Adding expect<T> - a value-or-error implementation

7 files changed, 1564 insertions, 0 deletions

diff --git a/src/common/CMakeLists.txt b/src/common/CMakeLists.txt
index f0df05b0d..606f6fc9a 100644
--- a/src/common/CMakeLists.txt
+++ b/src/common/CMakeLists.txt
@@ -33,6 +33,8 @@ set(common_sources
   command_line.cpp
   dns_utils.cpp
   download.cpp
+  error.cpp
+  expect.cpp
   util.cpp
   i18n.cpp
   password.cpp
@@ -54,6 +56,8 @@ set(common_private_headers
   common_fwd.h
   dns_utils.h
   download.h
+  error.h
+  expect.h
   http_connection.h
   int-util.h
   pod-class.h
diff --git a/src/common/error.cpp b/src/common/error.cpp
new file mode 100644
index 000000000..e091e4478
--- /dev/null
+++ b/src/common/error.cpp
@@ -0,0 +1,75 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#include "error.h"
+
+#include <string>
+
+namespace
+{
+    struct category final : std::error_category
+    {
+        virtual const char* name() const noexcept override final
+        {
+            return "common_category()";
+        }
+
+        virtual std::string message(int value) const override final
+        {
+            switch (common_error(value))
+            {
+                case common_error::kInvalidArgument:
+                    return make_error_code(std::errc::invalid_argument).message();
+                case common_error::kInvalidErrorCode:
+                    return "expect<T> was given an error value of zero";
+                default:
+                    break;
+            }
+            return "Unknown basic_category() value";
+        }
+
+        virtual std::error_condition default_error_condition(int value) const noexcept override final
+        {
+            // maps specific errors to generic `std::errc` cases.
+            switch (common_error(value))
+            {
+                case common_error::kInvalidArgument:
+                case common_error::kInvalidErrorCode:
+                    return std::errc::invalid_argument;
+                default:
+                    break;
+            }
+            return std::error_condition{value, *this};
+        }
+    };
+}
+
+std::error_category const& common_category() noexcept
+{
+    static const category instance{};
+    return instance;
+}
+
diff --git a/src/common/error.h b/src/common/error.h
new file mode 100644
index 000000000..6fef3eb4b
--- /dev/null
+++ b/src/common/error.h
@@ -0,0 +1,52 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <system_error>
+#include <type_traits>
+
+enum class common_error : int
+{
+    // 0 is reserved for no error, as per expect<T>
+    kInvalidArgument = 1, //!< A function argument is invalid
+    kInvalidErrorCode     //!< Default `std::error_code` given to `expect<T>`
+};
+
+std::error_category const& common_category() noexcept;
+
+inline std::error_code make_error_code(::common_error value) noexcept
+{
+    return std::error_code{int(value), common_category()};
+}
+
+namespace std
+{
+    template<>
+    struct is_error_code_enum<::common_error>
+      : true_type
+    {};
+}
diff --git a/src/common/expect.cpp b/src/common/expect.cpp
new file mode 100644
index 000000000..c86e23e95
--- /dev/null
+++ b/src/common/expect.cpp
@@ -0,0 +1,70 @@
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "expect.h"
+
+#include <easylogging++.h>
+#include <string>
+
+namespace detail
+{
+    namespace
+    {
+        std::string generate_error(const char* msg, const char* file, unsigned line)
+        {
+            std::string error_msg{};
+            if (msg)
+            {
+                error_msg.append(msg);
+                if (file)
+                    error_msg.append(" (");
+            }
+            if (file)
+            {
+                error_msg.append("thrown at ");
+
+                // remove path, get just filename + extension
+                char buff[256] = {0};
+                el::base::utils::File::buildBaseFilename(file, buff, sizeof(buff) - 1);
+                error_msg.append(buff);
+
+                error_msg.push_back(':');
+                error_msg.append(std::to_string(line));
+            }
+            if (msg && file)
+                error_msg.push_back(')');
+            return error_msg;
+        }
+    }
+
+    void expect::throw_(std::error_code ec, const char* msg, const char* file, unsigned line)
+    {
+        if (msg || file)
+            throw std::system_error{ec, generate_error(msg, file, line)};
+        throw std::system_error{ec};
+    }
+} // detail
diff --git a/src/common/expect.h b/src/common/expect.h
new file mode 100644
index 000000000..326242502
--- /dev/null
+++ b/src/common/expect.h
@@ -0,0 +1,447 @@
+// Copyright (c) 2018, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#pragma once
+
+#include <cassert>
+#include <system_error>
+#include <type_traits>
+#include <utility>
+
+#include "common/error.h"
+
+//! If precondition fails, return `::error::kInvalidArgument` in current scope.
