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ECMAScript spread param rules causing loss of type-safety (and slower compile time?) #7347

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@benlesh

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@benlesh

I think we're hitting a similar situation to #1336 with RxJS 5, which is causing some crazy workarounds for some minimal type-safety, which in turn is causing slower build times.

Ideally what we'd have is this:

// Observable.prototype.concat
concat<R>(...observables: ObservableInput<any>[], scheduler?: Scheduler): Observable<R>

But since that's illegal, we're forced to do this:

concat<R>(...args: Array<ObservableInput<any> | Scheduler>): Observable<R>

... that clearly doesn't give us what we want for type safety, so we end up defining a crazy list of common type signatures like:

/* tslint:disable:max-line-length */
export interface ConcatSignature<T> {
  (scheduler?: Scheduler): Observable<T>;
  <T2>(v2: ObservableInput<T2>, scheduler?: Scheduler): Observable<T | T2>;
  <T2, T3>(v2: ObservableInput<T2>, v3: ObservableInput<T3>, scheduler?: Scheduler): Observable<T | T2 | T3>;
  <T2, T3, T4>(v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, scheduler?: Scheduler): Observable<T | T2 | T3 | T4>;
  <T2, T3, T4, T5>(v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, v5: ObservableInput<T5>, scheduler?: Scheduler): Observable<T | T2 | T3 | T4 | T5>;
  <T2, T3, T4, T5, T6>(v2: ObservableInput<T2>, v3: ObservableInput<T3>, v4: ObservableInput<T4>, v5: ObservableInput<T5>, v6: ObservableInput<T6>, scheduler?: Scheduler): Observable<T | T2 | T3 | T4 | T5 | T6>;
  (...observables: Array<ObservableInput<T> | Scheduler>): Observable<T>;
  <R>(...observables: Array<ObservableInput<any> | Scheduler>): Observable<R>;
}
/* tslint:enable:max-line-length */

I believe this is directly related to compilation speed issues that @mhegazy wanted to discuss with me.

cc/ @kwonoj @david-driscoll @jayphelps

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