OptionSet with associated value for each option

But that does not compile. In fact, it looks like the filter property is not separately defined for each option, but rather for a whole option set.

This is because an OptionSet isn't really a set, per se. If I have the following OptionSet:

struct MyOptions: OptionSet {
    let rawValue: Int

    static let foo = MyOptions(1 << 0)
    static let bar = MyOptions(1 << 1)
}

and then I make the set like so:

let opts: MyOptions = [.foo, .bar]

I don't actually have a collection with two MyOptions instances in it. Instead, I have a new instance of MyOptions whose rawValue is set to (.foo.rawValue | .bar.rawValue)—i.e. 3. The original two MyOptions instances are discarded as soon as opts is made.

Similarly, your logger.outputs will be an instance of OutputOptions with rawValue 3 and the default value for filter.

Thus, it's not really possible to do what you want with an OptionSet.