add a test for untested Tensor.assign behavior (#12448)

* add a test for untested Tensor.assign behavior

* better

test/test_assign.py

@@ -119,6 +119,17 @@ class TestAssign(unittest.TestCase):
     new = a + old_a
     np.testing.assert_allclose(new.numpy(), 4)
 
+  def test_assign_changes_alt(self, realize=False):
+    a = Tensor(1).contiguous()
+    if realize: a.realize()
+    b = a.contiguous()    # b returns a new Tensor
+    b.assign(2)
+    b.realize()
+    self.assertNotEqual(a.item(), b.item())
+  # on a realized Tensor contiguous child changes the source
+  @unittest.expectedFailure
+  def test_assign_changes_realized_alt(self): return self.test_assign_changes_alt(realize=True)
+
   def test_assign_diamond_cycle(self):
     # NOTE: should *not* raise AssertionError from numpy
     with self.assertRaisesRegex(RuntimeError, "cycle"):