Stop OptimizeSwapBeforeMeasure recursing into control-flow blocks - #17012

Summary

Fixes #16853

OptimizeSwapBeforeMeasure removes a terminal SWAP inside a control-flow block even though the swapped state is observed by measurements after the block:

qc = QuantumCircuit(2, 2)
qc.x(0)
with qc.for_loop(range(1)):
    qc.swap(0, 1)
qc.measure(0, 0)
qc.measure(1, 1)

PassManager([OptimizeSwapBeforeMeasure()]).run(qc)
# loop body is now empty; the program measures '01' instead of '10'

Cause

The pass is decorated with control_flow.trivial_recurse, so each block is optimized as an independent DAGCircuit. Within a block the DAGOutNodes are only the block boundary, not the end of the program, but the eligibility check treats them as terminal and removes the swap. The same problem exists when the swap is followed only by measurements inside the block: retargeting the in-block measurements keeps the classical results, but the qubits remain live after the block in the swapped positions, so any later measurement of them outside the block is wrong.

Fix

Remove the recursion. The pass now only acts on the top level of the circuit and leaves control-flow blocks untouched, which is the conservative behaviour discussed in #7642 (a terminal swap of a complete program has no observable effect; a terminal swap of a block does). The pass is not part of the preset pass managers, so this only affects explicit use.

The two existing tests that asserted the recursion (test_if_else, test_nested_control_flow) are replaced by tests asserting that a block is left unchanged, plus the regression case from the issue, the "swap followed by in-block measurements" case, and a check that a top-level swap after a control-flow block is still optimized.

Details and comments

Verified with a BasicSimulator run on main vs this branch: the issue's reproducer returns {'10': 1024} before and after the pass with this change (was {'01': 1024} after).

AI/LLM disclosure

• No part of this submission is LLM generated.
• Some written text was generated by: Claude (Claude Code), edited and reviewed by me.
• Some submitted code was generated by: Claude (Claude Code), reviewed and tested by me.

🤖 Generated with Claude Code