`xslow` (via `XSlowGate`) can't be exported to QASM3 and is lost when exported as QPY

[francabrera]

Describe the bug

• Using the qasm3 exporter fails if the circuit contains a XSlowGate.
• QPY doesn't preserve the XSlowGate subclass. A circuit saved with XSlowGate comes back with a plain Gate named "xslow" after qpy.load.

Steps to reproduce

from qiskit import QuantumCircuit, qasm3
from qiskit_ibm_runtime.circuit.library import XSlowGate 

qc = QuantumCircuit(1, 1)
qc.append(XSlowGate(), [0])
qc.measure(0, 0)

qasm3.dumps(qc)

Fails with:

qiskit.qasm3.exceptions.QASM3ExporterError: "failed to export gate 'xslow' that has no definition"

Also this shows the XSlowGate class is lost at QPY serialization:

import io
from qiskit import QuantumCircuit, qasm3, qpy
from qiskit_ibm_runtime.circuit.library import XSlowGate

qc = QuantumCircuit(1, 1)
qc.append(XSlowGate(), [0])
qc.measure(0, 0)

buf = io.BytesIO()
qpy.dump(qc, buf)
buf.seek(0)
qc2 = qpy.load(buf)[0]

op = qc2.data[0].operation
print(type(op).__name__, op.definition)  # -> Gate None

Expected behavior

QASM3 export works.

Suggested solutions

• Add a placeholder body in XSlowGate.__init__:

class XSlowGate(Gate):
    def __init__(self) -> None:
        super().__init__("xslow", 1, [])
        self.definition = QuantumCircuit(1)   # empty placeholder body

• I believe QPY has no registration mechanism for third-party Gate subclasses; so maybe just documentation/caveat note in XSlowGate's docstring

Additional Information

• qiskit-ibm-runtime version:
• Python version:
• Operating system: