Restore covariant comparison to bluebird's Promise

Now that typescript@next checks variance more strictly for conditional
types, bluebird's Promise is invariant. This PR removes conditional
types from bluebird so that Promise compares covariantly again.

Note that I never figured out how to do this for `call`, so it returns
`Bluebird<any>` right now. I'm still working on that.

types/bluebird/index.d.ts

@@ -37,8 +37,6 @@
 
 type Constructor<E> = new (...args: any[]) => E;
 type CatchFilter<E> = ((error: E) => boolean) | (object & E);
-type IterableItem<R> = R extends Iterable<infer U> ? U : never;
-type IterableOrNever<R> = Extract<R, Iterable<any>>;
 type Resolvable<R> = R | PromiseLike<R>;
 type IterateFunction<T, R> = (item: T, index: number, arrayLength: number) => Resolvable<R>;
 
@@ -352,7 +350,7 @@ declare class Bluebird<R> implements PromiseLike<R>, Bluebird.Inspection<R> {
    * });
    * </code>
    */
-  call<U extends keyof R>(propertyName: U, ...args: any[]): Bluebird<R[U] extends (...args: any[]) => any ? ReturnType<R[U]> : never>;
+  call<U extends keyof R>(propertyName: U, ...args: any[]): Bluebird<any>;
 
   /**
    * This is a convenience method for doing:
@@ -562,12 +560,17 @@ declare class Bluebird<R> implements PromiseLike<R>, Bluebird.Inspection<R> {
   /**
    * Like calling `.then`, but the fulfillment value or rejection reason is assumed to be an array, which is flattened to the formal parameters of the handlers.
    */
-  spread<U>(fulfilledHandler: (...values: Array<IterableItem<R>>) => Resolvable<U>): Bluebird<U>;
+  spread<U, Q>(this: Bluebird<R & Iterable<Q>>, fulfilledHandler: (...values: Array<Q>) => Resolvable<U>): Bluebird<U>;
 
   /**
    * Same as calling `Promise.all(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  all(): Bluebird<IterableOrNever<R>>;
+  all(this: Bluebird<Iterable<unknown>>): Bluebird<R>;
+
+  /**
+   * Same as calling `Promise.all(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
+   */
+  all(): Bluebird<never>;
 
   /**
    * Same as calling `Promise.props(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
@@ -578,42 +581,59 @@ declare class Bluebird<R> implements PromiseLike<R>, Bluebird.Inspection<R> {
   /**
    * Same as calling `Promise.any(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  any(): Bluebird<IterableItem<R>>;
+  any<Q>(this: Bluebird<R & Iterable<Q>>): Bluebird<Q>;
+
+  /**
+   * Same as calling `Promise.any(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
+   */
+  any(): Bluebird<never>;
 
   /**
+   * Same as calling `Promise.some(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    * Same as calling `Promise.some(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  some(count: number): Bluebird<IterableOrNever<R>>;
+  some(this: Bluebird<Iterable<unknown>>, count: number): Bluebird<R>;
+
+  /**
+   * Same as calling `Promise.some(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
+   * Same as calling `Promise.some(thisPromise)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
+   */
+  some(count: number): Bluebird<never>;
 
   /**
    * Same as calling `Promise.race(thisPromise, count)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  race(): Bluebird<IterableItem<R>>;
+  race<Q>(this: Bluebird<R & Iterable<Q>>): Bluebird<Q>;
+
+  /**
+   * Same as calling `Promise.race(thisPromise, count)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
+   */
+  race(): Bluebird<never>;
 
   /**
    * Same as calling `Bluebird.map(thisPromise, mapper)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  map<U>(mapper: IterateFunction<IterableItem<R>, U>, options?: Bluebird.ConcurrencyOption): Bluebird<R extends Iterable<any> ? U[] : never>;
+  map<U, Q>(this: Bluebird<R & Iterable<Q>>, mapper: IterateFunction<Q, U>, options?: Bluebird.ConcurrencyOption): Bluebird<U[]>;
 
   /**
    * Same as calling `Promise.reduce(thisPromise, Function reducer, initialValue)`. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  reduce<U>(reducer: (memo: U, item: IterableItem<R>, index: number, arrayLength: number) => Resolvable<U>, initialValue?: U): Bluebird<R extends Iterable<any> ? U : never>;
+  reduce<U, Q>(this: Bluebird<R & Iterable<Q>>, reducer: (memo: U, item: Q, index: number, arrayLength: number) => Resolvable<U>, initialValue?: U): Bluebird<U>;
 
   /**
    * Same as calling ``Promise.filter(thisPromise, filterer)``. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  filter(filterer: IterateFunction<IterableItem<R>, boolean>, options?: Bluebird.ConcurrencyOption): Bluebird<IterableOrNever<R>>;
+  filter<Q>(this: Bluebird<R & Iterable<Q>>, filterer: IterateFunction<Q, boolean>, options?: Bluebird.ConcurrencyOption): Bluebird<R>;
 
   /**
    * Same as calling ``Bluebird.each(thisPromise, iterator)``. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  each(iterator: IterateFunction<IterableItem<R>, any>): Bluebird<IterableOrNever<R>>;
+  each<Q>(this: Bluebird<R & Iterable<Q>>, iterator: IterateFunction<Q, any>): Bluebird<R>;
 
   /**
    * Same as calling ``Bluebird.mapSeries(thisPromise, iterator)``. With the exception that if this promise is bound to a value, the returned promise is bound to that value too.
    */
-  mapSeries<U>(iterator: IterateFunction<IterableItem<R>, U>): Bluebird<R extends Iterable<any> ? U[] : never>;
+  mapSeries<U, Q>(this: Bluebird<R & Iterable<Q>>, iterator: IterateFunction<Q, U>): Bluebird<U[]>;
 
   /**
    * Cancel this `promise`. Will not do anything if this promise is already settled or if the cancellation feature has not been enabled