common: Implement a subset of P0323 (std::expected)

This implementation is based on and is a subset of the proposed implementation of std::expected
https://github.com/TartanLlama/expected/blob/master/include/tl/expected.hpp

src/common/CMakeLists.txt

@@ -55,6 +55,7 @@ add_library(common STATIC
     dynamic_library.h
     error.cpp
     error.h
+    expected.h
     fiber.cpp
     fiber.h
     fs/file.cpp

src/common/expected.h

@@ -0,0 +1,987 @@
+// Copyright 2021 yuzu Emulator Project
+// Licensed under GPLv2 or any later version
+// Refer to the license.txt file included.
+
+// This is based on the proposed implementation of std::expected (P0323)
+// https://github.com/TartanLlama/expected/blob/master/include/tl/expected.hpp
+
+#pragma once
+
+#include <type_traits>
+#include <utility>
+
+namespace Common {
+
+template <typename T, typename E>
+class Expected;
+
+template <typename E>
+class Unexpected {
+public:
+    Unexpected() = delete;
+
+    constexpr explicit Unexpected(const E& e) : m_val{e} {}
+
+    constexpr explicit Unexpected(E&& e) : m_val{std::move(e)} {}
+
+    constexpr E& value() & {
+        return m_val;
+    }
+
+    constexpr const E& value() const& {
+        return m_val;
+    }
+
+    constexpr E&& value() && {
+        return std::move(m_val);
+    }
+
+    constexpr const E&& value() const&& {
+        return std::move(m_val);
+    }
+
+private:
+    E m_val;
+};
+
+template <typename E>
+constexpr auto operator<=>(const Unexpected<E>& lhs, const Unexpected<E>& rhs) {
+    return lhs.value() <=> rhs.value();
+}
+
+struct unexpect_t {
+    constexpr explicit unexpect_t() = default;
+};
+
+namespace detail {
+
+struct no_init_t {
+    constexpr explicit no_init_t() = default;
+};
+
+/**
+ * This specialization is for when T is not trivially destructible,
+ * so the destructor must be called on destruction of `expected'
+ * Additionally, this requires E to be trivially destructible
+ */
+template <typename T, typename E, bool = std::is_trivially_destructible_v<T>>
+requires std::is_trivially_destructible_v<E>
+struct expected_storage_base {
+    constexpr expected_storage_base() : m_val{T{}}, m_has_val{true} {}
+
+    constexpr expected_storage_base(no_init_t) : m_has_val{false} {}
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<T, Args&&...>>* = nullptr>
+    constexpr expected_storage_base(std::in_place_t, Args&&... args)
+        : m_val{std::forward<Args>(args)...}, m_has_val{true} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr expected_storage_base(std::in_place_t, std::initializer_list<U> il, Args&&... args)
+        : m_val{il, std::forward<Args>(args)...}, m_has_val{true} {}
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<E, Args&&...>>* = nullptr>
+    constexpr explicit expected_storage_base(unexpect_t, Args&&... args)
+        : m_unexpect{std::forward<Args>(args)...}, m_has_val{false} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<E, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr explicit expected_storage_base(unexpect_t, std::initializer_list<U> il,
+                                             Args&&... args)
+        : m_unexpect{il, std::forward<Args>(args)...}, m_has_val{false} {}
+
+    ~expected_storage_base() {
+        if (m_has_val) {
+            m_val.~T();
+        }
+    }
+
+    union {
+        T m_val;
+        Unexpected<E> m_unexpect;
+    };
+
+    bool m_has_val;
+};
+
+/**
+ * This specialization is for when T is trivially destructible,
+ * so the destructor of `expected` can be trivial
+ * Additionally, this requires E to be trivially destructible
+ */
+template <typename T, typename E>
+requires std::is_trivially_destructible_v<E>
+struct expected_storage_base<T, E, true> {
+    constexpr expected_storage_base() : m_val{T{}}, m_has_val{true} {}
+
+    constexpr expected_storage_base(no_init_t) : m_has_val{false} {}
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<T, Args&&...>>* = nullptr>
+    constexpr expected_storage_base(std::in_place_t, Args&&... args)
+        : m_val{std::forward<Args>(args)...}, m_has_val{true} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr expected_storage_base(std::in_place_t, std::initializer_list<U> il, Args&&... args)
+        : m_val{il, std::forward<Args>(args)...}, m_has_val{true} {}
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<E, Args&&...>>* = nullptr>
+    constexpr explicit expected_storage_base(unexpect_t, Args&&... args)
+        : m_unexpect{std::forward<Args>(args)...}, m_has_val{false} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<E, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr explicit expected_storage_base(unexpect_t, std::initializer_list<U> il,
+                                             Args&&... args)
+        : m_unexpect{il, std::forward<Args>(args)...}, m_has_val{false} {}
