Conditions can sometimes cause stall when running on `AerSimulator`

[IlanIwumbwe]

Benny and I found this bug.

Informations

• Qiskit Aer version: 0.15.0
• Python version: 3.11.6
• Operating system: Ubuntu 23.10

What is the current behavior?

Running on AerSimulator stalls

Steps to reproduce the problem

Create a conditional that is guaranteed to be true, but does nothing.

qc = QuantumCircuit(3)
qc.h(0)
r = ClassicalRegister(1, "r")
qc.add_register(r)

with qc.if_test((r,0)):
    pass

qc.measure_active()

print(qc)

s = AerSimulator()
c = s.run(qc).result().get_counts()
print(c)

This also affects other control flow constructs that have the same characteristic.

What is the expected behavior?

Do nothing to the circuit, run the original circuit as normal and return a result

Suggested solutions

This may be an issue on qiskit's side, but maybe add a true_body of identity when creating the IfElseOp, and other relevant ops for the other control flow statements.