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.

Beginne damit, das Beispiel im verlinkten TypeScript Playground mit den gemeldeten Versionen zu reproduzieren, und bestätige den inferierten Typ von keyof typeof inst. Verfolge die Verarbeitung von keyof durch den Compiler für die gemischte anonyme Klasse und füge Testabdeckung für das Beispiel hinzu, sodass das Ergebnis 'a' | 'b' statt string | number ist.