+
+    ~expected_storage_base() = default;
+
+    union {
+        T m_val;
+        Unexpected<E> m_unexpect;
+    };
+
+    bool m_has_val;
+};
+
+template <typename T, typename E>
+struct expected_operations_base : expected_storage_base<T, E> {
+    using expected_storage_base<T, E>::expected_storage_base;
+
+    template <typename... Args>
+    void construct(Args&&... args) noexcept {
+        new (std::addressof(this->m_val)) T{std::forward<Args>(args)...};
+        this->m_has_val = true;
+    }
+
+    template <typename Rhs>
+    void construct_with(Rhs&& rhs) noexcept {
+        new (std::addressof(this->m_val)) T{std::forward<Rhs>(rhs).get()};
+        this->m_has_val = true;
+    }
+
+    template <typename... Args>
+    void construct_error(Args&&... args) noexcept {
+        new (std::addressof(this->m_unexpect)) Unexpected<E>{std::forward<Args>(args)...};
+        this->m_has_val = false;
+    }
+
+    void assign(const expected_operations_base& rhs) noexcept {
+        if (!this->m_has_val && rhs.m_has_val) {
+            geterr().~Unexpected<E>();
+            construct(rhs.get());
+        } else {
+            assign_common(rhs);
+        }
+    }
+
+    void assign(expected_operations_base&& rhs) noexcept {
+        if (!this->m_has_val && rhs.m_has_val) {
+            geterr().~Unexpected<E>();
+            construct(std::move(rhs).get());
+        } else {
+            assign_common(rhs);
+        }
+    }
+
+    template <typename Rhs>
+    void assign_common(Rhs&& rhs) {
+        if (this->m_has_val) {
+            if (rhs.m_has_val) {
+                get() = std::forward<Rhs>(rhs).get();
+            } else {
+                destroy_val();
+                construct_error(std::forward<Rhs>(rhs).geterr());
+            }
+        } else {
+            if (!rhs.m_has_val) {
+                geterr() = std::forward<Rhs>(rhs).geterr();
+            }
+        }
+    }
+
+    bool has_value() const {
+        return this->m_has_val;
+    }
+
+    constexpr T& get() & {
+        return this->m_val;
+    }
+
+    constexpr const T& get() const& {
+        return this->m_val;
+    }
+
+    constexpr T&& get() && {
+        return std::move(this->m_val);
+    }
+
+    constexpr const T&& get() const&& {
+        return std::move(this->m_val);
+    }
+
+    constexpr Unexpected<E>& geterr() & {
+        return this->m_unexpect;
+    }
+
+    constexpr const Unexpected<E>& geterr() const& {
+        return this->m_unexpect;
+    }
+
+    constexpr Unexpected<E>&& geterr() && {
+        return std::move(this->m_unexpect);
+    }
+
+    constexpr const Unexpected<E>&& geterr() const&& {
+        return std::move(this->m_unexpect);
+    }
+
+    constexpr void destroy_val() {
+        get().~T();
+    }
+};
+
+/**
+ * This manages conditionally having a trivial copy constructor
+ * This specialization is for when T is trivially copy constructible
+ * Additionally, this requires E to be trivially copy constructible
+ */
+template <typename T, typename E, bool = std::is_trivially_copy_constructible_v<T>>
+requires std::is_trivially_copy_constructible_v<E>
+struct expected_copy_base : expected_operations_base<T, E> {
+    using expected_operations_base<T, E>::expected_operations_base;
+};
+
+/**
+ * This specialization is for when T is not trivially copy constructible
+ * Additionally, this requires E to be trivially copy constructible
+ */
+template <typename T, typename E>
+requires std::is_trivially_copy_constructible_v<E>
+struct expected_copy_base<T, E, false> : expected_operations_base<T, E> {
+    using expected_operations_base<T, E>::expected_operations_base;
+
+    expected_copy_base() = default;
+
+    expected_copy_base(const expected_copy_base& rhs)
+        : expected_operations_base<T, E>{no_init_t{}} {
+        if (rhs.has_value()) {
+            this->construct_with(rhs);
+        } else {
+            this->construct_error(rhs.geterr());
+        }
+    }
+
+    expected_copy_base(expected_copy_base&&) = default;
+
+    expected_copy_base& operator=(const expected_copy_base&) = default;
+
+    expected_copy_base& operator=(expected_copy_base&&) = default;
+};
+
+/**
+ * This manages conditionally having a trivial move constructor
+ * This specialization is for when T is trivially move constructible
+ * Additionally, this requires E to be trivially move constructible
+ */
+template <typename T, typename E, bool = std::is_trivially_move_constructible_v<T>>
+requires std::is_trivially_move_constructible_v<E>
+struct expected_move_base : expected_copy_base<T, E> {
+    using expected_copy_base<T, E>::expected_copy_base;
+};
+
+/**
+ * This specialization is for when T is not trivially move constructible
+ * Additionally, this requires E to be trivially move constructible
+ */
+template <typename T, typename E>
