expected.hpp

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#pragma once

 
#include <exception>
#include <functional>
#include <system_error>
#include <type_traits>
#include <utility>
 
#if defined(__has_include)
#    if __has_include(<version>)
#        include <version>
#    elif __has_include(<experimental/version>)
#        include <experimental/version>
#    endif
#endif
 
#if defined(__cpp_lib_expected) && __cpp_lib_expected >= 202202L
#    include <expected>
#    define THREADSCHEDULE_HAS_STD_EXPECTED 1
#elif (defined(__cplusplus) && __cplusplus >= 202302L) || (defined(_MSVC_LANG) && _MSVC_LANG >= 202302L)
#    include <expected>
#    define THREADSCHEDULE_HAS_STD_EXPECTED 1
#else
#    define THREADSCHEDULE_HAS_STD_EXPECTED 0
#endif
 
// Exception handling control
// Automatically detects if exceptions are available using __cpp_exceptions
// When exceptions are disabled (e.g., with -fno-exceptions):
// - value() will call std::terminate() if accessed in an error state
// - Prefer value_or(), operator*, or check has_value() before accessing the
// value
// - Compatible with exception-free builds
#ifdef __cpp_exceptions
#    define THREADSCHEDULE_EXPECTED_THROW(ex) throw ex
#else
#    define THREADSCHEDULE_EXPECTED_THROW(ex) ::std::terminate()
#endif
 
namespace threadschedule
{
 
#if THREADSCHEDULE_HAS_STD_EXPECTED
template <typename E>
using unexpected = std::unexpected<E>;
using unexpect_t = std::unexpect_t;
inline constexpr unexpect_t unexpect{};
template <typename E>
using bad_expected_access = std::bad_expected_access<E>;
template <typename T, typename E = std::error_code>
using expected = std::expected<T, E>;
 
#else

struct unexpect_t
{
    explicit unexpect_t() = default;
};
inline constexpr unexpect_t unexpect{};
 
template <typename E>
class bad_expected_access;

template <>
class bad_expected_access<void> : public std::exception
{
  public:
    bad_expected_access() = default;
    [[nodiscard]] auto what() const noexcept -> char const* override
    {
        return "bad expected access";
    }
};

template <typename E>
class bad_expected_access : public bad_expected_access<void>
{
  public:
    explicit bad_expected_access(E e) : error_(std::move(e))
    {
    }
    [[nodiscard]] auto error() const& noexcept -> E const&
    {
        return error_;
    }
    auto error() & noexcept -> E&
    {
        return error_;
    }
    [[nodiscard]] auto error() const&& noexcept -> E const&&
    {
        return std::move(error_);
    }
    auto error() && noexcept -> E&&
    {
        return std::move(error_);
    }
 
  private:
    E error_;
};

template <typename E>
class unexpected
{
  public:
    constexpr explicit unexpected(E const& e) : error_(e)
    {
    }
    constexpr explicit unexpected(E&& e) : error_(std::move(e))
    {
    }
    [[nodiscard]] constexpr auto error() const& noexcept -> E const&
    {
        return error_;
    }
    constexpr auto error() & noexcept -> E&
    {
        return error_;
    }
    constexpr auto error() && noexcept -> E&&
    {
        return std::move(error_);
    }
 
  private:
    E error_;
};

template <typename T, typename E = std::error_code>
class expected
{
  public:
    using value_type = T;
    using error_type = E;
    using unexpected_type = unexpected<E>;
 
