I have been looking for a strict version of Extract<T, U>. For simplicity, let's call it ExtractStrict<T, U>. ExtractStrict would have one additional feature compared to Extract: ExtractStrict would throw an error if some element in U is not part of T.

I know of an implementation by @ChristianIvicevic which has been proposed for inclusion in Typescript itself: microsoft/TypeScript#31474 I've asked him if he'd be willing to submit it here instead.

Edit: I've found a use case in my code for a strict version of Exclude too (ExcludeStrict), so I've added it to the title.