+requires std::is_trivially_move_constructible_v<E>
+struct expected_move_base<T, E, false> : expected_copy_base<T, E> {
+    using expected_copy_base<T, E>::expected_copy_base;
+
+    expected_move_base() = default;
+
+    expected_move_base(const expected_move_base&) = default;
+
+    expected_move_base(expected_move_base&& rhs) noexcept(std::is_nothrow_move_constructible_v<T>)
+        : expected_copy_base<T, E>{no_init_t{}} {
+        if (rhs.has_value()) {
+            this->construct_with(std::move(rhs));
+        } else {
+            this->construct_error(std::move(rhs.geterr()));
+        }
+    }
+
+    expected_move_base& operator=(const expected_move_base&) = default;
+
+    expected_move_base& operator=(expected_move_base&&) = default;
+};
+
+/**
+ * This manages conditionally having a trivial copy assignment operator
+ * This specialization is for when T is trivially copy assignable
+ * Additionally, this requires E to be trivially copy assignable
+ */
+template <typename T, typename E,
+          bool = std::conjunction_v<std::is_trivially_copy_assignable<T>,
+                                    std::is_trivially_copy_constructible<T>,
+                                    std::is_trivially_destructible<T>>>
+requires std::conjunction_v<std::is_trivially_copy_assignable<E>,
+                            std::is_trivially_copy_constructible<E>,
+                            std::is_trivially_destructible<E>>
+struct expected_copy_assign_base : expected_move_base<T, E> {
+    using expected_move_base<T, E>::expected_move_base;
+};
+
+/**
+ * This specialization is for when T is not trivially copy assignable
+ * Additionally, this requires E to be trivially copy assignable
+ */
+template <typename T, typename E>
+requires std::conjunction_v<std::is_trivially_copy_assignable<E>,
+                            std::is_trivially_copy_constructible<E>,
+                            std::is_trivially_destructible<E>>
+struct expected_copy_assign_base<T, E, false> : expected_move_base<T, E> {
+    using expected_move_base<T, E>::expected_move_base;
+
+    expected_copy_assign_base() = default;
+
+    expected_copy_assign_base(const expected_copy_assign_base&) = default;
+
+    expected_copy_assign_base(expected_copy_assign_base&&) = default;
+
+    expected_copy_assign_base& operator=(const expected_copy_assign_base& rhs) {
+        this->assign(rhs);
+        return *this;
+    }
+
+    expected_copy_assign_base& operator=(expected_copy_assign_base&&) = default;
+};
+
+/**
+ * This manages conditionally having a trivial move assignment operator
+ * This specialization is for when T is trivially move assignable
+ * Additionally, this requires E to be trivially move assignable
+ */
+template <typename T, typename E,
+          bool = std::conjunction_v<std::is_trivially_move_assignable<T>,
+                                    std::is_trivially_move_constructible<T>,
+                                    std::is_trivially_destructible<T>>>
+requires std::conjunction_v<std::is_trivially_move_assignable<E>,
+                            std::is_trivially_move_constructible<E>,
+                            std::is_trivially_destructible<E>>
+struct expected_move_assign_base : expected_copy_assign_base<T, E> {
+    using expected_copy_assign_base<T, E>::expected_copy_assign_base;
+};
+
+/**
+ * This specialization is for when T is not trivially move assignable
+ * Additionally, this requires E to be trivially move assignable
+ */
+template <typename T, typename E>
+requires std::conjunction_v<std::is_trivially_move_assignable<E>,
+                            std::is_trivially_move_constructible<E>,
+                            std::is_trivially_destructible<E>>
+struct expected_move_assign_base<T, E, false> : expected_copy_assign_base<T, E> {
+    using expected_copy_assign_base<T, E>::expected_copy_assign_base;
+
+    expected_move_assign_base() = default;
+
+    expected_move_assign_base(const expected_move_assign_base&) = default;
+
+    expected_move_assign_base(expected_move_assign_base&&) = default;
+
+    expected_move_assign_base& operator=(const expected_move_assign_base&) = default;
+
+    expected_move_assign_base& operator=(expected_move_assign_base&& rhs) noexcept(
+        std::conjunction_v<std::is_nothrow_move_constructible<T>,
+                           std::is_nothrow_move_assignable<T>>) {
+        this->assign(std::move(rhs));
+        return *this;
+    }
+};
+
+/**
+ * expected_delete_ctor_base will conditionally delete copy and move constructors
+ * depending on whether T is copy/move constructible
+ * Additionally, this requires E to be copy/move constructible
+ */
+template <typename T, typename E, bool EnableCopy = std::is_copy_constructible_v<T>,
