Homogeneous arrays in TypeScript (+ check if a type is a union)

homogeneous-array.ts

type Singleton = string;
type Union = string | number;

type DistributedKeyOf<T> =
  T extends any
    ? keyof T
    : never;

type DistributedValueOf<T> = T[DistributedKeyOf<T>];

type IsSingleton<T> =
  unknown extends DistributedValueOf<T>
    ? false
    : true;

type T1 = IsSingleton<Singleton> // $ExpectType true
type T2 = IsSingleton<Union>     // $ExpectType false

class HomogeneousArray<T> extends Array<IsSingleton<T> extends true ? T : never> {}

const numbers: HomogeneousArray<number> = [1, 2, 3]; // ✔
const mixed: HomogeneousArray<string | number> = [1, '2', 3]; // ✘

Nice! Your solution is simpler and gives a better error message, too.