Warn if indexing bug would have been triggered.

theano/configdefaults.py

@@ -282,6 +282,13 @@ AddConfigVar('warn.sum_div_dimshuffle_bug',
              BoolParam(warn_default('0.3')),
              in_c_key=False)
 
+AddConfigVar('warn.subtensor_merge_bug',
+        "Warn if previous versions of Theano (before 0.5rc2) could have given "
+        "incorrect results when indexing into a subtensor with negative stride "
+        "(for instance, for instance, x[a:b:-1][c]).",
+        BoolParam(warn_default('0.5')),
+        in_c_key=False)
+
 AddConfigVar('compute_test_value',
         "If 'True', Theano will run each op at graph build time, using Constants, SharedVariables and the tag 'test_value' as inputs to the function. This helps the user track down problems in the graph before it gets optimized.",
         EnumStr('off', 'ignore', 'warn', 'raise'),

theano/tensor/opt.py

@@ -1682,6 +1682,12 @@ def merge_two_slices(slice1, len1, slice2, len2):
             # the k-th element from sl.start but the k-th element from
             # sl.stop backwards
             n_val = sl1.stop - 1 - sl2 * sl1.step
+            if config.warn.subtensor_merge_bug:
+                _logger.warn((
+                    'Your current code is fine, but Theano versions '
+                    'prior to 0.5rc2 might have given an incorrect result. '
+                    'To disable this warning, set the Theano flag '
+                    'warn.subtensor_merge_bug to False.'))
             # we need to pick either n_val or p_val and then follow same
             # steps as above for covering the index error cases
             val = T.switch(T.lt(reverse1, 0), n_val, p_val)