From 53e25fa0d8a3f21c44a4e884c0941f2c18863c8c Mon Sep 17 00:00:00 2001
From: ElePT <57907331+ElePT@users.noreply.github.com>
Date: Fri, 20 Jan 2023 23:45:01 +0100
Subject: [PATCH] Fix qpy for `MCX` gates (#9391)

* Add MCXGrayCode

* Add test and reno

* Add other mcx gates

* Fix test

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit ad952940e9367f1483e5e7f5e8e3203d8618301b)
---
 qiskit/qpy/binary_io/circuits.py              |  9 +++++++-
 .../fix-qpy-mcxgray-421cf8f673f24238.yaml     |  7 ++++++
 .../circuit/test_circuit_load_from_qpy.py     | 23 +++++++++++++++++--
 3 files changed, 36 insertions(+), 3 deletions(-)
 create mode 100644 releasenotes/notes/fix-qpy-mcxgray-421cf8f673f24238.yaml

diff --git a/qiskit/qpy/binary_io/circuits.py b/qiskit/qpy/binary_io/circuits.py
index 4053fb0a5758..527d7ec5442e 100644
--- a/qiskit/qpy/binary_io/circuits.py
+++ b/qiskit/qpy/binary_io/circuits.py
@@ -252,7 +252,14 @@ def _read_instruction(file_obj, circuit, registers, custom_operations, version,
     if gate_name in {"IfElseOp", "WhileLoopOp"}:
         gate = gate_class(condition_tuple, *params)
     elif version >= 5 and issubclass(gate_class, ControlledGate):
-        if gate_name in {"MCPhaseGate", "MCU1Gate"}:
+        if gate_name in {
+            "MCPhaseGate",
+            "MCU1Gate",
+            "MCXGrayCode",
+            "MCXGate",
+            "MCXRecursive",
+            "MCXVChain",
+        }:
             gate = gate_class(*params, instruction.num_ctrl_qubits)
         else:
             gate = gate_class(*params)
diff --git a/releasenotes/notes/fix-qpy-mcxgray-421cf8f673f24238.yaml b/releasenotes/notes/fix-qpy-mcxgray-421cf8f673f24238.yaml
new file mode 100644
index 000000000000..5f594a561522
--- /dev/null
+++ b/releasenotes/notes/fix-qpy-mcxgray-421cf8f673f24238.yaml
@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed a bug in QPY (:mod:`qiskit.qpy`) where circuits containing gates of class
+    :class:`.MCXGate`, :class:`.MCXGrayCode`, and :class:`MCXRecursive`, and
+    :class:`.MCXVChain` would fail to serialize.
+    See `#9390 <https://github.com/Qiskit/qiskit-terra/issues/9390>`__.
diff --git a/test/python/circuit/test_circuit_load_from_qpy.py b/test/python/circuit/test_circuit_load_from_qpy.py
index db4e62c5999a..349bca64f3f1 100644
--- a/test/python/circuit/test_circuit_load_from_qpy.py
+++ b/test/python/circuit/test_circuit_load_from_qpy.py
@@ -24,7 +24,18 @@
 from qiskit.circuit.quantumregister import Qubit
 from qiskit.circuit.random import random_circuit
 from qiskit.circuit.gate import Gate
-from qiskit.circuit.library import XGate, QFT, QAOAAnsatz, PauliEvolutionGate, DCXGate, MCU1Gate
+from qiskit.circuit.library import (
+    XGate,
+    QFT,
+    QAOAAnsatz,
+    PauliEvolutionGate,
+    DCXGate,
+    MCU1Gate,
+    MCXGate,
+    MCXGrayCode,
+    MCXRecursive,
+    MCXVChain,
+)
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.parameter import Parameter
 from qiskit.circuit.parametervector import ParameterVector
@@ -1043,9 +1054,17 @@ def test_open_controlled_gate(self):
 
     def test_standard_control_gates(self):
         """Test standard library controlled gates."""
-        qc = QuantumCircuit(3)
+        qc = QuantumCircuit(6)
         mcu1_gate = MCU1Gate(np.pi, 2)
+        mcx_gate = MCXGate(5)
+        mcx_gray_gate = MCXGrayCode(5)
+        mcx_recursive_gate = MCXRecursive(4)
+        mcx_vchain_gate = MCXVChain(3)
         qc.append(mcu1_gate, [0, 2, 1])
+        qc.append(mcx_gate, list(range(0, 6)))
+        qc.append(mcx_gray_gate, list(range(0, 6)))
+        qc.append(mcx_recursive_gate, list(range(0, 5)))
+        qc.append(mcx_vchain_gate, list(range(0, 5)))
         qc.mcp(np.pi, [0, 2], 1)
         qc.mct([0, 2], 1)
         qc.mcx([0, 2], 1)