fix has_decomposition for ControlledQubitUnitary (#4407)

* fix has_decomposition for ControlledQubitUnitary

* changelog

* add comment; add test when super returns False

doc/releases/changelog-dev.md

@@ -218,6 +218,10 @@
   trainable parameters of the expanded tape.
   [(#4365)](https://github.com/PennyLaneAI/pennylane/pull/4365)
 
+* `qml.ControlledQubitUnitary` no longer reports `has_decomposition` as `True` when it does
+  not really have a decomposition.
+  [(#4407)](https://github.com/PennyLaneAI/pennylane/pull/4407)
+
 <h3>Contributors ✍️</h3>
 
 This release contains contributions from (in alphabetical order):

pennylane/ops/op_math/controlled_ops.py

@@ -144,6 +144,19 @@ def _controlled(self, wire):
             work_wires=self.work_wires,
         )
 
+    @property
+    def has_decomposition(self):
+        if not super().has_decomposition:
+            return False
+        with qml.QueuingManager.stop_recording():
+            # we know this is using try-except as logical control, but are favouring
+            # certainty in it being correct over explicitness in an edge case.
+            try:
+                self.decomposition()
+            except qml.operation.DecompositionUndefinedError:
+                return False
+        return True
+
 
 class CY(ControlledOp):
     r"""CY(wires)

tests/ops/op_math/test_controlled_decompositions.py

@@ -224,15 +224,15 @@ def test_zyz_decomp_no_control_values(self, test_expand):
         decomp = (
             op.expand().expand().circuit if test_expand else op.decomposition()[0].decomposition()
         )
-        expected = qml.ops.ctrl_decomp_zyz(base, (0,))
+        expected = qml.ops.ctrl_decomp_zyz(base, (0,))  # pylint:disable=no-member
         assert equal_list(decomp, expected)
 
     @pytest.mark.xfail
     @pytest.mark.parametrize("test_expand", [False, True])
     def test_zyz_decomp_control_values(self, test_expand):
         """Test that the ZYZ decomposition is used for single qubit target operations
         when other decompositions aren't available and control values are present."""
-
+        # pylint:disable=no-member
         base = qml.QubitUnitary(
             np.array(
                 [
@@ -663,3 +663,21 @@ def test_decomposition_matrix(self, op, control_wires, tol):
         expected = expected_op.matrix()
 
         assert np.allclose(res, expected, atol=tol, rtol=tol)
+
+
+def test_ControlledQubitUnitary_has_decomposition_correct():
+    """Test that ControlledQubitUnitary reports has_decomposition=False if it is False"""
+    U = qml.Toffoli(wires=[0, 1, 2]).matrix()
+    op = qml.ControlledQubitUnitary(U, wires=[1, 2, 3], control_wires=[0])
+
+    assert not op.has_decomposition
+    with pytest.raises(qml.operation.DecompositionUndefinedError):
+        op.decomposition()
+
+
+def test_ControlledQubitUnitary_has_decomposition_super_False(mocker):
+    """Test that has_decomposition returns False if super() returns False"""
+    spy = mocker.spy(qml.QueuingManager, "stop_recording")
+    op = qml.ControlledQubitUnitary(np.diag((1.0,) * 8), wires=[2, 3, 4], control_wires=[0, 1])
+    assert not op.has_decomposition
+    spy.assert_not_called()