Control flow instructions don't work with `circuit_to_instruction`

[mtreinish]

Environment

• Qiskit Terra version: 1.0.0.dev0+ac7aca3
• Python version: 3.11
• Operating system: Linux

What is happening?

Trying to convert a circuit with a control flow operation in it to an instruction using qiskit.converters.circuit_to_instruction fails while processing the control flow condition. It is treating it like a c_if legacy style condition even for control flow ops and this causes it to fail with a NotImplementedError because the function is trying to call c_if.

How can we reproduce the issue?

>>> qc = QuantumCircuit(2)
>>> qc.h(0)
<qiskit.circuit.instructionset.InstructionSet object at 0x7f0813ebc280>
>>> qc.cx(0, 1)
<qiskit.circuit.instructionset.InstructionSet object at 0x7f0813ebef80>
>>> qc.measure_all()
>>> with qc.if_test((qc.clbits[0], 0)):
...     qc.x(0)
>>> circuit_to_instruction(qc)

What should happen?

We either should have a descriptive error saying we can't nest control flow operations in an instruction or update the .condition handling in circuit_to_instruction to understand a control flow operation.

Any suggestions?

No response

[jakelishman]

This needs to be an error in practice; there's no way to communicate the creg / variable requirements of an Instruction's inner definition up to the main circuit, so allowing arbitrary control-flow within a semi-opaque instruction wouldn't work.

Totally agree that the error message should be way better.

[jakelishman]

Really, circuit_to_instruction shouldn't even allow c_if in it - in practice, that hasn't ever worked fully correctly when registers are involved.

[maheshwaripranav]

@jakelishman This used to work well until version 0.45.0 Could you suggest what alternative should be used for this as I am writing programs to implement error correction and if_test functionality is much used in my program?

[jakelishman]

There should be no need to use circuit_to_instruction. If you want to build up circuit parts and then combine them into a larger circuit, you can use QuantumCircuit.compose to "inline" the smaller circuit into the bigger one. QuantumCircuit.append cannot really cope with complex classical data like control-flow constructs, because the necessary classical data available properly in the QuantumCircuit resource tracking through CircuitInstruction.

[jakelishman]

There's no particular guarantee that any classical handling was working correctly before if circuit_to_instruction was used (also via QuantumCircuit.append) - the circuit would silently not notice if the classical control flow was represented incorrectly.

[jakelishman]

We didn't get this done for 1.0, so we'll have to revisit for 1.1 and see what's reasonable for us to do within the stability policy.

[melechlapson]

@jakelishman @mtreinish I can work on this. I just need clarification if the fix would be to remove the c_if option altogether or add a check to make sure that it is not a control_flow operation and throw an Exception if it is