How to evaluate the return type of a function dynamically?

Question

I've got the following code:

// some interfaces to check
interface Car {
    name: string;
    color: string;
}

interface Bike {
    wheels: number;
}

// magic function that simplifies things
function getItems<Fn extends (...args: any[]) => ReturnType<Fn>>(  // <- this apparantly doesn't work (ReturnType<Fn>)
  getItemsFunc: Fn,
  item: ReturnType<Fn> | null
) {

  const getItem = () => {
    getItemsFunc().then((newItem) => (item = newItem));  // typescript complains here but should not complain here but in the last line
  };

   getItem();
}

// api call function that returns a promise. functions like this will be put into getItems()
function getCars(): Promise<Car[]> {
    // this would be an api call
    return new Promise(function (resolve) {
        resolve([{name: 'Mustang', color: 'red'}])
    })
}

// getItems will be used like this:
const cars: Car[] | null = null;
getItems(getCars, cars);

// i want typescript to complain about this:
const bikes: Bike[] | null = null;
getItems(getCars, bikes);  // <- getCars and bikes don't fit each other based on the types

How can I make typescript complain about the last line? But it should not complain about anything else.

I've put the code into the playground, see: Typescript Playground