`qasm3.loads` fails on circuits with `switch` due to unsupported `int` declaration

[YaegakiY]

Environment

• Qiskit version: 2.0.1
• Python version: 3.13.2
• Operating system: Windows 11, Version 24H2, Build 26100
• OpenQASM 3.0 version: 1.0.1

What is happening?

When using qiskit.qasm3.dumps on a circuit containing a switch statement, the resulting QASM code includes an int declaration (int switch_dummy;). However, when reloading it via qasm3.loads, the process fails with:

qiskit.qasm3.exceptions.QASM3ImporterError: "4,0: declarations of type 'int' are not supported"

How can we reproduce the issue?

To Reproduce:

from qiskit import QuantumCircuit, QuantumRegister, ClassicalRegister, transpile
from qiskit_aer import Aer

qreg = QuantumRegister(2) 
creg = ClassicalRegister(2) 
qc = QuantumCircuit(qreg, creg)

qc.h(qreg[0])
qc.cx(qreg[0], qreg[1])
qc.measure(qreg[0], creg[0])
qc.measure(qreg[1], creg[1])
with qc.switch(creg) as case:
	with case(0b00, 0b11):
		qc.x(1)
	with case(case.DEFAULT):
		qc.z(1)
qc.reset(qreg)
qc.measure(qreg[0], creg[0])
qc.measure(qreg[1], creg[1])

from qiskit.qasm3 import dumps, loads
code = dumps(qc)

print(code)

qasm3_to_qiskit = loads(code)

Generated QASM3 code:

OPENQASM 3.0;
include "stdgates.inc";
bit[2] c0;
int switch_dummy;
qubit[2] q0;
h q0[0];
cx q0[0], q0[1];
c0[0] = measure q0[0];
c0[1] = measure q0[1];
switch_dummy = c0;
switch (switch_dummy) {
  case 0, 3 {
    x q0[1];
  }
  default {
    z q0[1];
  }
}
reset q0[0];
reset q0[1];
c0[0] = measure q0[0];
c0[1] = measure q0[1];

What should happen?

The loads function should be able to parse the output from dumps.

Any suggestions?

No response

[jan-an]

Hello, I'd like to try working on this issue. Can you please assign this to me?

[YaegakiY]

Thanks for your interest! Sure, feel free to take it. I’m happy to see this get fixed. Let me know if you need any extra test cases or clarification.