Promise type inference issue with supertypes

[rkirov]

TypeScript Version: 2.1.4

Code

declare let p: Promise<number>;
declare let p2: Promise<number|string>;

// TS infers pfinal to be Promise<number> incorrectly.
// pfinal should be Promise<number|string>
let pfinal = p.then(_ => p2);

Expected behavior:
Pfinal should be Promise<number|string>

Actual behavior:
Pfinal is inferred to be Promise<number>

Note any choice of Promise<T> and Promise<S> where S is a supertype of T will trigger this error.

This happens because then function overload then(onfulfilled?: ((value: T) => T | PromiseLike<T>) | undefined | null, ...): Promise<T>; is matching before the one with the free type parameter TResult.

As far I can tell it is a consequence of the fix in #10524 (comment) and not a desired change.

//cc @calebegg

[aluanhaddad]

This is related to #13015, #13008 and in particular this comment from the latter #13008 (comment)
it seems the overload

then(
    onfulfilled?: ((value: T) => T | PromiseLike<T>) | undefined | null, 
    onrejected?: ((reason: any) => T | PromiseLike<T>) | undefined | null
): Promise<T>;

is not only redundant but is causing this specific bug.

[rkirov]

Agree, thanks for reference, closing in favor of #10977 as the canonical bug for this.