Fix Instruction.repeat with conditionals (backport #11940)
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* Fix `Instruction.repeat` with conditionals We can't put register conditionals within an `Instruction.definition` field; the data model of `QuantumCircuit` doesn't permit closing over registers from within definitions. This commit moves a condition to the outer `Instruction` that's returned. * Avoid 'to_mutable' if not needed * Add proviso on repeated conditionals in documentation * Update wording in release note Co-authored-by: Luciano Bello <[email protected]> --------- Co-authored-by: Luciano Bello <[email protected]> (cherry picked from commit 6ebb7aa)
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Changelog: Bugfix
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1ucian0
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Jun 4, 2024
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Summary
We can't put register conditionals within an
Instruction.definitionfield; the data model ofQuantumCircuitdoesn't permit closing over registers from within definitions. This commit moves a condition to the outerInstructionthat's returned.Details and comments
Fix #11935
Fix #11936 (though note that #11761 would then be triggered for OQ3).
Important
As implemented, this commit isn't necessarily a completely valid transformation, but I'm putting it up in this form for a bit of discussion.
The point is that an instruction can in theory affect its own condition; consider a
Measure().c_if(cr, value), where the measure instruction within the circuit is applied to a clbit that's incr. To go further, consider the custom instruction whose definition ish q[0]; measure q[0] -> c[0];- this is clearly not idempotent under ac-based condition (one could argue that the baremeasurewas, in the context of a noiseless abstract circuit, since the first measure would collapse the state).Given that
Instruction.repeatis only applied without a circuit context, and we're moving to removeInstruction.conditionfor these same sorts of reasons (in favour ofIfElseOp), I think this is a transformation we can accept, though perhaps with additional documentation.This is an automatic backport of pull request #11940 done by Mergify.