InverseCancellation does not run in classical blocks like CXCancellation

[1ucian0]

Environment

• Qiskit version: 2.0.0.dev0+5de0232

What is happening?

In #11212 we decided to "stop using CXCancellation as part of the preset pass managers as it's been superseded by InverseCancellation". However, CXCancellation runs inside flow controlled blocks but InverseCancellation does not.

How can we reproduce the issue?

1. Create a circuit with control flow:

from qiskit import QuantumCircuit
inner = QuantumCircuit(2,1)
inner.cx(0, 1)
inner.cx(0, 1)

qc = QuantumCircuit(2, 1)
qc.cx(0, 1)
qc.cx(0, 1)
qc.if_else((0, True), inner.copy(), inner.copy(), range(2), [0])
qc.draw()

2. Run InverseCancellation on it

from qiskit.transpiler.passes import InverseCancellation
from qiskit.circuit.library.standard_gates import CXGate
InverseCancellation([CXGate()])(qc).draw()

Notice that the CX outside got cancelled, but not those inside.

What should happen?

It should be like CXCancellation

from qiskit.transpiler.passes import CXCancellation
CXCancellation()(qc).draw()

Any suggestions?

Apply InverseCancellation recursively inside the code blocks.

[1ucian0]

The fix is rather straightforward. It seems like just adding the decorator @control_flow.trivial_recurse should work. Tagging as good first issue.

[haimeng-zhang]

I am in. Please assign me.