UserPasswordForm::validateForm

public UserPasswordForm::validateForm(array &$form, FormStateInterface $form_state)

public function validateForm(array &$form, FormStateInterface $form_state) {
  $name = trim($form_state->getValue('name'));
  // Try to load by email.
  $users = $this->userStorage->loadByProperties(array('mail' => $name));
  if (empty($users)) {
    // No success, try to load by name.
    $users = $this->userStorage->loadByProperties(array('name' => $name));
  }
  $account = reset($users);
  if ($account && $account->id()) {
    // Blocked accounts cannot request a new password.
    if (!$account->isActive()) {
      $form_state->setErrorByName('name', $this->t('%name is blocked or has not been activated yet.', array('%name' => $name)));
    }
    else {
      $form_state->setValueForElement(array('#parents' => array('account')), $account);
    }
  }
  else {
    $form_state->setErrorByName('name', $this->t('%name is not recognized as a username or an email address.', array('%name' => $name)));
  }
}