Merge pull request #7268 from Morph1984/expected-resultval

common, result: Implement a subset of std::expected and use it in ResultVal

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

src/core/file_sys/romfs_factory.cpp

@@ -39,13 +39,12 @@ void RomFSFactory::SetPackedUpdate(VirtualFile update_raw_file) {
 
 ResultVal<VirtualFile> RomFSFactory::OpenCurrentProcess(u64 current_process_title_id) const {
     if (!updatable) {
-        return MakeResult<VirtualFile>(file);
+        return file;
     }
 
     const PatchManager patch_manager{current_process_title_id, filesystem_controller,
                                      content_provider};
-    return MakeResult<VirtualFile>(
-        patch_manager.PatchRomFS(file, ivfc_offset, ContentRecordType::Program, update_raw));
+    return patch_manager.PatchRomFS(file, ivfc_offset, ContentRecordType::Program, update_raw);
 }
 
 ResultVal<VirtualFile> RomFSFactory::OpenPatchedRomFS(u64 title_id, ContentRecordType type) const {
@@ -58,8 +57,7 @@ ResultVal<VirtualFile> RomFSFactory::OpenPatchedRomFS(u64 title_id, ContentRecor
 
     const PatchManager patch_manager{title_id, filesystem_controller, content_provider};
 
-    return MakeResult<VirtualFile>(
-        patch_manager.PatchRomFS(nca->GetRomFS(), nca->GetBaseIVFCOffset(), type));
+    return patch_manager.PatchRomFS(nca->GetRomFS(), nca->GetBaseIVFCOffset(), type);
 }
 
 ResultVal<VirtualFile> RomFSFactory::OpenPatchedRomFSWithProgramIndex(
@@ -83,7 +81,7 @@ ResultVal<VirtualFile> RomFSFactory::Open(u64 title_id, StorageId storage,
         return ResultUnknown;
     }
 
-    return MakeResult<VirtualFile>(romfs);
+    return romfs;
 }
 
 std::shared_ptr<NCA> RomFSFactory::GetEntry(u64 title_id, StorageId storage,

src/core/file_sys/savedata_factory.cpp

@@ -94,7 +94,7 @@ ResultVal<VirtualDir> SaveDataFactory::Create(SaveDataSpaceId space,
         return ResultUnknown;
     }
 
-    return MakeResult<VirtualDir>(std::move(out));
+    return out;
 }
 
 ResultVal<VirtualDir> SaveDataFactory::Open(SaveDataSpaceId space,
@@ -115,7 +115,7 @@ ResultVal<VirtualDir> SaveDataFactory::Open(SaveDataSpaceId space,
         return ResultUnknown;
     }
 
-    return MakeResult<VirtualDir>(std::move(out));
+    return out;
 }
 
 VirtualDir SaveDataFactory::GetSaveDataSpaceDirectory(SaveDataSpaceId space) const {

src/core/file_sys/sdmc_factory.cpp

@@ -25,7 +25,7 @@ SDMCFactory::SDMCFactory(VirtualDir sd_dir_, VirtualDir sd_mod_dir_)
 SDMCFactory::~SDMCFactory() = default;
 
 ResultVal<VirtualDir> SDMCFactory::Open() const {
-    return MakeResult<VirtualDir>(sd_dir);
+    return sd_dir;
 }
 
 VirtualDir SDMCFactory::GetSDMCModificationLoadRoot(u64 title_id) const {

src/core/hle/kernel/k_page_table.cpp

@@ -859,7 +859,7 @@ ResultVal<VAddr> KPageTable::SetHeapSize(std::size_t size) {
         current_heap_addr = heap_region_start + size;
     }
 
-    return MakeResult<VAddr>(heap_region_start);
+    return heap_region_start;
 }
 
 ResultVal<VAddr> KPageTable::AllocateAndMapMemory(std::size_t needed_num_pages, std::size_t align,
@@ -893,7 +893,7 @@ ResultVal<VAddr> KPageTable::AllocateAndMapMemory(std::size_t needed_num_pages,
 
     block_manager->Update(addr, needed_num_pages, state, perm);
 
