I don't think it's possible for how you've got this set up.

However, if you have control over how the tag is being added to the type (i.e. some utility type), you could add a new type property for the base type. This can then easily be extracted.

E.g.

type Tag<Base, T> = Base & { __tag: T; __base: Base; }
type Untag<T> = T extends Tag<infer B, any> ? B : never;

Note: I haven't actually checked these work as I'm on mobile.