assign_parameters method is not working for controlled-U gate

[SimoneGasperini]

Environment

• Qiskit Terra version: 0.23.1
• Python version: 3.10.8

What is happening?

The QuantumCircuit.assign_parameters() method does not work properly with the parametric controlled-U gate. The values passed to be assigned to the parameters seem just to be ignored (in both cases with inplace=False or inplace=True).

How can we reproduce the issue?

from qiskit.circuit import QuantumCircuit, Parameter

chi = Parameter('$\\chi$')
theta = Parameter('$\\theta$')
phi = Parameter('$\\phi$')
lam = Parameter('$\\lambda$')
gamma = Parameter('$\\gamma$')

qc = QuantumCircuit(2)
qc.rx(chi, 1)
qc.cu(theta, phi, lam, gamma, 0, 1)

values = [4.68, 5.13, 1.06, 3.81, 3.83]
qc.assign_parameters(values, inplace=True)

qc.draw('mpl')

What should happen?

Each parameter of the controlled-U gate should be correctly bounded to the corresponding value passed to assign_parameters(), as it happens for any other parametric gate (Rx in the example above).

[jakelishman]

Thanks for the report - this is a known issue (#7326), so I'll just close this as duplicate. There's a potential fix available (#9118) that I need to review properly.

The root cause is mostly that CUGate is a badly behaved subclass of ControlledGate, and really, it's not even really the controlled version of UGate (because it's got an extra parameter).

[jakelishman]

Duplicate of #7326

[SimoneGasperini]

Yes, there is the extra gamma parameter serving as global phase.
Thank you and sorry for the duplicate, I didn't see that there was already an open issue on this.

[jakelishman]

No worries - it's really our fault that the issue's been open for so long.