Generating type definitions for mixin classes with protected members

TypeScript Version: 2.4.2

Code:

I'm using mixins as described by: https://github.com/Microsoft/TypeScript/pull/13743

export type Constructor<T> = new(...args: any[]) => T;
export function Unsubscriber<T extends Constructor<{}>>(Base: T)  {
  class Unsubscriber extends Base implements OnDestroy {
    protected unsubscribe: Subject<void> = new Subject();

    ngOnDestroy() {
      this.unsubscribe.next();
      this.unsubscribe.complete();
    }
  }
  return Unsubscriber;
}

If I compile this code with "declaration": true to get type definitions for my library, I get the following error:

ERROR in (truncated)/mixins.ts (...): Return type of exported function has or is using private name 'Unsubscriber'.

One solution is to add an interface...

export interface IUnsubscriber extends OnDestroy {
  unsubscribe: Subject<void>;
}

...and have my mixin function have a return type of Constructor<IUnsubscriber>. This works, but it forces me to make the properties/methods exposed by my mixin be public even in cases where I want them to be protected.

Short of adding protected members to interfaces (which I'm not sure is the right thing to do), this seems to be a limitation of the currently supported mixin strategy.

Start with the mixins.ts example in the issue and compile it with declaration generation enabled to reproduce the private-name error. Read the mixin and declaration-generation or type-checking paths involved, then verify that exported mixins with protected members produce usable type definitions without requiring those members to be public.