Allow for indexable types other than `string` and `number`

[aryzing]

Given the following definition of a user and a user map,

type UserId = string;

interface User {
  id: UserId;
  name: string;
  email: string;
}

interface UserMap {
  [id: string]: User;
}

let userMap: UserMap = {
  id0001: {
    id: 'id0001',
    name: 'person1',
    email: 'person1@example.com',
  },
};

We can insert another user into the map by using a variable of type UserId,

let anotherUserId: UserId = 'id0002';

userMap[anotherUserId] = {
  id: anotherUserId,
  name: 'person2',
  email: 'person2@example.com',
}

However, I was surprised to find out that the indexable type can't be defined using the UserId type,

interface UserMap {
  [id: UserId]: User // error!
}

I'm sure we would all appreciate the certainty of knowing that the value we're using as an index is of the proper type, to avoid situations like

let productId: ProductId = 'prod123'

userMap[productId] = {/*...*/}

Is this a viable feature to include in TypeScript?

[RyanCavanaugh]

See #7374 and its many linked duplicates

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.