Fix Instruction.repeat with conditionals

[jakelishman]

Summary

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.

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 in cr. To go further, consider the custom instruction whose definition is h q[0]; measure q[0] -> c[0]; - this is clearly not idempotent under a c-based condition (one could argue that the bare measure was, in the context of a noiseless abstract circuit, since the first measure would collapse the state).

Given that Instruction.repeat is only applied without a circuit context, and we're moving to remove Instruction.condition for these same sorts of reasons (in favour of IfElseOp), I think this is a transformation we can accept, though perhaps with additional documentation.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 8171222509

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 12 of 12 (100.0%) changed or added relevant lines in 1 file are covered.
• 44 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.04%) to 89.31%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/circuit/instruction.py	2	95.91%
crates/qasm2/src/lex.rs	4	92.19%
qiskit/qpy/binary_io/circuits.py	38	91.55%

Totals
Change from base Build 8117135103:	0.04%
Covered Lines:	59123
Relevant Lines:	66200

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💛 - Coveralls

[1ucian0]

The fix looks good to me. I just have a small non-blocking comment on the release note.

[jakelishman]

I've also updated the documentation of Instruction.repeat to include the warning against the potential non-idempotency of conditional instructions in a357a4d.

[1ucian0]

This seems (based on the presence of affected code) to affect stable/0.46. Shall we port it?

[1ucian0]

@Mergifyio backport stable/0.46

[mergify]

backport stable/0.46

✅ Backports have been created

• #12499 Fix Instruction.repeat with conditionals (backport #11940) has been created for branch stable/0.46