    // constructors
    template <typename U = T, typename std::enable_if_t<std::is_default_constructible_v<U>, int> = 0>
    constexpr expected() : has_(true)
    {
        new (&storage_.value_) T();
    }
 
    template <typename U = T, typename std::enable_if_t<!std::is_default_constructible_v<U>, int> = 0>
    constexpr expected() = delete;
 
    constexpr expected(expected const& other) : has_(other.has_)
    {
        if (has_)
            new (&storage_.value_) T(other.storage_.value_);
        else
            new (&storage_.error_) E(other.storage_.error_);
    }
 
    constexpr expected(expected&& other) noexcept(std::is_nothrow_move_constructible_v<T> &&
                                                  std::is_nothrow_move_constructible_v<E>)
        : has_(other.has_)
    {
        if (has_)
            new (&storage_.value_) T(std::move(other.storage_.value_));
        else
            new (&storage_.error_) E(std::move(other.storage_.error_));
    }
 
    template <typename U = T,
              typename = std::enable_if_t<
                  !std::is_same_v<std::decay_t<U>, expected> && !std::is_same_v<std::decay_t<U>, std::in_place_t> &&
                  !std::is_same_v<std::decay_t<U>, unexpected<E>> && std::is_constructible_v<T, U>>>
#    if __cplusplus >= 202002L
    constexpr explicit(!std::is_convertible_v<U, T>) expected(U&& value) : has_(true)
#    else
    constexpr expected(U&& value, std::enable_if_t<std::is_convertible_v<U, T>, int> /*unused*/ = 0) : has_(true)
    {
        new (&storage_.value_) T(std::forward<U>(value));
    }
 
    template <typename U = T,
              typename = std::enable_if_t<!std::is_same_v<std::decay_t<U>, expected> &&
                                          !std::is_same_v<std::decay_t<U>, std::in_place_t> &&
                                          !std::is_same_v<std::decay_t<U>, unexpected<E>> &&
                                          std::is_constructible_v<T, U> && !std::is_convertible_v<U, T>>>
    constexpr explicit expected(U&& value) : has_(true)
#    endif
    {
        new (&storage_.value_) T(std::forward<U>(value));
    }
 
    template <typename... Args, typename = std::enable_if_t<std::is_constructible_v<T, Args...>>>
    constexpr explicit expected(std::in_place_t /*unused*/, Args&&... args) : has_(true)
    {
        new (&storage_.value_) T(std::forward<Args>(args)...);
    }
 
    constexpr expected(unexpected<E> const& ue) : has_(false)
    {
        new (&storage_.error_) E(ue.error());
    }
 
    constexpr expected(unexpected<E>&& ue) : has_(false)
    {
        new (&storage_.error_) E(std::move(ue.error()));
    }
 
    template <typename... Args>
    constexpr explicit expected(unexpect_t /*unused*/, Args&&... args) : has_(false)
    {
        new (&storage_.error_) E(std::forward<Args>(args)...);
    }
 
    // assignment operators
    constexpr auto operator=(expected const& other) -> expected&
    {
        if (this == &other)
            return *this;
        this->~expected();
        new (this) expected(other);
        return *this;
    }
 
    constexpr auto operator=(expected&& other) noexcept(std::is_nothrow_move_constructible_v<T> &&
                                                        std::is_nothrow_move_constructible_v<E>) -> expected&
    {
        if (this == &other)
            return *this;
        this->~expected();
        new (this) expected(std::move(other));
        return *this;
    }
 
    template <typename U = T,
              typename = std::enable_if_t<!std::is_same_v<std::decay_t<U>, expected> && std::is_constructible_v<T, U>>>
    constexpr auto operator=(U&& value) -> expected&
    {
        if (has_)
        {
            storage_.value_.~T();
            new (&storage_.value_) T(std::forward<U>(value));
        }
        else
        {
            this->~expected();
            new (this) expected(std::forward<U>(value));
        }
        return *this;
    }
 
    constexpr auto operator=(unexpected<E> const& ue) -> expected&
    {
        if (!has_)
        {
            storage_.error_ = ue.error();
        }
        else
        {
            this->~expected();
            new (this) expected(ue);
        }
        return *this;
    }
 
    constexpr auto operator=(unexpected<E>&& ue) -> expected&
    {
        if (!has_)
        {
            storage_.error_ = std::move(ue.error());
        }
        else
        {
            this->~expected();
            new (this) expected(std::move(ue));
        }
        return *this;
    }
 
    ~expected()
    {
        if (has_)
            storage_.value_.~T();
        else
            storage_.error_.~E();
    }
 
