Cannot subscript a value of [AnyObject]? with an index of type Int

The problem isn't the cast, but the fact that self.objects seems to be an optional array: [AnyObject]?.
Therefore, if you want to access one of its values via a subscript, you have to unwrap the array first:

var user2: PFUser
if let userObject = self.objects?[indexPath.row] {
    user2 = userObject as! PFUser
} else {
    // Handle the case of `self.objects` being `nil`.
}

The expression self.objects?[indexPath.row] uses optional chaining to first unwrap self.objects, and then call its subscript.

---

As of Swift 2, you could also use the guard statement:

var user2: PFUser
guard let userObject = self.objects?[indexPath.row] else {
    // Handle the case of `self.objects` being `nil` and exit the current scope.
}
user2 = userObject as! PFUser

I ran into the same issue and resolved it like this:

let scope : String = searchBar.scopeButtonTitles![searchBar.selectedScopeButtonIndex] as! String

For your case, you might do:

var user2 : PFUser = self.objects![indexPath.row] as! PFUser