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(C++23)
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 , expected,
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(C++23)
 expected
() () [] 

namespace std {
  //   unexpected
  template<class E> class unexpected;

  //   bad_expected_access
  template<class E> class bad_expected_access;

  //  bad_expected_access  void
  template<> class bad_expected_access<void>;

  //     unexpected
  struct unexpect_t {
    explicit unexpect_t() = default;
  };
  inline constexpr unexpect_t unexpect{};

  //   expected
  template<class T, class E> class expected;

  //   expected   void
  template<class T, class E> requires is_void_v<T> class expected<T, E>;
}

std::unexpected

namespace std {
  template<class E>
  class unexpected {
  public:
    // 
    constexpr unexpected(const unexpected&) = default;
    constexpr unexpected(unexpected&&) = default;
    template<class... Args>
      constexpr explicit unexpected(in_place_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit unexpected(in_place_t, initializer_list<U>, Args&&...);
    template<class Err = E>
      constexpr explicit unexpected(Err&&);
    
    // 
    constexpr unexpected& operator=(const unexpected&) = default;
    constexpr unexpected& operator=(unexpected&&) = default;
  
    // 
    constexpr const E& error() const& noexcept;
    constexpr E& error() & noexcept;
    constexpr const E&& error() const&& noexcept;
    constexpr E&& error() && noexcept;

    // 
    constexpr void swap(unexpected& other) noexcept(/*   */);

    friend constexpr void swap(unexpected& x, unexpected& y) noexcept(noexcept(x.swap(y)));

    //  
    template<class E2>
      friend constexpr bool operator==(const unexpected&, const unexpected<E2>&);

  private:
    E unex;    //   
  };

  template<class E> 
    unexpected(E) -> unexpected<E>;
}

std::bad_expected_access

namespace std {
  template<class E>
  class bad_expected_access : public bad_expected_access<void> {
  public:

    //  
    explicit bad_expected_access(E);

    // 
    const char* what() const noexcept override;
    E& error() & noexcept;
    const E& error() const& noexcept;
    E&& error() && noexcept;
    const E&&  error() const&& noexcept;

  private:
    E unex;              //   
  };
}

std::bad_expected_access<void>

namespace std {
  template<>
  class bad_expected_access<void> : public exception {
  protected:
    // 
    bad_expected_access() noexcept;
    bad_expected_access(const bad_expected_access&);
    bad_expected_access(bad_expected_access&&);
    bad_expected_access& operator=(const bad_expected_access&);
    bad_expected_access& operator=(bad_expected_access&&);

    ~bad_expected_access();

  public:
    const char* what() const noexcept override;
  };
}

std::expected

namespace std {
  template<class T, class E>
  class expected {
  public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;

    template<class U>
    using rebind = expected<U, error_type>;

    // 
    constexpr expected();
    constexpr explicit(/*   */) 
      expected(const expected&);
    constexpr explicit(/*   */) 
      expected(expected&&) noexcept(/*   */);
    template<class U, class G>
      constexpr explicit(/*   */) expected(const expected<U, G>&);
    template<class U, class G>
      constexpr explicit(/*   */) expected(expected<U, G>&&);

    template<class U = T>
      constexpr explicit(/*   */) expected(U&& v);

    template<class G>
      constexpr expected(const unexpected<G>&);
    template<class G>
      constexpr expected(unexpected<G>&&);

    template<class... Args>
      constexpr explicit expected(in_place_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(in_place_t, initializer_list<U>, Args&&...);
    template<class... Args>
      constexpr explicit expected(unexpect_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);

    // 
    constexpr ~expected();

    // 
    constexpr expected& operator=(const expected&);
    constexpr expected& operator=(expected&&) noexcept(/*   */);
    template<class U = T> constexpr expected& operator=(U&&);
    template<class G>
      constexpr expected& operator=(const unexpected<G>&);
    template<class G>
      constexpr expected& operator=(unexpected<G>&&);

    template<class... Args>
      constexpr T& emplace(Args&&...) noexcept;
    template<class U, class... Args>
      constexpr T& emplace(initializer_list<U>, Args&&...) noexcept;

    // 
    constexpr void swap(expected&) noexcept(/*   */);
    friend constexpr void swap(expected&, expected&) noexcept(/*   */);

    // 
    constexpr const T* operator->() const noexcept;
    constexpr T* operator->() noexcept;
    constexpr const T& operator*() const& noexcept;
    constexpr T& operator*() & noexcept;
    constexpr const T&& operator*() const&& noexcept;
    constexpr T&& operator*() && noexcept;
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr const T& value() const&;
    constexpr T& value() &;
    constexpr const T&& value() const&&;
    constexpr T&& value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const&&;
    constexpr E&& error() &&;
    template<class U> constexpr T value_or(U&&) const&;
    template<class U> constexpr T value_or(U&&) &&;

    //  
    template<class T2, class E2>
      friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
    template<class T2>
      friend constexpr bool operator==(const expected&, const T2&);
    template<class E2>
      friend constexpr bool operator==(const expected&, const unexpected<E2>&);

  private:
    bool has_val;       //   
    union {
      T val;            //   
      E unex;           //   
    };
  };
}

std::expected void

namespace std {
  template<class T, class E> requires is_void_v<T>
  class expected<T, E> {
  public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;

    template<class U>
    using rebind = expected<U, error_type>;

    // 
    constexpr expected() noexcept;
    constexpr explicit(/*   */)
      expected(const expected&);
    constexpr explicit(/*   */)
      expected(expected&&) noexcept(/*   */);
    template<class U, class G>
      constexpr explicit(/*   */) expected(const expected<U, G>&);
    template<class U, class G>
      constexpr explicit(/*   */) expected(expected<U, G>&&);

    template<class G>
      constexpr expected(const unexpected<G>&);
    template<class G>
      constexpr expected(unexpected<G>&&);

    constexpr explicit expected(in_place_t) noexcept;
    template<class... Args>
      constexpr explicit expected(unexpect_t, Args&&...);
    template<class U, class... Args>
      constexpr explicit expected(unexpect_t, initializer_list<U>, Args&&...);

    // 
    constexpr ~expected();

    // 
    constexpr expected& operator=(const expected&);
    constexpr expected& operator=(expected&&) noexcept(/*   */);
    template<class G>
      constexpr expected& operator=(const unexpected<G>&);
    template<class G>
      constexpr expected& operator=(unexpected<G>&&);
    constexpr void emplace() noexcept;

    // 
    constexpr void swap(expected&) noexcept(/*   */);
    friend constexpr void swap(expected&, expected&) noexcept(/*   */);

    // 
    constexpr explicit operator bool() const noexcept;
    constexpr bool has_value() const noexcept;
    constexpr void operator*() const noexcept;
    constexpr void value() const&;
    constexpr void value() &&;
    constexpr const E& error() const&;
    constexpr E& error() &;
    constexpr const E&& error() const&&;
    constexpr E&& error() &&;

    //  
    template<class T2, class E2> requires is_void_v<T2>
      friend constexpr bool operator==(const expected& x, const expected<T2, E2>& y);
    template<class E2>
      friend constexpr bool operator==(const expected&, const unexpected<E2>&);

  private:
    bool has_val;        //   
    union {
      E unex;            //   
    };
  };
}
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