Eliminate need for in-repo just-extend types

[Silic0nS0ldier]

Ideally we'll defer to DefinityTyped or get it merged into the project in question directly (unlikely given past dialogue angus-c/just#126). Up until now technical limitations have blocked this.

Potential answer may be found in https://stackoverflow.com/a/49683575/4728587 however.

[Silic0nS0ldier]

Closer, but a TS performance bail out occurs at 8 inputs.

// Type definitions for just-extend 4.1
// Project: https://github.com/angus-c/just#readme
// Definitions by: Peter Safranek <https://github.com/pe8ter>
//                 Jordan Mele <https://github.com/Silic0nS0ldier>
// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
// TypeScript Version: 4.1

// Based on https://stackoverflow.com/a/49683575/4728587

type OptionalPropertyNames<T> =
  { [K in keyof T]-?: ({} extends { [P in K]: T[K] } ? K : never) }[keyof T];

type SpreadProperties<L, R, K extends keyof L & keyof R> =
  { [P in K]: L[P] | Exclude<R[P], undefined> };

type Id<T> = T extends infer U ? { [K in keyof U]: U[K] } : never

type SpreadTwo<L, R> = Id<
  & Pick<L, Exclude<keyof L, keyof R>>
  & Pick<R, Exclude<keyof R, OptionalPropertyNames<R>>>
  & Pick<R, Exclude<OptionalPropertyNames<R>, keyof L>>
  & SpreadProperties<L, R, OptionalPropertyNames<R> & keyof L>
>;

type Filter1<T> = T extends object ? T : {};
type Filter2<T> = T extends any[] ? {} : T;

type Filter<T> = Filter2<Filter1<T>>;

type Spread<A extends readonly [...any]> = A extends [infer L, ...infer R] ?
  SpreadTwo<Filter<L>, Spread<R>> : unknown

// caps out at 8 params, 9th results in TS bailout
declare function extend<A extends any[]>(...a: [...A]): Spread<A>;

// deep not implemented

export = extend;