Fix Barrier broadcast arguments (#11113) (#11114)

This commit fixes an issue with the Barrier class's
broadcast_arguments() method. Previously the Barrier class was
overriding broadcast_arguments(), however this custom implementation
resulted in an identical output to the Instruction's implementation
(its parent class) but stripped out all the error checking to determine
if the arguments aligned with the size of the instruction. This could
result in creating a corrupt circuit that had a mismatch between the
barrier width and the number of qargs. For example:

```
circuit = QuantumCircuit(1)
circuit.append(Barrier(42), [0])
```

would not error despite trying to add a 42 qubit barrier on qubit 0.
This would result in weird errors such as invalid qpy generation that
are confusing to debug. This commit fixes this by deleting the
broadcast_arguments() implementation for Barrier so it will just
depend on the inherited implementation from Instruction which will
return an identical result but check the instruction is valid.

(cherry picked from commit 5df2439)

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

qiskit/circuit/barrier.py

@@ -47,8 +47,5 @@ def inverse(self):
         """Special case. Return self."""
         return Barrier(self.num_qubits)
 
-    def broadcast_arguments(self, qargs, cargs):
-        yield [qarg for sublist in qargs for qarg in sublist], []
-
     def c_if(self, classical, val):
         raise QiskitError("Barriers are compiler directives and cannot be conditional.")

releasenotes/notes/fix-barrier-arg-list-check-ff69f37ede6bdf6c.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`.Barrier` class. When adding a
+    :class:`.Barrier` instance to a :class:`.QuantumCircuit` with the
+    :meth:`.QuantumCircuit.append` method previously there was no validation
+    that the size of the barrier matched the qargs specified.

test/python/circuit/test_circuit_operations.py

@@ -17,7 +17,7 @@
 from ddt import data, ddt
 
 from qiskit import BasicAer, ClassicalRegister, QuantumCircuit, QuantumRegister, execute
-from qiskit.circuit import Gate, Instruction, Measure, Parameter
+from qiskit.circuit import Gate, Instruction, Measure, Parameter, Barrier
 from qiskit.circuit.bit import Bit
 from qiskit.circuit.classicalregister import Clbit
 from qiskit.circuit.exceptions import CircuitError
@@ -156,6 +156,8 @@ def test_append_rejects_bad_arguments_opaque(self, bad_arg):
             qc.append(inst, bad_arg, [0, 1])
         with self.assertRaisesRegex(CircuitError, "The amount of clbit arguments"):
             qc.append(inst, [0, 1], bad_arg)
+        with self.assertRaisesRegex(CircuitError, "The amount of qubit arguments"):
+            qc.append(Barrier(4), bad_arg)
 
     def test_anding_self(self):
         """Test that qc &= qc finishes, which can be prone to infinite while-loops.