Fix QuantumCircuit.barrier argument conversion

qiskit/circuit/quantumcircuit.py

@@ -2852,23 +2852,12 @@ 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