[paldepind2]

I never understood why people are so keen to do that in TypeScript. With that definition a `UserID` can still be silently "coerced" to a `string` everywhere. So you only get halfway there to an encapsulated type.

I think it's a much better idea to do:

    type UserID = { readonly __tag: unique symbol }

Now clients of `UserID` no longer knows anything about the representation. Like with the original approach you need a bit of casting, but that can be neatly encapsulated as it would be in the original approach anyway.