enable controlled opaque gates.

[ewinston]

Summary

Previously attempting to create a controlled gate from an opaque gate would raise an error as described in #3565. This pr returns an opaque controlled gate for opaque gates.

fixes #3565
fixes #4010

Details and comments

This pr allows the identity gate to be controlled to handle controlled composite gates which contain the gate. The control operation applies identity gates to the controls as well.

[Cryoris]

I think this doesn't correctly fix #3565, or at least the example I tried:

from qiskit import transpile, QuantumCircuit
from qiskit.circuit import Gate

basis_gates = ['u1', 'u3', 'cx']
gate = Gate('my_gate', 1, [])
gate._definition = []

circuit = QuantumCircuit(2)
circuit.append(gate.control(), [0, 1], [])
transpile(circuit, basis_gates=basis_gates)

still resolves in

QiskitError: "Cannot unroll the circuit to the given basis, ['u1', 'u3', 'cx']. No rule to expand instruction cmy_gate.

@ewinston Are you actively working on this? In a few test cases for arithmetic circuits for the circuit library I had the case that I'm controlling empty gates. I could work around there by checking first if the definition is empty, but if this is on the way to be merged I can avoid that 🙂

[ewinston]

@Cryoris This seems to be due to the transpiler expecting opaque gates to have an empty list for definition instead of None. Controlled opaques where changed to match.

[ajavadia]

@ewinston can you update this branch?
also i noticed UnitaryGates don't return an instance of ControlledGate. they should I think

[ajavadia]

i'm good with this but will let @Cryoris or @1ucian0 do a final check. Thanks.

[CLAassistant]

All committers have signed the CLA.

[mtreinish]

Is this still active? I'm thinking this has been superseded by the AnnotedOperation work: #9846 which enables controlling arbitrary operations and leaves the synthesis to the transpiler.

[jakelishman]

Given the above comment, and the AnnotatedGate work which should supersede this, I'll mark this as closed now.