observable_constructor

  class Observable<T> implements Subscribable<T> {
  ...
  
    constructor(subscribe?: (this: Observable<T>, subscriber: Subscriber<T>) => TeardownLogic) {
      if (subscribe) {
        this._subscribe = subscribe;
      }
    }
  }
  
  ...
  
  type TeardownLogic = Unsubscribable | Function | void;