runtime: give Reflect.preventExtensions and Reflect.isExtensible Reflect-specific results and errors

[andrewtdiz]

Summary

Reflect.preventExtensions and Reflect.isExtensible are lowered to the same Perry HIR/runtime paths as the Object.* APIs, but the Reflect APIs have different observable behavior:

• Reflect.preventExtensions(obj) returns a boolean, while Object.preventExtensions(obj) returns the object.
• Reflect.preventExtensions(nonObject) throws TypeError, while the current shared helper returns the original value.
• Reflect.isExtensible(nonObject) throws TypeError, while Object.isExtensible(nonObject) returns false.
• The Reflect operations need proxy trap/invariant behavior rather than directly using the object flag helper.

Node behavior

Checked with Node v25.9.0:

const out = {};

const o = {};
out.objBefore = Reflect.isExtensible(o); // true
Object.preventExtensions(o);
out.objAfter = Reflect.isExtensible(o); // false

const preventProxyFalse = new Proxy({}, {
  preventExtensions() { return false; },
});
out.preventProxyFalse = Reflect.preventExtensions(preventProxyFalse); // false

const preventTarget = {};
const preventProxyTrue = new Proxy(preventTarget, {
  preventExtensions(target) {
    Object.preventExtensions(target);
    return true;
  },
});
out.preventProxyTrue = Reflect.preventExtensions(preventProxyTrue); // true
out.preventProxyTrueExtensible = Reflect.isExtensible(preventTarget); // false

for (const value of [1, null, undefined]) {
  Reflect.preventExtensions(value); // TypeError: Reflect.preventExtensions called on non-object
  Reflect.isExtensible(value); // TypeError: Reflect.isExtensible called on non-object
}

Node also enforces proxy invariants for Reflect.isExtensible; for example, an isExtensible trap returning false for an extensible target throws a TypeError.

Perry implementation

Object.preventExtensions and Object.isExtensible lower to the expected Object-specific expressions:

• crates/perry-hir/src/lower/expr_call/native_module.rs:717 lowers Object.preventExtensions to Expr::ObjectPreventExtensions.
• crates/perry-hir/src/lower/expr_call/native_module.rs:737 lowers Object.isExtensible to Expr::ObjectIsExtensible.

The Reflect lowering reuses those same Object expressions:

• crates/perry-hir/src/lower/expr_call/native_module.rs:1082 lowers Reflect.isExtensible(...) to Expr::ObjectIsExtensible(...).
• crates/perry-hir/src/lower/expr_call/native_module.rs:1086 lowers Reflect.preventExtensions(...) to Expr::ObjectPreventExtensions(...).

The shared runtime helpers implement Object behavior:

• crates/perry-runtime/src/object/object_ops.rs:1367 documents js_object_prevent_extensions as returning the object, and the function returns obj_value after setting OBJ_FLAG_NO_EXTEND.
• crates/perry-runtime/src/object/object_ops.rs:1418 documents js_object_is_extensible; it returns false for non-objects instead of throwing.

That is correct for Object.isExtensible(1) and Object.preventExtensions(obj), but it is observably wrong for the Reflect entry points.

Expected fix direction

Reflect.preventExtensions and Reflect.isExtensible should have Reflect-specific lowering/runtime helpers that:

• throw TypeError for non-object targets,
• return booleans for Reflect.preventExtensions,
• preserve the existing Object API return values/primitive behavior,
• invoke proxy traps and enforce proxy invariants for the Reflect operations.

Duplicate search

Searched existing issues/PRs for:

• Reflect.preventExtensions
• Reflect.isExtensible
• preventExtensions isExtensible
• Reflect preventExtensions returns object
• Reflect isExtensible non-object

No existing issue covered this specific Reflect/Object semantic split.