Type of reverse mappings of numeric enums is a `string` instead of `keyof typeof Enum`

TypeScript Version: 4.0.0-dev.20200526

Search Terms: enum numeric enum reverse enum

Code

enum Enum {
  one = 1,
  two = 2,
}

const a: keyof typeof Enum = Enum[Enum.one]; // (1)
const b = Enum[0]; // (2)
console.log(a, b);

Expected behavior: When a numeric enum is indexed by itself the value should be of type keyof typeof Enum: No error on line (1). Error Property '0' does not exist on type 'typeof Enum' on line (2).

Actual behavior: When a numeric enum is indexed by a number (including itself) the value is of type string: Error Type 'string' is not assignable to type '"one" | "two"' on line (1). No error on line (2).

Playground Link: Provided

Related Issues:

[markm77]

I can certainly agree that enum reverse mappings should have type keyof typeof Enum rather than string. This loses information and can necessitate an unnecessary cast.

[majellin24]

I would like to see this feature as well