Union to Tuple

union-to-tuple.ts

type Specify<T> = T extends any ? (k: T) => void : never

// Real intersection: UnionToIntersection<`c${string}` | 'car'> will become `c${string}`
type UnionToIntersection<T> = Specify<T> extends (a: infer A) => void ? A : never;

// Transforms 'a' | 'b' into ((f: "a") => void) & ((f: "b") => void)
type UnionToOverloads<U> = UnionToIntersection<Specify<U>>;

// Still figuring out why this happens
// ((f: "a") => void) & ((f: "b") => void) will become "b"
type LastOfUnion<U> = UnionToOverloads<U> extends (a: infer A) => void ? A : never;

// Only unions won't conform to this
type IsUnion<T> = [T] extends [UnionToIntersection<T>] ? false : true;

type PopUnion<T> = {
  result: LastOfUnion<T>,
  rest: Exclude<T, LastOfUnion<T>>
}

// Grabs the last of union as a way to infer last and pops it with exclude
type UnionToTuple<T, output extends unknown[] = []> = IsUnion<T> extends true
  ? UnionToTuple<PopUnion<T>['rest'], [PopUnion<T>['result'], ...output]>
  : [T, ...output];