keyof of 'mixed' class equals to `string | number`

TypeScript Version: 3.4.3 or 3.5.0-dev.20190427

Search Terms: keyof, anonymous class

Code

type AnyClass = new (...args: any[]) => any;

const mixin = <T extends AnyClass>(Class: T) => class extends Class {a: string};
const Cl = mixin(class {b = 123});
const inst = new Cl();

inst. //  <- IntelliSense works fine here and suggests 'a' and 'b' properties

type Keys = keyof typeof inst; // Keys = string | number

Expected behavior:
Keys to be equal to 'a' | 'b'

Actual behavior:
Keys to be equal to string | number

Playground Link: ts-playground

Description:
I'm sorry if that is a known bug ot expected behaviour. I did not find a duplicate.
Class C is a result of extension of some other class A provided as a function argument and class B which is declared inside the function body. Such class C works fine for the most part, but keyof InstanceType<typeof C> returns string | number.

While IntelliSense works fine:

I'll be glad if you suggest any workarounds for this issue.

Inizia riproducendo l'esempio nel TypeScript Playground collegato con le versioni indicate e conferma il tipo inferito di keyof typeof inst. Traccia la gestione di keyof da parte del compilatore per la classe anonima mista e aggiungi la copertura per l'esempio, in modo che il risultato sia 'a' | 'b' anziché string | number.