[cpp] HXCPP_GC_GENERATIONAL emits an invalid write barrier for cpp.Function fields

[Tutez64]

When HXCPP_GC_GENERATIONAL is enabled, assigning a cpp.Function instance field emits a generational write barrier even though cpp::Function is a native function-pointer wrapper rather than a GC object.

This affects Haxe 4.3.7 and the current development branch (2f724abd255c5c0204d350c55bc64068c18c4ad1).

Minimal reproduction:

class Main {
	static function main():Void {
		var holder = new NativeCallbackHolder(cpp.Callable.fromStaticFunction(increment));
		if (holder.callback.call(41) != 42) {
			throw "Invalid native callback result";
		}
	}

	static function increment(value:Int):Int {
		return value + 1;
	}
}

class NativeCallbackHolder {
	public var callback:cpp.Function<Int->Int, cpp.abi.Abi>;

	public function new(callback:cpp.Function<Int->Int, cpp.abi.Abi>) {
		this.callback = callback;
	}
}

-main Main
-D HXCPP_GC_GENERATIONAL
-cpp out

The generated setter uses HX_OBJ_WB_SET for the field. That macro expects an object-like value with an mPtr member, which cpp::Function<int(int)> does not have, so the generated C++ fails to compile:

error: cpp::Function<int(int)> has no member named mPtr

Without HXCPP_GC_GENERATIONAL, the same program compiles and runs successfully.

Expected behavior: cpp.Function fields should not receive GC write barriers, and the program should compile and return the expected callback result.