Nullable types can't be reconciled when they are parameterized keys

[johnfn]

TypeScript Version: 2.6.2

Code

type Rect = {
 width: number | undefined;
}

class HasARect<T extends Rect> {
 rect: T;

 get<K extends keyof T>(prop: K): T[K] {
 return this.rect[prop];
 }

 getWidth(r: T): number {
 return this.get("width") || 0; // Type 'undefined' is not assignable to type 'number'.
 }
}

Expected behavior:

This is not a type error.

Actual behavior:

Type 'T["width"]' is not assignable to type 'number'.
 Type 'number | undefined' is not assignable to type 'number'.
 Type 'undefined' is not assignable to type 'number'.

[mhegazy]

narrowing does not occur on generic types, we need a higher order type operator to be applied here instead. see #15376 (comment) for more info.

duplicate of #14366

[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.