Qasm3 exporter failing when if branch does not contain all clbits #8903
Description
Environment
- Qiskit Terra version: 0.39.0
- Python version: 3.10.6
- Operating system: MacOs 12.6 (21G115)
What is happening?
In some cases, the qasm3 exporter gives an error if the circuit has more clbits than needed in the if_else statement.
How can we reproduce the issue?
Run the following:
backend_wellington = provider.get_backend('ibm_wellington')
cr = ClassicalRegister(1) # if >1 breaks
cr2 = ClassicalRegister(1) # if this reg exists, breaks
qr = QuantumRegister(5)
qc = QuantumCircuit(qr, cr, cr2)
with qc.if_test((cr[0], 1)) as _:
qc.cnot(0,1)
qc.cnot(0,2)
qc.cnot(0,3)
qc.cnot(0,4)
qc_trans = transpile(qc, backend=backend_wellington, basis_gates=backend_wellington.configuration().basis_gates+['if_else'], optimization_level=1)
print(qasm3.Exporter().dumps(qc))
print(qasm3.Exporter().dumps(qc_trans))
Now, qc gets correctly exported, while qc_trans provides the following error:
QASM3ExporterError: 'Tried to push a scope whose circuit needs 2 clbits, but only provided 1 clbits to create the mapping.'
What should happen?
The QASM3 string should be output for both cases.
Any suggestions?
No response