feat: add generic type parameter to using for the resource

This overload alleviates the observableFactory from needing to cast its
parameter to a more specific type.

Author: jennings

api_guard/dist/types/index.d.ts

@@ -841,7 +841,8 @@ export interface UnsubscriptionError extends Error {
 
 export declare const UnsubscriptionError: UnsubscriptionErrorCtor;
 
-export declare function using<T extends ObservableInput<any>>(resourceFactory: () => Unsubscribable | void, observableFactory: (resource: Unsubscribable | void) => T | void): Observable<ObservedValueOf<T>>;
+export declare function using<T extends ObservableInput<any>, R extends Unsubscribable = Unsubscribable>(resourceFactory: () => R, observableFactory: (resource: R) => T | void): Observable<ObservedValueOf<T>>;
+export declare function using<T extends ObservableInput<any>>(resourceFactory: () => void, observableFactory: () => T | void): Observable<ObservedValueOf<T>>;
 
 export declare type ValueFromArray<A extends readonly unknown[]> = A extends Array<infer T> ? T : never;
 

spec-dtslint/observables/using-spec.ts

@@ -1,4 +1,4 @@
-import { using } from 'rxjs';
+import { using, Subscription, of} from 'rxjs';
 import { a$, b$ } from '../helpers';
 
 it('should infer with a simple factory', () => {
@@ -7,4 +7,8 @@ it('should infer with a simple factory', () => {
 
 it('should infer with a factory that returns a union', () => {
   const o = using(() => {}, () => Math.random() < 0.5 ? a$ : b$); // $ExpectType Observable<A | B>
-});
+});
+
+it('should infer resource types', () => {
+  const o = using(() => new Subscription(), (sub) => of(sub)); // $ExpectType Observable<Subscription>
+});

src/internal/observable/using.ts

@@ -31,9 +31,17 @@ import { EMPTY } from './empty';
  * @return {Observable<T>} An Observable that behaves the same as Observable returned by `observableFactory`, but
  * which - when completed, errored or unsubscribed - will also call `unsubscribe` on created resource object.
  */
+export function using<T extends ObservableInput<any>, R extends Unsubscribable = Unsubscribable>(
+  resourceFactory: () => R,
+  observableFactory: (resource: R) => T | void
+): Observable<ObservedValueOf<T>>;
 export function using<T extends ObservableInput<any>>(
-  resourceFactory: () => Unsubscribable | void,
-  observableFactory: (resource: Unsubscribable | void) => T | void
+  resourceFactory: () => void,
+  observableFactory: () => T | void
+): Observable<ObservedValueOf<T>>;
+export function using<T extends ObservableInput<any>, R extends Unsubscribable = Unsubscribable>(
+  resourceFactory: () => R | void,
+  observableFactory: (resource: R | void) => T | void
 ): Observable<ObservedValueOf<T>> {
   return new Observable<ObservedValueOf<T>>((subscriber) => {
     const resource = resourceFactory();