+#define MONERO_PRECOND(...)                            \
+    do                                                 \
+    {                                                  \
+        if (!( __VA_ARGS__ ))                          \
+            return {::common_error::kInvalidArgument}; \
+    } while (0)
+
+//! Check `expect<void>` and return errors in current scope.
+#define MONERO_CHECK(...)                           \
+    do                                              \
+    {                                               \
+        const ::expect<void> result = __VA_ARGS__ ; \
+        if (!result)                                \
+            return result.error();                  \
+    } while (0)
+
+/*! Get `T` from `expect<T>` by `std::move` as-if by function call.
+    `expect<void>` returns nothing.
+
+    \throw std::system_error with `expect<T>::error()`, filename and line
+        number when `expect<T>::has_error() == true`.*/
+#define MONERO_UNWRAP(...)                                        \
+    ::detail::expect::unwrap( __VA_ARGS__ , nullptr, __FILE__ , __LINE__ )
+
+/* \throw std::system_error with `code` and `msg` as part of the details. The
+filename and line number will automatically be injected into the explanation
+string. `code` can be any enum convertible to `std::error_code`. */
+#define MONERO_THROW(code, msg) \
+    ::detail::expect::throw_( code , msg , __FILE__ , __LINE__ )
+
+
+template<typename> class expect;
+
+namespace detail
+{
+    // Shortens the characters in the places that `enable_if` is used below.
+    template<bool C>
+    using enable_if = typename std::enable_if<C>::type;
+ 
+    struct expect
+    {
+        //! \throw std::system_error with `ec`, optional `msg` and/or optional `file` + `line`.
+        static void throw_(std::error_code ec, const char* msg, const char* file, unsigned line);
+
+        //! If `result.has_error()` call `throw_`. Otherwise, \return `*result` by move.
+        template<typename T>
+        static T unwrap(::expect<T>&& result, const char* error_msg, const char* file, unsigned line)
+        {
+            if (!result)
+                throw_(result.error(), error_msg, file, line);
+            return std::move(*result);
+        }
+
+        //! If `result.has_error()` call `throw_`.
+        static void unwrap(::expect<void>&& result, const char* error_msg, const char* file, unsigned line);
+    };
+}
+
+/*!
+    `expect<T>` is a value or error implementation, similar to Rust std::result
+    or various C++ proposals (boost::expected, boost::outcome). This
+    implementation currently has a strict error type, `std::error_code`, and a
+    templated value type `T`. `expect<T>` is implicitly convertible from `T`
+    or `std::error_code`, and one `expect<T>` object type is implicitly
+    convertible to another `expect<U>` object iff the destination value type
+    can be implicitly constructed from the source value type (i.e.
+    `struct U { ... U(T src) { ...} ... };`).
+
+    `operator==` and `operator!=` are the only comparison operators provided;
+    comparison between different value types is allowed provided the two values
+    types have a `operator==` defined between them (i.e.
+    `assert(expect<int>{100} == expect<short>{100});`). Comparisons can also be
+    done against `std::error_code` objects or error code enums directly (i.e.
+    `assert(expect<int>{make_error_code(common_error::kInvalidArgument)} == error::kInvalidArgument)`).
+    Comparison of default constructed `std::error_code` will always fail.
+    "Generic" comparisons can be done with `std::error_condition` via the `matches`
+    method only (i.e.
