C++20 implementation of std::expected which passes MSVC's unit tests

breaded_expected.hpp

#pragma once

#include <cassert>
#include <concepts>
#include <memory>
#include <type_traits>
#include <utility>

namespace breaded {

#pragma region Forward Declarations

template <class E>
class bad_expected_access;

template <class E>
class unexpected;

template <class T, class E>
class expected;

#pragma endregion

// https://en.cppreference.com/w/cpp/utility/expected/unexpect_t
struct unexpect_t {
    constexpr explicit unexpect_t() = default;
};

// https://en.cppreference.com/w/cpp/utility/expected/unexpect_t
constexpr unexpect_t unexpect{};

#pragma region Traits
namespace details {

struct in_place_from_return_t {
    constexpr explicit in_place_from_return_t() = default;
};
constexpr in_place_from_return_t in_place_from_return{};

struct unexpect_from_return_t {
    constexpr explicit unexpect_from_return_t() = default;
};
constexpr unexpect_from_return_t unexpect_from_return{};

template <class T>
struct IsSpecializationOfUnexpected : std::false_type {};
template <class T, class E>
struct IsSpecializationOfUnexpected<breaded::expected<T, E>> : std::true_type {};
template <class T>
constexpr bool kIsSpecializationOfUnexpected = IsSpecializationOfUnexpected<T>::value;

template <class T>
struct IsSpecializationOfExpected : std::false_type {};
template <class T, class E>
struct IsSpecializationOfExpected<breaded::expected<T, E>> : std::true_type {};
template <class T>
constexpr bool kIsSpecializationOfExpected = IsSpecializationOfExpected<T>::value;

// https://en.cppreference.com/w/cpp/named_req/Destructible
// This isn't quite the same as std::is_destructible_v<T>
// This does not allow arrays or reference types.
template <class T>
concept NamedRequirementDestructible =
    std::is_nothrow_destructible_v<T> && !std::is_array_v<T> && !std::is_reference_v<T>;

template <typename T>
concept NotTag = !std::is_same_v<std::remove_cvref_t<T>, std::in_place_t> &&
                 !std::is_same_v<std::remove_cvref_t<T>, breaded::unexpect_t>;

template <typename E>
concept ValidExpectedErrorType =
    std::is_object_v<E> && !std::is_const_v<E> && !std::is_volatile_v<E> && !kIsSpecializationOfExpected<E>;

template <class T>
concept ValidExpectedValueType =
    NotTag<T> && !std::is_reference_v<T> && !std::is_function_v<T> && !kIsSpecializationOfUnexpected<T> &&
    (std::is_void_v<T> || NamedRequirementDestructible<T>);

} // namespace details
#pragma endregion

#pragma region bad_expected_access

// https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access
template <>
class bad_expected_access<void> : public std::exception {
public:
    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::what
    const char* what() const noexcept override { return "Attempted to access the value of an errored expected type"; }

protected:
    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::bad_expected_access
    explicit bad_expected_access()                                   = default;
    bad_expected_access(const bad_expected_access& other)            = default;
    bad_expected_access(bad_expected_access&& other)                 = default;
    ~bad_expected_access() override                                  = default;
    bad_expected_access& operator=(const bad_expected_access& other) = default;
    bad_expected_access& operator=(bad_expected_access&& other)      = default;
};

// https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access
template <class E>
class bad_expected_access : public breaded::bad_expected_access<void> {
public:
    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::bad_expected_access
    explicit bad_expected_access(E e) : error_{std::move(e)} {}

    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::error
    [[nodiscard]] const E& error() const& noexcept { return error_; }

    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::error
    [[nodiscard]] E& error() & noexcept { return error_; }

    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::error
    [[nodiscard]] const E&& error() const&& noexcept { return std::move(error_); }

    // https://en.cppreference.com/w/cpp/utility/expected/bad_expected_access#std::bad_expected_access::error
    [[nodiscard]] E&& error() && noexcept { return std::move(error_); }

private:
    E error_;
};

#pragma endregion

#pragma region Unexpected

// https://en.cppreference.com/w/cpp/utility/expected/unexpected
template <class E>
class unexpected {
    static_assert(details::ValidExpectedErrorType<E>);

public:
    unexpected() = delete;

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#ctor
    constexpr unexpected(const unexpected&) noexcept(std::is_nothrow_copy_constructible_v<E>) = default;

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#ctor
    constexpr unexpected(unexpected&&) noexcept(std::is_nothrow_move_constructible_v<E>) = default;

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#ctor
    template <class Err = E>
        requires(!std::is_same_v<std::remove_cvref_t<Err>, unexpected> &&
                 !std::is_same_v<std::remove_cvref_t<Err>, std::in_place_t> && std::is_constructible_v<E, Err>)
    constexpr explicit unexpected(Err&& e) noexcept(std::is_nothrow_constructible_v<E, Err>)
        : error_(std::forward<Err>(e)) {}

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#ctor
    template <class... Args>
        requires(std::is_constructible_v<E, Args...>)
    constexpr explicit unexpected(std::in_place_t, Args&&... args) noexcept(std::is_nothrow_constructible_v<E, Args...>)
        : error_(std::forward<Args>(args)...) {}

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#ctor
    template <class U, class... Args>
        requires(std::is_constructible_v<E, std::initializer_list<U>&, Args...>)
    constexpr explicit unexpected(std::in_place_t, std::initializer_list<U> il, Args&&... args) noexcept(
        std::is_nothrow_constructible_v<E, std::initializer_list<U>&, Args...>)
        : error_(il, std::forward<Args>(args)...) {}

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#error
    [[nodiscard]] constexpr const E& error() const& noexcept { return error_; }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#error
    [[nodiscard]] constexpr E& error() & noexcept { return error_; }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#error
    [[nodiscard]] constexpr const E&& error() const&& noexcept { return std::move(error_); }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#error
    [[nodiscard]] constexpr E&& error() && noexcept { return std::move(error_); }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#swap
    constexpr void swap(unexpected& other) noexcept(std::is_nothrow_swappable_v<E>) {
        // Prefer custom swap friend function, and use move-based std::swap as a fallback
        // https://en.cppreference.com/w/cpp/language/adl
        using std::swap;
        swap(error_, other.error_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#operator.3D.3D.28std::unexpected.29
    template <class E2>
    [[nodiscard]] friend constexpr bool operator==(const unexpected& x,
                                                   const breaded::unexpected<E2>& y) noexcept(noexcept(x.error() ==
                                                                                                       y.error())) {
        return x.error() == y.error();
    }

    // https://en.cppreference.com/w/cpp/utility/expected/unexpected#swap.28std::unexpected.29
    friend constexpr void swap(unexpected& x, unexpected& y) noexcept(std::is_nothrow_swappable_v<E>) { x.swap(y); }

private:
    E error_;
};

