Return type not inferred correctly in static method

[SylvainEstevez]

TypeScript Version:
1.8.7

Code

'use strict';

class User {
  firstName: string;
  lastName: string;  

  static getInstance() {
    return new this();
  }
}

class StaffUser extends User {  
  position: string;
}

let staffUser = StaffUser.getInstance();

console.log(staffUser.position); // error TS2339: Property 'position' does not exist on type 'User'

Expected behavior:

staffUser should be of type StaffUser, not simply User.

Actual behavior:
The compiler doesn't guess that getInstance in StaffUser returns instances of StaffUser.

[vladima]

duplicate of #5863