Trying to control a custom gate calls unroll and raises an error

[ianarawjo]

Information

• Qiskit Terra version: 0.18.3
• Python version: 3.8.8
• Operating system: Mac OS Big Sur 11.3.1

What is the current behavior?

Using .control(2) method on a custom Gate() produces an error.

Error output:

File "/opt/anaconda3/lib/python3.8/site-packages/qiskit/circuit/gate.py", line 119, in control
return add_control(self, num_ctrl_qubits, label, ctrl_state)
File "/opt/anaconda3/lib/python3.8/site-packages/qiskit/circuit/add_control.py", line 59, in add_control
cgate = control(operation, num_ctrl_qubits=num_ctrl_qubits, label=label, ctrl_state=ctrl_state)
File "/opt/anaconda3/lib/python3.8/site-packages/qiskit/circuit/add_control.py", line 113, in control
unrolled_gate = _unroll_gate(operation, basis_gates=basis)
File "/opt/anaconda3/lib/python3.8/site-packages/qiskit/circuit/add_control.py", line 268, in _unroll_gate
opqc = dag_to_circuit(unroller.run(dag))
File "/opt/anaconda3/lib/python3.8/site-packages/qiskit/transpiler/passes/basis/unroller.py", line 76, in run
raise QiskitError(
qiskit.exceptions.QiskitError: "Error decomposing node of instruction 'A': 'NoneType' object has no attribute 'global_phase'. Unable to define instruction 'A' in the given basis."

Steps to reproduce the problem

Code:

gateA = Gate(name='A', num_qubits=1, params=[])
ccA = gateA.control(2)

What is the expected behavior?

I expected that qiskit would allow the to-be-defined gate A to be added to the circuit, since I do not explicitly call "unroll." If I define the gate using 'ControlledGate' directly, where base_gate=gateA as init param, there's no error and I can add it to the circuit.

Suggested solutions

Waiting for developers to reply --possible this is a technical limitation of the way qiskit is implemented.

[ecpeterson]

There's an effort currently underway (see, e.g., #7087) which includes an aim to decouple gates-as-circuit elements from gates-as-definitions. This bug belongs to that same class of issues.