`.next()` is available in observables returned by `.pipe()`

Describe the bug

The following code shows a weird behavior of .pipe():

const RxJs = require('rxjs');

const mySubj$ = new RxJs.Subject();

const doubled$ = mySubj$.pipe(
  RxJs.map((v) => v * 2),
);

mySubj$.subscribe((value) => {
  console.log('Original value:', value);
});

doubled$.subscribe((value) => {
  console.log('Doubled value:', value);
});

mySubj$.next(1); // Logs: 
// ✅ `Original value: 1`
// ✅ `Doubled value: 2`

doubled$.next(3); // Logs:
// ❌ `Original value: 3`
// ❌ `Doubled value: 6`

There's an inconsistency in the APIs exposed by doubled$:

• .subscribe() applies the observable returned by the .pipe()
• .next() applies to source of the .pipe() (mySubj$).

Expected behavior

I would expect .next() to fail with not a function.

According to the types, .pipe() returns an Observable (where .next() is not available).

Reproduction code

const RxJs = require('rxjs');

const mySubj$ = new RxJs.Subject();

const doubled$ = mySubj$.pipe(
  RxJs.map((v) => v * 2),
);

mySubj$.subscribe((value) => {
  console.log('Original value:', value);
});

doubled$.subscribe((value) => {
  console.log('Doubled value:', value);
});

mySubj$.next(1); // Logs: 
// ✅ `Original value: 1`
// ✅ `Doubled value: 2`

doubled$.next(3); // Logs:
// ❌ `Original value: 3`
// ❌ `Doubled value: 6`

Reproduction URL

No response

Version

7.8.2

Environment

No response

Additional context

Currently, this is the workaround to achieve this:

const doubled$ = mySubj$.asObservable().pipe(

Start by tracing how Subject, asObservable(), and pipe() expose the returned observable at runtime, alongside the Observable and Subject types. Reproduce the example on version 7.8.2 and inspect existing tests for Subject and pipe behavior. Done means the piped observable no longer exposes or forwards .next(), while direct Subject usage and the documented asObservable() workaround remain consistent.