`insertion_sort_transformer` causes "Measurement key missing" with classical control

[YaegakiY]

Describe the issue

Using insertion_sort_transformer on a circuit that contains a CircuitOperation controlled by a measurement key results in a ValueError. The error disappears when using defer_measurements, or if the transformer is not applied.

Explain how to reproduce the bug or problem

To reproduce:

import cirq
from cirq.transformers import *

q = cirq.LineQubit.range(3)
circuit = cirq.Circuit()


sub_circuit = cirq.Circuit() 

sub_circuit.append(cirq.X(q[0]).controlled_by(q[1]))

sub_op = cirq.CircuitOperation(sub_circuit.freeze()) 
circuit.append(cirq.measure(q[2], key="c"))
circuit.append(sub_op.with_classical_controls("c")) 

circuit.append(cirq.measure(q, key="m"))

# circuit = defer_measurements(circuit)

circuit = insertion_sort_transformer(circuit)
simulator = cirq.Simulator() 
result = simulator.run(circuit, repetitions=500) 
print(result.histogram(key='m'))

Traceback (most recent call last): File "C:\Users\temp_test.py", line 22, in result = simulator.run(circuit, repetitions=500) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\work\sampler.py", line 68, in run return self.run_sweep(program, param_resolver, repetitions)[0] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\sim\simulator.py", line 59, in run_sweep return list(self.run_sweep_iter(program, params, repetitions)) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\sim\simulator.py", line 90, in run_sweep_iter records = self._run( ^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\sim\simulator_base.py", line 249, in _run for step_result in self._core_iterator( ^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\sim\simulator_base.py", line 209, in _core_iterator protocols.act_on(op, sim_state) File "C:\Users\venv\Lib\site-packages\cirq\protocols\act_on_protocol.py", line 143, in act_on result = action_act_on(sim_state) if is_op else action_act_on(sim_state, qubits) ^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\ops\classically_controlled_operation.py", line 205, in act_on if all(c.resolve(sim_state.classical_data) for c in self._conditions): ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\ops\classically_controlled_operation.py", line 205, in if all(c.resolve(sim_state.classical_data) for c in self._conditions): ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ File "C:\Users\venv\Lib\site-packages\cirq\value\condition.py", line 110, in resolve raise ValueError(f'Measurement key {self.key} missing when testing classical control') ValueError: Measurement key c missing when testing classical control

Tell us the version of Cirq where this happens

1.6.0.dev20250702012506

[daxfohl]

Confirmed. It looks like the insertion sort transformer skips any measurement key commutativity checks if the qubits are disjoint. https://github.com/quantumlib/cirq/blob/9c376c4e72ec924ee909a6c31147d703e69f1078/cirq-core/cirq/transformers/insertion_sort.py#L57. If I understand the intent correctly it looks like all those special cases are just for performance, not required for logic? If so, I think the best option would be to simply remove them. commutes itself also has similar optimizations in place, but includes measurement key checks in its implementation, so it should be able to guarantee correctness but without being much of a perf hit.

cc @NoureldinYosri

[daxfohl]

Here's also a simpler repro that only requires two gates and no subcircuits.

    q0, q1 = cirq.LineQubit.range(2)
    circuit = cirq.Circuit(
        cirq.measure(q1, key="c"),
        cirq.X(q0).with_classical_controls("c")
    )

    print(repr(circuit))
    circuit = insertion_sort_transformer(circuit)
    print(repr(circuit))
    cirq.Simulator().run(circuit)