Only apply MCMT plugin on MCMTGate

qiskit/transpiler/passes/synthesis/hls_plugins.py

@@ -420,6 +420,7 @@
     C3XGate,
     C4XGate,
     PauliEvolutionGate,
+    MCMTGate,
     ModularAdderGate,
     HalfAdderGate,
     FullAdderGate,
@@ -1156,6 +1157,9 @@ class MCMTSynthesisDefault(HighLevelSynthesisPlugin):
 
     def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
         # first try to use the V-chain synthesis if enough auxiliary qubits are available
+        if not isinstance(high_level_object, MCMTGate):
+            return None
+
         if (
             decomposition := MCMTSynthesisVChain().run(
                 high_level_object, coupling_map, target, qubits, **options
@@ -1170,6 +1174,9 @@ class MCMTSynthesisNoAux(HighLevelSynthesisPlugin):
     """A V-chain based synthesis for ``MCMTGate``."""
 
     def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
+        if not isinstance(high_level_object, MCMTGate):
+            return None
+
         base_gate = high_level_object.base_gate
         ctrl_state = options.get("ctrl_state", None)
 
@@ -1195,6 +1202,9 @@ class MCMTSynthesisVChain(HighLevelSynthesisPlugin):
     """A V-chain based synthesis for ``MCMTGate``."""
 
     def run(self, high_level_object, coupling_map=None, target=None, qubits=None, **options):
+        if not isinstance(high_level_object, MCMTGate):
+            return None
+
         if options.get("num_clean_ancillas", 0) < high_level_object.num_ctrl_qubits - 1:
             return None  # insufficient number of auxiliary qubits
 

releasenotes/notes/fix-mcmt-to-gate-ec84e1c625312444.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed a bug where any instruction called ``"mcmt"`` would be passed into the high-level
+    synthesis routine for a :class:`.MCMTGate`, which causes a failure or invalid result.
+    In particular, this could happen accidentally when handling the :class:`.MCMT` _circuit_,
+    named ``"mcmt"``, and implicitly converting it into an instruction e.g. when appending 
+    it to a circuit.
+    Fixed `#13563 <https://github.com/Qiskit/qiskit/issues/13563>`__.

test/python/circuit/library/test_mcmt.py

@@ -285,6 +285,19 @@ def test_gate_with_parameters_vchain(self):
         self.assertEqual(circuit.num_parameters, 1)
         self.assertIs(circuit.parameters[0], theta)
 
+    def test_mcmt_circuit_as_gate(self):
+        """Test the MCMT plugin is only triggered for the gate, not the same-named circuit.
+
+        Regression test of #13563.
+        """
+        circuit = QuantumCircuit(2)
+        gate = RYGate(0.1)
+        mcmt = MCMT(gate=gate, num_ctrl_qubits=1, num_target_qubits=1)
+        circuit.append(mcmt, circuit.qubits)  # append the MCMT circuit as gate called "MCMT"
+
+        transpiled = transpile(circuit, basis_gates=["u", "cx"])
+        self.assertTrue(Operator(transpiled).equiv(gate.control(1)))
+
 
 if __name__ == "__main__":
     unittest.main()