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

type A1 = First<[3, 2, 1]>;
type B1 = 3;
type C1 = Expect<Equal<A1, B1>>;

type A2 = First<[() => 1, { a: string }]>;
type B2 = () => 1;
type C2 = Expect<Equal<A2, B2>>;

type A3 = First<[]>;
type B3 = never;
type C3 = Expect<Equal<A3, B3>>;

type A4 = First<[undefined]>;
type B4 = undefined;
type C4 = Expect<Equal<A4, B4>>;

// @ts-expect-error(2344)
type E1 = First<'notArray'>;

// @ts-expect-error(2344)
type E2 = First<{ 0: 'arrayLike' }>;

// start with `= T[0]`
// ============= Your Code Here =============
type First<T extends unknown[]> =
  T extends []
  ? never
  : T[0]
;

// ============== Alternatives ==============
type First<T extends unknown[]> =
  T['length'] extends 0
  ? never
  : T[0]
;

type First<T extends unknown[]> =
  T extends never[] // also `[never]`
  ? never
  : T[0]
;

type First<T extends unknown[]> =
  T extends [infer Head, ...infer _]
  ? Head
  : never
;

/* @nagq */
type First<T extends unknown[]> =
  T[0] extends T[number]
  ? T[0]
  : never;

type First<T extends unknown[]> =
  '0' extends keyof T
  ? T[0]
  : never;

type First<T extends unknown[]> =
  '0' extends keyof T
  ? T[0] extends infer U
    ? U
    : never
  : never;

type First<T extends unknown[]> =
  T[number] extends never
  ? never
  : T[0];

type First<T extends unknown[]> =
  T extends [infer Head, ...any[]]
  ? Head
  : never;

type First<T extends unknown[]> =
  T extends [infer Head, ...any]
  ? Head
  : never;

type First<T extends unknown[]> =
  T extends [any, ...any[]]
  ? T[0]
  : never;

type First<T extends unknown[]> =
  T extends [T[0], ...any]
  ? T[0]
  : never;

type First<T extends unknown[]> =
  T[Extract<keyof T, '0'>]

// ================== NOPE ==================
type Nope0<T extends unknown[]> =
  T[0] extends undefined
  ? never
  : T[0]
;