    // observers
    constexpr auto operator->() const noexcept -> T const*
    {
        return &storage_.value_;
    }
 
    constexpr auto operator->() noexcept -> T*
    {
        return &storage_.value_;
    }
 
    constexpr auto operator*() const& noexcept -> T const&
    {
        return storage_.value_;
    }
 
    constexpr auto operator*() & noexcept -> T&
    {
        return storage_.value_;
    }
 
    constexpr auto operator*() const&& noexcept -> T const&&
    {
        return std::move(storage_.value_);
    }
 
    constexpr auto operator*() && noexcept -> T&&
    {
        return std::move(storage_.value_);
    }
 
    constexpr explicit operator bool() const noexcept
    {
        return has_;
    }
 
    [[nodiscard]] constexpr auto has_value() const noexcept -> bool
    {
        return has_;
    }
 
    [[nodiscard]] constexpr auto value() const& -> T const&
    {
        if (!has_)
            THREADSCHEDULE_EXPECTED_THROW(bad_expected_access<E>(storage_.error_));
        return storage_.value_;
    }
 
    constexpr auto value() & -> T&
    {
        if (!has_)
            THREADSCHEDULE_EXPECTED_THROW(bad_expected_access<E>(storage_.error_));
        return storage_.value_;
    }
 
    [[nodiscard]] constexpr auto value() const&& -> T const&&
    {
        if (!has_)
            THREADSCHEDULE_EXPECTED_THROW(bad_expected_access<E>(std::move(storage_.error_)));
        return std::move(storage_.value_);
    }
 
    constexpr auto value() && -> T&&
    {
        if (!has_)
            THREADSCHEDULE_EXPECTED_THROW(bad_expected_access<E>(std::move(storage_.error_)));
        return std::move(storage_.value_);
    }
 
    [[nodiscard]] constexpr auto error() const& noexcept -> E const&
    {
        return storage_.error_;
    }
 
    constexpr auto error() & noexcept -> E&
    {
        return storage_.error_;
    }
 
    [[nodiscard]] constexpr auto error() const&& noexcept -> E const&&
    {
        return std::move(storage_.error_);
    }
 
    constexpr auto error() && noexcept -> E&&
    {
        return std::move(storage_.error_);
    }
 
    template <typename U>
    constexpr auto value_or(U&& default_value) const& -> T
    {
        return has_ ? storage_.value_ : static_cast<T>(std::forward<U>(default_value));
    }
 
    template <typename U>
    constexpr auto value_or(U&& default_value) && -> T
    {
        return has_ ? std::move(storage_.value_) : static_cast<T>(std::forward<U>(default_value));
    }
 
    // emplace
    template <typename... Args>
    constexpr auto emplace(Args&&... args) -> T&
    {
        this->~expected();
        new (this) expected(std::in_place, std::forward<Args>(args)...);
        return storage_.value_;
    }
 
    // swap
    constexpr void swap(expected& other) noexcept(std::is_nothrow_move_constructible_v<T> &&
                                                  std::is_nothrow_move_constructible_v<E> &&
                                                  std::is_nothrow_swappable_v<T> && std::is_nothrow_swappable_v<E>)
    {
        if (has_ && other.has_)
        {
            using std::swap;
            swap(storage_.value_, other.storage_.value_);
        }
        else if (!has_ && !other.has_)
        {
            using std::swap;
            swap(storage_.error_, other.storage_.error_);
        }
        else
        {
            expected temp(std::move(other));
            other.~expected();
            new (&other) expected(std::move(*this));
            this->~expected();
            new (this) expected(std::move(temp));
        }
    }
 
