Construct a type in which only a single member of `T` is valid at a time.

OneOf.ts

/**
 * Construct a type in which only a single member of `T` is valid at a time.
 *
 * @example
 * ```ts
 * type U = OneOf<
 *   | {
 *       a: string;
 *     }
 *   | {
 *       b: string;
 *       c: number;
 *     }
 * >;
 * // {
 * //   a: string;
 * //   b?: undefined;
 * //   c?: undefined;
 * // } | {
 * //   a?: undefined;
 * //   b: string;
 * //   c: number;
 * // }
 * ```
 */
type OneOf<T extends Record<PropertyKey, any>> = UnionKey<T> extends infer K extends
  PropertyKey
  ? T extends T
    ? Eval<
        T & {
          [_ in Exclude<K, keyof T>]?: never;
        }
      >
    : never
  : never;
  
type Eval<T> = { [K in keyof T]: T[K] } & {};
type UnionKey<T> = T extends T ? keyof T : never;