+    `assert(expect<int>{make_error_code{common_error::kInvalidErrorCode}.matches(std::errc::invalid_argument))`),
+    `operator==` and `operator!=` will not work with `std::errc` or
+    `std::error_condition`. A comparison with `matches` is more expensive
+    because an equivalency between error categories is computed, but is
+    recommended when an error can be one of several categories (this is going
+    to be the case in nearly every situation when calling a function from
+    another C++ struct/class).
+
+    `expect<void>` is a special case with no stored value. It is used by
+    functions that can fail, but otherwise would return `void`. It is useful
+    for consistency; all macros, standalone functions, and comparison operators
+    work with `expect<void>`.
+
+    \note See `src/common/error.h` for creating a custom error enum.
+ */
+template<typename T>
+class expect
+{
+    static_assert(std::is_nothrow_destructible<T>(), "T must have a nothrow destructor");
+
+    template<typename U>
+    static constexpr bool is_convertible() noexcept
+    {
+        return std::is_constructible<T, U>() &&
+            std::is_convertible<U, T>();
+    }
+
+    // MEMBERS
+    std::error_code code_;
+    typename std::aligned_storage<sizeof(T), alignof(T)>::type storage_;
+    // MEMBERS
+
+    T& get() noexcept
+    {
+        assert(has_value());
+        return *reinterpret_cast<T*>(std::addressof(storage_));
+    }
+
+    T const& get() const noexcept
+    {
+        assert(has_value());
+        return *reinterpret_cast<T const*>(std::addressof(storage_));
+    }
+
+    template<typename U>
+    void store(U&& value) noexcept(std::is_nothrow_constructible<T, U>())
+    {
+        new (std::addressof(storage_)) T{std::forward<U>(value)};
+        code_ = std::error_code{};
+    }
+
+    void maybe_throw() const
+    {
+        if (has_error())
+            ::detail::expect::throw_(error(), nullptr, nullptr, 0);
+    }
+
+public:
+    using value_type = T;
+    using error_type = std::error_code;
+
+    expect() = delete;
+
+    /*! Store an error, `code`, in the `expect` object. If `code` creates a
+    `std::error_code` object whose `.value() == 0`, then `error()` will be set
+    to `::common_error::kInvalidErrorCode`. */
+    expect(std::error_code const& code) noexcept
+      : code_(code), storage_()
+    {
+        if (!has_error())
+            code_ = ::common_error::kInvalidErrorCode;
+    }
+
+    //! Store a value, `val`, in the `expect` object.
+    expect(T val) noexcept(std::is_nothrow_move_constructible<T>())
+      : code_(), storage_()
+    {
+        store(std::move(val));
+    }
+
+    expect(expect const& src) noexcept(std::is_nothrow_copy_constructible<T>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(src.get());
+    }
+
+    //! Copy conversion from `U` to `T`.
+    template<typename U, typename = detail::enable_if<is_convertible<U const&>()>>
+    expect(expect<U> const& src) noexcept(std::is_nothrow_constructible<T, U const&>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(*src);
+    }
+
+    expect(expect&& src) noexcept(std::is_nothrow_move_constructible<T>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(std::move(src.get()));
+    }
+
+    //! Move conversion from `U` to `T`.
+    template<typename U, typename = detail::enable_if<is_convertible<U>()>>
+    expect(expect<U>&& src) noexcept(std::is_nothrow_constructible<T, U>())
+      : code_(src.error()), storage_()
+    {
+        if (src.has_value())
+            store(std::move(*src));
+    }
+
+    ~expect() noexcept
+    {
+        if (has_value())
+            get().~T();
+    }
+
+    expect& operator=(expect const& src) noexcept(std::is_nothrow_copy_constructible<T>() && std::is_nothrow_copy_assignable<T>())
+    {
+        if (this != std::addressof(src))
+        {
+            if (has_value() && src.has_value())
+                get() = src.get();
+            else if (has_value())
+                get().~T();
+            else if (src.has_value())
+                store(src.get());
+            code_ = src.error();
+        }
+        return *this;
+    }
+
+    /*! Move `src` into `this`. If `src.has_value() && addressof(src) != this`
+    then `src.value() will be in a "moved from state". */
+    expect& operator=(expect&& src) noexcept(std::is_nothrow_move_constructible<T>() && std::is_nothrow_move_assignable<T>())
+    {
+        if (this != std::addressof(src))
+        {
+            if (has_value() && src.has_value())
+                get() = std::move(src.get());
+            else if (has_value())
+                get().~T();
+            else if (src.has_value())
+                store(std::move(src.get()));
+            code_ = src.error();
+        }
+        return *this;
+    }
+
+    //! \return True if `this` is storing a value instead of an error.
