Feat: fromPromise support promise cancelable?

[imcuttle]

rxjs/src/internal/observable/from.ts

Lines 191 to 205 in 0d78d21

	function fromPromise<T>(promise: PromiseLike<T>) {
	return new Observable((subscriber: Subscriber<T>) => {
	promise
	.then(
	(value) => {
	if (!subscriber.closed) {
	subscriber.next(value);
	subscriber.complete();
	}
	},
	(err: any) => subscriber.error(err)
	)
	.then(null, reportUnhandledError);
	});
	}

I saw the soure code of fromPromise to Observable, Why not support promise cancelable?

 function fromPromise<T>(promise: PromiseLike<T>) { 
   return new Observable((subscriber: Subscriber<T>) => { 
     promise 
       .then( 
         (value) => { 
           if (!subscriber.closed) { 
             subscriber.next(value); 
             subscriber.complete(); 
           } 
         }, 
         (err: any) => subscriber.error(err) 
       ) 
       .then(null, reportUnhandledError); 
   }); 
   return () => {
     // See https://github.com/sindresorhus/p-cancelable/blob/master/index.js
     if (promise && typeof promise.cacel === 'function') {
       if (!promise.isCanceled) {
            promise.cacel();
       }
     }
   };
 } 

[kwonoj]

There is discussion around to support spec-compliant abort signal (#5863). Rxjs don't support non-spec compliant cancel from 3rd party libraries.