KeyError: Clbit(ClassicalRegister(2, 'cr'), 1) during transpile of a conditional circuit

[jiannanWang]

Environment

• Qiskit version: 1.0.1
• Python version: 3.10.13
• Operating system: Ubuntu 22.04.3 LTS

What is happening?

Running below code results in the exception copied in the "What should happen" section. I would expect the code to run without error.

How can we reproduce the issue?

import qiskit
from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister, transpile
from qiskit.circuit.library.standard_gates import CXGate
from qiskit.providers.basic_provider import BasicSimulator


qr = QuantumRegister(2, name='qr')
cr = ClassicalRegister(2, name='cr')
qc = QuantumCircuit(qr, cr, name='qc')
qc.append(CXGate().c_if(cr, 0).repeat(1), qargs=qr)

backend = BasicSimulator()
qc = transpile(qc, backend)

What should happen?

Traceback (most recent call last):
  File "/mnt/tests/reproduction/reproduce_KeyError:Clbit(ClassicalRegister(2,'cr'),1).py", line 13, in <module>
    qc = transpile(qc, backend)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/compiler/transpiler.py", line 425, in transpile
    out_circuits = pm.run(circuits, callback=callback)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/passmanager.py", line 397, in run
    return super().run(circuits, output_name, callback, num_processes=num_processes)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/passmanager.py", line 420, in wrapper
    return meth(*meth_args, **meth_kwargs)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/passmanager.py", line 182, in run
    return super().run(
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/passmanager/passmanager.py", line 229, in run
    out_program = _run_workflow(
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/passmanager/passmanager.py", line 292, in _run_workflow
    passmanager_ir, final_state = flow_controller.execute(
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/passmanager/base_tasks.py", line 218, in execute
    passmanager_ir, state = next_task.execute(
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/basepasses.py", line 195, in execute
    new_dag, state = super().execute(
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/passmanager/base_tasks.py", line 98, in execute
    ret = self.run(passmanager_ir)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/passes/synthesis/high_level_synthesis.py", line 379, in run
    decomposition, modified = self._recursively_handle_op(node.op, qubits)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/transpiler/passes/synthesis/high_level_synthesis.py", line 467, in _recursively_handle_op
    dag = circuit_to_dag(definition, copy_operations=False)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/converters/circuit_to_dag.py", line 92, in circuit_to_dag
    dagcircuit.apply_operation_back(op, instruction.qubits, instruction.clbits, check=False)
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/dagcircuit/dagcircuit.py", line 682, in apply_operation_back
    [self.output_map[bit]._node_id for bits in (qargs, all_cbits) for bit in bits],
  File "/root/anaconda3/envs/qiskit1.0.1/lib/python3.10/site-packages/qiskit/dagcircuit/dagcircuit.py", line 682, in <listcomp>
    [self.output_map[bit]._node_id for bits in (qargs, all_cbits) for bit in bits],
KeyError: Clbit(ClassicalRegister(2, 'cr'), 0)

Any suggestions?

No response

[1ucian0]

I confirmed this is indeed a bug. The problem is the conditional in combination with the repeat, indeed.

[jakelishman]

I've put up #11940 which is one way of fixing this, but it's not 100% clear to me that the route I went with #11940 is fully acceptable.