Laravel 5.5 cannot make validator in a form Request class

I want to check if a form input 'departement' is filled only if two 'villes' have the same name.

within controller this code wokds perfectly :

 $rules=[  'nom' => 'required', 'ville'=> 'required|exists:villes,nom'];
 $messages = [
      'depart.required' => 'Deux villes portent le même nom, preciser le 
 département'];
  $validator = Validator::make($request->All(), $rules,$messages);
  $validator->sometimes('depart_id', 'required|exists:departs,id', function 
  ($input) {
      return Ville::where('nom',$input->ville)->count()>1;
  });
  if ($validator->fails()) {
    return redirect('admin/etab/create')
            ->withErrors($validator)
            ->withInput();
    }

I put the same code in a Form Request class:

public function rules()
{
  $rules=[  'nom' => 'required', 'ville'=> 'required|exists:villes,nom'];
  $messages = [
      'depart.required' => 'Deux villes portent le même nom, preciser le 
  département',
  ];
  $validator = Validator::make($this->All(), $rules,$messages);
  $validator->sometimes('depart_id', 'required|exists:departs,id', function 
  ($input) {
      return Ville::where('nom',$input->ville)->count()>1;
  });
  return $validator;
}

I get "Type error: Argument 2 passed to Illuminate\Validation\Factory::make() must be of the type array, object given," I think error message is inadequate but I cannot find why this way does not work

Thanks ......

via Chebli Mohamed