// https://en.cppreference.com/w/cpp/utility/expected/unexpected#Deduction_guides
template <class E>
unexpected(E) -> unexpected<E>;

#pragma endregion

namespace details {

template <class NewType, class OldType, class... Args>
constexpr bool kUnionAssignable =
    std::is_constructible_v<NewType, Args...> &&
    (std::is_nothrow_constructible_v<NewType, Args...> || std::is_nothrow_move_constructible_v<NewType> ||
     std::is_nothrow_move_constructible_v<OldType>);

template <class NewType, class OldType, class... Args>
constexpr bool kNothrowUnionAssignable =
    kUnionAssignable<NewType, OldType, Args...> && std::is_nothrow_constructible_v<NewType, Args...>;

// This function allows assigning to an inactive union member,
// while preserving the old active member if the assignment throws an exception.
// This is required for std::expected since it can never be value-less
//
// "If an exception is thrown, the old value is retained; *this does not become valueless."
// https://en.cppreference.com/w/cpp/utility/expected/operator%3D#Helper_function_template
template <class NewType, class OldType, class... Args>
constexpr void UnionAssign(NewType& newVal, OldType& oldVal,
                           Args&&... args) noexcept(kNothrowUnionAssignable<NewType, OldType, Args...>)
    requires(kUnionAssignable<NewType, OldType, Args...>)
{
    if constexpr (std::is_nothrow_constructible_v<NewType, Args...>) {
        std::destroy_at(std::addressof(oldVal));
        std::construct_at(std::addressof(newVal), std::forward<Args>(args)...);
    } else if constexpr (std::is_nothrow_move_constructible_v<NewType>) {
        NewType temp(std::forward<Args>(args)...);
        std::destroy_at(std::addressof(oldVal));
        std::construct_at(std::addressof(newVal), std::move(temp));
    } else if constexpr (std::is_nothrow_move_constructible_v<OldType>) {
        OldType temp(std::move(oldVal));
        std::destroy_at(std::addressof(oldVal));
        try {
            std::construct_at(std::addressof(newVal), std::forward<Args>(args)...);
        } catch (...) {
            std::construct_at(std::addressof(oldVal), std::move(temp));
            throw;
        }
    } else {
        static_assert(sizeof(NewType) == 0,
                      "Sanity check failed. Cannot call without fulfilling one of these conditions");
    }
}

template <class T, class U>
constexpr bool kUnionSwappable = std::is_move_constructible_v<T> && std::is_move_constructible_v<U> &&
                                 (std::is_nothrow_move_constructible_v<T> || std::is_nothrow_move_constructible_v<U>);

template <class T, class U>
constexpr bool kNothrowUnionSwappable =
    std::is_nothrow_move_constructible_v<T> && std::is_nothrow_move_constructible_v<U>;

// https://en.cppreference.com/w/cpp/utility/expected/swap
template <class T, class U>
constexpr void UnionSwap(T& dstT, U& dstU, T&& srcT, U&& srcU) noexcept(kNothrowUnionSwappable<T, U>)
    requires(kUnionSwappable<T, U>)
{
    if constexpr (std::is_nothrow_move_constructible_v<T>) {
        T tmp(std::move(srcT));
        std::destroy_at(std::addressof(srcT));
        if constexpr (std::is_nothrow_move_constructible_v<U>) {
            std::construct_at(std::addressof(dstU), std::move(srcU));
        } else {
            try {
                std::construct_at(std::addressof(dstU), std::move(srcU));
            } catch (...) {
                std::construct_at(std::addressof(srcT), std::move(tmp));
                throw;
            }
        }
        std::destroy_at(std::addressof(srcU));
        std::construct_at(std::addressof(dstT), std::move(tmp));
    } else {
        static_assert(std::is_nothrow_move_constructible_v<U>,
                      "Either T or U must be nothrow_move_constructible to enforce strong exception safety");
        UnionSwap(dstU, dstT, std::move(srcU), std::move(srcT));
    }
}

} // namespace details

#pragma region Expected T
// https://en.cppreference.com/w/cpp/utility/expected
template <class T, class E>
class expected {
    static_assert(details::ValidExpectedValueType<T>);
    static_assert(details::ValidExpectedErrorType<E>);
    static_assert(!std::is_void_v<T>, "Sanity check failed. This should be handled in a specialization");

    template <typename Self, typename F>
    struct AndThenReturn {
        using type = std::remove_cvref_t<std::invoke_result_t<F, decltype(*std::declval<Self>())>>;
    };

    template <typename Self, typename F>
    struct TransformReturn {
        using URaw = std::invoke_result_t<F, decltype(*std::declval<Self>())>;
        using U    = std::remove_cvref_t<URaw>;
        using type = breaded::expected<U, E>;
    };

    template <typename Self, typename F>
    struct OrElseReturn {
        using type = std::remove_cvref_t<std::invoke_result_t<F, decltype(std::declval<Self>().error())>>;
    };

    template <typename Self, typename F>
    struct TransformErrorReturn {
        using GRaw = std::invoke_result_t<F, decltype(std::declval<Self>().error())>;
        using G    = std::remove_cvref_t<GRaw>;
        using type = breaded::expected<T, G>;
    };

public:
#pragma region Member types
    // https://en.cppreference.com/w/cpp/utility/expected#Member_types

    using value_type      = T;
    using error_type      = E;
    using unexpected_type = breaded::unexpected<E>;
#pragma endregion

#pragma region Member alias templates
    // https://en.cppreference.com/w/cpp/utility/expected#Member_alias_templates

    template <class U>
    using rebind = breaded::expected<U, error_type>;
#pragma endregion

private:
    template <typename U>
    static constexpr bool kSameAsThis = std::is_same_v<expected, std::remove_cvref_t<U>>;

    static constexpr bool kValueTypeIsBool = std::is_same_v<bool, std::remove_cvref_t<T>>;

    template <class U>
    static constexpr bool kExpectedConvertibleToValueType =
        std::is_constructible_v<T, U&> || std::is_constructible_v<T, U> || std::is_constructible_v<T, const U&> ||
        std::is_constructible_v<T, const U> || std::is_convertible_v<U&, T> || std::is_convertible_v<U, T> ||
        std::is_convertible_v<const U&, T> || std::is_convertible_v<const U, T>;

    template <class U>
    static constexpr bool kExpectedConvertibleToErrorType =
        std::is_constructible_v<unexpected_type, U&> || std::is_constructible_v<unexpected_type, U> ||
        std::is_constructible_v<unexpected_type, const U&> || std::is_constructible_v<unexpected_type, const U>;

public:
#pragma region Member functions
    // https://en.cppreference.com/w/cpp/utility/expected#Member_functions

#pragma region constructor
    // Default value constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected() noexcept(std::is_nothrow_default_constructible_v<T>)
        requires(std::is_default_constructible_v<T>)
        : value_(), hasValue_(true) {}

