laravel 5.3 aliases not found => undefined class - php

Question

I created a project in laravel 5.2, and moving this to laravel 5.3
For this, I created a brand new laravel 5.3 project folder where I am copying views and controllers to the new project. When trying to get Auth working, I see a lot of "undefined class" errors.
By checking laravel documentation, I saw this, where I am stuck now
https://laravel.com/docs/5.3/authentication#retrieving-the-authenticated-user
so, in app.php I see
'Auth' => Illuminate\Support\Facades\Auth::class,
when in my controller, I get "Undefined class Auth" on this
use Auth;
this "undefined class" is solved by changing that line to
use Illuminate\Support\Facades\Auth;
first thing, why can't I just use "use Auth;" ?
In my controller file, I get "undefined class" error on DB, Auth and File
use DB;
use Auth;
use Illuminate\Support\Facades\Redirect;
use Illuminate\Http\Request;
use File;
in my app.php file, these names are aliased as
'App' => Illuminate\Support\Facades\App::class,
'DB' => Illuminate\Support\Facades\DB::class,
'Auth' => Illuminate\Support\Facades\Auth::class,
so I wonder what could be wrong with the aliases ?
Next to this, when trying to get the authenticated user, as described in the documentation, this line is enough
Auth::user()
Here, "user" throws this error:
Method 'user' not found in \Illuminate\Support\Facades\Auth
I checked around using google, comparing files in my Laravel 5.2 against my Laravel 5.3 folders, but did not see any solutions. Maybe someone here has the answer? It worked in my Laravel 5.2 project as described in the link above.
thank you
UPDATE
just try to rebuild the project by these commands in this order:
composer update
php artisan config:clear
composer dump-autoload
but still no progress