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

type A1 = GetOptional<{ foo: number; bar?: string }>;
type B1 = { bar?: string };
type C1 = Expect<Equal<A1, B1>>;

type A2 = GetOptional<{ foo: undefined; bar?: undefined }>;
type B2 = { bar?: undefined };
type C2 = Expect<Equal<A2, B2>>;

// ============= Your Code Here =============
type GetOptional<T> = {
  [
    P in keyof T as
      T[P] extends Required<T>[P]
      ? never
      : P
  ]: T[P];
};

// ============== Alternatives ==============
// @Jack-Works
type GetRequired<T> = {
  [
    P in keyof T as
      T[P] extends Required<T>[P]
      ? P
      : never
  ]: T[P];
};
type GetOptional<T> = Omit<T, keyof GetRequired<T>>


// sneak peak from @yi-tuan:
type GetOptional<T> = Pick<T, OptionalKeys<T>>;

// @dqn
type GetOptional<
  T,
  U extends Required<T> = Required<T>,
  K extends keyof T = keyof T
> =
  Pick<
    T,
    K extends keyof T
    ? T[K] extends U[K]
      ? never
      : K
    : never
  >;