    // Non-trivial copy constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected(const expected& other) noexcept(std::is_nothrow_copy_constructible_v<T> &&
                                                       std::is_nothrow_copy_constructible_v<E>)
        requires(std::is_copy_constructible_v<T> && std::is_copy_constructible_v<E> &&
                 !(std::is_trivially_copy_constructible_v<T> && std::is_trivially_copy_constructible_v<E>))
    {
        constructFromExpected(other);
    }

    // Trivial copy constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    expected(const expected& other)
        requires(std::is_copy_constructible_v<T> && std::is_copy_constructible_v<E> &&
                 (std::is_trivially_copy_constructible_v<T> && std::is_trivially_copy_constructible_v<E>))
    = default;

    // This constructor is defined as deleted unless is_copy_constructible_v<T> is true
    // and is_copy_constructible_v<E> is true
    // https://github.com/cplusplus/draft/blob/d29ef68d3c7a1c2317f4670f08bbc8a63828ce35/source/utilities.tex#L7423
    expected(const expected&) = delete;

    // Non-trivial move constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected(expected&& other) noexcept(std::is_nothrow_move_constructible_v<T> &&
                                                  std::is_nothrow_move_constructible_v<E>)
        requires(std::is_move_constructible_v<T> && std::is_move_constructible_v<E> &&
                 !(std::is_trivially_move_constructible_v<T> && std::is_trivially_move_constructible_v<E>))
    {
        constructFromExpected(std::move(other));
    }

    // Trivial move constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    expected(expected&& other)
        requires(std::is_move_constructible_v<T> && std::is_move_constructible_v<E> &&
                 (std::is_trivially_move_constructible_v<T> && std::is_trivially_move_constructible_v<E>))
    = default;

    // Converting copy construct from expected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class G>
        requires(!kSameAsThis<breaded::expected<U, G>> && std::is_constructible_v<T, const U&> &&
                 (kValueTypeIsBool || !kExpectedConvertibleToValueType<breaded::expected<U, G>>) &&
                 !kExpectedConvertibleToErrorType<breaded::expected<U, G>>)
    constexpr explicit(!std::is_convertible_v<const U&, T> || !std::is_convertible_v<const G&, E>)
        expected(const breaded::expected<U, G>& other) noexcept(std::is_nothrow_constructible_v<T, const U&> &&
                                                                std::is_nothrow_constructible_v<E, const G&>) {
        constructFromExpected(other);
    }

    // Converting move construct from expected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class G>
        requires(!kSameAsThis<breaded::expected<U, G>> && std::is_constructible_v<T, U> &&
                 (kValueTypeIsBool || !kExpectedConvertibleToValueType<breaded::expected<U, G>>) &&
                 !kExpectedConvertibleToErrorType<breaded::expected<U, G>>)
    constexpr explicit(!std::is_convertible_v<U, T> || !std::is_convertible_v<G, E>)
        expected(breaded::expected<U, G>&& other) noexcept(std::is_nothrow_constructible_v<T, U> &&
                                                           std::is_nothrow_constructible_v<E, G>) {
        constructFromExpected(std::move(other));
    }

    // Constructor from value
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U = T>
        requires(details::NotTag<T> && !kSameAsThis<U> && std::is_constructible_v<T, U> &&
                 !details::kIsSpecializationOfUnexpected<std::remove_cvref_t<U>> &&
                 (!kValueTypeIsBool || !details::kIsSpecializationOfUnexpected<std::remove_cvref_t<U>>))
    constexpr explicit(!std::is_convertible_v<U, T>) expected(U&& v) noexcept(std::is_nothrow_constructible_v<T, U>)
        : value_(std::forward<U>(v)), hasValue_(true) {}

    // Copy constructor from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class G>
        requires(std::is_constructible_v<E, const G&>)
    constexpr explicit(!std::is_convertible_v<const G&, E>)
        expected(const breaded::unexpected<G>& e) noexcept(std::is_nothrow_constructible_v<E, const G&>)
        : error_(e.error()), hasValue_(false) {}

    // Move constructor from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class G>
        requires(std::is_constructible_v<E, G>)
    constexpr explicit(!std::is_convertible_v<G, E>)
        expected(breaded::unexpected<G>&& e) noexcept(std::is_nothrow_constructible_v<E, G>)
        : error_(std::move(e).error()), hasValue_(false) {}

    // In-place value constructor from args
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class... Args>
        requires(std::is_constructible_v<T, Args...>)
    constexpr explicit expected(std::in_place_t, Args&&... args) noexcept(std::is_nothrow_constructible_v<T, Args...>)
        : value_(std::forward<Args>(args)...), hasValue_(true) {}

    // In-place value constructor from initializer list
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class... Args>
        requires(std::is_constructible_v<T, std::initializer_list<U>&, Args...>)
    constexpr explicit expected(std::in_place_t, std::initializer_list<U> il, Args&&... args) noexcept(
        std::is_nothrow_constructible_v<T, std::initializer_list<U>&, Args...>)
        : value_(il, std::forward<Args>(args)...), hasValue_(true) {}

    // In-place error constructor from args
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class... Args>
        requires(std::is_constructible_v<E, Args...>)
    constexpr explicit expected(breaded::unexpect_t,
                                Args&&... args) noexcept(std::is_nothrow_constructible_v<E, Args...>)
        : error_(std::forward<Args>(args)...), hasValue_(false) {}

    // In-place error constructor from initializer list
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class... Args>
        requires(std::is_constructible_v<E, std::initializer_list<U>&, Args...>)
    constexpr explicit expected(breaded::unexpect_t, std::initializer_list<U> il, Args&&... args) noexcept(
        std::is_nothrow_constructible_v<E, std::initializer_list<U>&, Args...>)
        : error_(il, std::forward<Args>(args)...), hasValue_(false) {}

#pragma endregion

    // https://en.cppreference.com/w/cpp/utility/expected/~expected#Main_template_destructor
    constexpr ~expected() noexcept { destroy(); }

    // https://en.cppreference.com/w/cpp/utility/expected/~expected#Main_template_destructor
    ~expected()
        requires(std::is_trivially_destructible_v<T> && std::is_trivially_destructible_v<E>)
    = default;

#pragma region Assignment

    // Non-trivial copy assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    constexpr expected& operator=(const expected& other) noexcept(std::is_nothrow_copy_assignable_v<T> &&
                                                                  std::is_nothrow_copy_assignable_v<E> &&
                                                                  details::kNothrowUnionAssignable<T, E, const T&> &&
                                                                  details::kNothrowUnionAssignable<E, T, const E&>)
        requires(std::is_copy_assignable_v<T> && std::is_copy_assignable_v<E> &&
                 details::kUnionAssignable<T, E, const T&> && details::kUnionAssignable<E, T, const E&> &&
                 !(std::is_trivially_copy_assignable_v<T> && std::is_trivially_copy_assignable_v<E>))
    {
        assignFromExpected(other);
        return *this;
    }

