Comment by Ari_Rahikkala

I used to think that this was only ever going to be possible with some sort of incomprehensible type trickery that I would never be able to understand. Then TypeScript came along and showed me that no, actually, in a structural type system, it's just about as simple as you could imagine:

    type Foo = { kind: 'A', aItem: number } | { kind: 'B', bItem: string } | { kind: 'C' };
    
    type SubsetFoo = Foo & { kind: 'B' | 'C' }
    type SupersetFoo = Foo | { kind: 'D', dItem: boolean };

I'm sure there are imperfections here. For a start, SubsetFoo's normalized form looks rather ugly when you mouse over it in VSCode. But it does get you the niceties of exhaustiveness checking and type-aware suggestions with control flow awareness, etc..