-    return MakeResult<VAddr>(addr);
+    return addr;
 }
 
 ResultCode KPageTable::LockForDeviceAddressSpace(VAddr addr, std::size_t size) {

src/core/hle/result.h

@@ -4,11 +4,10 @@
 
 #pragma once
 
-#include <new>
-#include <utility>
 #include "common/assert.h"
 #include "common/bit_field.h"
 #include "common/common_types.h"
+#include "common/expected.h"
 
 // All the constants in this file come from http://switchbrew.org/index.php?title=Error_codes
 
@@ -155,203 +154,130 @@ constexpr ResultCode ResultSuccess(0);
 constexpr ResultCode ResultUnknown(UINT32_MAX);
 
 /**
- * This is an optional value type. It holds a `ResultCode` and, if that code is a success code,
- * also holds a result of type `T`. If the code is an error code then trying to access the inner
- * value fails, thus ensuring that the ResultCode of functions is always checked properly before
- * their return value is used.  It is similar in concept to the `std::optional` type
- * (http://en.cppreference.com/w/cpp/experimental/optional) originally proposed for inclusion in
- * C++14, or the `Result` type in Rust (http://doc.rust-lang.org/std/result/index.html).
+ * This is an optional value type. It holds a `ResultCode` and, if that code is ResultSuccess, it
+ * also holds a result of type `T`. If the code is an error code (not ResultSuccess), then trying
+ * to access the inner value with operator* is undefined behavior and will assert with Unwrap().
+ * Users of this class must be cognizant to check the status of the ResultVal with operator bool(),
+ * Code(), Succeeded() or Failed() prior to accessing the inner value.
  *
  * An example of how it could be used:
  * \code
  * ResultVal<int> Frobnicate(float strength) {
  *     if (strength < 0.f || strength > 1.0f) {
  *         // Can't frobnicate too weakly or too strongly
- *         return ResultCode(ErrorDescription::OutOfRange, ErrorModule::Common,
- *             ErrorSummary::InvalidArgument, ErrorLevel::Permanent);
+ *         return ResultCode{ErrorModule::Common, 1};
  *     } else {
  *         // Frobnicated! Give caller a cookie
- *         return MakeResult<int>(42);
+ *         return 42;
  *     }
  * }
  * \endcode
  *
  * \code
- * ResultVal<int> frob_result = Frobnicate(0.75f);
+ * auto frob_result = Frobnicate(0.75f);
  * if (frob_result) {
  *     // Frobbed ok
  *     printf("My cookie is %d\n", *frob_result);
  * } else {
- *     printf("Guess I overdid it. :( Error code: %ux\n", frob_result.code().hex);
+ *     printf("Guess I overdid it. :( Error code: %ux\n", frob_result.Code().raw);
  * }
  * \endcode
  */
 template <typename T>
 class ResultVal {
 public:
-    /// Constructs an empty `ResultVal` with the given error code. The code must not be a success
-    /// code.
-    ResultVal(ResultCode error_code = ResultUnknown) : result_code(error_code) {
-        ASSERT(error_code.IsError());
-    }
+    constexpr ResultVal() : expected{} {}
+
+    constexpr ResultVal(ResultCode code) : expected{Common::Unexpected(code)} {}
+
+    template <typename U>
+    constexpr ResultVal(U&& val) : expected{std::forward<U>(val)} {}
 
-    /**
-     * Similar to the non-member function `MakeResult`, with the exception that you can manually
-     * specify the success code. `success_code` must not be an error code.
-     */
     template <typename... Args>
-    [[nodiscard]] static ResultVal WithCode(ResultCode success_code, Args&&... args) {
-        ResultVal<T> result;
-        result.emplace(success_code, std::forward<Args>(args)...);
-        return result;
-    }
+    constexpr ResultVal(Args&&... args) : expected{std::in_place, std::forward<Args>(args)...} {}
 
-    ResultVal(const ResultVal& o) noexcept : result_code(o.result_code) {
-        if (!o.empty()) {
-            new (&object) T(o.object);
-        }
-    }
+    ~ResultVal() = default;
 
-    ResultVal(ResultVal&& o) noexcept : result_code(o.result_code) {
-        if (!o.empty()) {
-            new (&object) T(std::move(o.object));
-        }
-    }
+    constexpr ResultVal(const ResultVal&) = default;
+    constexpr ResultVal(ResultVal&&) = default;
 