    // Trivial copy assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    expected& operator=(const expected& other)
        requires(std::is_copy_assignable_v<T> && std::is_copy_assignable_v<E> &&
                 details::kUnionAssignable<T, E, const T&> && details::kUnionAssignable<E, T, const E&> &&
                 (std::is_trivially_copy_assignable_v<T> && std::is_trivially_copy_assignable_v<E>))
    = default;

    // This operator is defined as deleted unless is_copy_constructible_v<T> is true and is_copy_assignable_v<T> is true
    // and is_copy_assignable_v<E> is true and is_copy_constructible_v<E> is true and
    // is_nothrow_move_constructible_v<T> || is_nothrow_move_constructible_v<E> is true
    // https://github.com/cplusplus/draft/blob/d29ef68d3c7a1c2317f4670f08bbc8a63828ce35/source/utilities.tex#L7796
    expected& operator=(const expected&) = delete;

    // Non-trivial move assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    constexpr expected& operator=(expected&& other) noexcept(std::is_nothrow_move_assignable_v<T> &&
                                                             std::is_nothrow_move_assignable_v<E> &&
                                                             details::kNothrowUnionAssignable<T, E, T> &&
                                                             details::kNothrowUnionAssignable<E, T, E>)
        requires(std::is_move_assignable_v<T> && std::is_move_assignable_v<E> && details::kUnionAssignable<T, E, T> &&
                 details::kUnionAssignable<E, T, E> &&
                 !(std::is_trivially_move_assignable_v<T> && std::is_trivially_move_assignable_v<E>))
    {
        assignFromExpected(std::move(other));
        return *this;
    }

    // Trivial move assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    expected& operator=(expected&& other)
        requires(std::is_move_assignable_v<T> && std::is_move_assignable_v<E> && details::kUnionAssignable<T, E, T> &&
                 details::kUnionAssignable<E, T, E> &&
                 (std::is_trivially_move_assignable_v<T> && std::is_trivially_move_assignable_v<E>))
    = default;

    // Assignment from value
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    template <class U = T>
    constexpr expected& operator=(U&& v) noexcept(std::is_nothrow_assignable_v<T, U> &&
                                                  details::kNothrowUnionAssignable<T, E, U>)
        requires(!kSameAsThis<U> && !details::kIsSpecializationOfExpected<U> && std::is_constructible_v<T, U> &&
                 std::is_assignable_v<T&, U> && details::kUnionAssignable<T, E, U>)
    {
        if (hasValue_) {
            value_ = std::forward<U>(v);
        } else {
            details::UnionAssign(value_, error_, std::forward<U>(v));
            hasValue_ = true;
        }
        return *this;
    }

    // Copy assignment from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    template <class G>
    constexpr expected& operator=(const breaded::unexpected<G>& other) noexcept(
        std::is_nothrow_assignable_v<E, const G&> && details::kNothrowUnionAssignable<E, T, const G&>)
        requires(std::is_assignable_v<E, const G&> && details::kUnionAssignable<E, T, const G&>)
    {
        if (hasValue_) {
            details::UnionAssign(error_, value_, other.error());
            hasValue_ = false;
        } else {
            error_ = other.error();
        }
        return *this;
    }

    // Move assignment from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    template <class G>
    constexpr expected& operator=(breaded::unexpected<G>&& other) noexcept(std::is_nothrow_assignable_v<E, G> &&
                                                                           details::kNothrowUnionAssignable<E, T, G>)
        requires(std::is_assignable_v<E, G> && details::kUnionAssignable<E, T, G>)
    {
        if (hasValue_) {
            details::UnionAssign(error_, value_, std::move(other).error());
            hasValue_ = false;
        } else {
            error_ = std::move(other).error();
        }
        return *this;
    }
#pragma endregion

#pragma region Observers

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    constexpr const T* operator->() const noexcept {
        assert(hasValue_ && "Attempted to call expected::operator-> when has_value() == false");
        return std::addressof(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    constexpr T* operator->() noexcept {
        assert(hasValue_ && "Attempted to call expected::operator-> when has_value() == false");
        return std::addressof(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    [[nodiscard]] constexpr const T& operator*() const& noexcept {
        assert(hasValue_ && "Attempted to call expected::operator* when has_value() == false");
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    [[nodiscard]] constexpr T& operator*() & noexcept {
        assert(hasValue_ && "Attempted to call expected::operator* when has_value() == false");
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    [[nodiscard]] constexpr const T&& operator*() const&& noexcept {
        assert(hasValue_ && "Attempted to call expected::operator* when has_value() == false");
        return std::move(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    [[nodiscard]] constexpr T&& operator*() && noexcept {
        assert(hasValue_ && "Attempted to call expected::operator* when has_value() == false");
        return std::move(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_bool
    [[nodiscard]] constexpr explicit operator bool() const noexcept { return hasValue_; }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_bool
    [[nodiscard]] constexpr bool has_value() const noexcept { return hasValue_; }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr T& value() & noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::as_const(error_));
        }
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr const T& value() const& noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::as_const(error_));
        }
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr T&& value() && noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::move(error_));
        }
        return std::move(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr const T&& value() const&& noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::move(error_));
        }
        return std::move(value_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr E& error() & noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return error_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr const E& error() const& noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return error_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr E&& error() && noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return std::move(error_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr const E&& error() const&& noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return std::move(error_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value_or
    template <class U>
        requires(std::is_copy_constructible_v<T> && std::is_convertible_v<U, T>)
    [[nodiscard]] constexpr T value_or(U&& default_value) const& noexcept(std::is_nothrow_copy_constructible_v<T> &&
                                                                          std::is_nothrow_convertible_v<U, T>) {
        if (hasValue_) {
            return value_;
        }
        return std::forward<U>(default_value);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value_or
    template <class U>
        requires(std::is_move_constructible_v<T> && std::is_convertible_v<U, T>)
    [[nodiscard]] constexpr T value_or(U&& default_value) && noexcept(std::is_nothrow_move_constructible_v<T> &&
                                                                      std::is_nothrow_convertible_v<U, T>) {
        if (hasValue_) {
            return std::move(value_);
        }
        return std::forward<U>(default_value);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error_or
    template <class G = E>
        requires(std::is_copy_constructible_v<E> && std::is_convertible_v<G, E>)
    [[nodiscard]] constexpr E error_or(G&& default_value) const& noexcept(std::is_nothrow_copy_constructible_v<E> &&
                                                                          std::is_nothrow_convertible_v<G, E>) {
        if (!hasValue_) {
            return error_;
        }
        return std::forward<G>(default_value);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error_or
    template <class G = E>
        requires(std::is_move_constructible_v<E> && std::is_convertible_v<G, E>)
    [[nodiscard]] constexpr E error_or(G&& default_value) && noexcept(std::is_nothrow_move_constructible_v<E> &&
                                                                      std::is_nothrow_convertible_v<G, E>) {
        if (!hasValue_) {
            return std::move(error_);
        }
        return std::forward<G>(default_value);
    }

