update assert_valid to catch CNOT decomposition

pennylane/ops/functions/assert_valid.py

@@ -56,6 +56,9 @@ def _check_decomposition(op, skip_wire_mapping):
 
  assert isinstance(decomp, list), "decomposition must be a list"
  assert isinstance(compute_decomp, list), "decomposition must be a list"
+ assert op.__class__ not in [
+ decomp_op.__class__ for decomp_op in decomp
+ ], "an operator should not be included in its own decomposition"
 
  for o1, o2, o3 in zip(decomp, compute_decomp, processed_queue):
  assert o1 == o2, "decomposition must match compute_decomposition"

tests/ops/functions/test_assert_valid.py

@@ -80,6 +80,17 @@ def compute_decomposition(wires):
  with pytest.raises(AssertionError, match="If has_decomposition is False"):
  assert_valid(BadDecomp(wires=0), skip_pickle=True)
 
+ def test_decomposition_must_not_contain_op(self):
+ """Test that decomposition op an operator doesn't include the operator itself"""
+
+ class BadDecomp(Operator):
+ @staticmethod
+ def compute_decomposition(wires):
+ return [BadDecomp(wires)]
+
+ with pytest.raises(AssertionError, match="should not be included in its own decomposition"):
+ assert_valid(BadDecomp(wires=0), skip_pickle=True)
+
 
 class TestBadMatrix:
  """Tests involving matrix validation."""