Error inferring types of promise function

TypeScript Version: nightly (Version 2.5.0-dev.20170816)

Code

(based on DefinitelyTyped/types/bluebird/index.d.ts)

declare function props<K, V>(x: PromiseLike<Map<K, PromiseLike<V> | V>>): Promise<Map<K, V>>;
declare const input: Promise<Map<number, Promise<string>>>;
const out: Promise<Map<number, string>> = props(input);

Expected behavior:

No error.

Actual behavior:

src/a.ts(3,7): error TS2322: Type 'Promise<Map<number, Promise<string>>>' is not assignable to type 'Promise<Map<number, string>>'.
  Type 'Map<number, Promise<string>>' is not assignable to type 'Map<number, string>'.
    Type 'Promise<string>' is not assignable to type 'string'.

It works if I explicitly specify props<number, string>(input).
The error only reproduces with "lib": ["es6"].

从 issue 中的 TypeScript 复现开始，使用所示的 lib: ["es6"] 设置，并将 props(input) 的推断结果与显式指定类型的调用进行比较。当无需显式类型参数时，推断结果可赋值给 Promise<Map<number, string>>，且报告的错误不再出现时，该 issue 即完成。