-    ~ResultVal() {
-        if (!empty()) {
-            object.~T();
-        }
-    }
+    ResultVal& operator=(const ResultVal&) = default;
+    ResultVal& operator=(ResultVal&&) = default;
 
-    ResultVal& operator=(const ResultVal& o) noexcept {
-        if (this == &o) {
-            return *this;
-        }
-        if (!empty()) {
-            if (!o.empty()) {
-                object = o.object;
-            } else {
-                object.~T();
-            }
-        } else {
-            if (!o.empty()) {
-                new (&object) T(o.object);
-            }
-        }
-        result_code = o.result_code;
-
-        return *this;
+    [[nodiscard]] constexpr explicit operator bool() const noexcept {
+        return expected.has_value();
     }
 
-    ResultVal& operator=(ResultVal&& o) noexcept {
-        if (this == &o) {
-            return *this;
-        }
-        if (!empty()) {
-            if (!o.empty()) {
-                object = std::move(o.object);
-            } else {
-                object.~T();
-            }
-        } else {
-            if (!o.empty()) {
-                new (&object) T(std::move(o.object));
-            }
-        }
-        result_code = o.result_code;
-
-        return *this;
+    [[nodiscard]] constexpr ResultCode Code() const {
+        return expected.has_value() ? ResultSuccess : expected.error();
     }
 
-    /**
-     * Replaces the current result with a new constructed result value in-place. The code must not
-     * be an error code.
-     */
-    template <typename... Args>
-    void emplace(ResultCode success_code, Args&&... args) {
-        ASSERT(success_code.IsSuccess());
-        if (!empty()) {
-            object.~T();
-        }
-        new (&object) T(std::forward<Args>(args)...);
-        result_code = success_code;
+    [[nodiscard]] constexpr bool Succeeded() const {
+        return expected.has_value();
     }
 
-    /// Returns true if the `ResultVal` contains an error code and no value.
-    [[nodiscard]] bool empty() const {
-        return result_code.IsError();
+    [[nodiscard]] constexpr bool Failed() const {
+        return !expected.has_value();
     }
 
-    /// Returns true if the `ResultVal` contains a return value.
-    [[nodiscard]] bool Succeeded() const {
-        return result_code.IsSuccess();
+    [[nodiscard]] constexpr T* operator->() {
+        return std::addressof(expected.value());
     }
-    /// Returns true if the `ResultVal` contains an error code and no value.
-    [[nodiscard]] bool Failed() const {
-        return empty();
+
+    [[nodiscard]] constexpr const T* operator->() const {
+        return std::addressof(expected.value());
     }
 
-    [[nodiscard]] ResultCode Code() const {
-        return result_code;
+    [[nodiscard]] constexpr T& operator*() & {
+        return *expected;
     }
 
-    [[nodiscard]] const T& operator*() const {
-        return object;
+    [[nodiscard]] constexpr const T& operator*() const& {
+        return *expected;
     }
-    [[nodiscard]] T& operator*() {
-        return object;
+
+    [[nodiscard]] constexpr T&& operator*() && {
+        return *expected;
     }
-    [[nodiscard]] const T* operator->() const {
-        return &object;
+
+    [[nodiscard]] constexpr const T&& operator*() const&& {
+        return *expected;
     }
-    [[nodiscard]] T* operator->() {
-        return &object;
+
+    [[nodiscard]] constexpr T& Unwrap() & {
+        ASSERT_MSG(Succeeded(), "Tried to Unwrap empty ResultVal");
+        return expected.value();
     }
 
-    /// Returns the value contained in this `ResultVal`, or the supplied default if it is missing.
-    template <typename U>
-    [[nodiscard]] T ValueOr(U&& value) const {
-        return !empty() ? object : std::move(value);
+    [[nodiscard]] constexpr const T& Unwrap() const& {
+        ASSERT_MSG(Succeeded(), "Tried to Unwrap empty ResultVal");
+        return expected.value();
     }
 
-    /// Asserts that the result succeeded and returns a reference to it.
-    [[nodiscard]] T& Unwrap() & {
+    [[nodiscard]] constexpr T&& Unwrap() && {
         ASSERT_MSG(Succeeded(), "Tried to Unwrap empty ResultVal");
-        return **this;
+        return std::move(expected.value());
     }
 
