fix gh-5907. Make OpFromGraph.grad handle compute_test_value correctly.

theano/compile/builders.py

@@ -271,6 +271,7 @@ class OpFromGraph(gof.Op):
         is_inline = self.is_inline
         return '%(name)s{inline=%(is_inline)s}' % locals()
 
+    @theano.configparser.change_flags(compute_test_value='off')
     def _recompute_grad_op(self):
         '''
         converts self._grad_op from user supplied form to type(self) instance

theano/compile/tests/test_builders.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import, print_function, division
 from functools import partial
 import numpy as np
 
+import theano
 from theano import config, shared
 
 from theano.gradient import DisconnectedType
@@ -313,3 +314,14 @@ class T_OpFromGraph(unittest_tools.InferShapeTester):
                                 [np.ones([3, 4], dtype=config.floatX),
                                  np.ones([3, 4], dtype=config.floatX)],
                                 OpFromGraph)
+
+    @theano.configparser.change_flags(compute_test_value='raise')
+    def test_compute_test_value(self):
+        x = T.scalar('x')
+        x.tag.test_value = np.array(1.)
+        op = OpFromGraph([x], [x ** 3])
+        y = T.scalar('y')
+        y.tag.test_value = np.array(1.)
+        f = op(y)
+        grad_f = T.grad(f, y)
+        assert grad_f.tag.test_value is not None