import type { Equal, Expect } from './test-utils'

type A1 = Concat<[], []>;
type B1 = [];
type C1 = Expect<Equal<A1, B1>>;

type A2 = Concat<[], [1]>;
type B2 = [1];
type C2 = Expect<Equal<A2, B2>>;

type A3 = Concat<[1, 2], [3, 4]>;
type B3 = [1, 2, 3, 4];
type C3 = Expect<Equal<A3, B3>>;

type A4 = Concat<['1', 2], [false, boolean, '4']>;
type B4 = ['1', 2, false, boolean, '4'];
type C4 = Expect<Equal<A4, B4>>;

// ============= Your Code Here =============
type Concat<
  T extends unknown[],
  U extends unknown[]
> = [...T, ...U];

// ============== Alternatives ==============
// this is bad
// because it won't error if you pass a non-array
type Concat<T, U> =
  T extends unknown[]
  ? U extends unknown[]
    ? [...T, ...U]
    : never
  : never;