+    explicit operator bool() const noexcept { return has_value(); }
+
+    //! \return True if `this` is storing an error instead of a value.
+    bool has_error() const noexcept { return bool(code_); }
+
+    //! \return True if `this` is storing a value instead of an error.
+    bool has_value() const noexcept { return !has_error(); }
+
+    //! \return Error - always safe to call. Empty when `!has_error()`.
+    std::error_code error() const noexcept { return code_; }
+
+    //! \return Value if `has_value()` otherwise \throw `std::system_error{error()}`.
+    T& value() &
+    {
+        maybe_throw();
+        return get();
+    }
+
+    //! \return Value if `has_value()` otherwise \throw `std::system_error{error()}`.
+    T const& value() const &
+    {
+        maybe_throw();
+        return get();
+    }
+
+    /*! Same as other overloads, but expressions such as `foo(bar().value())`
+    will automatically perform moves with no copies. */
+    T&& value() &&
+    {
+        maybe_throw();
+        return std::move(get());
+    }
+
+    //! \return Value, \pre `has_value()`.
+    T* operator->() noexcept { return std::addressof(get()); }
+    //! \return Value, \pre `has_value()`.
+    T const* operator->() const noexcept { return std::addressof(get()); }
+    //! \return Value, \pre `has_value()`.
+    T& operator*() noexcept { return get(); }
+    //! \return Value, \pre `has_value()`.
+    T const& operator*() const noexcept { return get(); }
+
+    /*! 
+        \note This function is `noexcept` when `U == T` is `noexcept`.
+        \return True if `has_value() == rhs.has_value()` and if values or errors are equal.
+    */
+    template<typename U>
+    bool equal(expect<U> const& rhs) const noexcept(noexcept(*std::declval<expect<T>>() == *rhs))
+    {
+        return has_value() && rhs.has_value() ?
+            get() == *rhs : error() == rhs.error();
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool equal(std::error_code const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+
+    /*!
+        \note This function is `noexcept` when `U == T` is `noexcept`.
+        \return False if `has_error()`, otherwise `value() == rhs`.
+    */
+    template<typename U, typename = detail::enable_if<!std::is_constructible<std::error_code, U>::value>>
+    bool equal(U const& rhs) const noexcept(noexcept(*std::declval<expect<T>>() == rhs))
+    {
+        return has_value() && get() == rhs;
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool matches(std::error_condition const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+};
+
+template<>
+class expect<void>
+{
+    std::error_code code_;
+
+public:
+    using value_type = void;
+    using error_type = std::error_code;
+
+    //! Create a successful object.
+    expect() = default;
+
+    expect(std::error_code const& code) noexcept
+      : code_(code)
+    {
+        if (!has_error())
+            code_ = ::common_error::kInvalidErrorCode;
+    }
+
+    expect(expect const&) = default;
+    ~expect() = default;
+    expect& operator=(expect const&) = default;
+
+    //! \return True if `this` is storing a value instead of an error.
+    explicit operator bool() const noexcept { return !has_error(); }
+
+    //! \return True if `this` is storing an error instead of a value.
+    bool has_error() const noexcept { return bool(code_); }
+
+    //! \return Error - alway
+    std::error_code error() const noexcept { return code_; }
+
+    //! \return `error() == rhs.error()`.
