Incorrect return types for Promise#catch and 0- and 2- arg forms of Promise#then

[jeffreymorlan]

Expression (p: Promise)	Correct type	Current type
p.then()	Promise	Promise<{}>
p.then(b => 1)	Promise	Promise
p.then(b => 1, e => 'error')	Promise<number	string>
p.then(b => 1, e => {})	Promise<number	void>
p.then(b => 1, e => {throw Error()})	Promise	Promise
p.then(b => 1, e => Promise.reject(Error()))	Promise	Promise
p.catch(e => 'error')	Promise<boolean	string>
p.catch(e => {})	Promise<boolean	void>
p.catch(e => {throw Error()})	Promise	Promise
p.catch(e => Promise.reject(Error()))	Promise	Promise

Using overloading and the new never type, PromiseLike/Promise could be typed to handle all these cases correctly:

interface PromiseLike<T> {
    then(): PromiseLike<T>;
    then<R1>(onfulfilled: (value: T) => R1 | PromiseLike<R1>): PromiseLike<R1>;
    then<R1, R2>(onfulfilled: (value: T) => R1 | PromiseLike<R1>,
                 onrejected: (reason: any) => R2 | PromiseLike<R2>): PromiseLike<R1 | R2>;
}

interface Promise<T> {
    then(): Promise<T>;
    then<R1>(onfulfilled: (value: T) => R1 | PromiseLike<R1>): Promise<R1>;
    then<R1, R2>(onfulfilled: (value: T) => R1 | PromiseLike<R1>,
                 onrejected: (reason: any) => R2 | PromiseLike<R2>): Promise<R1 | R2>;
    catch<R2>(onrejected: (reason: any) => R2 | PromiseLike<R2>): Promise<T | R2>;
}

declare var Promise: {
    reject(reason: any): Promise<never>; // Returned promise is never fulfilled
};

[mhegazy]

//cc: @rbuckton

[rbuckton]

There are a few things we need to fix in the checker for properly handling never in these cases with async functions, and there's a breaking change in the definition for PromiseLike and Promise that needs to be resolved, but this seems like the right direction.