Fix internal transpiler error in CommutativeCancellation with classical conditions #8556
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Thanks for the super speedy fix! I shrunk the regression test down to a minimal reproducer, and corrected the comments in the commute method to reflect the changes, but since this is just restoring the previous behaviour of CommutationAnalysis with regard to conditions, I think we're good to go.
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Summary
This is a fix for #8553, due to a bug I have introduced in #8184. Previously, when checking whether two gates commute, the commutation checking returned False when at least one of the gates was conditional, and it should stay this way for now. This is definitely sub-optimal, and in the future we should think of ways to improve this.
The code in #8184 was handling this incorrectly though, since it returned True for gates with different quantum and classical bits, without looking at conditions, the fix immediately returns False as soon as one of the gates is conditional.
However, if we do generalize commutation checking to pairs of commuting conditional gates, we would need to also update the
CommutativeCancellationpass to avoid throwing errors for conditional gates.Details and comments