+    bool equal(expect const& rhs) const noexcept
+    {
+        return error() == rhs.error();
+    }
+
+    //! \return `has_error() && error() == rhs`.
+    bool equal(std::error_code const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+
+    //! \return False if `has_value()`, otherwise `error() == rhs`.
+    bool matches(std::error_condition const& rhs) const noexcept
+    {
+        return has_error() && error() == rhs;
+    }
+};
+
+//! \return An `expect<void>` object with `!has_error()`.
+inline expect<void> success() noexcept { return expect<void>{}; }
+
+template<typename T, typename U>
+inline
+bool operator==(expect<T> const& lhs, expect<U> const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator==(expect<T> const& lhs, U const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator==(T const& lhs, expect<U> const& rhs) noexcept(noexcept(rhs.equal(lhs)))
+{
+    return rhs.equal(lhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(expect<T> const& lhs, expect<U> const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return !lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(expect<T> const& lhs, U const& rhs) noexcept(noexcept(lhs.equal(rhs)))
+{
+    return !lhs.equal(rhs);
+}
+
+template<typename T, typename U>
+inline
+bool operator!=(T const& lhs, expect<U> const& rhs) noexcept(noexcept(rhs.equal(lhs)))
+{
+    return !rhs.equal(lhs);
+}
+
+namespace detail
+{
+    inline void expect::unwrap(::expect<void>&& result, const char* error_msg, const char* file, unsigned line)
+    {
+        if (!result)
+            throw_(result.error(), error_msg, file, line);
+    }
+}
+
diff --git a/tests/unit_tests/CMakeLists.txt b/tests/unit_tests/CMakeLists.txt
index 076a4a873..0fa22b4e1 100644
--- a/tests/unit_tests/CMakeLists.txt
+++ b/tests/unit_tests/CMakeLists.txt
@@ -47,6 +47,7 @@ set(unit_tests_sources
   epee_boosted_tcp_server.cpp
   epee_levin_protocol_handler_async.cpp
   epee_utils.cpp
+  expect.cpp
   fee.cpp
   json_serialization.cpp
   get_xtype_from_string.cpp
diff --git a/tests/unit_tests/expect.cpp b/tests/unit_tests/expect.cpp
new file mode 100644
index 000000000..efa843496
--- /dev/null
+++ b/tests/unit_tests/expect.cpp
@@ -0,0 +1,915 @@
+// Copyright (c) 2018, The Monero Project
+//
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include <gtest/gtest.h>
+
+#include <boost/algorithm/string/predicate.hpp>
+#include <boost/utility/string_ref.hpp>
+#include <string>
+#include <system_error>
+#include <type_traits>
+
+#include "common/expect.h"
+
+namespace
+{
+    struct move_only;
+    struct throw_construct;
+    struct throw_copies;
+    struct throw_moves;
+
+    struct move_only
+    {
+        move_only() = default;
+        move_only(move_only const&) = delete;
+        move_only(move_only&&) = default;
+        ~move_only() = default;
+        move_only& operator=(move_only const&) = delete;
+        move_only& operator=(move_only&&) = default;
+    };
+
+    struct throw_construct
+    {
+        throw_construct() {}
+        throw_construct(int) {}
+        throw_construct(throw_construct const&) = default;
+        throw_construct(throw_construct&&) = default;
+        ~throw_construct() = default;
+        throw_construct& operator=(throw_construct const&) = default;
+        throw_construct& operator=(throw_construct&&) = default;
+    };
+
+    struct throw_copies
+    {
+        throw_copies() noexcept {}
+        throw_copies(throw_copies const&) {}
+        throw_copies(throw_copies&&) = default;
+        ~throw_copies() = default;
+        throw_copies& operator=(throw_copies const&) { return *this; }
+        throw_copies& operator=(throw_copies&&) = default;
+        bool operator==(throw_copies const&) noexcept { return true; }
+        bool operator==(throw_moves const&) noexcept { return true; }
+    };
+
+    struct throw_moves
+    {
+        throw_moves() noexcept {}
+        throw_moves(throw_moves const&) = default;
+        throw_moves(throw_moves&&) {}
+        ~throw_moves() = default;
