Here's an useful pattern I just discovered in #godot . I was wondering why the class has #enum s if the function asks for #bitflag s. The reason I discovered, that like this, we can quickly identify and chain the flags we want.
For example, FLAG_RELATIVE_PATHS + FLAG_BUNDLE_RESOURCES (EDIT: Or better, use | as suggested below by @gertkeno ). It's more clear than doing math magic and hope it's correct. Just assign enum keys values that are power of 2 for it to work.