Deprecation of MCMT in favor of MCMTGate (backport #13584) (#13810)

* Deprecation of MCMT in favor of MCMTGate (#13584)

* remove pending deprecations in mcmt

* reno

* testing a regression

(cherry picked from commit 28a33d7)

# Conflicts:
#	test/python/circuit/library/test_mcmt.py

* Fix conflict

---------

Co-authored-by: Luciano Bello <bel@zurich.ibm.com>
Co-authored-by: Elena Peña Tapia <57907331+ElePT@users.noreply.github.com>

qiskit/circuit/library/generalized_gates/mcmt.py

@@ -49,7 +49,7 @@ class MCMT(QuantumCircuit):
     :class:`~qiskit.circuit.library.MCMTVChain`.
     """
 
-    @deprecate_func(since="1.3", additional_msg="Use MCMTGate instead.", pending=True)
+    @deprecate_func(since="1.4", additional_msg="Use MCMTGate instead.")
     def __init__(
         self,
         gate: Gate | Callable[[QuantumCircuit, circuit.Qubit, circuit.Qubit], circuit.Instruction],
@@ -76,15 +76,15 @@ def __init__(
             warnings.warn(
                 "Passing a callable to MCMT is pending deprecation since Qiskit 1.3. Pass a "
                 "gate instance or the gate name instead, e.g. pass 'h' instead of QuantumCircuit.h.",
-                category=PendingDeprecationWarning,
+                category=DeprecationWarning,
                 stacklevel=2,
             )
             gate = gate.__name__
         elif isinstance(gate, QuantumCircuit):
             warnings.warn(
                 "Passing a QuantumCircuit is pending deprecation since Qiskit 1.3. Pass a gate "
                 "or turn the circuit into a gate using the ``to_gate`` method, instead.",
-                category=PendingDeprecationWarning,
+                category=DeprecationWarning,
                 stacklevel=2,
             )
             gate = gate.to_gate()
@@ -157,9 +157,8 @@ class MCMTVChain(MCMT):
     """
 
     @deprecate_func(
-        since="1.3",
+        since="1.4",
         additional_msg="Use MCMTGate with the V-chain synthesis plugin instead.",
-        pending=True,
     )
     def __init__(
         self,
@@ -276,7 +275,7 @@ def _identify_base_gate(gate):
             warnings.warn(
                 "Passing a controlled gate to MCMT is pending deprecation since Qiskit 1.3. Pass a "
                 "single-qubit gate instance or the gate name instead, e.g. pass 'h' instead of 'ch'.",
-                category=PendingDeprecationWarning,
+                category=DeprecationWarning,
                 stacklevel=2,
             )
             base_gate = gate.base_gate

releasenotes/notes/followup_13150-5bd0c77248601e1a.yaml

@@ -0,0 +1,8 @@
+---
+deprecations_circuits:
+  - |
+    The Multiple-Control-Multiple-Target in :class:`~qiskit.circuit.library.generalized_gates.MCMT` is now deprecated
+    and replaced by :class:`.MCMTGate`, which is a proper :class:`.Gate` subclass. Using
+    a gate instead of a circuit allows the compiler to reason about the object at a higher
+    level of abstraction and allows for multiple synthesis plugins.
+  

test/python/circuit/library/test_mcmt.py

@@ -51,7 +51,8 @@ class TestMCMT(QiskitTestCase):
     def test_mcmt_as_normal_control(self, mcmt_class):
         """Test that the MCMT can act as normal control gate."""
         qc = QuantumCircuit(2)
-        mcmt = mcmt_class(gate=CHGate(), num_ctrl_qubits=1, num_target_qubits=1)
+        with self.assertWarns(DeprecationWarning):
+            mcmt = mcmt_class(gate=CHGate(), num_ctrl_qubits=1, num_target_qubits=1)
         qc = qc.compose(mcmt, [0, 1])
 
         ref = QuantumCircuit(2)
@@ -65,20 +66,23 @@ def test_mcmt_as_normal_control(self, mcmt_class):
     def test_missing_qubits(self):
         """Test that an error is raised if qubits are missing."""
         with self.subTest(msg="no control qubits"):
-            with self.assertRaises(AttributeError):
-                _ = MCMT(XGate(), num_ctrl_qubits=0, num_target_qubits=1)
+            with self.assertWarns(DeprecationWarning):
+                with self.assertRaises(AttributeError):
+                    _ = MCMT(XGate(), num_ctrl_qubits=0, num_target_qubits=1)
 
         with self.subTest(msg="no target qubits"):
-            with self.assertRaises(AttributeError):
-                _ = MCMT(ZGate(), num_ctrl_qubits=4, num_target_qubits=0)
+            with self.assertWarns(DeprecationWarning):
+                with self.assertRaises(AttributeError):
+                    _ = MCMT(ZGate(), num_ctrl_qubits=4, num_target_qubits=0)
 
     def test_different_gate_types(self):
         """Test the different supported input types for the target gate."""
         x_circ = QuantumCircuit(1)
         x_circ.x(0)
         for input_gate in [x_circ, QuantumCircuit.cx, QuantumCircuit.x, "cx", "x", CXGate()]:
             with self.subTest(input_gate=input_gate):
-                mcmt = MCMT(input_gate, 2, 2)
+                with self.assertWarns(DeprecationWarning):
+                    mcmt = MCMT(input_gate, 2, 2)
                 if isinstance(input_gate, QuantumCircuit):
                     self.assertEqual(mcmt.gate.definition[0].operation, XGate())
                     self.assertEqual(len(mcmt.gate.definition), 1)
@@ -89,13 +93,15 @@ def test_mcmt_v_chain_ancilla_test(self):
         """Test too few and too many ancillas for the MCMT V-chain mode."""
         with self.subTest(msg="insufficient number of auxiliary qubits on gate"):
             qc = QuantumCircuit(5)
-            mcmt = MCMTVChain(ZGate(), 3, 1)
+            with self.assertWarns(DeprecationWarning):
+                mcmt = MCMTVChain(ZGate(), 3, 1)
             with self.assertRaises(QiskitError):
                 qc.append(mcmt, range(5))
 
         with self.subTest(msg="too many auxiliary qubits on gate"):
             qc = QuantumCircuit(9)
-            mcmt = MCMTVChain(ZGate(), 3, 1)
+            with self.assertWarns(DeprecationWarning):
+                mcmt = MCMTVChain(ZGate(), 3, 1)
             with self.assertRaises(QiskitError):
                 qc.append(mcmt, range(9))
 
@@ -135,7 +141,8 @@ def test_mcmt_v_chain_simulation(self, cgate, num_controls, num_targets):
             for i in subset:
                 qc.x(controls[i])
 
-            mcmt = MCMTVChain(cgate, num_controls, num_targets)
+            with self.assertWarns(DeprecationWarning):
+                mcmt = MCMTVChain(cgate, num_controls, num_targets)
             qc.compose(mcmt, qubits, inplace=True)
 
             for i in subset: