Deepcopy of control flow with standalone clbit results in invalid circuit

[mtreinish]

Environment

• Qiskit Terra version: 0.24.1 (but also on main)
• Python version: 3.11
• Operating system: Linux

What is happening?

When running a deepcopy of a circuit object using a clbit as a standalone bit as the condition for a control flow operation results in the deepcopied circuit being invalid so that qasm3 or qpy serialization, and visualization will fail because the internal state of the circuit is corrupted and not valid in the copy.

How can we reproduce the issue?

import copy

from qiskit.circuit import QuantumCircuit, Qubit, Clbit, ClassicalRegister, QuantumRegister, Gate, Parameter

bits = [Qubit(), Qubit(), Clbit()]
qc = QuantumCircuit(bits)
with qc.if_test((qc.clbits[0], 1)):
    qc.x(0)

copy_qc = copy.deepcopy(qc)
print(copy_qc)

What should happen?

It should print the visualization of the circuit. This will stack trace though during the circuit to dag conversion. You can also replace print(copy_qc) with something like:

import io
from qiskit.qpy import dump

with io.BytesIO() as fd:
    dump(copy_qc, fd)

this will also fail on a lookup of the clbit.

Any suggestions?

I think the special __deepcopy__ handling in the qiskit.circuit.Instruction class is not valid for the ControlFlowOp when the .condition attribute is a clbit. It might just be simplest to revert to the default deepcopy behavior for ControlFlowOp.

[jakelishman]

It's the Bit instances that are the problem here, really - a deepcopy of an immutable object should return itself. If we make Bit.__deepcopy__ and __copy__ return self, it should all automatically fix itself.

The handling would fail for a deep-copied regular instruction with a single clbit condition as well, there's just not really any point in our tests where we'd try and and see the failure.

[mtreinish]

Yeah that's fair, adding a Bit.__deepcopy__ to return self would fix this. That's easy enough to add.

[eendebakpt]

Perhaps #10385 is related. There also a deepcopy of an internal structure results in an exception.