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.

Commencez par reproduire l’exemple dans le TypeScript Playground lié avec les versions indiquées et confirmez le type inféré de keyof typeof inst. Suivez le traitement de keyof par le compilateur pour la classe anonyme mixte et ajoutez une couverture pour l’exemple afin que le résultat soit 'a' | 'b' plutôt que string | number.