Function has_value

common/constant.h:225–227

  }

  ABSL_MUST_USE_RESULT bool has_value() const {
    return !absl::holds_alternative<absl::monostate>(kind());
  }

  ABSL_MUST_USE_RESULT bool has_null_value() const {
    return absl::holds_alternative<std::nullptr_t>(kind());