keyof behavior in 2.9

[domoritz]

TypeScript Version: 2.9

Code

export interface Dict<T> {
  [key: string]: T;
}

type a = Dict<boolean>;

type b = keyof a;

Expected behavior:

b should be string

Actual behavior:

b is string | number

Maybe there is a simple fix to this but I cannot find anything. My temporary fix is to use "keyofStringsOnly": true.

Playground Link:

http://www.typescriptlang.org/play/#src=export%20interface%20Dict%3CT%3E%20%7B%0D%0A%20%20%5Bkey%3A%20string%5D%3A%20T%3B%0D%0A%7D%0D%0A%0D%0Atype%20a%20%3D%20Dict%3Cboolean%3E%3B%0D%0A%0D%0Atype%20b%20%3D%20keyof%20a%3B

Related Issues:

#24560 may be the same but I cannot tell.

[domoritz]

My use case works with Extract.

export const keys = Object.keys as <T>(o: T) => (Extract<keyof T, string>)[];

#12253