`compose` fails when `front=True` is provided and when the circuit to be composed has a different number of qubits

[tnemoz]

Environment

• Qiskit version: 2.4.0.dev0
• Python version: 3.14.3
• Operating system: Arch Linux

What is happening?

When using the compose method of a QuantumCircuit with front=True on another, smaller, QuantumCircuit, compose fails with

ValueError: 'qubits' argument is the wrong length

How can we reproduce the issue?

The following code allows to reproduce the issue.

from qiskit import QuantumCircuit

qc_base = QuantumCircuit(2)
qc_base.h(0)
qc_x = QuantumCircuit(1)
qc_x.x(0)

qc_base.compose(qc_x, qubits=[0], front=True).draw()

What should happen?

The compose method should not raise an error.

Any suggestions?

The previous code runs when using Qiskit 2.3.0. It would seem that this bug was introduced in #15696, where compose was changed.

[jakelishman]

Thanks for the report! We're unlikely to have a fix posted before 2.4.0rc1 tomorrow, but we'll try and get this fixed by 2.4.0 final in two weeks' time.