`ForLoopOp` with reused Parameter causes "circuit parameter table is inconsistent" on `assign_parameters`

[huhu37]

Environment

• Qiskit version: 2.3.0
• Python version: 3.11.14
• Operating system: Ubuntu 24.04.3

What is happening?

When using qiskit.circuit.controlflow.ForLoopOp, calling QuantumCircuit.assign_parameters on the outer circuit raises a runtime error:
RuntimeError: internal error: circuit parameter table is inconsistent,
while the same code executes without error in Qiskit version 2.2.3.

How can we reproduce the issue?

from qiskit.circuit import QuantumCircuit, Parameter
from qiskit.circuit.controlflow import ForLoopOp

theta = Parameter('θ')
body = QuantumCircuit(1)
body.rx(theta, 0)
indexset = range(3)
loop_parameter = theta
for_loop_op = ForLoopOp(indexset, loop_parameter, body)
qc = QuantumCircuit(1)
qc.append(for_loop_op, [0])
# This line triggers the error
qc.assign_parameters({theta: 3.14159 / 2}, inplace=True)

What should happen?

The parameter assignment succeeds, correctly binding theta throughout the circuit, or a clear user-facing error is raised earlier.

Any suggestions?

No response

[jakelishman]

Huh, this seems like it should really have been an error on all Qiskit versions, and the error shouldn't get as far as the internal logic error - the loop parameter should be scoped in a way that assign_parameters can't attempt to bind it at all, because it already is bound by the loop body.

By the way, just checking you know you can do:

qc = QuantumCircuit(1)
with qc.for_loop(range(3)) as theta:
    qc.rx(theta, 0)

[huhu37]

Thank you for your reply and explanation. If using ForLoopOp is not safe in this case, I will construct the circuit following the recommended approach.

[jakelishman]

The way you're constructing the ForLoopOp is correct, it's just really fiddly because you're building the internal representation very manually. Using the with qc.for_loop stuff is much much easier and less wordy.

What's your goal with calling assign_parameters on the loop variable of a for loop? It seems to me that that shouldn't have any meaning, so I'm not sure what your use case is here.