Error inferring tuple type arguments

TypeScript Version: nightly (2.4.0-dev.20170608)

Code

interface IPromise<T> {
 then<U>(onFulfill: (value: T) => IWhenable<U>): IPromise<U>;
}

interface QPromise<T> {
 then<U>(onFulfill: (value: T) => IWhenable<U>): IPromise<U>;
}

type IWhenable<T> = IPromise<T> | T;

declare function all<A, B>(promises: IWhenable<[IWhenable<A>, IWhenable<B>]>): IPromise<[A, B]>;

declare const prom1: QPromise<[QPromise<string>, QPromise<number>]>;
const x1: IPromise<[string, number]> = all(prom1);

const prom2: IPromise<[IPromise<string>, IPromise<number>]> = prom1;
const x2: IPromise<[string, number]> = all(prom2);

Expected behavior:

Since IPromise and QPromise have identical bodies, I would expect these to both work.

Actual behavior:

x1 fails, x2 succeeds.

src/a.ts(14,7): error TS2322: Type 'IPromise<[QPromise<string>, QPromise<number>]>' is not assignable to type 'IPromise<[string, number]>'.
 Type '[QPromise<string>, QPromise<number>]' is not assignable to type '[string, number]'.
 Type 'QPromise<string>' is not assignable to type 'string'.

[mhegazy]

Seems like a duplicate of #19194

Mohamed Hegazy (@mhegazy) I don't think this is a duplicate -- that issue references the actual Promise definitions from lib, but this is a type inference issue that just happens to use a promise-like type as an example. Here's a simpler repro:

interface IPromise<T> {
 then(onFulfill: (value: T) => void): void;
}
interface QPromise<T> {
 then(onFulfill: (value: T) => void): void;
}
type IWhenable<T> = IPromise<T> | T;

declare function unwrapTwice<T>(p: IWhenable<IWhenable<T>>): T;
declare const qq: QPromise<QPromise<string>>;
const x: string = unwrapTwice(qq); // Error: unwraps once...