Fix an unchecked condition in InverseCancellation (#11834) (#11867)

* fix an unchecked condition in self_inverse inversecancellation

* add release note for the bug in inversecancellation

* reformat reverse_cancellation.py to follow the standards

* update fix-inverse-cancellation release note

* add a test for InverseCancellation with self-inverse

* fix a typo in test_inverse_cancellation.py

Co-authored-by: Matthew Treinish <[email protected]>

* Update qiskit/transpiler/passes/optimization/inverse_cancellation.py

* Update releasenotes/notes/fix-inverse-cancellation-self-inverse-e09a5553331e1b0b.yaml

* Revert "Update qiskit/transpiler/passes/optimization/inverse_cancellation.py"

This had a logical error in it and doesn't work.

This reverts commit 0048e8e.

---------

Co-authored-by: Matthew Treinish <[email protected]>
(cherry picked from commit 8509d94)

Co-authored-by: Seyed Sajad Kahani <[email protected]>

qiskit/transpiler/passes/optimization/inverse_cancellation.py

@@ -111,19 +111,18 @@ def _run_on_self_inverse(self, dag: DAGCircuit):
             for gate_cancel_run in gate_runs:
                 partitions = []
                 chunk = []
-                for i in range(len(gate_cancel_run) - 1):
+                max_index = len(gate_cancel_run) - 1
+                for i in range(len(gate_cancel_run)):
                     if gate_cancel_run[i].op == gate:
                         chunk.append(gate_cancel_run[i])
                     else:
                         if chunk:
                             partitions.append(chunk)
                             chunk = []
                         continue
-                    if gate_cancel_run[i].qargs != gate_cancel_run[i + 1].qargs:
+                    if i == max_index or gate_cancel_run[i].qargs != gate_cancel_run[i + 1].qargs:
                         partitions.append(chunk)
                         chunk = []
-                chunk.append(gate_cancel_run[-1])
-                partitions.append(chunk)
                 # Remove an even number of gates from each chunk
                 for chunk in partitions:
                     if len(chunk) % 2 == 0:

releasenotes/notes/fix-inverse-cancellation-self-inverse-e09a5553331e1b0b.yaml

@@ -0,0 +1,8 @@
+---
+
+fixes:
+  - |
+    Fixed an issue in the :class:`.InverseCancellation` transpiler pass where in some cases it
+    would incorrectly cancel a self-inverse parameterized gate even if the parameter value
+    didn't match.
+    Fixed  `#11815 <https://github.com/Qiskit/qiskit/issues/11815>`__

test/python/transpiler/test_inverse_cancellation.py

@@ -358,8 +358,10 @@ def test_parameterized_self_inverse(self):
         new_circ = inverse_pass(qc)
         self.assertEqual(new_circ, QuantumCircuit(1))
 
-    def test_parameterized_self_inverse_not_equal_parameter(self):
-        """Test that a parameterized self inverse gate doesn't cancel incorrectly."""
+    def test_parameterized_self_inverse_not_equal_parameter_1(self):
+        """Test that a parameterized self inverse gate doesn't cancel incorrectly.
+        This test, checks three gates with the same name but the middle one has a
+        different parameter."""
         qc = QuantumCircuit(1)
         qc.rz(0, 0)
         qc.rz(3.14159, 0)
@@ -368,6 +370,16 @@ def test_parameterized_self_inverse_not_equal_parameter(self):
         new_circ = inverse_pass(qc)
         self.assertEqual(new_circ, qc)
 
+    def test_parameterized_self_inverse_not_equal_parameter_2(self):
+        """Test that a parameterized self inverse gate doesn't cancel incorrectly.
+        This test, checks two gates with the same name but different parameters."""
+        qc = QuantumCircuit(1)
+        qc.rz(0, 0)
+        qc.rz(3.14159, 0)
+        inverse_pass = InverseCancellation([RZGate(0)])
+        new_circ = inverse_pass(qc)
+        self.assertEqual(qc, new_circ)
+
     def test_controlled_gate_open_control_does_not_cancel(self):
         """Test that a controlled gate with an open control doesn't cancel."""
         qc = QuantumCircuit(2)