As you’ve used it, combineLatest isn’t really an operator; it’s an observable creator. And it can be imported directly, like this:

import { combineLatest } from "rxjs/observable/combineLatest";
const c = combineLatest(a, b);

Also, if a is an observable, there is already a combineLatest lettable operator that you could use like this:

import { combineLatest } from "rxjs/operators/combineLatest";
const c = a.pipe(combineLatest(b));