function getDate(): Promise<Date> {
    return Promise.resolve(new Date());
}

function getFoo(): Promise<{}> {
    return Promise.resolve({});
}

function doFoo() : Promise<number> {

    // Error: Type 'Promise<Date>' is not assignable to type 'Promise<number>'.
    return (
        getDate()
            .then(() => {

                return (
                    getFoo()
                        .then(() => {

                            return 10;
                        })
                );
            })
    );
}