Enums can not be used for index signature types

[ChiriVulpes]

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

[zpdDG4gta8XKpMCd]

can we allow string & Whatever for index signatures too?

[RyanCavanaugh]

Approved.

Note that this will not cause nominal enforcement of enum index expressions!

Specifically, [index in UserInterfaceElement]: ColorInfo; will eagerly reduce to "0" | "1" : ColorInfo, so indexing by any other 0- or 1-valued enum value will not cause an error.