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:

Start with the provided Playground link and the minimal numeric-enum reproduction in the issue. Verify the indexed access types against the expected behavior: Enum[Enum.one] should be keyof typeof Enum, while Enum[0] should report that property 0 does not exist on typeof Enum.