#pragma endregion

#pragma region Monadic operations

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<expected&, F>::type and_then(F&& f) &
        requires(std::is_constructible_v<E, E&>)
    {
        return andThenImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<const expected&, F>::type and_then(F&& f) const&
        requires(std::is_constructible_v<E, const E&>)
    {
        return andThenImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<expected&&, F>::type and_then(F&& f) &&
        requires(std::is_constructible_v<E, E &&>)
    {
        return andThenImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<const expected&&, F>::type and_then(F&& f) const&&
        requires(std::is_constructible_v<E, const E &&>)
    {
        return andThenImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<expected&, F>::type transform(F&& f) &
        requires(std::is_constructible_v<E, E&>)
    {
        return transformImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<const expected&, F>::type transform(F&& f) const&
        requires(std::is_constructible_v<E, const E&>)
    {
        return transformImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<expected&&, F>::type transform(F&& f) &&
        requires(std::is_constructible_v<E, E &&>)
    {
        return transformImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<const expected&&, F>::type transform(F&& f) const&&
        requires(std::is_constructible_v<E, const E &&>)
    {
        return transformImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<expected&, F>::type or_else(F&& f) &
        requires(std::is_constructible_v<T, T&>)
    {
        return orElseImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<const expected&, F>::type or_else(F&& f) const&
        requires(std::is_constructible_v<T, const T&>)
    {
        return orElseImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<expected&&, F>::type or_else(F&& f) &&
        requires(std::is_constructible_v<T, T &&>)
    {
        return orElseImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<const expected&&, F>::type or_else(F&& f) const&&
        requires(std::is_constructible_v<T, const T &&>)
    {
        return orElseImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<expected&, F>::type transform_error(F&& f) &
        requires(std::is_constructible_v<T, T&>)
    {
        return transformErrorImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<const expected&, F>::type transform_error(F&& f) const&
        requires(std::is_constructible_v<T, const T&>)
    {
        return transformErrorImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<expected&&, F>::type transform_error(F&& f) &&
        requires(std::is_constructible_v<T, T &&>)
    {
        return transformErrorImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<const expected&&, F>::type transform_error(F&& f) const&&
        requires(std::is_constructible_v<T, const T &&>)
    {
        return transformErrorImpl(std::move(*this), std::forward<F>(f));
    }

#pragma endregion

#pragma region Modifiers

    // https://en.cppreference.com/w/cpp/utility/expected/emplace
    template <class... Args>
    constexpr T& emplace(Args&&... args) noexcept
        requires(std::is_nothrow_constructible_v<T, Args...>)
    {
        destroy();
        std::construct_at(std::addressof(value_), std::forward<Args>(args)...);
        hasValue_ = true;
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/emplace
    template <class U, class... Args>
    constexpr T& emplace(std::initializer_list<U> il, Args&&... args) noexcept
        requires(std::is_nothrow_constructible_v<T, std::initializer_list<U>&, Args...>)
    {
        destroy();
        std::construct_at(std::addressof(value_), il, std::forward<Args>(args)...);
        hasValue_ = true;
        return value_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/swap
    constexpr void swap(expected& other) noexcept(std::is_nothrow_swappable_v<T> && std::is_nothrow_swappable_v<E> &&
                                                  details::kNothrowUnionSwappable<T, E>)
        requires(std::is_swappable_v<T> && std::is_swappable_v<E> && details::kUnionSwappable<T, E>)
    {
        // Prefer custom swap friend function, and use move-based std::swap as a fallback
        // https://en.cppreference.com/w/cpp/language/adl
        using std::swap;
        if (hasValue_) {
            if (other.hasValue_) {
                swap(value_, other.value_);
            } else {
                details::UnionSwap(error_, other.value_, std::move(other.error_), std::move(value_));
                hasValue_       = false;
                other.hasValue_ = true;
            }
        } else {
            if (other.hasValue_) {
                other.swap(*this);
            } else {
                swap(error_, other.error_);
            }
        }
    }

#pragma endregion Modifiers
#pragma endregion Member functions

#pragma region Non-member functions

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    template <class T2, class E2>
        requires(!std::is_void_v<T2>)
    [[nodiscard]] friend constexpr bool operator==(const expected& lhs, const breaded::expected<T2, E2>& rhs) noexcept(
        noexcept(*lhs == *rhs) && noexcept(lhs.error() == rhs.error())) {
        if (lhs.has_value()) {
            if (rhs.has_value()) {
                return *lhs == *rhs;
            } else {
                return false;
            }
        } else {
            if (rhs.has_value()) {
                return false;
            } else {
                return lhs.error() == rhs.error();
            }
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    template <class T2>
    [[nodiscard]] friend constexpr bool operator==(const expected& x, const T2& val) noexcept(noexcept(*x == val)) {
        if (x.hasValue_) {
            return x.value_ == val;
        } else {
            return false;
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    template <class E2>
    [[nodiscard]] friend constexpr bool operator==(const expected& x,
                                                   const breaded::unexpected<E2>& e) noexcept(noexcept(x.error() ==
                                                                                                       e.error())) {
        if (x.hasValue_) {
            return false;
        } else {
            return x.error_ == e.error();
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/swap2
    friend constexpr void swap(expected& lhs, expected& rhs) noexcept(noexcept(lhs.swap(rhs)))
        requires(std::is_swappable_v<T> && std::is_swappable_v<E> && details::kUnionSwappable<T, E>)
    {
        lhs.swap(rhs);
    }
#pragma endregion

    // ⚠️ Implementation detail.
    // A constructor to initialize via RVO in the monadic function implementation
    // This works even for non-moveable types
    template <class F, class... Args>
        requires(std::is_invocable_v<F, Args...>)
    constexpr explicit expected(details::in_place_from_return_t, F&& f, Args&&... args)
        : value_(std::invoke(std::forward<F>(f), std::forward<Args>(args)...)), hasValue_(true) {}

