Fix wrong argument supplied to _identity_op()

[mstnb]

Summary

This fixes a typo in the CommutationChecker where 2**num_qubits was passed to _identity_op instead of num_qubits.

Details and comments

Fixes #9197.

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[jakelishman]

This looks correct, thanks for the quick response! Please could you also add a test in the file test/python/circuit/test_commutation_checker.py that we get a correct result - something like

from qiskit.circuit import CommutationChecker, Qubit
from qiskit.circuit.library import XGate

qargs = [Qubit() for _ in [None]*8]
CommutationChecker().commute(XGate(), qargs[:1], [], XGate().control(7), qargs, [])

should be False, for example, and will completely blow up Numpy without your fix. You could write a test about that.

We also need to add a short release note - if you've done pip install reno already, you should be able to run reno new --edit fix-commutation-checker-size and remove everything but the fixes section (and write a sentence there briefly explaining what was fixed).

[coveralls]

Pull Request Test Coverage Report for Build 4307516998

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.343%

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Totals
Change from base Build 4307516345:	0.0%
Covered Lines:	67530
Relevant Lines:	79128

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💛 - Coveralls

[mstnb]

Okay, is there any naming convention for these test cases? And I guess all the methods in the QiskitTestCase are called automatically so I can simply add a new method?

[jakelishman]

Sorry for the delay in responding here. As long as the method name starts with test_ (and doesn't clash with another), yeah, it gets called automatically - QiskitTestCase is a subclass of Python's built-in unittest.TestCase, so all the assertion methods on those classes should work as well.

[mstnb]

Hi, I didn't get to work on this any sooner. I now added a simple unit test and release note. I wanted to merge the upstream changes to my repo to be able to push my changes. Apparently I pressed the wrong button now, sorry for that.
I messed something up on my local repo, trying to get that fixed.
I think I merged upstream to the wrong branch, now I have a detached head on my main branch. Haha can you help me out there so I don't break it even more?

[jakelishman]

A "detached HEAD" state in git means you've just got a commit checked out that isn't the end point of any branch. If you've not got any changes in your working directory, you can just git checkout <branch> where <branch> is wherever you want to get back to.

If you've got the changes you want to push in your local copy right now, but git is saying weird things, then you might want to try copying the files with changes in to somewhere safe, then running

git fetch origin
git checkout bug-9197-fix
git merge origin/bug-9197-fix

Ideally this will just update your local branch with the changes that have happened to the remote on GitHub, and then you'll just be able to run git push to update GitHub with the changes you've made locally. The point of copying the files is just so you don't lose your changes if something goes wrong.

Don't worry - there's nothing you can do that can break our side of things, and the worst that can possibly happen is that your branch gets too messed up, and then we can close this and you can just make a new pull request.

[mtreinish]

To unblock this since the same bug being fixed by this was reported again in #9693 I pushed up the test proposed by @jakelishman and also added a release note.