Fix for-in missing inherited enumerable index properties placed on Array.prototype#2655
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lahma merged 1 commit intoJul 13, 2026
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July 13, 2026 07:47
…ray.prototype Array.prototype is itself array-backed, so script-placed elements (Array.prototype[5] = x) live in its dense element store. The for-in key-collection shortcut for builtin-shaped hosts handed out the shared builtin name list, which only covers shape slots - the own index keys were invisible, so `for (var k in [1, 2])` never yielded the inherited enumerable index. The shortcut now steps aside when an array-backed host carries any own index-keyed element (a present dense element or a materialized per-index descriptor); the fallback path collects indices and builtin names correctly. The extra cost for pristine prototypes is one type test plus a scan of an empty backing store. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Bug
Array.prototypeis itself array-backed, so script-placed elements live in its dense element store. The for-in key-collection shortcut for builtin-shaped hosts (GetForInStringKeys) handed out the shared builtin name list, which only covers shape slots — own index keys on the prototype were invisible to enumeration. Verified against pristinemain.Fix
The shortcut steps aside when an array-backed host carries any own index-keyed element (new
ArrayInstance.HasAnyOwnIndex(): a present dense element or a materialized per-index descriptor); the fallback path collects indices and builtin names correctly. For pristine prototypes — the overwhelmingly common case — the added cost is one type test plus a scan of an empty backing store, and the shared-name-list shortcut still applies.Found by a new enumeration test written while working on the for-in array fast path (follow-up PR); regression test included.
Gate
Jint.Tests, PublicInterface, CommonScripts all green; Test262 full run green except the four known load-dependent
annexB RegExp *-escape-BMPslow tests, which pass in 5s when run in isolation (they time out at 30s only under full-suite CPU starvation — flake hardening for those is coming separately).🤖 Generated with Claude Code