Missing type error for invalid use of #private field

Bug Report

ES #private fields are not source-positional, but scoped to each instance of a class. (That is unfortunate, in my humble opinion). However TypeScript is not detecting this case.

🔎 Search Terms

typescript no type error private field mixin

🕗 Version & Regression Information

4.3-beta (I didn't try earlier)

⏯ Playground Link

Playground link with relevant code

💻 Code

type Ctor<T = {}> = new (...args: any[]) => T

function FooMixin<C extends Ctor>(Base: C) {
  return class Foo extends Base {
    #foo = 123
    test(foo: Foo) {
      console.log(foo.#foo)
    }
  }
}

class Cat extends FooMixin(Object) {}
class Dog extends FooMixin(Object) {}

const c = new Cat
c.test(new Dog) // Should there be a type error here, perhaps?

🙁 Actual behavior

No type error, but only a runtime error.

🙂 Expected behavior

There should perhaps also be a type error, if that's possible.

Mixin docs

The TypeScript Mixin documentation advertises that people can use #private fields with class-factory mixins:

// Mixins may not declare private/protected properties
// however, you can use ES2020 private fields

This may lead people to the try them and they'll have this issue.

Beginne mit dem verlinkten TypeScript Playground und reproduziere das Mixin-Beispiel mit #private-Feldern, FooMixin, Cat und Dog. Untersuche das Verhalten der Typprüfung im Zusammenhang mit der Identität privater Felder und bestimme, welche Diagnose, falls überhaupt, für c.test(new Dog) ausgegeben werden sollte; die Aufgabe ist erst erledigt, wenn eine abgestimmte Semantik und entsprechende Regressionstests vorhanden sind.