TypeScript Version: 2.1.4

export interface UserInterfaceColors {
    [index in UserInterfaceElement]: ColorInfo;
}
export interface ColorInfo {
    r: number;
    g: number;
    b: number;
    a: number;
}
export enum UserInterfaceElement {
    ActiveTitleBar = 0,
    InactiveTitleBar = 1,
}

Expected behavior:
No errors will be thrown.

Actual behavior:
index in UserInterfaceElement throws the error Type 'UserInterfaceElement' is not assignable to type 'string'

This is a regression caused by #12425. The change makes sense, but there are very valid use cases for supporting enums in index signatures.

For another use case, in a project I'm working on, we use an enum to list all types of items. We have a function that takes an object with the enum values as keys, and data as values.

function loadItemData (data: { [index in ItemType]: string }): void;

A workaround is to use index: number, but then it's not enforced that you have to use a valid ItemType as a key.

Original Issue: #2491