    // ⚠️ Implementation detail.
    // A constructor to initialize via RVO in the monadic function implementation
    // This works even for non-moveable types
    template <class F, class... Args>
        requires(std::is_invocable_v<F, Args...>)
    constexpr explicit expected(details::unexpect_from_return_t, F&& f, Args&&... args)
        : error_(std::invoke(std::forward<F>(f), std::forward<Args>(args)...)), hasValue_(false) {}

private:
    template <class Other>
    constexpr void constructFromExpected(Other&& other) {
        if (other.has_value()) {
            std::construct_at(std::addressof(value_), *std::forward<Other>(other));
        } else {
            std::construct_at(std::addressof(error_), std::forward<Other>(other).error());
        }
        hasValue_ = other.has_value();
    }

    template <class Other>
    constexpr void assignFromExpected(Other&& other) {
        if (other.has_value()) {
            if (hasValue_) {
                // If this->has_value() equals other.has_value(), assigns the value contained in other.
                value_ = *std::forward<Other>(other);
            } else {
                details::UnionAssign(value_, error_, *std::forward<Other>(other));
            }
        } else {
            if (hasValue_) {
                details::UnionAssign(error_, value_, std::forward<Other>(other).error());
            } else {
                // If this->has_value() equals other.has_value(), assigns the value contained in other.
                error_ = std::forward<Other>(other).error();
            }
        }
        // If no exception was thrown, after assignment, has_value() is equal to other.has_value().
        hasValue_ = other.has_value();
    }

    constexpr void destroy() noexcept {
        if (hasValue_) {
            std::destroy_at(std::addressof(value_));
        } else {
            std::destroy_at(std::addressof(error_));
        }
    }

    template <class Self, class F>
    constexpr static AndThenReturn<Self, F>::type andThenImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return std::invoke(std::forward<F>(f), *std::forward<Self>(self));
        } else {
            return AndThenReturn<Self, F>::type(breaded::unexpect, std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static TransformReturn<Self, F>::type transformImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return TransformReturn<Self, F>::type(details::in_place_from_return, std::forward<F>(f),
                                                  *std::forward<Self>(self));
        } else {
            return TransformReturn<Self, F>::type(breaded::unexpect, std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static OrElseReturn<Self, F>::type orElseImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return OrElseReturn<Self, F>::type(*std::forward<Self>(self));
        } else {
            return std::invoke(std::forward<F>(f), std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static TransformErrorReturn<Self, F>::type transformErrorImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return TransformErrorReturn<Self, F>::type(*std::forward<Self>(self));
        } else {
            return TransformErrorReturn<Self, F>::type(details::unexpect_from_return, std::forward<F>(f),
                                                       std::forward<Self>(self).error());
        }
    }

    union {
        T value_;
        E error_;
    };
    bool hasValue_;
};

#pragma endregion

#pragma region Expected void
template <class T, class E>
    requires(std::is_void_v<T>)
class expected<T, E> {
    // partial specialization of expected for void types
    // https://github.com/cplusplus/draft/blob/d29ef68d3c7a1c2317f4670f08bbc8a63828ce35/source/utilities.tex#L6909

    static_assert(details::ValidExpectedErrorType<E>);

    template <typename Self, typename F>
    struct AndThenReturn {
        using type = std::remove_cvref_t<std::invoke_result_t<F>>;
    };

    template <typename Self, typename F>
    struct TransformReturn {
        using URaw = std::invoke_result_t<F>;
        using U    = std::remove_cvref_t<URaw>;
        using type = breaded::expected<U, E>;
    };

    template <typename Self, typename F>
    struct OrElseReturn {
        using type = std::remove_cvref_t<std::invoke_result_t<F, decltype(std::declval<Self>().error())>>;
    };

    template <typename Self, typename F>
    struct TransformErrorReturn {
        using GRaw = std::invoke_result_t<F, decltype(std::declval<Self>().error())>;
        using G    = std::remove_cvref_t<GRaw>;
        using type = breaded::expected<T, G>;
    };

public:
#pragma region Member types
    using value_type      = T;
    using error_type      = E;
    using unexpected_type = breaded::unexpected<E>;
#pragma endregion

#pragma region Member alias templates
    // Member alias templates
    template <class U>
    using rebind = breaded::expected<U, error_type>;
#pragma endregion

private:
    template <class U>
    static constexpr bool kExpectedConvertibleToErrorType =
        std::is_constructible_v<unexpected_type, U&> || std::is_constructible_v<unexpected_type, U> ||
        std::is_constructible_v<unexpected_type, const U&> || std::is_constructible_v<unexpected_type, const U>;

public:
    // Default empty constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected() noexcept : hasValue_(true) {}

    // Non-trivial copy constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected(const expected& other) noexcept(std::is_nothrow_copy_constructible_v<E>)
        requires(std::is_copy_constructible_v<E> && !std::is_trivially_copy_constructible_v<E>)
    {
        constructFromExpected(other);
    }

    // Trivial copy constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    expected(const expected& other)
        requires(std::is_copy_constructible_v<E> && std::is_trivially_copy_constructible_v<E>)
    = default;

    // This constructor is defined as deleted unless is_copy_constructible_v<E> is true
    // https://github.com/cplusplus/draft/blob/d29ef68d3c7a1c2317f4670f08bbc8a63828ce35/source/utilities.tex#L3373
    expected(const expected&) = delete;

    // Non-trivial move constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr expected(expected&& other) noexcept(std::is_nothrow_move_constructible_v<E>)
        requires(std::is_move_constructible_v<E> && !std::is_trivially_move_constructible_v<E>)
    {
        constructFromExpected(std::move(other));
    }

    // Trivial move constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    expected(expected&& other)
        requires(std::is_move_constructible_v<E> && std::is_trivially_move_constructible_v<E>)
    = default;

    // Converting copy constructor from expected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class G>
        requires(!std::is_same_v<expected, breaded::expected<U, G>> && std::is_void_v<U> &&
                 std::is_constructible_v<E, const G&> && !kExpectedConvertibleToErrorType<breaded::expected<U, G>>)
    constexpr explicit(!std::is_convertible_v<const G&, E>)
        expected(const breaded::expected<U, G>& other) noexcept(std::is_nothrow_constructible_v<E, const G&>) {
        constructFromExpected(other);
    }

    // Converting move constructor from expected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class G>
        requires(!std::is_same_v<expected, breaded::expected<U, G>> && std::is_void_v<U> &&
                 std::is_constructible_v<E, G> && !kExpectedConvertibleToErrorType<breaded::expected<U, G>>)
    constexpr explicit(!std::is_convertible_v<G, E>)
        expected(breaded::expected<U, G>&& other) noexcept(std::is_nothrow_constructible_v<E, G>) {
        constructFromExpected(std::move(other));
    }

    // Copy constructor from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class G>
        requires(std::is_constructible_v<E, const G&>)
    constexpr explicit(!std::is_convertible_v<const G&, E>)
        expected(const breaded::unexpected<G>& e) noexcept(std::is_nothrow_constructible_v<E, const G&>)
        : error_(e.error()), hasValue_(false) {}

