Repository: Qiskit/qiskit (python)

Issue:
Barrier instruction ignores number of qubits when passed qubit operands as set
### Environment

- **Qiskit Terra version**: 0.45
- **Python version**: 3.9.*
- **Operating system**: Linux



### What is happening?

When a circuit is created with integer qubit operands and a barrier is applied with those operands as a set the barrier instruction is created for a single qubit operand.

### How can we reproduce the issue?

```python
from qiskit import QuantumCircuit
qc = QuantumCircuit(2)
qc.barrier({0, 1})
print(qc.data)
```
gives
```
>>> [CircuitInstruction(operation=Instruction(name='barrier', num_qubits=1, num_clbits=0, params=[]), qubits=(Qubit(QuantumRegister(2, 'q'), 0), Qubit(QuantumRegister(2, 'q'), 1)), clbits=())]
```

### What should happen?

I would expect (and the behaviour was this way in 0.44)

```
>>> [CircuitInstruction(operation=Instruction(name='barrier', num_qubits=2, num_clbits=0, params=[]), qubits=(Qubit(QuantumRegister(2, 'q'), 0), Qubit(QuantumRegister(2, 'q'), 1)), clbits=())]
```

Passing a list gives the expected behaviour
```python
qc.barrier([0, 1])
```
### Any suggestions?

_No response_

Pull request description:
Fix `QuantumCircuit.barrier` argument conversion
### Summary

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.

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### Details and comments

Fix #11208.


Patch:
diff --git a/qiskit/circuit/quantumcircuit.py b/qiskit/circuit/quantumcircuit.py
index bc89e9740..c9f9b04fa 100644
--- a/qiskit/circuit/quantumcircuit.py
+++ b/qiskit/circuit/quantumcircuit.py
@@ -2852,23 +2852,12 @@ class QuantumCircuit:
         """
         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(
diff --git a/releasenotes/notes/fix-circuit-barrier-c696eabae1dcc6c2.yaml b/releasenotes/notes/fix-circuit-barrier-c696eabae1dcc6c2.yaml
new file mode 100644
index 000000000..b16792f8f
--- /dev/null
+++ b/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 patch:
diff --git a/test/python/circuit/test_circuit_operations.py b/test/python/circuit/test_circuit_operations.py
index 11770d896..3f773fb09 100644
--- a/test/python/circuit/test_circuit_operations.py
+++ b/test/python/circuit/test_circuit_operations.py
@@ -420,6 +420,27 @@ class TestCircuitOperations(QiskitTestCase):
         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