Angular Error TS2554:Expected 0 arguments, but got x on piped operators

I have an Error TS2554: Expected 0 arguments, but got 4 in the pipe of the observable getHappyDays()

getHappyDays() Observable returns a Observable<HttpResponse<IHappyDays>> | Observable<HttpErrorResponse> , I've included a stackblitze to better showcase the problem

resolve(route: ActivatedRouteSnapshot, state: RouterStateSnapshot):
Observable<IHappyDays> | Observable<never> {

return this.happyService.getHappyDays()
.pipe(//=>error thrown here
first(),
mergeMap((res) => {
return of(res.body)
})
)
}


https://stackblitz.com/edit/angular-3iujhb // in happy-resolver.service

Continue reading...