Improve the error message when a saved function captures a symbolic Tensor without a constant value.

PiperOrigin-RevId: 253624252

tensorflow/python/saved_model/save.py

@@ -262,8 +262,13 @@ class _SaveableView(object):
         if (tensor_util.is_tensor(capture)
             and capture.dtype not in _UNCOPIABLE_DTYPES
             and capture not in self.captured_tensor_node_ids):
-          copied_tensor = constant_op.constant(
-              tensor_util.constant_value(capture))
+          capture_constant_value = tensor_util.constant_value(capture)
+          if capture_constant_value is None:
+            raise ValueError(
+                ("Attempted to save a function {} which references a symbolic "
+                 "Tensor {} that is not a simple constant. This is not "
+                 "supported.").format(concrete_function.name, capture))
+          copied_tensor = constant_op.constant(capture_constant_value)
           node_id = len(self.nodes)
           node = _CapturedConstant(
               eager_tensor=capture, graph_tensor=copied_tensor)

tensorflow/python/saved_model/save_test.py

@@ -124,6 +124,23 @@ class SaveTest(test.TestCase):
     root = util.Checkpoint(model=sequential.Sequential([core.Dense(2)]))
     save.save(root, os.path.join(self.get_temp_dir(), "saved_model"))
 
+  def test_captured_symbolic_tensor_exception(self):
+    root = module.Module()
+    symbolic_tensor = []
+
+    @def_function.function
+    def captured_intermediate(x):
+      symbolic_tensor.append(math_ops.add(x, x, name="a_tensor"))
+      return symbolic_tensor[-1] * 2
+
+    captured_intermediate(constant_op.constant(1.))
+
+    root.f = def_function.function(lambda: symbolic_tensor[-1],
+                                   input_signature=[])
+    with self.assertRaisesRegexp(ValueError, "a_tensor"):
+      save.save(root, os.path.join(self.get_temp_dir(), "saved_model"),
+                signatures=root.f)
+
   def test_version_information_included(self):
     root = tracking.AutoTrackable()
     save_dir = os.path.join(self.get_temp_dir(), "saved_model")