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).

Start with the linked TypeScript Playground and reproduce the two constructor-access checks in the supplied mixin and direct-extension examples. Trace constructor accessibility for the class expression returned by the mixin, then add or update a regression test so new NonSubclassableThroughMixin() receives the same private-constructor diagnostic as the direct subclass.