Homogeneous arrays in TypeScript

homogenous-array.ts

/**
 * @author https://stackoverflow.com/users/2887218/jcalz
 * @see https://stackoverflow.com/a/50375286/10325032
 */
type UnionToIntersection<Union> =
  (Union extends any
    ? (argument: Union) => void
    : never
  ) extends (argument: infer Intersection) => void
      ? Intersection
      : never;

type IsSingleton<T> =
  [T] extends [UnionToIntersection<T>]
    ? true
    : false

type SingletonOnly<T> =
  IsSingleton<T> extends true
    ? T
    : never

interface HomogenousArray<T> extends Array<SingletonOnly<T>> { }

declare function foo<T>(...items: HomogenousArray<T>): T;

foo(1, 2);     // ✅ Good!
foo(1, 'foo'); // ⛔️ Compile-time error (expected all members to be of the same type)

Could have just used:

declare function foo<T>(...items: [T, ...T[]]): T;

foo(1, 2);     // ✅ Good!
foo(1, 'foo'); // ⛔️ Compile-time error (expected all members to be of the same type)