    // Move constructor from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class G>
        requires(std::is_constructible_v<E, G>)
    constexpr explicit(!std::is_convertible_v<G, E>)
        expected(breaded::unexpected<G>&& e) noexcept(std::is_nothrow_constructible_v<E, G>)
        : error_(std::move(e).error()), hasValue_(false) {}

    // In-place empty constructor
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    constexpr explicit expected(std::in_place_t) noexcept : hasValue_(true) {}

    // In-place error constructor from args
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class... Args>
        requires(std::is_constructible_v<E, Args...>)
    constexpr explicit expected(breaded::unexpect_t,
                                Args&&... args) noexcept(std::is_nothrow_constructible_v<E, Args...>)
        : error_(std::forward<Args>(args)...), hasValue_(false) {}

    // In-place error constructor from initializer list
    // https://en.cppreference.com/w/cpp/utility/expected/expected
    template <class U, class... Args>
        requires(std::is_constructible_v<E, std::initializer_list<U>&, Args...>)
    constexpr explicit expected(breaded::unexpect_t, std::initializer_list<U> il, Args&&... args) noexcept(
        std::is_nothrow_constructible_v<E, std::initializer_list<U>&, Args...>)
        : error_(il, std::forward<Args>(args)...), hasValue_(false) {}

    // Non-trivial destructor
    // https://en.cppreference.com/w/cpp/utility/expected/~expected#void_partial_specialization_destructor
    constexpr ~expected() noexcept { destroy(); }

    // Trivial destructor
    // https://en.cppreference.com/w/cpp/utility/expected/~expected#void_partial_specialization_destructor
    ~expected()
        requires(std::is_trivially_destructible_v<E>)
    = default;

#pragma region Assignment

    // Non-trivial copy assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    constexpr expected& operator=(const expected& other) noexcept(std::is_nothrow_copy_assignable_v<E> &&
                                                                  std::is_nothrow_copy_constructible_v<E>)
        requires(std::is_copy_assignable_v<E> && std::is_copy_constructible_v<E> &&
                 !std::is_trivially_copy_assignable_v<E>)
    {
        assignFromExpected(other);
        return *this;
    }

    // Trivial copy assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    expected& operator=(const expected& other)
        requires(std::is_copy_assignable_v<E> && std::is_copy_constructible_v<E> &&
                 std::is_trivially_copy_assignable_v<E>)
    = default;

    // This operator is defined as deleted unless is_copy_assignable_v<E> is true and is_copy_constructible_v<E> is true
    // https://github.com/cplusplus/draft/blob/d29ef68d3c7a1c2317f4670f08bbc8a63828ce35/source/utilities.tex#L9007
    expected& operator=(const expected&) = delete;

    // Non-trivial move assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    constexpr expected& operator=(expected&& other) noexcept(std::is_nothrow_move_assignable_v<E> &&
                                                             std::is_nothrow_move_constructible_v<E>)
        requires(std::is_move_assignable_v<E> && std::is_move_constructible_v<E> &&
                 !std::is_trivially_move_assignable_v<E>)
    {
        assignFromExpected(std::move(other));
        return *this;
    }

    // Trivial move assignment
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    expected& operator=(expected&& other)
        requires(std::is_move_assignable_v<E> && std::is_move_constructible_v<E> &&
                 std::is_trivially_move_assignable_v<E>)
    = default;

    // Copy assignment from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    template <class G>
    constexpr expected& operator=(const breaded::unexpected<G>& other) noexcept(
        std::is_nothrow_constructible_v<E, const G&> && std::is_nothrow_assignable_v<E, const G&>)
        requires(std::is_constructible_v<E, const G&> && std::is_assignable_v<E, const G&>)
    {
        if (hasValue_) {
            std::construct_at(std::addressof(error_), other.error());
            hasValue_ = false;
        } else {
            error_ = other.error();
        }
        return *this;
    }

    // Move assignment from unexpected
    // https://en.cppreference.com/w/cpp/utility/expected/operator%3D
    template <class G>
    constexpr expected& operator=(breaded::unexpected<G>&& other) noexcept(std::is_nothrow_constructible_v<E, G> &&
                                                                           std::is_nothrow_assignable_v<E, G>)
        requires(std::is_constructible_v<E, G> && std::is_assignable_v<E, G>)
    {
        if (hasValue_) {
            std::construct_at(std::addressof(error_), std::move(other).error());
            hasValue_ = false;
        } else {
            error_ = std::move(other).error();
        }
        return *this;
    }
#pragma endregion

#pragma region Observers

    // https://en.cppreference.com/w/cpp/utility/expected/operator*
    constexpr void operator*() const noexcept {
        assert(hasValue_ && "Attempted to call expected::operator* when has_value() == false");
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_bool
    [[nodiscard]] constexpr explicit operator bool() const noexcept { return hasValue_; }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_bool
    [[nodiscard]] constexpr bool has_value() const noexcept { return hasValue_; }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr void value() const& noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::as_const(error_));
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/value
    constexpr void value() && noexcept(false) {
        if (!hasValue_) {
            throw breaded::bad_expected_access(std::move(error_));
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr E& error() & noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return error_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr const E& error() const& noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return error_;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr E&& error() && noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return std::move(error_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error
    [[nodiscard]] constexpr const E&& error() const&& noexcept {
        // The behavior is undefined if this->has_value() is true.
        assert(!hasValue_ && "Attempted to call expected::error when has_value() == true");
        return std::move(error_);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error_or
    template <class G = E>
        requires(std::is_copy_constructible_v<E> && std::is_convertible_v<G, E>)
    [[nodiscard]] constexpr E error_or(G&& default_value) const& noexcept(std::is_nothrow_copy_constructible_v<E> &&
                                                                          std::is_nothrow_convertible_v<G, E>) {
        if (!hasValue_) {
            return error_;
        }
        return std::forward<G>(default_value);
    }

    // https://en.cppreference.com/w/cpp/utility/expected/error_or
    template <class G = E>
        requires(std::is_move_constructible_v<E> && std::is_convertible_v<G, E>)
    [[nodiscard]] constexpr E error_or(G&& default_value) && noexcept(std::is_nothrow_move_constructible_v<E> &&
                                                                      std::is_nothrow_convertible_v<G, E>) {
        if (!hasValue_) {
            return std::move(error_);
        }
        return std::forward<G>(default_value);
    }

