astype("int32")
+    _sum = f(_seq)
+    print("sum %f" % _sum)
+    assert _sum == 2