union-to-tuple.ts

type UnionToIntersection<U> =
  (U extends U ? (arg: U) => unknown : never) extends 
  ((arg: infer I) => unknown) ? I : never

type LastOfUnion<U> = 
  UnionToIntersection<
  (U extends unknown ? (arg: U) => unknown : never)
  > extends (arg: infer I) => unknown ? I : never

type IsAny<T> = [T] extends [never] ? 
  false :
  T extends (1 & T) ? 
  true : unknown extends T ? 
  true : false

type IsUnion<T, Copy = T> = 
  [T] extends [never] ? 
  false :
  T extends T ?
  [Copy] extends [T] ?
  // using isAny here because of 
  // specific type-cases
  IsAny<T> extends true ?
  true :
  false :
  true :
  false

type UnionToTupleImpl<U, R extends unknown[] = []> =
 [U] extends [never] ? 
 R :
 UnionToTupleImpl<Exclude<U, LastOfUnion<U>>, [...R, LastOfUnion<U>]>

type UnionToTuple<U> =
  [U] extends [never] ? 
  never :
  IsUnion<U> extends false ? 
  U :
  UnionToTupleImpl<U>

/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type ExtractValuesOfTuple<T extends any[]> = T[keyof T & number]

type cases = [
  Expect<Equal<UnionToTuple<'a' | 'b'>['length'], 2>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<'a' | 'b'>>, 'a' | 'b'>>,
  // Expect<Equal<ExtractValuesOfTuple<UnionToTuple<'a'>>, 'a'>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<any>>, any>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<undefined | void | 1>>, void | 1>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<any | 1>>, any | 1>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<any | 1>>, any>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<'d' | 'f' | 1 | never>>, 'f' | 'd' | 1>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<[{ a: 1 }] | 1>>, [{ a: 1 }] | 1>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<never>>, never>>,
  Expect<Equal<ExtractValuesOfTuple<UnionToTuple<'a' | 'b' | 'c' | 1 | 2 | 'd' | 'e' | 'f' | 'g'>>, 'f' | 'e' | 1 | 2 | 'g' | 'c' | 'd' | 'a' | 'b'>>,
]