message: refactor(switchScan): avoid using deprecated switchMap signature
diff: import { ObservableInput, ObservedValueOf, OperatorFunction } from '../types';
  import { switchMap } from './switchMap';
  +import { tap } from './tap';
  import { operate } from '../util/lift';
  // TODO: Generate a marble diagram for these docs.
  @@ -30,17 +31,20 @@ export function switchScan<T, R, O extends ObservableInput<any>>(
  // accumulator function at each new value from the source.
  let state = seed;
  - // Use `switchMap` on our `source` to do the work of creating
  - // this operator. Note the backwards order here of `switchMap()(source)`
  - // to avoid needing to use `pipe` unnecessarily
  + source
  + .pipe(
  + // Use `switchMap` on our `source` to do the work of creating this operator.
  switchMap(
  // On each value from the source, call the accumulator with
  // our previous state, the value and the index.
  - (value: T, index) => accumulator(state, value, index),
  - // Using the deprecated result selector here as a dirty trick
  - // to update our state with the flattened value.
  - (_, innerValue) => ((state = innerValue), innerValue)
  - )(source).subscribe(subscriber);
  + (value, index) => accumulator(state, value, index)
  + ),
  + tap((innerValue) => {
  + // Update our state with the flattened value.
  + state = innerValue;
  + })
  + )
  + .subscribe(subscriber);
  return () => {
  // Release state on finalization
  