QuantumCircuit.compose does not raise an error with invalid qubits argument

[georgios-ts]

Information

• Qiskit Terra version: master @ b1c9a23
• Python version: 3.7.1
• Operating system: osx

What is the current behavior?

QuantumCircuit.compose runs normally if qubits argument has duplicate values. If you try to compose two circuits after converting to dag, DAGCircuit.compose raises an error. Introduced after #4762 .

Steps to reproduce the problem

n = 3
qc = QuantumCircuit(n)
qc.h(range(n))

m = 2
circ = QuantumCircuit(m)
circ.z(range(m))
circ.cx(0, 1)

new_circuit = qc.compose(circ, qubits=[1, 1])
print(new_circuit)


     ┌───┐          
q_0: ┤ H ├──────────
     ├───┤┌───┐┌───┐
q_1: ┤ H ├┤ Z ├┤ Z ├
     ├───┤└───┘└───┘
q_2: ┤ H ├──────────
     └───┘   

But

dag_qc   = circuit_to_dag(qc)
dag_circ = circuit_to_dag(circ)

dag = dag_qc.compose(dag_circ, 
                     qubits=[1, 1], inplace=False)


raises DAGCircuitError

What is the expected behavior?

As duplicate values in qubits is not a valid input, QuantumCircuit.compose should raise an error.

Suggested solutions

Similar to DAGCircuit.compose

        # Check the edge_map for duplicate values
        if len(set(edge_map.values())) != len(edge_map):
              raise CircuitError("duplicates in wire_map")

[Cryoris]

I'm a little hesitant to adding checks in the compose method because it might slow down the usage and this use-case is obviously a wrong input, so that users won't be surprised if it doesn't work. But if the DAGCircuitError is too cryptic, we could improve it there? This would not give us additional overhead when users use compose correctly but would still give a proper error message.

Edit: Maybe it would be better to actually do include the check in compose. If we notice it causes performance issues we can also add a flag that deactivates the checks or use a private method like _append?

[mrvee-qC]

Update for Santa:elf:- QGoGP

✅ Error reproducible! Has the same 'duplicates in wire map' DAGCircuitError raised as mentioned by OP

from qiskit.converters import circuit_to_dag
from qiskit import QuantumCircuit

n = 3
qc = QuantumCircuit(n)
qc.h(range(n))

m = 2
circ = QuantumCircuit(m)
circ.z(range(m))
circ.cx(0, 1)

new_circuit = qc.compose(circ, qubits=[1, 1])
print(new_circuit)
dag_qc   = circuit_to_dag(qc)
dag_circ = circuit_to_dag(circ)

dag = dag_qc.compose(dag_circ, 
                     qubits=[1, 1], inplace=False)

Screenshot

Will update comment/add more comments if cause is found/fixed!

Python version 3.9.7
qiskit-terra version: 0.19.1