public function getDocTypes() {
    // No type specified means type is mixed.
    $types = ['mixed'];
    // Use types from the doc comment if available.
    $doc_comment = $this->getFunction()->getDocComment();
    if (!$doc_comment) {
      return $types;
    }
    $param_tag = $doc_comment->getParameter($this->getName());
    if (!$param_tag) {
      return $types;
    }
    $types = Types::normalize($param_tag->getTypes());
    if (empty($types)) {
      $types[] = 'mixed';
    }
    return $types;
  }