Fix accessing properties defined by a bound function

[lkosson]

When an object property is defined as a getter, which in turn is a bound function, accessing the property results in InvalidCastException in ObjectInstance.UnwrapFromGetter, because getter variable is Jint.Native.Function.BindFunction instead of Jint.Native.Function.Function.

Explicit test for BindFunction is not ideal and perhaps ICallable should be used, but I'm not sure how to properly pass it to Engine.

Test code to reproduce the behavior:

Console.WriteLine(new Engine().Evaluate("""
	const holder = {
		x: 42,
		getter() { return this.x; }
	};

	const target = {};

	Object.defineProperty(target, "prop", { get: holder.getter.bind(holder) });

	target.prop;
"""));

Before fix: throws InvalidCastException
After fix: prints out 42

[lahma]

Let's see copilot's review, also each bug fic should contain also a test case proving the fix.

[Copilot]

Pull request overview

Fixes a runtime crash when an accessor property’s getter is a bound function, which currently triggers an InvalidCastException during property access in ObjectInstance.UnwrapFromGetter.

Changes:

• Adds special-case handling for Function.BindFunction in ObjectInstance.UnwrapFromGetter to avoid casting it to Function.Function.

---

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[Copilot]

bindFunction._engine.Call(bindFunction, thisObject) resolves to Engine.Call(JsValue, params JsValue[] arguments), meaning thisObject is passed as a function argument and the getter is invoked with this=undefined. That changes observable behavior (e.g., arguments.length inside the getter becomes 1) and isn’t the correct way to invoke an accessor getter.

Call the overload that sets the receiver as this and passes no arguments (e.g., Call(callable, thisObject, Arguments.Empty) / Call((ICallable)callable, thisObject, Arguments.Empty, ...)).

Suggested change

	if (getter is Function.BindFunction bindFunction) return bindFunction._engine.Call(bindFunction, thisObject);
	var functionInstance = (Function.Function) getter;
	return functionInstance._engine.Call(functionInstance, thisObject);
	if (getter is Function.BindFunction bindFunction) return bindFunction._engine.Call(bindFunction, thisObject, Arguments.Empty);
	var functionInstance = (Function.Function) getter;
	return functionInstance._engine.Call(functionInstance, thisObject, Arguments.Empty);

[Copilot]

This accessor invocation path is still tightly coupled to specific concrete function types (BindFunction vs Function.Function) and will continue to throw InvalidCastException for any other callable getter that isn’t Function.Function. Elsewhere in this file (e.g., TryGetValue) getters are treated as ICallable.

Consider switching UnwrapFromGetter to use IsCallable/ICallable (and then invoking via Engine.Call) so all valid callable getter types are supported without adding more special-cases over time.

Suggested change

	if (getter is Function.BindFunction bindFunction) return bindFunction._engine.Call(bindFunction, thisObject);
	var functionInstance = (Function.Function) getter;
	return functionInstance._engine.Call(functionInstance, thisObject);
	if (!getter.IsCallable())
	{
	return Undefined;
	}
	var callable = (ICallable) getter;
	return callable.Call(thisObject, Arguments.Empty);

[Copilot]

This change fixes a specific regression scenario (accessor getter is a bound function), but the PR doesn’t add a regression test to ensure it stays fixed. Please add a test that defines an accessor via Object.defineProperty(..., { get: someFn.bind(...) }) and asserts the property access returns the expected value (and ideally that arguments.length inside the getter remains 0).

[lahma]

Created issue #2300 so we can process this more easily.

[lahma]

A more robust fix was made in #2301 and now fixed in main.