multiple using of tap() causes a false error

[its-dibo]

Describe the bug

the following code is correct, but will generate a false error

import { Observable, of, tap } from 'rxjs';

let test$: Observable<number> = of(1).pipe(
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
  tap(() => 1),
);

Error:

Type 'Observable<unknown>' is not assignable to type 'Observable<number>'.
  Type 'unknown' is not assignable to type 'number'.ts(2322)

commenting any of tap() blocks causes the error to be disappeared.

also, explicitly set the type again resolves the error. but TS can infer the type easily

of(1).pipe( ... ) as Observable<number>

Expected behavior

no error

Reproduction code

No response

Reproduction URL

https://stackblitz.com/edit/rxjs-jvajmd?devtoolsheight=60&file=index.ts,tsconfig.json

Version

next

Environment

No response

Additional context

No response

[martinsaison]

According to the unit tests for pipe(), type should not be enforced beyond the 9th argument.
But the signature for pipe() with more than 9 arguments returns an Observable<unknown>. It should rather return a Observable<any> as the type safety of "unknown" can't be verified here at compilation time anyway.

In observable.ts:

 pipe<A, B, C, D, E, F, G, H, I>(
    op1: UnaryFunction<Observable<T>, A>,
    op2: UnaryFunction<A, B>,
    op3: UnaryFunction<B, C>,
    op4: UnaryFunction<C, D>,
    op5: UnaryFunction<D, E>,
    op6: UnaryFunction<E, F>,
    op7: UnaryFunction<F, G>,
    op8: UnaryFunction<G, H>,
    op9: UnaryFunction<H, I>,
    ...operations: OperatorFunction<any, any>[]
  ): Observable<any>; // <--- instead of Observable<unknown>

(* edited because markdown was removing my text between < > )