#pragma endregion

#pragma region Monadic operations

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<expected&, F>::type and_then(F&& f) &
        requires(std::is_constructible_v<E, E&>)
    {
        return andThenImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<const expected&, F>::type and_then(F&& f) const&
        requires(std::is_constructible_v<E, const E&>)
    {
        return andThenImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<expected&&, F>::type and_then(F&& f) &&
        requires(std::is_constructible_v<E, E &&>)
    {
        return andThenImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/and_then
    template <class F>
    constexpr AndThenReturn<const expected&&, F>::type and_then(F&& f) const&&
        requires(std::is_constructible_v<E, const E &&>)
    {
        return andThenImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<expected&, F>::type transform(F&& f) &
        requires(std::is_constructible_v<E, E&>)
    {
        return transformImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<const expected&, F>::type transform(F&& f) const&
        requires(std::is_constructible_v<E, const E&>)
    {
        return transformImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<expected&&, F>::type transform(F&& f) &&
        requires(std::is_constructible_v<E, E &&>)
    {
        return transformImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform
    template <class F>
    constexpr TransformReturn<const expected&&, F>::type transform(F&& f) const&&
        requires(std::is_constructible_v<E, const E &&>)
    {
        return transformImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<expected&, F>::type or_else(F&& f) & {
        return orElseImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<const expected&, F>::type or_else(F&& f) const& {
        return orElseImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<expected&&, F>::type or_else(F&& f) && {
        return orElseImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/or_else
    template <class F>
    constexpr OrElseReturn<const expected&&, F>::type or_else(F&& f) const&& {
        return orElseImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<expected&, F>::type transform_error(F&& f) & {
        return transformErrorImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<const expected&, F>::type transform_error(F&& f) const& {
        return transformErrorImpl(*this, std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<expected&&, F>::type transform_error(F&& f) && {
        return transformErrorImpl(std::move(*this), std::forward<F>(f));
    }

    // https://en.cppreference.com/w/cpp/utility/expected/transform_error
    template <class F>
    constexpr TransformErrorReturn<const expected&&, F>::type transform_error(F&& f) const&& {
        return transformErrorImpl(std::move(*this), std::forward<F>(f));
    }

#pragma endregion

#pragma region Modifiers

    // https://en.cppreference.com/w/cpp/utility/expected/emplace
    constexpr void emplace() noexcept {
        destroy();
        hasValue_ = true;
    }

    // https://en.cppreference.com/w/cpp/utility/expected/swap
    constexpr void swap(expected& other) noexcept(std::is_nothrow_swappable_v<E> &&
                                                  std::is_nothrow_move_constructible_v<E>)
        requires(std::is_swappable_v<E> && std::is_move_constructible_v<E>)
    {
        // Prefer custom swap friend function, and use move-based std::swap as a fallback
        // https://en.cppreference.com/w/cpp/language/adl
        using std::swap;
        if (hasValue_) {
            if (!other.hasValue_) {
                std::construct_at(std::addressof(error_), std::move(other.error_));
                std::destroy_at(std::addressof(other.error_));
                hasValue_       = false;
                other.hasValue_ = true;
            }
        } else {
            if (other.hasValue_) {
                other.swap(*this);
            } else {
                swap(error_, other.error_);
            }
        }
    }

#pragma endregion

#pragma region Non-member functions

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    template <class T2, class E2>
        requires(std::is_void_v<T2>)
    [[nodiscard]] friend constexpr bool operator==(const expected& lhs, const breaded::expected<T2, E2>& rhs) noexcept(
        noexcept(lhs.error() == rhs.error())) {
        if (lhs.hasValue_) {
            return rhs.hasValue_;
        } else {
            if (rhs.hasValue_) {
                return false;
            } else {
                return lhs.error_ == rhs.error_;
            }
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    template <class E2>
    [[nodiscard]] friend constexpr bool operator==(const expected& x,
                                                   const breaded::unexpected<E2>& e) noexcept(noexcept(x.error() ==
                                                                                                       e.error())) {
        if (x.hasValue_) {
            return false;
        } else {
            return x.error_ == e.error();
        }
    }

    // https://en.cppreference.com/w/cpp/utility/expected/operator_cmp
    [[nodiscard]] friend constexpr void swap(expected& lhs, expected& rhs) noexcept(noexcept(lhs.swap(rhs)))
        requires(std::is_swappable_v<E> && std::is_move_constructible_v<E>)
    {
        lhs.swap(rhs);
    }

#pragma endregion

    // ⚠️ Implementation detail.
    // A constructor to initialize via RVO in the monadic function implementation
    // This works even for non-moveable types
    template <class F, class... Args>
        requires(std::is_invocable_v<F, Args...>)
    constexpr explicit expected(details::in_place_from_return_t, F&& f, Args&&... args) : hasValue_(true) {
        std::invoke(std::forward<F>(f), std::forward<Args>(args)...);
    }

    // ⚠️ Implementation detail.
    // A constructor to initialize via RVO in the monadic function implementation
    // This works even for non-moveable types
    template <class F, class... Args>
        requires(std::is_invocable_v<F, Args...>)
    constexpr explicit expected(details::unexpect_from_return_t, F&& f, Args&&... args)
        : error_(std::invoke(std::forward<F>(f), std::forward<Args>(args)...)), hasValue_(false) {}

private:
    template <class Other>
    constexpr void constructFromExpected(Other&& other) {
        if (!other.has_value()) {
            std::construct_at(std::addressof(error_), std::forward<Other>(other).error());
        }
        hasValue_ = other.has_value();
    }

    template <class Other>
    constexpr void assignFromExpected(Other&& other) {
        if (other.has_value()) {
            std::destroy_at(std::addressof(error_));
        } else {
            if (hasValue_) {
                std::construct_at(std::addressof(error_), std::forward<Other>(other).error());
            } else {
                // If this->has_value() equals other.has_value(), assigns the value contained in other.
                error_ = std::forward<Other>(other).error();
            }
        }
        // If no exception was thrown, after assignment, has_value() is equal to other.has_value().
        hasValue_ = other.has_value();
    }

    constexpr void destroy() noexcept {
        if (!hasValue_) {
            std::destroy_at(std::addressof(error_));
        }
    }

    template <class Self, class F>
    constexpr static AndThenReturn<Self, F>::type andThenImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return std::invoke(std::forward<F>(f));
        } else {
            return AndThenReturn<Self, F>::type(breaded::unexpect, std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static TransformReturn<Self, F>::type transformImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return TransformReturn<Self, F>::type(details::in_place_from_return, std::forward<F>(f));
        } else {
            return TransformReturn<Self, F>::type(breaded::unexpect, std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static OrElseReturn<Self, F>::type orElseImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return OrElseReturn<Self, F>::type();
        } else {
            return std::invoke(std::forward<F>(f), std::forward<Self>(self).error());
        }
    }

    template <class Self, class F>
    constexpr static TransformErrorReturn<Self, F>::type transformErrorImpl(Self&& self, F&& f) {
        if (self.hasValue_) {
            return TransformErrorReturn<Self, F>::type();
        } else {
            return TransformErrorReturn<Self, F>::type(details::unexpect_from_return, std::forward<F>(f),
                                                       std::forward<Self>(self).error());
        }
    }

    union {
        E error_;
    };
    bool hasValue_;
};

#pragma endregion

}; // namespace breaded