qpy roundtrip for FourierChecking fails with TypeError: DiagonalGate.__init__() takes 2 positional arguments but 5 were given

[rht]

Environment

• Qiskit Terra version: 0.24.1
• Python version: 3.10
• Operating system: NixOS 23.11

What is happening?

This is likely because the named gate (DiagonalGate) is not explicitly supported in the gates in the following code branch: https://github.com/Qiskit/qiskit-terra/blob/e9f8b7c50968501e019d0cb426676ac606eb5a10/qiskit/qpy/binary_io/circuits.py#L282-L289

How can we reproduce the issue?

import io
from qiskit import qpy
import qiskit.circuit.library as qcl

def encode(qc):
    with io.BytesIO() as f:
        qpy.dump(qc, f)
        # Rewind back to the beginning of the "file".
        f.seek(0)
        serialized = f.read()
    return serialized

def decode(data):
    with io.BytesIO(data) as f:
        circuits = qpy.load(f)
    return circuits[0]

# Works
decode(encode(qcl.QFT(3)))
# Doesn't work
decode(encode(qcl.FourierChecking([1, -1, -1, -1], [1, 1, -1, -1])))

What should happen?

The error message should have said that the DiagonalGate is not supported when it is not covered by the if-else checks in the _read_instruction.

Any suggestions?

No response

[rht]

The traceback:

Traceback (most recent call last):
  File "/qiskit_bug_report.py", line 21, in <module>
    decode(encode(qcl.FourierChecking([1, -1, -1, -1], [1, 1, -1, -1])))
  File "/qiskit_bug_report.py", line 15, in decode
    circuits = qpy.load(f)
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/interface.py", line 269, in load
    loader(
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/binary_io/circuits.py", line 932, in read_circuit
    custom_operations = _read_custom_operations(file_obj, version, vectors)
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/binary_io/circuits.py", line 423, in _read_custom_operations
    definition_circuit = common.data_from_binary(
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/common.py", line 261, in data_from_binary
    obj = deserializer(container, **kwargs)
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/binary_io/circuits.py", line 934, in read_circuit
    _read_instruction(file_obj, circ, out_registers, custom_operations, version, vectors)
  File "/home/rht/code/venv/lib/python3.10/site-packages/qiskit/qpy/binary_io/circuits.py", line 289, in _read_instruction
    gate = gate_class(*params)
TypeError: DiagonalGate.__init__() takes 2 positional arguments but 5 were given

[jakelishman]

Thanks for the report. The fix should be a case of adding the string "diagonal" to the check of line 282 in your code link - DiagonalGate is an old gate that has its constructor and params fields behave in slightly different ways to most Qiskit gates.

[Raghav-Bell]

Let me resolve it by adding diagonal