+          bool EnableMove = std::is_move_constructible_v<T>>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>>
+struct expected_delete_ctor_base {
+    expected_delete_ctor_base() = default;
+    expected_delete_ctor_base(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base(expected_delete_ctor_base&&) noexcept = default;
+    expected_delete_ctor_base& operator=(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base& operator=(expected_delete_ctor_base&&) noexcept = default;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>>
+struct expected_delete_ctor_base<T, E, true, false> {
+    expected_delete_ctor_base() = default;
+    expected_delete_ctor_base(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base(expected_delete_ctor_base&&) noexcept = delete;
+    expected_delete_ctor_base& operator=(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base& operator=(expected_delete_ctor_base&&) noexcept = default;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>>
+struct expected_delete_ctor_base<T, E, false, true> {
+    expected_delete_ctor_base() = default;
+    expected_delete_ctor_base(const expected_delete_ctor_base&) = delete;
+    expected_delete_ctor_base(expected_delete_ctor_base&&) noexcept = default;
+    expected_delete_ctor_base& operator=(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base& operator=(expected_delete_ctor_base&&) noexcept = default;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>>
+struct expected_delete_ctor_base<T, E, false, false> {
+    expected_delete_ctor_base() = default;
+    expected_delete_ctor_base(const expected_delete_ctor_base&) = delete;
+    expected_delete_ctor_base(expected_delete_ctor_base&&) noexcept = delete;
+    expected_delete_ctor_base& operator=(const expected_delete_ctor_base&) = default;
+    expected_delete_ctor_base& operator=(expected_delete_ctor_base&&) noexcept = default;
+};
+
+/**
+ * expected_delete_assign_base will conditionally delete copy and move assignment operators
+ * depending on whether T is copy/move constructible + assignable
+ * Additionally, this requires E to be copy/move constructible + assignable
+ */
+template <
+    typename T, typename E,
+    bool EnableCopy = std::conjunction_v<std::is_copy_constructible<T>, std::is_copy_assignable<T>>,
+    bool EnableMove = std::conjunction_v<std::is_move_constructible<T>, std::is_move_assignable<T>>>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>,
+                            std::is_copy_assignable<E>, std::is_move_assignable<E>>
+struct expected_delete_assign_base {
+    expected_delete_assign_base() = default;
+    expected_delete_assign_base(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base(expected_delete_assign_base&&) noexcept = default;
+    expected_delete_assign_base& operator=(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base& operator=(expected_delete_assign_base&&) noexcept = default;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>,
+                            std::is_copy_assignable<E>, std::is_move_assignable<E>>
+struct expected_delete_assign_base<T, E, true, false> {
+    expected_delete_assign_base() = default;
+    expected_delete_assign_base(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base(expected_delete_assign_base&&) noexcept = default;
+    expected_delete_assign_base& operator=(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base& operator=(expected_delete_assign_base&&) noexcept = delete;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>,
+                            std::is_copy_assignable<E>, std::is_move_assignable<E>>
+struct expected_delete_assign_base<T, E, false, true> {
+    expected_delete_assign_base() = default;
+    expected_delete_assign_base(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base(expected_delete_assign_base&&) noexcept = default;
+    expected_delete_assign_base& operator=(const expected_delete_assign_base&) = delete;
+    expected_delete_assign_base& operator=(expected_delete_assign_base&&) noexcept = default;
+};
+
+template <typename T, typename E>
+requires std::conjunction_v<std::is_copy_constructible<E>, std::is_move_constructible<E>,
+                            std::is_copy_assignable<E>, std::is_move_assignable<E>>
+struct expected_delete_assign_base<T, E, false, false> {
+    expected_delete_assign_base() = default;
+    expected_delete_assign_base(const expected_delete_assign_base&) = default;
+    expected_delete_assign_base(expected_delete_assign_base&&) noexcept = default;
+    expected_delete_assign_base& operator=(const expected_delete_assign_base&) = delete;
+    expected_delete_assign_base& operator=(expected_delete_assign_base&&) noexcept = delete;
+};
+
+/**
+ * This is needed to be able to construct the expected_default_ctor_base which follows,
+ * while still conditionally deleting the default constructor.
