Feature Request: Process of Elimination for Unions

[Gaelan]

This should compile without errors:

declare function getDBObjectById(id: number): DBObject;
declare function bar(arg: DBObject): void;
function foo(arg: number | DBObject) {
  if (arg instanceof DBObject) {
    bar(arg)
  } else {
    bar(getDBObjectById(arg) // error TS2345: Argument of type 'number | DBObject' is not assignable to parameter of type 'number'.
                             // Type 'DBObject' is not assignable to type 'number'.
  }
}

[mhegazy]

looks like a duplicate of #1719

[mhegazy]

also note that typeof arg !== "number" will work as you expect.