Method Borrow

common/memory.h:1289–1294

struct Borrowable {
 protected:
  Borrowed<const T> Borrow() const noexcept {
    static_assert(std::is_base_of_v<Data, T>, "T must be derived from Data");
    const T* const that = static_cast<const T*>(this);
    return Borrowed<const T>(Borrower(static_cast<const Data*>(that)->owner_),
                             that);
  }

  Borrowed<T> Borrow() noexcept {
    static_assert(std::is_base_of_v<Data, T>, "T must be derived from Data");