+ */
+struct default_constructor_tag {
+    constexpr explicit default_constructor_tag() = default;
+};
+
+/**
+ * expected_default_ctor_base will ensure that expected
+ * has a deleted default constructor if T is not default constructible
+ * This specialization is for when T is default constructible
+ */
+template <typename T, typename E, bool Enable = std::is_default_constructible_v<T>>
+struct expected_default_ctor_base {
+    constexpr expected_default_ctor_base() noexcept = default;
+    constexpr expected_default_ctor_base(expected_default_ctor_base const&) noexcept = default;
+    constexpr expected_default_ctor_base(expected_default_ctor_base&&) noexcept = default;
+    expected_default_ctor_base& operator=(expected_default_ctor_base const&) noexcept = default;
+    expected_default_ctor_base& operator=(expected_default_ctor_base&&) noexcept = default;
+
+    constexpr explicit expected_default_ctor_base(default_constructor_tag) {}
+};
+
+template <typename T, typename E>
+struct expected_default_ctor_base<T, E, false> {
+    constexpr expected_default_ctor_base() noexcept = delete;
+    constexpr expected_default_ctor_base(expected_default_ctor_base const&) noexcept = default;
+    constexpr expected_default_ctor_base(expected_default_ctor_base&&) noexcept = default;
+    expected_default_ctor_base& operator=(expected_default_ctor_base const&) noexcept = default;
+    expected_default_ctor_base& operator=(expected_default_ctor_base&&) noexcept = default;
+
+    constexpr explicit expected_default_ctor_base(default_constructor_tag) {}
+};
+
+template <typename T, typename E, typename U>
+using expected_enable_forward_value =
+    std::enable_if_t<std::is_constructible_v<T, U&&> &&
+                     !std::is_same_v<std::remove_cvref_t<U>, std::in_place_t> &&
+                     !std::is_same_v<Expected<T, E>, std::remove_cvref_t<U>> &&
+                     !std::is_same_v<Unexpected<E>, std::remove_cvref_t<U>>>;
+
+template <typename T, typename E, typename U, typename G, typename UR, typename GR>
+using expected_enable_from_other = std::enable_if_t<
+    std::is_constructible_v<T, UR> && std::is_constructible_v<E, GR> &&
+    !std::is_constructible_v<T, Expected<U, G>&> && !std::is_constructible_v<T, Expected<U, G>&&> &&
+    !std::is_constructible_v<T, const Expected<U, G>&> &&
+    !std::is_constructible_v<T, const Expected<U, G>&&> &&
+    !std::is_convertible_v<Expected<U, G>&, T> && !std::is_convertible_v<Expected<U, G>&&, T> &&
+    !std::is_convertible_v<const Expected<U, G>&, T> &&
+    !std::is_convertible_v<const Expected<U, G>&&, T>>;
+
+} // namespace detail
+
+template <typename T, typename E>
+class Expected : private detail::expected_move_assign_base<T, E>,
+                 private detail::expected_delete_ctor_base<T, E>,
+                 private detail::expected_delete_assign_base<T, E>,
+                 private detail::expected_default_ctor_base<T, E> {
+public:
+    using value_type = T;
+    using error_type = E;
+    using unexpected_type = Unexpected<E>;
+
+    constexpr Expected() = default;
+    constexpr Expected(const Expected&) = default;
+    constexpr Expected(Expected&&) = default;
+    Expected& operator=(const Expected&) = default;
+    Expected& operator=(Expected&&) = default;
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<T, Args&&...>>* = nullptr>
+    constexpr Expected(std::in_place_t, Args&&... args)
+        : impl_base{std::in_place, std::forward<Args>(args)...},
+          ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<T, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr Expected(std::in_place_t, std::initializer_list<U> il, Args&&... args)
+        : impl_base{std::in_place, il, std::forward<Args>(args)...},
+          ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename G = E, std::enable_if_t<std::is_constructible_v<E, const G&>>* = nullptr,
+              std::enable_if_t<!std::is_convertible_v<const G&, E>>* = nullptr>
+    constexpr explicit Expected(const Unexpected<G>& e)
+        : impl_base{unexpect_t{}, e.value()}, ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename G = E, std::enable_if_t<std::is_constructible_v<E, const G&>>* = nullptr,
+              std::enable_if_t<std::is_convertible_v<const G&, E>>* = nullptr>
+    constexpr Expected(Unexpected<G> const& e)
+        : impl_base{unexpect_t{}, e.value()}, ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename G = E, std::enable_if_t<std::is_constructible_v<E, G&&>>* = nullptr,
+              std::enable_if_t<!std::is_convertible_v<G&&, E>>* = nullptr>
