Type safety for enum property names

Suggestion

Enums work in two directions, where an enum property value can be retrieved via AnEnum.aProperty and a property name can be retrieved via AnEnum[aValue]. In TypeScript 3.4.3, the type of the second expression is string. Instead, it should be equivalent to keyof typeof AnEnum | undefined.

Use Cases

While the nameof operator (suggested in #1579) does not exist yet, a construct such as AnEnum[AnEnum.aProperty] can currently be used to safely refer to an enum property name. Unfortunately, since the type of that expression is string it requires a cast when assigning it to a type safe target.

Examples

enum AnEnum { aProperty = 1, anotherProperty = 23, yetAnotherOne = 42 };
let target: keyof typeof AnEnum;

…

// Compilation error here: Type 'string' is not assignable to type '"aProperty" | "anotherProperty" | "yetAnotherOne"'.
target = AnEnum[AnEnum.aProperty];

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

Start with the TypeScript 3.4.3 example and trace how the checker types the indexed access in AnEnum[AnEnum.aProperty]. Done means the expression is accepted when assigned to keyof typeof AnEnum, includes undefined as appropriate, and does not change emitted JavaScript.