-    [[nodiscard]] T&& Unwrap() && {
+    [[nodiscard]] constexpr const T&& Unwrap() const&& {
         ASSERT_MSG(Succeeded(), "Tried to Unwrap empty ResultVal");
-        return std::move(**this);
+        return std::move(expected.value());
     }
 
-private:
-    // A union is used to allocate the storage for the value, while allowing us to construct and
-    // destruct it at will.
-    union {
-        T object;
-    };
-    ResultCode result_code;
-};
+    template <typename U>
+    [[nodiscard]] constexpr T ValueOr(U&& v) const& {
+        return expected.value_or(v);
+    }
 
-/**
- * This function is a helper used to construct `ResultVal`s. It receives the arguments to construct
- * `T` with and creates a success `ResultVal` contained the constructed value.
- */
-template <typename T, typename... Args>
-[[nodiscard]] ResultVal<T> MakeResult(Args&&... args) {
-    return ResultVal<T>::WithCode(ResultSuccess, std::forward<Args>(args)...);
-}
+    template <typename U>
+    [[nodiscard]] constexpr T ValueOr(U&& v) && {
+        return expected.value_or(v);
+    }
 
-/**
- * Deducible overload of MakeResult, allowing the template parameter to be ommited if you're just
- * copy or move constructing.
- */
-template <typename Arg>
-[[nodiscard]] ResultVal<std::remove_cvref_t<Arg>> MakeResult(Arg&& arg) {
-    return ResultVal<std::remove_cvref_t<Arg>>::WithCode(ResultSuccess, std::forward<Arg>(arg));
-}
+private:
+    // TODO: Replace this with std::expected once it is standardized in the STL.
+    Common::Expected<T, ResultCode> expected;
+};
 
 /**
  * Check for the success of `source` (which must evaluate to a ResultVal). If it succeeds, unwraps

src/core/hle/service/filesystem/filesystem.cpp

@@ -226,11 +226,10 @@ ResultVal<FileSys::VirtualFile> VfsDirectoryServiceWrapper::OpenFile(const std::
     }
 
     if (mode == FileSys::Mode::Append) {
-        return MakeResult<FileSys::VirtualFile>(
-            std::make_shared<FileSys::OffsetVfsFile>(file, 0, file->GetSize()));
+        return std::make_shared<FileSys::OffsetVfsFile>(file, 0, file->GetSize());
     }
 
-    return MakeResult<FileSys::VirtualFile>(file);
+    return file;
 }
 
 ResultVal<FileSys::VirtualDir> VfsDirectoryServiceWrapper::OpenDirectory(const std::string& path_) {
@@ -240,7 +239,7 @@ ResultVal<FileSys::VirtualDir> VfsDirectoryServiceWrapper::OpenDirectory(const s
         // TODO(DarkLordZach): Find a better error code for this
         return FileSys::ERROR_PATH_NOT_FOUND;
     }
-    return MakeResult(dir);
+    return dir;
 }
 
 ResultVal<FileSys::EntryType> VfsDirectoryServiceWrapper::GetEntryType(
@@ -252,12 +251,12 @@ ResultVal<FileSys::EntryType> VfsDirectoryServiceWrapper::GetEntryType(
     auto filename = Common::FS::GetFilename(path);
     // TODO(Subv): Some games use the '/' path, find out what this means.
     if (filename.empty())
-        return MakeResult(FileSys::EntryType::Directory);
+        return FileSys::EntryType::Directory;
 
     if (dir->GetFile(filename) != nullptr)
-        return MakeResult(FileSys::EntryType::File);
+        return FileSys::EntryType::File;
     if (dir->GetSubdirectory(filename) != nullptr)
-        return MakeResult(FileSys::EntryType::Directory);
+        return FileSys::EntryType::Directory;
     return FileSys::ERROR_PATH_NOT_FOUND;
 }
 
@@ -270,7 +269,7 @@ ResultVal<FileSys::FileTimeStampRaw> VfsDirectoryServiceWrapper::GetFileTimeStam
     if (GetEntryType(path).Failed()) {
         return FileSys::ERROR_PATH_NOT_FOUND;
     }
-    return MakeResult(dir->GetFileTimeStamp(Common::FS::GetFilename(path)));
+    return dir->GetFileTimeStamp(Common::FS::GetFilename(path));
 }
 
