Fix DiagonalGate QPY serialisation

[Raghav-Bell]

Added "diagonal" in line 282

Summary

Diagonal gate is causing error as discussed in issue #10364 . This PR closes the issue #10364

Details and comments

As suggested by @jakelishman , i have added "diagonal" at line 282.

Fix #10364.

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[jakelishman]

Thanks for looking at this! Please can you add a test case and a "bug fix" release note (see the contributing guide in the repo root for information on that).

[Raghav-Bell]

@jakelishman I have added release notes using reno but i am new to tox can you please help me in adding test cases.

[rht]

@Raghav-Bell you can do so by adding FourierChecking to https://github.com/Qiskit/qiskit-terra/blob/8a8609f93a600019bf208610a348adcbe61e3acd/test/qpy_compat/test_qpy.py#L577 (or a simpler circuit that has DiagonalGate). You should go through the functions generate_*_circuit. Read https://github.com/Qiskit/qiskit-terra/blob/main/CONTRIBUTING.md#test for running the tests locally.

[jakelishman]

Thanks @rht! The test/qpy_compat place is the backwards-compatibility tests for QPY; those are actually a little tricky to run locally, but it would be good to add a case to them (protected on Terra version >= (0, 25, 0), since that's when we'll release the fix). You can also add a regular unit test to test/python/qpy/test_circuit_load_from_qpy.py, which are all just regular unittest tests.

[jakelishman]

(btw, the lint is failing with a style error - if you pip install tox, you can tox -e black to fix it up automatically.)

[Raghav-Bell]

I have tested for diagonal gate test using tox -ep310 -- generate* and tox -ep310 -- diag* both ends succesfully. Complete tests take alot time ,hence i hv dropped .

[jakelishman]

Thanks for this! I fixed up the release note a little and moved the test into the regular unit tests (there's a complication with DiagonalGate itself that will make it hard to use our QPY backwards-compatibility tests for now).

[mtreinish]

LGTM, this seems straightforward enough

[coveralls]

Pull Request Test Coverage Report for Build 5510413486

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 17 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.005%) to 85.981%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	80.0%
crates/qasm2/src/lex.rs	3	91.65%
crates/qasm2/src/parse.rs	12	96.65%

Totals
Change from base Build 5490783039:	-0.005%
Covered Lines:	71634
Relevant Lines:	83314

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💛 - Coveralls