+        throw_moves& operator=(throw_moves const&) = default;
+        throw_moves& operator=(throw_moves&&) { return *this; }
+        bool operator==(throw_moves const&) { return true; }
+        bool operator==(throw_copies const&) { return true; }
+    };
+
+    template<typename T>
+    void construction_bench()
+    {
+        EXPECT_TRUE(std::is_copy_constructible<expect<T>>());
+        EXPECT_TRUE(std::is_move_constructible<expect<T>>());
+        EXPECT_TRUE(std::is_copy_assignable<expect<T>>());
+        EXPECT_TRUE(std::is_move_assignable<expect<T>>());
+        EXPECT_TRUE(std::is_destructible<expect<T>>());
+    }
+
+    template<typename T>
+    void noexcept_bench()
+    {
+        EXPECT_TRUE(std::is_nothrow_copy_constructible<expect<T>>());
+        EXPECT_TRUE(std::is_nothrow_move_constructible<expect<T>>());
+        EXPECT_TRUE(std::is_nothrow_copy_assignable<expect<T>>());
+        EXPECT_TRUE(std::is_nothrow_move_assignable<expect<T>>());
+        EXPECT_TRUE(std::is_nothrow_destructible<expect<T>>());
+
+        EXPECT_TRUE(noexcept(bool(std::declval<expect<T>>())));
+        EXPECT_TRUE(noexcept(std::declval<expect<T>>().has_error()));
+        EXPECT_TRUE(noexcept(std::declval<expect<T>>().error()));
+        EXPECT_TRUE(noexcept(std::declval<expect<T>>().equal(std::declval<expect<T>>())));
+        EXPECT_TRUE(noexcept(std::declval<expect<T>>() == std::declval<expect<T>>()));
+        EXPECT_TRUE(noexcept(std::declval<expect<T>>() != std::declval<expect<T>>()));
+    }
+
+    template<typename T>
+    void conversion_bench()
+    {
+        EXPECT_TRUE((std::is_convertible<std::error_code, expect<T>>()));
+        EXPECT_TRUE((std::is_convertible<std::error_code&&, expect<T>>()));
+        EXPECT_TRUE((std::is_convertible<std::error_code&, expect<T>>()));
+        EXPECT_TRUE((std::is_convertible<std::error_code const&, expect<T>>()));
+
+        EXPECT_TRUE((std::is_constructible<expect<T>, std::error_code>()));
+        EXPECT_TRUE((std::is_constructible<expect<T>, std::error_code&&>()));
+        EXPECT_TRUE((std::is_constructible<expect<T>, std::error_code&>()));
+        EXPECT_TRUE((std::is_constructible<expect<T>, std::error_code const&>()));
+    }
+}
+
+
+TEST(Expect, Constructions)
+{
+    construction_bench<void>();
+    construction_bench<int>();
+
+    EXPECT_TRUE(std::is_constructible<expect<void>>());
+
+    EXPECT_TRUE((std::is_constructible<expect<throw_construct>, expect<int>>()));
+
+    EXPECT_TRUE(std::is_move_constructible<expect<move_only>>());
+    EXPECT_TRUE(std::is_move_assignable<expect<move_only>>());
+}
+
+TEST(Expect, Conversions)
+{
+    struct implicit { implicit(int) {} };
+    struct explicit_only { explicit explicit_only(int) {} };
+
+    conversion_bench<void>();
+    conversion_bench<int>();
+
+    EXPECT_TRUE((std::is_convertible<int, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<int&&, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<int&, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<int const, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<expect<unsigned>, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<expect<unsigned>&&, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<expect<unsigned>&, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<expect<unsigned> const&, expect<int>>()));
+    EXPECT_TRUE((std::is_convertible<expect<int>, expect<implicit>>()));
+    EXPECT_TRUE((std::is_convertible<expect<int>&&, expect<implicit>>()));
+    EXPECT_TRUE((std::is_convertible<expect<int>&, expect<implicit>>()));
+    EXPECT_TRUE((std::is_convertible<expect<int> const&, expect<implicit>>()));
+    EXPECT_TRUE(!(std::is_convertible<expect<int>, expect<explicit_only>>()));
+    EXPECT_TRUE(!(std::is_convertible<expect<int>&&, expect<explicit_only>>()));
+    EXPECT_TRUE(!(std::is_convertible<expect<int>&, expect<explicit_only>>()));
+    EXPECT_TRUE(!(std::is_convertible<expect<int> const&, expect<explicit_only>>()));
+