+    constexpr explicit Expected(Unexpected<G>&& e) noexcept(std::is_nothrow_constructible_v<E, G&&>)
+        : impl_base{unexpect_t{}, std::move(e.value())}, ctor_base{
+                                                             detail::default_constructor_tag{}} {}
+
+    template <typename G = E, std::enable_if_t<std::is_constructible_v<E, G&&>>* = nullptr,
+              std::enable_if_t<std::is_convertible_v<G&&, E>>* = nullptr>
+    constexpr Expected(Unexpected<G>&& e) noexcept(std::is_nothrow_constructible_v<E, G&&>)
+        : impl_base{unexpect_t{}, std::move(e.value())}, ctor_base{
+                                                             detail::default_constructor_tag{}} {}
+
+    template <typename... Args, std::enable_if_t<std::is_constructible_v<E, Args&&...>>* = nullptr>
+    constexpr explicit Expected(unexpect_t, Args&&... args)
+        : impl_base{unexpect_t{}, std::forward<Args>(args)...},
+          ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_constructible_v<E, std::initializer_list<U>&, Args&&...>>* =
+                  nullptr>
+    constexpr explicit Expected(unexpect_t, std::initializer_list<U> il, Args&&... args)
+        : impl_base{unexpect_t{}, il, std::forward<Args>(args)...},
+          ctor_base{detail::default_constructor_tag{}} {}
+
+    template <typename U, typename G,
+              std::enable_if_t<!(std::is_convertible_v<U const&, T> &&
+                                 std::is_convertible_v<G const&, E>)>* = nullptr,
+              detail::expected_enable_from_other<T, E, U, G, const U&, const G&>* = nullptr>
+    constexpr explicit Expected(const Expected<U, G>& rhs)
+        : ctor_base{detail::default_constructor_tag{}} {
+        if (rhs.has_value()) {
+            this->construct(*rhs);
+        } else {
+            this->construct_error(rhs.error());
+        }
+    }
+
+    template <typename U, typename G,
+              std::enable_if_t<(std::is_convertible_v<U const&, T> &&
+                                std::is_convertible_v<G const&, E>)>* = nullptr,
+              detail::expected_enable_from_other<T, E, U, G, const U&, const G&>* = nullptr>
+    constexpr Expected(const Expected<U, G>& rhs) : ctor_base{detail::default_constructor_tag{}} {
+        if (rhs.has_value()) {
+            this->construct(*rhs);
+        } else {
+            this->construct_error(rhs.error());
+        }
+    }
+
+    template <typename U, typename G,
+              std::enable_if_t<!(std::is_convertible_v<U&&, T> && std::is_convertible_v<G&&, E>)>* =
+                  nullptr,
+              detail::expected_enable_from_other<T, E, U, G, U&&, G&&>* = nullptr>
+    constexpr explicit Expected(Expected<U, G>&& rhs)
+        : ctor_base{detail::default_constructor_tag{}} {
+        if (rhs.has_value()) {
+            this->construct(std::move(*rhs));
+        } else {
+            this->construct_error(std::move(rhs.error()));
+        }
+    }
+
+    template <typename U, typename G,
+              std::enable_if_t<(std::is_convertible_v<U&&, T> && std::is_convertible_v<G&&, E>)>* =
+                  nullptr,
+              detail::expected_enable_from_other<T, E, U, G, U&&, G&&>* = nullptr>
+    constexpr Expected(Expected<U, G>&& rhs) : ctor_base{detail::default_constructor_tag{}} {
+        if (rhs.has_value()) {
+            this->construct(std::move(*rhs));
+        } else {
+            this->construct_error(std::move(rhs.error()));
+        }
+    }
+
+    template <typename U = T, std::enable_if_t<!std::is_convertible_v<U&&, T>>* = nullptr,
+              detail::expected_enable_forward_value<T, E, U>* = nullptr>
+    constexpr explicit Expected(U&& v) : Expected{std::in_place, std::forward<U>(v)} {}
+
+    template <typename U = T, std::enable_if_t<std::is_convertible_v<U&&, T>>* = nullptr,
+              detail::expected_enable_forward_value<T, E, U>* = nullptr>
+    constexpr Expected(U&& v) : Expected{std::in_place, std::forward<U>(v)} {}
+
+    template <typename U = T, typename G = T,
+              std::enable_if_t<std::is_nothrow_constructible_v<T, U&&>>* = nullptr,
+              std::enable_if_t<(
+                  !std::is_same_v<Expected<T, E>, std::remove_cvref_t<U>> &&
+                  !std::conjunction_v<std::is_scalar<T>, std::is_same<T, std::remove_cvref_t<U>>> &&
+                  std::is_constructible_v<T, U> && std::is_assignable_v<G&, U> &&
+                  std::is_nothrow_move_constructible_v<E>)>* = nullptr>
+    Expected& operator=(U&& v) {
+        if (has_value()) {
+            val() = std::forward<U>(v);
+        } else {
+            err().~Unexpected<E>();
+            new (valptr()) T{std::forward<U>(v)};
