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Summary
Fixes #13079.
Detailed Comments
The problem was due to first simplifying a controlled-gate by removing its controls (in the example: the control qubit of a CX-gate is at
|1>, allowing to replace the CX-gate by an X-gate), then incorrectly keeping track of the simplified gate (in order to store the new X-gate for later purposes, the code mutated the original CX-gate in some weird way).The code was already using
substitute_node_with_dagto replace the controlled-gate by the base-gate (I don't really like this, but I don't think we can usesubstitute_nodeas the simplified node has fewer qubits than the original node). To fix the problem, we can instead read off the simplified gate fromsubstitute_node_with_dag's output.While this fixes the immediate problem, the code in
HoareOptimizeris very messy and quite possibly contains many other sources of bugs (for instance, I did not like the idea of iterating over the DAG's topological nodes, while removing and simplifying some of the nodes in-place) and not optimized for performance. A complete rewrite while porting to Rust would be ideal :).This is an automatic backport of pull request #13083 done by Mergify.