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Commit Message

inference: Cache tombstoned const-prop results to prevent non-termination (#61264)

When a const-prop frame encounters a cycle and gets poisoned,
`finishinfer!` marks the result as tombstoned and sets `cache_mode =
CACHE_MODE_NULL`, which prevents `promotecache!` from pushing the result
to the local inference cache.
Meanwhile, `constprop_cache_lookup` (added in #57545) skips tombstoned
entries entirely. This combination means the same const-prop is
re-attempted on every cycle iteration, causing inference to never
terminate when `aggressive_constant_propagation = true`.

Fix this by:
- In `const_prop_call`, explicitly pushing tombstoned results to the
inference cache after a successful but limited `typeinf`, and returning
`nothing` to fall back to the regular inference result.
- In `constprop_cache_lookup`, no longer skipping tombstoned entries so
they can be found on subsequent lookups, preventing re-attempts.
- In `const_prop_call` cache-hit path, checking `inf_result.tombstone`
and returning `nothing` (same as the existing cycle-hit handling).
- Removing the now-unnecessary tombstone check from
`OverlayCodeCache.get`, where the subsequent `overridden_by_const` and
`isdefined` checks already filter out such entries.

Note that #57545 added `tombstone && continue` to `cache_lookup` to
prevent `LimitedAccuracy` results from propagating to callers via
`const_prop_result`. This change removes that skip but achieves the same
protection by explicitly checking `inf_result.tombstone` in
`const_prop_call` and returning `nothing` instead of using the result.

Fixes #61257

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Run Details

12 of 14 new or added lines in 3 files covered. (85.71%)

89 existing lines in 8 files now uncovered.

63673 of 82827 relevant lines covered (76.87%)

23446802.16 hits per line