+            this->m_has_val = true;
+        }
+
+        return *this;
+    }
+
+    template <typename U = T, typename G = T,
+              std::enable_if_t<!std::is_nothrow_constructible_v<T, U&&>>* = nullptr,
+              std::enable_if_t<(
+                  !std::is_same_v<Expected<T, E>, std::remove_cvref_t<U>> &&
+                  !std::conjunction_v<std::is_scalar<T>, std::is_same<T, std::remove_cvref_t<U>>> &&
+                  std::is_constructible_v<T, U> && std::is_assignable_v<G&, U> &&
+                  std::is_nothrow_move_constructible_v<E>)>* = nullptr>
+    Expected& operator=(U&& v) {
+        if (has_value()) {
+            val() = std::forward<U>(v);
+        } else {
+            auto tmp = std::move(err());
+            err().~Unexpected<E>();
+            new (valptr()) T{std::forward<U>(v)};
+            this->m_has_val = true;
+        }
+
+        return *this;
+    }
+
+    template <typename G = E, std::enable_if_t<std::is_nothrow_copy_constructible_v<G> &&
+                                               std::is_assignable_v<G&, G>>* = nullptr>
+    Expected& operator=(const Unexpected<G>& rhs) {
+        if (!has_value()) {
+            err() = rhs;
+        } else {
+            this->destroy_val();
+            new (errptr()) Unexpected<E>{rhs};
+            this->m_has_val = false;
+        }
+
+        return *this;
+    }
+
+    template <typename G = E, std::enable_if_t<std::is_nothrow_move_constructible_v<G> &&
+                                               std::is_move_assignable_v<G>>* = nullptr>
+    Expected& operator=(Unexpected<G>&& rhs) noexcept {
+        if (!has_value()) {
+            err() = std::move(rhs);
+        } else {
+            this->destroy_val();
+            new (errptr()) Unexpected<E>{std::move(rhs)};
+            this->m_has_val = false;
+        }
+
+        return *this;
+    }
+
+    template <typename... Args,
+              std::enable_if_t<std::is_nothrow_constructible_v<T, Args&&...>>* = nullptr>
+    void emplace(Args&&... args) {
+        if (has_value()) {
+            val() = T{std::forward<Args>(args)...};
+        } else {
+            err().~Unexpected<E>();
+            new (valptr()) T{std::forward<Args>(args)...};
+            this->m_has_val = true;
+        }
+    }
+
+    template <typename... Args,
+              std::enable_if_t<!std::is_nothrow_constructible_v<T, Args&&...>>* = nullptr>
+    void emplace(Args&&... args) {
+        if (has_value()) {
+            val() = T{std::forward<Args>(args)...};
+        } else {
+            auto tmp = std::move(err());
+            err().~Unexpected<E>();
+            new (valptr()) T{std::forward<Args>(args)...};
+            this->m_has_val = true;
+        }
+    }
+
+    template <typename U, typename... Args,
+              std::enable_if_t<std::is_nothrow_constructible_v<T, std::initializer_list<U>&,
+                                                               Args&&...>>* = nullptr>
+    void emplace(std::initializer_list<U> il, Args&&... args) {
+        if (has_value()) {
+            T t{il, std::forward<Args>(args)...};
+            val() = std::move(t);
+        } else {
+            err().~Unexpected<E>();
+            new (valptr()) T{il, std::forward<Args>(args)...};
+            this->m_has_val = true;
+        }
+    }
+
+    template <typename U, typename... Args,
+              std::enable_if_t<!std::is_nothrow_constructible_v<T, std::initializer_list<U>&,
+                                                                Args&&...>>* = nullptr>
+    void emplace(std::initializer_list<U> il, Args&&... args) {
+        if (has_value()) {
+            T t{il, std::forward<Args>(args)...};
+            val() = std::move(t);
+        } else {
+            auto tmp = std::move(err());
+            err().~Unexpected<E>();
+            new (valptr()) T{il, std::forward<Args>(args)...};
+            this->m_has_val = true;
+        }
+    }
+
+    constexpr T* operator->() {
+        return valptr();
+    }
+
+    constexpr const T* operator->() const {
+        return valptr();
+    }
+
+    template <typename U = T>
+    constexpr U& operator*() & {
+        return val();
+    }
+
+    template <typename U = T>
+    constexpr const U& operator*() const& {
+        return val();
+    }
+
+    template <typename U = T>
+    constexpr U&& operator*() && {
+        return std::move(val());
+    }
+
+    template <typename U = T>
+    constexpr const U&& operator*() const&& {
+        return std::move(val());
+    }
+
+    constexpr bool has_value() const noexcept {
+        return this->m_has_val;
+    }
+
