import { Unreachable } from "../type";
import { Required } from ".";
/**
 * gets the optional keys of `T`
 *
 * @remarks
 * does not capture index signatures.
 *
 * @remarks
 * may fail to capture keys that are also assignable to an index signature
 *
 * @since 0.0.2
 *
 * @example
 * type e0 = OptionalKeys<{}> // never
 * type e1 = OptionalKeys<{ a?: 0 }> // "a"
 * type e2 = OptionalKeys<{ a?: 0, b: 1 | undefined, c?: 2 }> // "a" | "c"
 *
 * type w0 = OptionalKeys<{ [x: number]: string, 0?: '' }>  // never // should've captured 0!!
 */
export type OptionalKeys<T> = Required<T> extends (infer U extends Record<keyof T, any>) ? {
    [K in keyof T]-?: T[K] extends U[K] ? never : K;
}[keyof T] : Unreachable;
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