delete of a NON-WRITABLE data property leaves the key un-re-addable: the next o.a = 9 is silently dropped (pre-existing on v0.5.1618) #10840

Found while writing the differential probe for the property-read guard removals (stacked on #10824/#10826/#10828/#10833). Reproduces on pristine main v0.5.1618 (/root/claude-nanbox baseline binary), so it is not caused by any of those PRs.

A data property that was made non-writable with Object.defineProperty, then deleted, cannot be re-added: the later plain assignment is silently dropped. The same sequence with a writable data descriptor, or with an accessor, works.

Reproduction

const o: any = { a: 1, b: 2 };
Object.defineProperty(o, "a", { value: 7, writable: false, configurable: true });
delete o.a;
o.a = 9;
console.log(o.a, "a" in o, JSON.stringify(Object.keys(o)));

Expected (node 26.8.1)

9 true ["b","a"]

Actual (perry v0.5.1618 main, also the v0.5.1622 stack)

undefined false ["b"]

The delete itself is observed correctly (o.a reads undefined, "a" in o is false), and Object.keys after the failed re-add shows the key was never added — so it is the write after the delete that is refused, as if the deleted key's writable: false attribute still applied.

Variants (same binary)

Likely the per-key attribute survives the delete in the descriptor side table (or the write path's non-writable check consults it by key without confirming the key is still own), so the re-add is treated as a rejected write to a read-only property. Lane 1's area (object/descriptor_state.rs / delete_rest.rs); not touched by the read-path work.