[cpp] Avoid write barriers for cpp.Function fields

src/generators/cpp/cppContext.ml

@@ -89,7 +89,10 @@ let hash_keys hash =
   !key_list
 
 let is_gc_element ctx member_type =
-  Gctx.defined ctx.ctx_common Define.HxcppGcGenerational && (is_object_element member_type)
+  Gctx.defined ctx.ctx_common Define.HxcppGcGenerational &&
+  match member_type with
+  | CppAst.TCppFunction _ -> false
+  | _ -> is_object_element member_type
 
 let strip_file ctx file = match Gctx.defined ctx Define.AbsolutePath with
   | true -> Path.get_full_path file

tests/misc/cpp/projects/Issue13001/Main.hx

@@ -0,0 +1,20 @@
+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;
+	}
+}

tests/misc/cpp/projects/Issue13001/compile.hxml

@@ -0,0 +1,3 @@
+-main Main
+-D HXCPP_GC_GENERATIONAL
+-cpp out