Mapped inference types do not work as I understand

[AlexStacker]

Directly show my code, the text description may not be clear.
From the callback function used in the parameters

TypeScript Version: 2.6.2

Code

interface MapType { 
    a: {
        w: number;
        h: number;
    }
    b: {
        s: string;
        n: boolean;
    }
    q: {
        s: string;
        n: boolean;
    }
}

type NS<T, P extends keyof T> = {
    [U in keyof T[P]]: T[P][U] 
}

let n: NS<MapType, 'a' | 'b'>
n.s //Property 's' does not exist on type 'NS<M, "a" | "b">'.
let m: NS<MapType, 'a'>
m.h // (property) h: number

Expected behavior:

let n: NS<MapType, 'a' | 'b'>
n.s // string
n.n // boolean
n.w // number
n.h // number

Actual behavior:

let n: NS<MapType, 'a' | 'b'>
n.s //Property 's' does not exist on type 'NS<M, "a" | "b">'.

Playground Link:

Related Issues:
I think this is very helpful to me,But did not solve my problem.
17930

[mhegazy]

I think you want NS<MapType, "a"> & NS<MapType, "b">

[mhegazy]

You might have better luck sharing this on StackOverflow. This is not a support forum, it is one for reporting compiler/language bugs and suggestions.
If you believe you are running into a compiler bug, please share a minimal repro project that demonstrates the issue you are running into and why you believe this is a compiler bug and not a declaration file issue or library usage problem.

[AlexStacker]

Here's the code that can work:

  type NS<T, P extends keyof T> = {
    [U in keyof T]: T[P]
  }

but I want to get their flattened values because I have this code:

function getFieds({
    a: true,
    q: true
  }, function(res) {
    res.s // string
    res.n // boolean
    res.w // number
    res.h // number
  });

or

function getFieds(['a', 'q'], function(res) {
    res.s // string
    res.n // boolean
    res.w // number
    res.h // number
  });

The first argument's subkey is unknown. I need to write its declaration file for this js function.But I think this should also work it:

type NS<T, P extends keyof T> = {
    [U in keyof T[P]]: T[P][U] 
}

function getFieds<T>(options: T, callback: (res: NS<MapType , keyof T>) => void ): void;

It typescript can solve this problem it?

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.