NonNullable isn't narrowing down object values' types for optional properties

TypeScript Version: 3.2.0-dev.20181106

Search Terms:
NonNullable object
NonNullable object values

Code

Run the following code via tsc --no-emit --strict test.ts:

interface P {
  color?: 'red' | 'green';
}

type RequiredP = {
  [K in keyof P]: NonNullable<P[K]>;
}

declare const p: RequiredP;
const color: 'red' | 'green' = p.color;

Expected behavior:

The resulting type should not allow undefined for the value at a property color.

Actual behavior:

undefined is still allowed. Using the NonNullable type doesn't seem to have any effect.

Playground Link: Note: you need to enable strictNullChecks manually!
https://www.typescriptlang.org/play/#src=interface%20P%20%7B%0D%0A%20%20color%3F%3A%20'red'%20%7C%20'green'%3B%0D%0A%7D%0D%0A%0D%0Atype%20RequiredP%20%3D%20%7B%0D%0A%20%20%5BK%20in%20keyof%20P%5D%3A%20NonNullable%3CP%5BK%5D%3E%3B%0D%0A%7D%0D%0A%0D%0Adeclare%20const%20p%3A%20RequiredP%3B%0D%0Aconst%20color%3A%20'red'%20%7C%20'green'%20%3D%20p.color%3B%0D%0A

Related Issues:

TypeScript 3.2.0-dev.20181106 を使い、tsc --no-emit --strict test.ts で例を再現してから、現在のコンパイラおよびリンク先の Playground と比較します。マップ型、オプショナルプロパティ、NonNullable がどのように扱われるかを調査します。宣言された color の代入で undefined が拒否され、型 'red' | 'green' が維持されれば完了です。