What happens if you register a custom class with a built-in parent? The check for typename T::self_type should not be a valid type, should this also be added to the GDEXTENSION_CLASS_ALIAS and Wrapped like it is int the engine (With Object)?

I.e. add typedef Wrapped self_type; to Wrapped, and add the define to the other macro, just for completeness (or just to Wrapped, that would ensure there's a self_type to check against)

What happens if you register a custom class with a built-in parent?

Well, you'd get other compiler errors even without the static_assert():

/home/dsnopek/Sync/Projects/default/godot-cpp/include/godot_cpp/core/class_db.hpp:196:41: error: 'has_get_property_list' is not a member of 'ExampleNoGDCLASS'
  196 |                 T::has_get_property_list() ? T::get_property_list_bind : nullptr, // GDExtensionClassGetPropertyList get_property_list_func;
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~^~
/home/dsnopek/Sync/Projects/default/godot-cpp/include/godot_cpp/core/class_db.hpp:205:20: error: 'create' is not a member of 'ExampleNoGDCLASS'
  205 |                 T::create, // GDExtensionClassCreateInstance create_instance_func; /* this one is mandatory */
      |                    ^~~~~~
/home/dsnopek/Sync/Projects/default/godot-cpp/include/godot_cpp/core/class_db.hpp:206:20: error: 'free' is not a member of 'ExampleNoGDCLASS'
  206 |                 T::free, // GDExtensionClassFreeInstance free_instance_func; /* this one is mandatory */
      |                    ^~~~
/home/dsnopek/Sync/Projects/default/godot-cpp/include/godot_cpp/core/class_db.hpp:207:20: error: 'recreate' is not a member of 'ExampleNoGDCLASS'
  207 |                 T::recreate, // GDExtensionClassRecreateInstance recreate_instance_func;

Because native classes with GDEXTENSION_CLASS() (as opposed to GDCLASS()) are missing a number of other things that are needed to register a class.

However, I suppose it'd be nice to have the static_assert() message in there too, because it's more helpful? Or, we could even add stub implementations of the missing methods to GDEXTENSION_CLASS() so there's only the helpful message?

I'd say that sounds good! At minimum adding the definition to Wrapped to get a clean error message, and see if that resolves the rest, as in catches the error and stops there, it will give you something descriptive you can solve otherwise it gets cryptic

Just for completeness I'd add:

typedef Wrapped self_type;

To wrapped, though extending wrapped directly shouldn't happen it'd ensure the check doesn't break (and has parity with the engine)

I added the stub methods as well as self_type on native classes, and so in my latest push, you'll get the same helpful error (rather than the cryptic ones I posted above) if you try to register a class declared like:

class ExampleNoGDCLASS : public Object {
};

Cherry-picked for 4.1 in PR #1281