ValueError: Cannot output operation as QASM: cirq.global_phase_operation

[rht]

Description of the issue

Circuits containing cirq.global_phase_operation can't be converted to QASM

How to reproduce the issue

import cirq
import numpy as np

c = cirq.Circuit([cirq.global_phase_operation(np.exp(1j * 5))])
cirq.qasm(c)

Traceback (most recent call last):
  File "/workspace/delete_global_phase_problem.py", line 5, in <module>
    cirq.qasm(c)
  File "/workspace/venv/lib/python3.11/site-packages/cirq/protocols/qasm.py", line 165, in qasm
    result = method(**kwargs)
             ^^^^^^^^^^^^^^^^
  File "/workspace/venv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 1315, in _qasm_
    return self.to_qasm()
           ^^^^^^^^^^^^^^
  File "/workspace/venv/lib/python3.11/site-packages/cirq/circuits/circuit.py", line 1359, in to_qasm
    return str(self._to_qasm_output(header, precision, qubit_order))
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/venv/lib/python3.11/site-packages/cirq/circuits/qasm_output.py", line 245, in __str__
    self._write_qasm(lambda s: output.append(s))
  File "/workspace/venv/lib/python3.11/site-packages/cirq/circuits/qasm_output.py", line 299, in _write_qasm
    self._write_operations(self.operations, output, output_line_gap)
  File "/workspace/venv/lib/python3.11/site-packages/cirq/circuits/qasm_output.py", line 326, in _write_operations
    decomposed = protocols.decompose(
                 ^^^^^^^^^^^^^^^^^^^^
  File "/workspace/venv/lib/python3.11/site-packages/cirq/protocols/decompose_protocol.py", line 315, in decompose
    return [*_decompose_dfs(val, args)]
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/venv/lib/python3.11/site-packages/cirq/protocols/decompose_protocol.py", line 223, in _decompose_dfs
    raise error
ValueError: Cannot output operation as QASM: cirq.global_phase_operation((0.28366218546322625-0.9589242746631385j))

Cirq version
1.3.0

[NoureldinYosri]

cirq-cynk: global phase is a non qubit operation. AI: check if qasm has a way to represent non qubit operations. if yes add support for it.

short term solution: drop the operation and print a warning.

[tanujkhattar]

Qiskit/qiskit#7167 (comment) suggests that OpenQASM 2 doesn't support global phase operations. In the short term, we should just add a warning and ignore global phase operations when converting to OpenQASM.

We should also add support for OpenQASM 3 in Cirq. This has been requested for a while now - #4509

[kenya-sk]

@NoureldinYosri @tanujkhattar
I'm interested in this issue. Can I try to implement a warning process?

[NoureldinYosri]

hi @kenya-sk , thanks for offering to help with this. I assigned the task to you