Fix Proxy getPrototypeOf/setPrototypeOf traps with null prototypes#2668
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Two latent bugs in JsProxy's prototype traps when JS null is involved: 1. [[GetPrototypeOf]]: after correctly validating that the trap result is object-or-null, the code did `(ObjectInstance) handlerProto` -- when a spec-legal trap returns null (`getPrototypeOf: () => null`), handlerProto is the JsNull singleton (not an ObjectInstance), so the extensible-target path crashed with InvalidCastException. On the non-extensible path, `ReferenceEquals(handlerProto, _target.Prototype)` compared JsNull against a CLR-null Prototype -> false -> spurious TypeError even though both are conceptually null (SameValue true). 2. [[SetPrototypeOf]]: same ReferenceEquals shape -- `Object.setPrototypeOf(proxy, null)` on a non-extensible target whose prototype is already null compared JsNull against CLR null and threw a spurious TypeError where the spec (SameValue(V, targetProto)) requires success. Fix: normalize the JsNull sentinel to CLR null once, then compare/return that. All script-visible callers of SetPrototypeOf (Object.setPrototypeOf, Reflect.setPrototypeOf, __proto__ setter, __proto__ in object literals) already validate object-or-null before calling, so the `as` cast is safe. test262 does not cover trap-returns-null on these paths, which is why this stayed latent. All five expectations cross-checked against Node v24.18.0 (null / null / true null / true / TypeError on non-extensible mismatch). New tests verified to fail (3/5, incl. the InvalidCastException repro) without the fix; regression tests confirm the non-extensible mismatch invariant still throws. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EsHuKuE4UihKZp5HYapDHE
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Two latent bugs in the prototype traps, both around JS
nullvs CLRnull:JsProxy.GetPrototypeOfcast the trap result straight toObjectInstance, so a spec-legal null-returning trap crashed withInvalidCastExceptioninstead of returningnull:JsNull) against_target.Prototype(CLRnull) withReferenceEquals, so a null-prototype non-extensible target + null trap result wrongly threw; same pattern inSetPrototypeOfbrokeObject.setPrototypeOf(proxy, null)for non-extensible null-proto targets. Per spec the comparison is SameValue, which succeeds when both are null.Fix: normalize the JS value to
ObjectInstance?once after the object-or-null validation, then compare/return that. Trap fetch order, invariant semantics and messages unchanged.test262 never exercises a null trap result in these paths (its getPrototypeOf suite only returns objects — verified file-by-file), hence latent. Expectations cross-checked against node 24 / V8. Negative validation: without the fix, the new tests reproduce the exact
InvalidCastException.🤖 Generated with Claude Code
https://claude.ai/code/session_01EsHuKuE4UihKZp5HYapDHE