PromiseLike.then returns `PromiseLike<Promise<T>>` instead of `PromiseLike<T>`

TypeScript Version: nightly (2.6.0-dev.20170817)

Code

declare const p: PromiseLike<string>;
const x: PromiseLike<number> = p.then(() => Promise.resolve(1));

Expected behavior:

No error.

Actual behavior:

src/a.ts(2,7): error TS2322: Type 'PromiseLike<Promise<number>>' is not assignable to type 'PromiseLike<number>'.
  Type 'Promise<number>' is not assignable to type 'number'.

Discovered in DefinitelyTyped/types/xadesjs.

[spion]

FYI this is now also happening in mainline TS 2.6.2

[jakebailey]

Old issue, but this was fixed back in #40002.