 FileSystemController::FileSystemController(Core::System& system_) : system{system_} {}
@@ -395,7 +394,7 @@ ResultVal<FileSys::VirtualDir> FileSystemController::OpenSaveDataSpace(
         return FileSys::ERROR_ENTITY_NOT_FOUND;
     }
 
-    return MakeResult(save_data_factory->GetSaveDataSpaceDirectory(space));
+    return save_data_factory->GetSaveDataSpaceDirectory(space);
 }
 
 ResultVal<FileSys::VirtualDir> FileSystemController::OpenSDMC() const {
@@ -421,7 +420,7 @@ ResultVal<FileSys::VirtualDir> FileSystemController::OpenBISPartition(
         return FileSys::ERROR_INVALID_ARGUMENT;
     }
 
-    return MakeResult<FileSys::VirtualDir>(std::move(part));
+    return part;
 }
 
 ResultVal<FileSys::VirtualFile> FileSystemController::OpenBISPartitionStorage(
@@ -437,7 +436,7 @@ ResultVal<FileSys::VirtualFile> FileSystemController::OpenBISPartitionStorage(
         return FileSys::ERROR_INVALID_ARGUMENT;
     }
 
-    return MakeResult<FileSys::VirtualFile>(std::move(part));
+    return part;
 }
 
 u64 FileSystemController::GetFreeSpaceSize(FileSys::StorageId id) const {

src/core/hle/service/glue/glue_manager.cpp

@@ -26,7 +26,7 @@ ResultVal<ApplicationLaunchProperty> ARPManager::GetLaunchProperty(u64 title_id)
         return ERR_NOT_REGISTERED;
     }
 
-    return MakeResult<ApplicationLaunchProperty>(iter->second.launch);
+    return iter->second.launch;
 }
 
 ResultVal<std::vector<u8>> ARPManager::GetControlProperty(u64 title_id) const {
@@ -39,7 +39,7 @@ ResultVal<std::vector<u8>> ARPManager::GetControlProperty(u64 title_id) const {
         return ERR_NOT_REGISTERED;
     }
 
-    return MakeResult<std::vector<u8>>(iter->second.control);
+    return iter->second.control;
 }
 
 ResultCode ARPManager::Register(u64 title_id, ApplicationLaunchProperty launch,

src/core/hle/service/ldr/ldr.cpp

@@ -335,7 +335,7 @@ public:
             CASCADE_CODE(result);
 
             if (ValidateRegionForMap(page_table, addr, size)) {
-                return MakeResult<VAddr>(addr);
+                return addr;
             }
         }
 
@@ -371,7 +371,7 @@ public:
             }
 
             if (ValidateRegionForMap(page_table, addr, size)) {
-                return MakeResult<VAddr>(addr);
+                return addr;
             }
         }
 

src/core/hle/service/mii/mii_manager.cpp

@@ -443,14 +443,14 @@ ResultVal<std::vector<MiiInfoElement>> MiiManager::GetDefault(SourceFlag source_
     std::vector<MiiInfoElement> result;
 
     if ((source_flag & SourceFlag::Default) == SourceFlag::None) {
-        return MakeResult(std::move(result));
+        return result;
     }
 
     for (std::size_t index = BaseMiiCount; index < DefaultMiiCount; index++) {
         result.emplace_back(BuildDefault(index), Source::Default);
     }
 
-    return MakeResult(std::move(result));
+    return result;
 }
 
 ResultCode MiiManager::GetIndex([[maybe_unused]] const MiiInfo& info, u32& index) {

src/core/hle/service/ns/ns.cpp

@@ -414,7 +414,7 @@ ResultVal<u8> IApplicationManagerInterface::GetApplicationDesiredLanguage(
     for (const auto lang : *priority_list) {
         const auto supported_flag = GetSupportedLanguageFlag(lang);
         if (supported_languages == 0 || (supported_languages & supported_flag) == supported_flag) {
-            return MakeResult(static_cast<u8>(lang));
+            return static_cast<u8>(lang);
         }
     }
 
@@ -448,7 +448,7 @@ ResultVal<u64> IApplicationManagerInterface::ConvertApplicationLanguageToLanguag
         return ERR_APPLICATION_LANGUAGE_NOT_FOUND;
     }
 
