Fix QuantumCircuit.barrier argument conversion (#11272)

* Fix `QuantumCircuit.barrier` argument conversion

The manual argument conversion within `QuantumCircuit.barrier` made it
inconsistent with the rest of the circuit methods, and it would silently
generate invalid circuit output when given iterables outside an expected
set.  This switches the method to use the standard argument conversion,
bringing it inline with the rest of the circuit methods.

* Include link to issue

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

---------

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
(cherry picked from commit 252dbd5)

qiskit/circuit/quantumcircuit.py

@@ -2850,23 +2850,12 @@ def barrier(self, *qargs: QubitSpecifier, label=None) -> InstructionSet:
         """
         from .barrier import Barrier
 
-        qubits: list[QubitSpecifier] = []
-
-        if not qargs:  # None
-            qubits.extend(self.qubits)
-
-        for qarg in qargs:
-            if isinstance(qarg, QuantumRegister):
-                qubits.extend([qarg[j] for j in range(qarg.size)])
-            elif isinstance(qarg, list):
-                qubits.extend(qarg)
-            elif isinstance(qarg, range):
-                qubits.extend(list(qarg))
-            elif isinstance(qarg, slice):
-                qubits.extend(self.qubits[qarg])
-            else:
-                qubits.append(qarg)
-
+        qubits = (
+            # This uses a `dict` not a `set` to guarantee a deterministic order to the arguments.
+            list({q: None for qarg in qargs for q in self.qbit_argument_conversion(qarg)})
+            if qargs
+            else self.qubits.copy()
+        )
         return self.append(Barrier(len(qubits), label=label), qubits, [])
 
     def delay(

releasenotes/notes/fix-circuit-barrier-c696eabae1dcc6c2.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    :meth:`.QuantumCircuit.barrier` will now generate correct output when given a :class:`set` as
+    one of its inputs.  Previously, it would append an invalid operation onto the circuit, though in
+    practice this usually would not cause observable problems.
+    Fixed `#11208 <https://github.com/Qiskit/qiskit/issues/11208>`__

test/python/circuit/test_circuit_operations.py

@@ -420,6 +420,27 @@ def test_clear_circuit(self):
         self.assertEqual(len(qc.data), 0)
         self.assertEqual(len(qc._parameter_table), 0)
 
+    def test_barrier(self):
+        """Test multiple argument forms of barrier."""
+        qr1, qr2 = QuantumRegister(3, "qr1"), QuantumRegister(4, "qr2")
+        qc = QuantumCircuit(qr1, qr2)
+        qc.barrier()  # All qubits.
+        qc.barrier(0, 1)
+        qc.barrier([4, 2])
+        qc.barrier(qr1)
+        qc.barrier(slice(3, 5))
+        qc.barrier({1, 4, 2}, range(5, 7))
+
+        expected = QuantumCircuit(qr1, qr2)
+        expected.append(Barrier(expected.num_qubits), expected.qubits.copy(), [])
+        expected.append(Barrier(2), [expected.qubits[0], expected.qubits[1]], [])
+        expected.append(Barrier(2), [expected.qubits[2], expected.qubits[4]], [])
+        expected.append(Barrier(3), expected.qubits[0:3], [])
+        expected.append(Barrier(2), [expected.qubits[3], expected.qubits[4]], [])
+        expected.append(Barrier(5), [expected.qubits[x] for x in [1, 2, 4, 5, 6]], [])
+
+        self.assertEqual(qc, expected)
+
     def test_measure_active(self):
         """Test measure_active
         Applies measurements only to non-idle qubits. Creates a ClassicalRegister of size equal to