Decompose a TypeScript union type into specific types

Issue

I wonder whether it's possible to “split” union types into the specific subtypes in TypeScript. This is the code I tried to use, it should be obvious what I'm trying to achieve from the snippet:

type SplitType<T> =
  T extends (infer A)|(infer B)
  ? Something<A, B>
  : T;

In this example Something<A, B> could be [A, B], or a completely different type. This would mean that SplitType<string> would just output a string, but SplitType<number|string> would mean [number, string].

Is something like that possible in TypeScript? And if not, is there a feature that will allow this in the future (eg. variadic types)?

Solution

With Matt McCutchen's answer, and the recursive conditional types since v4.1:

type UnionToParm<U> = U extends any ? (k: U) => void : never;
type UnionToSect<U> = UnionToParm<U> extends ((k: infer I) => void) ? I : never;
type ExtractParm<F> = F extends { (a: infer A): void } ? A : never;

type SpliceOne<Union> = Exclude<Union, ExtractOne<Union>>;
type ExtractOne<Union> = ExtractParm<UnionToSect<UnionToParm<Union>>>;

type ToTuple<Union> = ToTupleRec<Union, []>;
type ToTupleRec<Union, Rslt extends any[]> = 
    SpliceOne<Union> extends never ? [ExtractOne<Union>, ...Rslt]
    : ToTupleRec<SpliceOne<Union>, [ExtractOne<Union>, ...Rslt]>
;

type test = ToTuple<5 | 6 | "l">;

Answered By - SE12938683