Fix replace_block_with_op on operations with wrong number of qubits

qiskit/dagcircuit/dagcircuit.py

@@ -1342,6 +1342,13 @@ def replace_block_with_op(
         block_cargs.sort(key=wire_pos_map.get)
         new_node = DAGOpNode(op, block_qargs, block_cargs, dag=self)
 
+        # check the op to insert matches the number of qubits we put it on
+        if op.num_qubits != len(block_qargs):
+            raise DAGCircuitError(
+                f"Number of qubits in the replacement operation ({op.num_qubits}) is not equal to "
+                f"the number of qubits in the block ({len(block_qargs)})!"
+            )
+
         try:
             new_node._node_id = self._multi_graph.contract_nodes(
                 block_ids, new_node, check_cycle=cycle_check

qiskit/transpiler/passes/routing/commuting_2q_gate_routing/pauli_2q_evolution_commutation.py

@@ -51,7 +51,11 @@ def run(self, dag: DAGCircuit) -> DAGCircuit:
                     sub_dag = self._decompose_to_2q(dag, node.op)
 
                     block_op = Commuting2qBlock(set(sub_dag.op_nodes()))
-                    wire_order = {wire: idx for idx, wire in enumerate(dag.qubits)}
+                    wire_order = {
+                        wire: idx
+                        for idx, wire in enumerate(sub_dag.qubits)
+                        if wire not in sub_dag.idle_wires()
+                    }
                     dag.replace_block_with_op([node], block_op, wire_order)
 
         return dag

releasenotes/notes/raise-on-illegal-replace-block-50cef8da757a580a.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Previously, :meth:`.DAGCircuit.replace_block_with_op` allowed to place an
+    ``n``-qubit operation onto a block of ``m`` qubits, leaving the DAG in an
+    invalid state. This behavior has been fixed, and the attempt will raise
+    a :class:`.DAGCircuitError`.

test/python/dagcircuit/test_dagcircuit.py

@@ -2903,6 +2903,22 @@ def test_single_node_block(self):
         self.assertEqual(expected_dag.count_ops(), dag.count_ops())
         self.assertIsInstance(new_node.op, XGate)
 
+    def test_invalid_replacement_size(self):
+        """Test inserting an operation on a wrong number of qubits raises."""
+
+        # two X gates, normal circuit
+        qc = QuantumCircuit(2)
+        qc.x(range(2))
+
+        # mutilate the DAG
+        dag = circuit_to_dag(qc)
+        to_replace = list(dag.op_nodes())
+        new_node = XGate()
+        idx_map = {node.qargs[0]: i for i, node in enumerate(to_replace)}
+
+        with self.assertRaises(DAGCircuitError):
+            dag.replace_block_with_op(to_replace, new_node, idx_map)
+
     def test_replace_control_flow_block(self):
         """Test that we can replace a block of control-flow nodes with a single one."""
         body = QuantumCircuit(1)