Bug fix in HoareOptimizer (#13083)

* bug fix, test, reno

* suggestions from code review; lint

qiskit/transpiler/passes/optimization/hoare_opt.py

@@ -203,20 +203,24 @@ def _traverse_dag(self, dag):
         """
         import z3
 
-        for node in dag.topological_op_nodes():
+        # Pre-generate all DAG nodes, since we later iterate over them, while
+        # potentially modifying and removing some of them.
+        nodes = list(dag.topological_op_nodes())
+        for node in nodes:
             gate = node.op
-            ctrlqb, ctrlvar, trgtqb, trgtvar = self._seperate_ctrl_trgt(node)
+            _, ctrlvar, trgtqb, trgtvar = self._seperate_ctrl_trgt(node)
 
             ctrl_ones = z3.And(*ctrlvar)
 
-            remove_ctrl, new_dag, qb_idx = self._remove_control(gate, ctrlvar, trgtvar)
+            remove_ctrl, new_dag, _ = self._remove_control(gate, ctrlvar, trgtvar)
 
             if remove_ctrl:
-                dag.substitute_node_with_dag(node, new_dag)
-                gate = gate.base_gate
-                node.op = gate.to_mutable()
-                node.name = gate.name
-                node.qargs = tuple((ctrlqb + trgtqb)[qi] for qi in qb_idx)
+                # We are replacing a node by a new node over a smaller number of qubits.
+                # This can be done using substitute_node_with_dag, which furthermore returns
+                # a mapping from old node ids to new nodes.
+                mapped_nodes = dag.substitute_node_with_dag(node, new_dag)
+                node = next(iter(mapped_nodes.values()))
+                gate = node.op
                 _, ctrlvar, trgtqb, trgtvar = self._seperate_ctrl_trgt(node)
 
                 ctrl_ones = z3.And(*ctrlvar)

releasenotes/notes/fix-hoare-opt-56d1ca6a07f07a2d.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`.HoareOptimizer` where a controlled gate was simplified
+    by removing its controls but the new gate was not handled correctly.

test/python/transpiler/test_hoare_opt.py

@@ -661,6 +661,41 @@ def test_multiple_pass(self):
 
         self.assertEqual(result2, circuit_to_dag(expected))
 
+    def test_remove_control_then_identity(self):
+        """This first simplifies a gate by removing its control, then removes the
+        simplified gate by canceling it with another gate.
+        See: https://github.com/Qiskit/qiskit-terra/issues/13079
+        """
+        #      ┌───┐┌───┐┌───┐
+        # q_0: ┤ X ├┤ X ├┤ X ├
+        #      └─┬─┘└───┘└─┬─┘
+        # q_1: ──■─────────┼──
+        #      ┌───┐       │
+        # q_2: ┤ X ├───────■──
+        #      └───┘
+        circuit = QuantumCircuit(3)
+        circuit.cx(1, 0)
+        circuit.x(2)
+        circuit.x(0)
+        circuit.cx(2, 0)
+
+        simplified = HoareOptimizer()(circuit)
+
+        # The CX(1, 0) gate is removed as the control qubit q_1 is initially 0.
+        # The CX(2, 0) gate is first replaced by X(0) gate as the control qubit q_2 is at 1,
+        # then the two X(0) gates are removed.
+        #
+        # q_0: ─────
+        #
+        # q_1: ─────
+        #      ┌───┐
+        # q_2: ┤ X ├
+        #      └───┘
+        expected = QuantumCircuit(3)
+        expected.x(2)
+
+        self.assertEqual(simplified, expected)
+
 
 if __name__ == "__main__":
     unittest.main()