Method operator()

base/attribute.cc:82–87

      : output_(*output), type_(type) {}

  absl::Status operator()(const Kind& ignored) const {
    // Attributes are represented as a variant, with illegal attribute
    // qualifiers represented with their type as the first alternative.
    return absl::InvalidArgumentError(
        absl::StrCat("Unsupported attribute qualifier ", KindToString(type_)));
  }

  absl::Status operator()(int64_t index) {
    absl::StrAppend(&output_, index);