Resolution of `Record`-typed objects that extend `string` resolve to `any`

[alecf]

I'm using a style of nominal typing as described here: https://spin.atomicobject.com/2018/01/15/typescript-flexible-nominal-typing/

But I find when I have a object of type Record<NominalStringType, OtherType>, then resolving a key in that object always resolves to any
TypeScript Version: 4.0.3

Search Terms: Record Flavor Nominal String

Code

interface Flavoring<FlavorT> {
  _type?: FlavorT;
}
type FlavorString<FlavorT> = string & Flavoring<FlavorT>;
type MyString = FlavorString<'MyString'>;

// Create a record type using this nominal type
type RS<T extends string> = Record<T, number>;

// define an instance of this
const r: RS<MyString> = { foo: 1};

// `v` is typed as 'any' even though we know `Record<T, number>[K]` should be a number
const v = r['foo'];

// we can also try with an instance of the nominal type:
const foo: MyString = 'foo';

// `v2` still is type as `any`
const v2 = r[foo];

Expected behavior:

v and v2 above should be typed as a number

Actual behavior:

v and v2 are typed as any

Playground Link:

https://www.typescriptlang.org/play?#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

Related Issues:

This looks possibly related: #36981

[andrewbranch]

Duplicate of #40193

[typescript-bot]

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