rxjs.ts

const nums$ = fromEvent(document, "click").pipe(
  mapTo(((x: number) => x + 1)),
  scan((acc, whatever) => acc, 0)
);

/*
Argument of type 'MonoTypeOperatorFunction<number>' is not assignable to parameter of type 'OperatorFunction<(x: number) => number, number>'.
  Types of parameters 'source' and 'source' are incompatible.
    Type 'Observable<(x: number) => number>' is not assignable to type 'Observable<number>'.
      Type '(x: number) => number' is not assignable to type 'number'.
*/