Fix SabreSwap with classically conditioned gates (backport #8041) #8058
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* Fix SabreSwap with classically conditioned gates The calculation of the expected number of predecessors for a gate to be considered "resolved" in `SabreSwap` was not accounting for wires stemming from classical conditions. The tracking of the _actual_ number of predecessor requirements satisfied did correctly account for this, so in certain circumstances the actual count could jump from "too low" to "too high" without passing through "just right", and the gate would never get added to the circuit. * Use `successors` to calculate required predecessors This unifies the calculation of what is considered a required predecessor by using the same `SabreSwap._successors` function for both aspects of the comparison: counting the required predecessors and counting the actual number of applied predecessors. This simplifies the logic, since now an update in one place is sufficient, and the wires are read directly from the DAG, rather than using assumptions about the nodes. * Fix incorrect import * Fix decremented typo Co-authored-by: Matthew Treinish <mtreinish@kortar.org> Co-authored-by: Matthew Treinish <mtreinish@kortar.org> Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com> (cherry picked from commit 4410a9b)
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jakelishman
approved these changes
May 12, 2022
jakelishman
added
Changelog: Bugfix
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This is an automatic backport of pull request #8041 done by Mergify.
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