-    return MakeResult(static_cast<u64>(*language_code));
+    return static_cast<u64>(*language_code);
 }
 
 IApplicationVersionInterface::IApplicationVersionInterface(Core::System& system_)

src/core/hle/service/sm/sm.cpp

@@ -87,7 +87,7 @@ ResultVal<Kernel::KPort*> ServiceManager::GetServicePort(const std::string& name
     auto handler = it->second;
     port->GetServerPort().SetSessionHandler(std::move(handler));
 
-    return MakeResult(port);
+    return port;
 }
 
 /**
@@ -165,7 +165,7 @@ ResultVal<Kernel::KClientSession*> SM::GetServiceImpl(Kernel::HLERequestContext&
 
     LOG_DEBUG(Service_SM, "called service={} -> session={}", name, session->GetId());
 
-    return MakeResult(session);
+    return session;
 }
 
 void SM::RegisterService(Kernel::HLERequestContext& ctx) {

src/core/hle/service/spl/spl_module.cpp

@@ -120,40 +120,40 @@ ResultVal<u64> Module::Interface::GetConfigImpl(ConfigItem config_item) const {
         return ResultSecureMonitorNotImplemented;
     case ConfigItem::ExosphereApiVersion:
         // Get information about the current exosphere version.
-        return MakeResult((u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MAJOR} << 56) |
-                          (u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MINOR} << 48) |
-                          (u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MICRO} << 40) |
-                          (static_cast<u64>(HLE::ApiVersion::GetTargetFirmware())));
+        return (u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MAJOR} << 56) |
+               (u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MINOR} << 48) |
+               (u64{HLE::ApiVersion::ATMOSPHERE_RELEASE_VERSION_MICRO} << 40) |
+               (static_cast<u64>(HLE::ApiVersion::GetTargetFirmware()));
     case ConfigItem::ExosphereNeedsReboot:
         // We are executing, so we aren't in the process of rebooting.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereNeedsShutdown:
         // We are executing, so we aren't in the process of shutting down.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereGitCommitHash:
         // Get information about the current exosphere git commit hash.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereHasRcmBugPatch:
         // Get information about whether this unit has the RCM bug patched.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereBlankProdInfo:
         // Get whether this unit should simulate a "blanked" PRODINFO.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereAllowCalWrites:
         // Get whether this unit should allow writing to the calibration partition.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereEmummcType:
         // Get what kind of emummc this unit has active.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExospherePayloadAddress:
         // Gets the physical address of the reboot payload buffer, if one exists.
         return ResultSecureMonitorNotInitialized;
     case ConfigItem::ExosphereLogConfiguration:
         // Get the log configuration.
-        return MakeResult(u64{0});
+        return u64{0};
     case ConfigItem::ExosphereForceEnableUsb30:
         // Get whether usb 3.0 should be force-enabled.
-        return MakeResult(u64{0});
+        return u64{0};
     default:
         return ResultSecureMonitorInvalidArgument;
     }

src/core/hle/service/vi/vi.cpp

@@ -1284,15 +1284,15 @@ private:
     static ResultVal<ConvertedScaleMode> ConvertScalingModeImpl(NintendoScaleMode mode) {
         switch (mode) {
         case NintendoScaleMode::None:
-            return MakeResult(ConvertedScaleMode::None);
+            return ConvertedScaleMode::None;
         case NintendoScaleMode::Freeze:
-            return MakeResult(ConvertedScaleMode::Freeze);
+            return ConvertedScaleMode::Freeze;
         case NintendoScaleMode::ScaleToWindow:
-            return MakeResult(ConvertedScaleMode::ScaleToWindow);
+            return ConvertedScaleMode::ScaleToWindow;
         case NintendoScaleMode::ScaleAndCrop:
-            return MakeResult(ConvertedScaleMode::ScaleAndCrop);
+            return ConvertedScaleMode::ScaleAndCrop;
         case NintendoScaleMode::PreserveAspectRatio:
-            return MakeResult(ConvertedScaleMode::PreserveAspectRatio);
+            return ConvertedScaleMode::PreserveAspectRatio;
         default:
             LOG_ERROR(Service_VI, "Invalid scaling mode specified, mode={}", mode);
             return ERR_OPERATION_FAILED;