+    constexpr explicit operator bool() const noexcept {
+        return this->m_has_val;
+    }
+
+    template <typename U = T>
+    constexpr U& value() & {
+        return val();
+    }
+
+    template <typename U = T>
+    constexpr const U& value() const& {
+        return val();
+    }
+
+    template <typename U = T>
+    constexpr U&& value() && {
+        return std::move(val());
+    }
+
+    template <typename U = T>
+    constexpr const U&& value() const&& {
+        return std::move(val());
+    }
+
+    constexpr E& error() & {
+        return err().value();
+    }
+
+    constexpr const E& error() const& {
+        return err().value();
+    }
+
+    constexpr E&& error() && {
+        return std::move(err().value());
+    }
+
+    constexpr const E&& error() const&& {
+        return std::move(err().value());
+    }
+
+    template <typename U>
+    constexpr T value_or(U&& v) const& {
+        static_assert(std::is_copy_constructible_v<T> && std::is_convertible_v<U&&, T>,
+                      "T must be copy-constructible and convertible from U&&");
+        return bool(*this) ? **this : static_cast<T>(std::forward<U>(v));
+    }
+
+    template <typename U>
+    constexpr T value_or(U&& v) && {
+        static_assert(std::is_move_constructible_v<T> && std::is_convertible_v<U&&, T>,
+                      "T must be move-constructible and convertible from U&&");
+        return bool(*this) ? std::move(**this) : static_cast<T>(std::forward<U>(v));
+    }
+
+private:
+    static_assert(!std::is_reference_v<T>, "T must not be a reference");
+    static_assert(!std::is_same_v<T, std::remove_cv_t<std::in_place_t>>,
+                  "T must not be std::in_place_t");
+    static_assert(!std::is_same_v<T, std::remove_cv_t<unexpect_t>>, "T must not be unexpect_t");
+    static_assert(!std::is_same_v<T, std::remove_cv_t<Unexpected<E>>>,
+                  "T must not be Unexpected<E>");
+    static_assert(!std::is_reference_v<E>, "E must not be a reference");
+
+    T* valptr() {
+        return std::addressof(this->m_val);
+    }
+
+    const T* valptr() const {
+        return std::addressof(this->m_val);
+    }
+
+    Unexpected<E>* errptr() {
+        return std::addressof(this->m_unexpect);
+    }
+
+    const Unexpected<E>* errptr() const {
+        return std::addressof(this->m_unexpect);
+    }
+
+    template <typename U = T>
+    constexpr U& val() {
+        return this->m_val;
+    }
+
+    template <typename U = T>
+    constexpr const U& val() const {
+        return this->m_val;
+    }
+
+    constexpr Unexpected<E>& err() {
+        return this->m_unexpect;
+    }
+
+    constexpr const Unexpected<E>& err() const {
+        return this->m_unexpect;
+    }
+
+    using impl_base = detail::expected_move_assign_base<T, E>;
+    using ctor_base = detail::expected_default_ctor_base<T, E>;
+};
+
+template <typename T, typename E, typename U, typename F>
+constexpr bool operator==(const Expected<T, E>& lhs, const Expected<U, F>& rhs) {
+    return (lhs.has_value() != rhs.has_value())
+               ? false
+               : (!lhs.has_value() ? lhs.error() == rhs.error() : *lhs == *rhs);
+}
+
+template <typename T, typename E, typename U, typename F>
+constexpr bool operator!=(const Expected<T, E>& lhs, const Expected<U, F>& rhs) {
+    return !operator==(lhs, rhs);
+}
+
+template <typename T, typename E, typename U>
+constexpr bool operator==(const Expected<T, E>& x, const U& v) {
+    return x.has_value() ? *x == v : false;
+}
+
+template <typename T, typename E, typename U>
+constexpr bool operator==(const U& v, const Expected<T, E>& x) {
+    return x.has_value() ? *x == v : false;
+}
+
+template <typename T, typename E, typename U>
+constexpr bool operator!=(const Expected<T, E>& x, const U& v) {
+    return !operator==(x, v);
+}
+
+template <typename T, typename E, typename U>
+constexpr bool operator!=(const U& v, const Expected<T, E>& x) {
+    return !operator==(v, x);
+}
+
+template <typename T, typename E>
+constexpr bool operator==(const Expected<T, E>& x, const Unexpected<E>& e) {
+    return x.has_value() ? false : x.error() == e.value();
+}
+
+template <typename T, typename E>
+constexpr bool operator==(const Unexpected<E>& e, const Expected<T, E>& x) {
+    return x.has_value() ? false : x.error() == e.value();
+}
+
+template <typename T, typename E>
+constexpr bool operator!=(const Expected<T, E>& x, const Unexpected<E>& e) {
+    return !operator==(x, e);
+}
+
+template <typename T, typename E>
+constexpr bool operator!=(const Unexpected<E>& e, const Expected<T, E>& x) {
+    return !operator==(e, x);
+}
+
+} // namespace Common