UserLoginForm::submitForm

public UserLoginForm::submitForm(array &$form, FormStateInterface $form_state)

public function submitForm(array &$form, FormStateInterface $form_state) {
  $account = $this->userStorage->load($form_state->get('uid'));

  // A destination was set, probably on an exception controller,
  if (!$this->getRequest()->request->has('destination')) {
    $form_state->setRedirect(
    'entity.user.canonical', 
    array('user' => $account->id())
    );
  }
  else {
    $this->getRequest()->query->set('destination', $this->getRequest()->request->get('destination'));
  }

  user_login_finalize($account);
}