Mixin classes with private/protected constructors are subclassable

Bug Report

🔎 Search Terms

mixin, private constructor

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about Classes.
Versions tried:

• 3.7.4
• 4.1.3
• Nightly

⏯ Playground Link

Playground link with relevant code

💻 Code

function PreventSubclassingMixin<TBase extends new(...args: any[]) => any>(Base: TBase) {
  return class NonSubclassable extends Base {
    private constructor(...args: any[]) {
      super();
    }
  }
}

class Base {
  constructor() {}
}

class NonSubclassableThroughExtension extends Base {
  private constructor() {
    super();
  }
}

const NonSubclassableThroughMixin = PreventSubclassingMixin(Base);

new NonSubclassableThroughExtension(); // Constructor of class 'NonSubclassableThroughExtension' is private and only accessible within the class declaration.(2673)
new NonSubclassableThroughMixin(); // Doesn't throw an error

🙁 Actual behavior

NonSubclassableThroughExtension correctly throw an error — I've extended a superclass Base, which has a publicly accessible constructor, and provided a private constructor in the derived class. I can't access the derived class' constructor anymore:

new NonSubclassableThroughExtension(); // Constructor of class 'NonSubclassableThroughExtension' is private and only accessible within the class declaration.(2673)

The PreventSubclassingMixin supposedly works the same — it creates a class expression that extends the Base class with a public constructor and defines a private constructor, and returns it. The returned derived mixin class is supposedly identical to the example above. However, if I apply the mixin to the Base: const NonSubclassableThroughMixin = PreventSubclassingMixin(Base), and try to initialize it, Typescript won't throw any error.

🙂 Expected behavior

new NonSubclassableThroughMixin() should throw a Constructor of class 'NonSubclassableThroughExtension' is private and only accessible within the class declaration.(2673).

Beginne mit dem verknüpften TypeScript Playground und reproduziere die beiden Konstruktorzugriffsprüfungen in den bereitgestellten Mixin- und Direkt-Erweiterungsbeispielen. Verfolge die Konstruktorzugänglichkeit für den vom Mixin zurückgegebenen Klassenausdruck und füge dann einen Regressionstest hinzu oder aktualisiere ihn, sodass new NonSubclassableThroughMixin() dieselbe Diagnose für einen privaten Konstruktor erhält wie die direkte Unterklasse.