    // monadic operations
    template <typename F>
    constexpr auto and_then(F&& f) &
    {
        using U = std::invoke_result_t<F, T&>;
        if (has_)
            return std::invoke(std::forward<F>(f), storage_.value_);
        return U(unexpect, storage_.error_);
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) const&
    {
        using U = std::invoke_result_t<F, T const&>;
        if (has_)
            return std::invoke(std::forward<F>(f), storage_.value_);
        return U(unexpect, storage_.error_);
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) &&
    {
        using U = std::invoke_result_t<F, T&&>;
        if (has_)
            return std::invoke(std::forward<F>(f), std::move(storage_.value_));
        return U(unexpect, std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) const&&
    {
        using U = std::invoke_result_t<F, T const&&>;
        if (has_)
            return std::invoke(std::forward<F>(f), std::move(storage_.value_));
        return U(unexpect, std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) &
    {
        using U = std::invoke_result_t<F, E&>;
        if (has_)
            return U(storage_.value_);
        return std::invoke(std::forward<F>(f), storage_.error_);
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) const&
    {
        using U = std::invoke_result_t<F, E const&>;
        if (has_)
            return U(storage_.value_);
        return std::invoke(std::forward<F>(f), storage_.error_);
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) &&
    {
        using U = std::invoke_result_t<F, E&&>;
        if (has_)
            return U(std::move(storage_.value_));
        return std::invoke(std::forward<F>(f), std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) const&&
    {
        using U = std::invoke_result_t<F, E const&&>;
        if (has_)
            return U(std::move(storage_.value_));
        return std::invoke(std::forward<F>(f), std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto transform(F&& f) &
    {
        using U = std::remove_cv_t<std::invoke_result_t<F, T&>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f), storage_.value_));
        return expected<U, E>(unexpect, storage_.error_);
    }
 
    template <typename F>
    constexpr auto transform(F&& f) const&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F, T const&>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f), storage_.value_));
        return expected<U, E>(unexpect, storage_.error_);
    }
 
    template <typename F>
    constexpr auto transform(F&& f) &&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F, T&&>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f), std::move(storage_.value_)));
        return expected<U, E>(unexpect, std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto transform(F&& f) const&&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F, T const&&>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f), std::move(storage_.value_)));
        return expected<U, E>(unexpect, std::move(storage_.error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) &
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E&>>;
        if (has_)
            return expected<T, G>(storage_.value_);
        return expected<T, G>(unexpect, std::invoke(std::forward<F>(f), storage_.error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) const&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E const&>>;
        if (has_)
            return expected<T, G>(storage_.value_);
        return expected<T, G>(unexpect, std::invoke(std::forward<F>(f), storage_.error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) &&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E&&>>;
        if (has_)
            return expected<T, G>(std::move(storage_.value_));
        return expected<T, G>(unexpect, std::invoke(std::forward<F>(f), std::move(storage_.error_)));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) const&&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E const&&>>;
        if (has_)
            return expected<T, G>(std::move(storage_.value_));
        return expected<T, G>(unexpect, std::invoke(std::forward<F>(f), std::move(storage_.error_)));
    }
 
    // equality operators
    template <typename T2, typename E2>
    constexpr friend auto operator==(expected const& lhs, expected<T2, E2> const& rhs) -> bool
    {
        if (lhs.has_value() != rhs.has_value())
            return false;
        if (lhs.has_value())
            return *lhs == *rhs;
        return lhs.error() == rhs.error();
    }
 
    template <typename T2, typename E2>
    constexpr friend auto operator!=(expected const& lhs, expected<T2, E2> const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename T2, typename = std::enable_if_t<!std::is_same_v<expected, std::decay_t<T2>>>>
    constexpr friend auto operator==(expected const& lhs, const T2& rhs) -> bool
    {
        return lhs.has_value() && *lhs == rhs;
    }
 
    template <typename T2, typename = std::enable_if_t<!std::is_same_v<expected, std::decay_t<T2>>>>
    constexpr friend auto operator==(const T2& lhs, expected const& rhs) -> bool
    {
        return rhs.has_value() && lhs == *rhs;
    }
 
