Fix qpy custom ControlledGate with overloaded _define()

qiskit/qpy/binary_io/circuits.py

@@ -632,6 +632,9 @@ def _write_custom_operation(file_obj, name, operation, custom_operations):
         # state is open, and the definition setter (during a subsequent read) uses the "fully
         # excited" control definition only.
         has_definition = True
+        # Build internal definition to support overloaded subclasses by
+        # calling definition getter on object
+        operation.definition  # pylint: disable=pointless-statement
         data = common.data_to_binary(operation._definition, write_circuit)
         size = len(data)
         num_ctrl_qubits = operation.num_ctrl_qubits

releasenotes/notes/fix-qpy-custom-controlled-gate-a9355df1a88a83a5.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed an issue in QPY serialization (:func:`~.qpy.dump`) when a custom
+    :class:`~.ControlledGate` subclass that overloaded the ``_define()``
+    method to provide a custom definition for the operation. Previously,
+    this case of operation was not serialized correctly because it wasn't
+    accounting for using the potentially ``_define()`` method to provide
+    a definition.
+    Fixes `#8794 <https://github.com/Qiskit/qiskit-terra/issues/8794>`__

test/python/circuit/test_circuit_load_from_qpy.py

@@ -34,6 +34,7 @@
 from qiskit.test import QiskitTestCase
 from qiskit.circuit.qpy_serialization import dump, load
 from qiskit.quantum_info.random import random_unitary
+from qiskit.circuit.controlledgate import ControlledGate
 
 
 class TestLoadFromQPY(QiskitTestCase):
@@ -1054,3 +1055,31 @@ def test_standard_control_gates(self):
         qpy_file.seek(0)
         new_circuit = load(qpy_file)[0]
         self.assertEqual(qc, new_circuit)
+
+    def test_controlled_gate_subclass_custom_definition(self):
+        """Test controlled gate with overloaded definition.
+
+        Reproduce from: https://github.com/Qiskit/qiskit-terra/issues/8794
+        """
+
+        class CustomCXGate(ControlledGate):
+            """Custom CX with overloaded _define."""
+
+            def __init__(self, label=None, ctrl_state=None):
+                super().__init__(
+                    "cx", 2, [], label, num_ctrl_qubits=1, ctrl_state=ctrl_state, base_gate=XGate()
+                )
+
+            def _define(self) -> None:
+                qc = QuantumCircuit(2, name=self.name)
+                qc.cx(0, 1)
+                self.definition = qc
+
+        qc = QuantumCircuit(2)
+        qc.append(CustomCXGate(), [0, 1])
+        qpy_file = io.BytesIO()
+        dump(qc, qpy_file)
+        qpy_file.seek(0)
+        new_circ = load(qpy_file)[0]
+        self.assertEqual(qc, new_circ)
+        self.assertEqual(qc.decompose(), new_circ.decompose())