feat(combineLatest): add N-args signature for observable inputs

When using combineLatest with an array of more than six
observable inputs, the result observable has a union type
of all observed types. Now, by using the 'as const' assertion,
the output observable has a correct tuple type that mirrors the
input observables.

spec-dtslint/observables/combineLatest-spec.ts

@@ -92,6 +92,10 @@ it('should have basic support for 7 or more params', () => {
   const o = combineLatest([a$, b$, c$, d$, e$, f$, g$]); // $ExpectType Observable<(A | B | C | D | E | F | G)[]>
 });
 
+it('should have full support for 7 or more params with readonly tuples', () => {
+  const o = combineLatest([a$, b$, c$, d$, e$, f$, g$] as const); // $ExpectType Observable<readonly [A, B, C, D, E, F, G]>
+});
+
 it('should handle an array of Observables', () => {
   const o = combineLatest([a$, a$, a$, a$, a$, a$, a$, a$, a$, a$, a$]); // $ExpectType Observable<A[]>
 });

src/internal/observable/combineLatest.ts

@@ -1,5 +1,5 @@
 import { Observable } from '../Observable';
-import { ObservableInput, SchedulerLike, ObservedValueOf } from '../types';
+import { ObservableInput, SchedulerLike, ObservedValueOf, ObservedValueTupleFromArray } from '../types';
 import { isScheduler  } from '../util/isScheduler';
 import { isArray  } from '../util/isArray';
 import { Subscriber } from '../Subscriber';
@@ -67,6 +67,7 @@ export function combineLatest<O1 extends ObservableInput<any>, O2 extends Observ
 export function combineLatest<O1 extends ObservableInput<any>, O2 extends ObservableInput<any>, O3 extends ObservableInput<any>, O4 extends ObservableInput<any>, O5 extends ObservableInput<any>>(sources: [O1, O2, O3, O4, O5]): Observable<[ObservedValueOf<O1>, ObservedValueOf<O2>, ObservedValueOf<O3>, ObservedValueOf<O4>, ObservedValueOf<O5>]>;
 export function combineLatest<O1 extends ObservableInput<any>, O2 extends ObservableInput<any>, O3 extends ObservableInput<any>, O4 extends ObservableInput<any>, O5 extends ObservableInput<any>, O6 extends ObservableInput<any>>(sources: [O1, O2, O3, O4, O5, O6]): Observable<[ObservedValueOf<O1>, ObservedValueOf<O2>, ObservedValueOf<O3>, ObservedValueOf<O4>, ObservedValueOf<O5>, ObservedValueOf<O6>]>;
 export function combineLatest<O extends ObservableInput<any>>(sources: O[]): Observable<ObservedValueOf<O>[]>;
+export function combineLatest<O extends readonly ObservableInput<any>[]>(sources: O): Observable<ObservedValueTupleFromArray<O>>;
 
 // Standard calls
 /** @deprecated Pass arguments in a single array instead `combineLatest([a, b, c])` */

src/internal/types.ts

@@ -165,7 +165,7 @@ export type ObservedValuesFromArray<X> = ObservedValueUnionFromArray<X>;
  * of `[string, number]`.
  */
 export type ObservedValueTupleFromArray<X> =
-  X extends Array<ObservableInput<any>>
+  X extends readonly ObservableInput<any>[]
     ? { [K in keyof X]: ObservedValueOf<X[K]> }
     : never;
 
@@ -201,4 +201,4 @@ export type Tail<X extends any[]> =
  * If you have `T extends Array<any>`, and pass a `string[]` to it,
  * `ValueFromArray<T>` will return the actual type of `string`.
  */
-export type ValueFromArray<A> = A extends Array<infer T> ? T : never;
+export type ValueFromArray<A> = A extends Array<infer T> ? T : never;