    template <typename T2, typename = std::enable_if_t<!std::is_same_v<expected, std::decay_t<T2>>>>
    constexpr friend auto operator!=(expected const& lhs, const T2& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename T2, typename = std::enable_if_t<!std::is_same_v<expected, std::decay_t<T2>>>>
    constexpr friend auto operator!=(const T2& lhs, expected const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename E2>
    constexpr friend auto operator==(expected const& lhs, unexpected<E2> const& rhs) -> bool
    {
        return !lhs.has_value() && lhs.error() == rhs.error();
    }
 
    template <typename E2>
    constexpr friend auto operator==(unexpected<E2> const& lhs, expected const& rhs) -> bool
    {
        return !rhs.has_value() && lhs.error() == rhs.error();
    }
 
    template <typename E2>
    constexpr friend auto operator!=(expected const& lhs, unexpected<E2> const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename E2>
    constexpr friend auto operator!=(unexpected<E2> const& lhs, expected const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
  private:
    bool has_;
    union Storage {
        Storage() {};
        ~Storage() {};
        T value_;
        E error_;
    } storage_;
};

template <typename E>
class expected<void, E>
{
  public:
    using value_type = void;
    using error_type = E;
    using unexpected_type = unexpected<E>;
 
    constexpr expected() : has_(true)
    {
    }
 
    constexpr expected(expected const& other) = default;
    constexpr expected(expected&& other) = default;
 
    template <typename... Args>
    constexpr explicit expected(unexpect_t /*unused*/, Args&&... args)
        : has_(false), error_(std::forward<Args>(args)...)
    {
    }
 
    constexpr expected(unexpected<E> const& ue) : has_(false), error_(ue.error())
    {
    }
 
    constexpr expected(unexpected<E>&& ue) : has_(false), error_(std::move(ue.error()))
    {
    }
 
    constexpr auto operator=(expected const& other) -> expected& = default;
    constexpr auto operator=(expected&& other) -> expected& = default;
 
    constexpr auto operator=(unexpected<E> const& ue) -> expected&
    {
        has_ = false;
        error_ = ue.error();
        return *this;
    }
 
    constexpr auto operator=(unexpected<E>&& ue) -> expected&
    {
        has_ = false;
        error_ = std::move(ue.error());
        return *this;
    }
 
    constexpr explicit operator bool() const noexcept
    {
        return has_;
    }
 
    [[nodiscard]] constexpr auto has_value() const noexcept -> bool
    {
        return has_;
    }
 
    constexpr void value() const
    {
        if (!has_)
            THREADSCHEDULE_EXPECTED_THROW(bad_expected_access<E>(error_));
    }
 
    [[nodiscard]] constexpr auto error() const& noexcept -> E const&
    {
        return error_;
    }
 
    constexpr auto error() & noexcept -> E&
    {
        return error_;
    }
 
    [[nodiscard]] constexpr auto error() const&& noexcept -> E const&&
    {
        return std::move(error_);
    }
 
    constexpr auto error() && noexcept -> E&&
    {
        return std::move(error_);
    }
 
    constexpr void emplace()
    {
        has_ = true;
    }
 
    constexpr void swap(expected& other) noexcept(std::is_nothrow_move_constructible_v<E> &&
                                                  std::is_nothrow_swappable_v<E>)
    {
        if (has_ && other.has_)
        {
            // both have values, nothing to swap
        }
        else if (!has_ && !other.has_)
        {
            using std::swap;
            swap(error_, other.error_);
        }
        else
        {
            std::swap(has_, other.has_);
            std::swap(error_, other.error_);
        }
    }
 
    // monadic operations
    template <typename F>
    constexpr auto and_then(F&& f) &
    {
        using U = std::invoke_result_t<F>;
        if (has_)
            return std::invoke(std::forward<F>(f));
        return U(unexpect, error_);
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) const&
    {
        using U = std::invoke_result_t<F>;
        if (has_)
            return std::invoke(std::forward<F>(f));
        return U(unexpect, error_);
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) &&
    {
        using U = std::invoke_result_t<F>;
        if (has_)
            return std::invoke(std::forward<F>(f));
        return U(unexpect, std::move(error_));
    }
 
    template <typename F>
    constexpr auto and_then(F&& f) const&&
    {
        using U = std::invoke_result_t<F>;
        if (has_)
            return std::invoke(std::forward<F>(f));
        return U(unexpect, std::move(error_));
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) &
    {
        using U = std::invoke_result_t<F, E&>;
        if (has_)
            return U();
        return std::invoke(std::forward<F>(f), error_);
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) const&
    {
        using U = std::invoke_result_t<F, E const&>;
        if (has_)
            return U();
        return std::invoke(std::forward<F>(f), error_);
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) &&
    {
        using U = std::invoke_result_t<F, E&&>;
        if (has_)
            return U();
        return std::invoke(std::forward<F>(f), std::move(error_));
    }
 
    template <typename F>
    constexpr auto or_else(F&& f) const&&
    {
        using U = std::invoke_result_t<F, E const&&>;
        if (has_)
            return U();
        return std::invoke(std::forward<F>(f), std::move(error_));
    }
 
    template <typename F>
    constexpr auto transform(F&& f) &
    {
        using U = std::remove_cv_t<std::invoke_result_t<F>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f)));
        return expected<U, E>(unexpect, error_);
    }
 
    template <typename F>
    constexpr auto transform(F&& f) const&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f)));
        return expected<U, E>(unexpect, error_);
    }
 
    template <typename F>
    constexpr auto transform(F&& f) &&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f)));
        return expected<U, E>(unexpect, std::move(error_));
    }
 
    template <typename F>
    constexpr auto transform(F&& f) const&&
    {
        using U = std::remove_cv_t<std::invoke_result_t<F>>;
        if (has_)
            return expected<U, E>(std::in_place, std::invoke(std::forward<F>(f)));
        return expected<U, E>(unexpect, std::move(error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) &
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E&>>;
        if (has_)
            return expected<void, G>();
        return expected<void, G>(unexpect, std::invoke(std::forward<F>(f), error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) const&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E const&>>;
        if (has_)
            return expected<void, G>();
        return expected<void, G>(unexpect, std::invoke(std::forward<F>(f), error_));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) &&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E&&>>;
        if (has_)
            return expected<void, G>();
        return expected<void, G>(unexpect, std::invoke(std::forward<F>(f), std::move(error_)));
    }
 
    template <typename F>
    constexpr auto transform_error(F&& f) const&&
    {
        using G = std::remove_cv_t<std::invoke_result_t<F, E const&&>>;
        if (has_)
            return expected<void, G>();
        return expected<void, G>(unexpect, std::invoke(std::forward<F>(f), std::move(error_)));
    }
 
    // equality operators
    template <typename E2>
    constexpr friend auto operator==(expected const& lhs, expected<void, E2> const& rhs) -> bool
    {
        if (lhs.has_value() != rhs.has_value())
            return false;
        if (lhs.has_value())
            return true;
        return lhs.error() == rhs.error();
    }
 
    template <typename E2>
    constexpr friend auto operator!=(expected const& lhs, expected<void, E2> const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename E2>
    constexpr friend auto operator==(expected const& lhs, unexpected<E2> const& rhs) -> bool
    {
        return !lhs.has_value() && lhs.error() == rhs.error();
    }
 
    template <typename E2>
    constexpr friend auto operator==(unexpected<E2> const& lhs, expected const& rhs) -> bool
    {
        return !rhs.has_value() && lhs.error() == rhs.error();
    }
 
    template <typename E2>
    constexpr friend auto operator!=(expected const& lhs, unexpected<E2> const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
    template <typename E2>
    constexpr friend auto operator!=(unexpected<E2> const& lhs, expected const& rhs) -> bool
    {
        return !(lhs == rhs);
    }
 
  private:
    bool has_;
    E error_{};
};
 
#endif // std::expected fallback
 
// swap for expected
template <typename T, typename E>
constexpr void swap(expected<T, E>& lhs, expected<T, E>& rhs) noexcept(noexcept(lhs.swap(rhs)))
{
    lhs.swap(rhs);
}
 
} // namespace threadschedule