+    EXPECT_TRUE((std::is_constructible<expect<int>, int>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, int&&>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, int&>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, int const&>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, expect<unsigned>>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, expect<unsigned>&&>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, expect<unsigned>&>()));
+    EXPECT_TRUE((std::is_constructible<expect<int>, expect<unsigned> const&>()));
+    EXPECT_TRUE((std::is_constructible<expect<implicit>, expect<int>>()));
+    EXPECT_TRUE((std::is_constructible<expect<implicit>, expect<int>&&>()));
+    EXPECT_TRUE((std::is_constructible<expect<implicit>, expect<int>&>()));
+    EXPECT_TRUE((std::is_constructible<expect<implicit>, expect<int> const&>()));
+    EXPECT_TRUE(!(std::is_constructible<expect<explicit_only>, expect<int>>()));
+    EXPECT_TRUE(!(std::is_constructible<expect<explicit_only>, expect<int>&&>()));
+    EXPECT_TRUE(!(std::is_constructible<expect<explicit_only>, expect<int>&>()));
+    EXPECT_TRUE(!(std::is_constructible<expect<explicit_only>, expect<int> const&>()));
+
+    EXPECT_EQ(expect<int>{expect<short>{100}}.value(), 100);
+
+    expect<std::string> val1{std::string{}};
+    expect<const char*> val2{"foo"};
+
+    EXPECT_EQ(val1.value(), std::string{});
+    EXPECT_EQ(val2.value(), std::string{"foo"});
+
+    const expect<std::string> val3{val2};
+
+    EXPECT_EQ(val1.value(), std::string{});
+    EXPECT_EQ(val2.value(), std::string{"foo"});
+    EXPECT_EQ(val3.value(), std::string{"foo"});
+
+    val1 = val2;
+
+    EXPECT_EQ(val1.value(), "foo");
+    EXPECT_EQ(val2.value(), std::string{"foo"});
+    EXPECT_EQ(val3.value(), "foo");
+}
+
+TEST(Expect, NoExcept)
+{
+    noexcept_bench<void>();
+    noexcept_bench<int>();
+
+    EXPECT_TRUE(std::is_nothrow_constructible<expect<void>>());
+
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<int>, int>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<int>, expect<unsigned>>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<int>, expect<unsigned>&&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<int>, expect<unsigned>&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<int>, expect<unsigned> const&>()));
+
+    EXPECT_TRUE(noexcept(expect<int>{std::declval<expect<unsigned>&&>()}));
+    EXPECT_TRUE(noexcept(expect<int>{std::declval<expect<unsigned> const&>()}));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>().has_value()));
+    EXPECT_TRUE(noexcept(*std::declval<expect<int>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>().equal(std::declval<expect<unsigned>>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<unsigned>>().equal(std::declval<expect<int>>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>().equal(0)));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>() == std::declval<expect<unsigned>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<unsigned>>() == std::declval<expect<int>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>() == 0));
+    EXPECT_TRUE(noexcept(0 == std::declval<expect<int>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>() != std::declval<expect<unsigned>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<unsigned>>() != std::declval<expect<int>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<int>>() != 0));
+    EXPECT_TRUE(noexcept(0 != std::declval<expect<int>>()));
+
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, std::error_code>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, std::error_code&&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, std::error_code&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, std::error_code const&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, throw_construct>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, throw_construct&&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, throw_construct&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_construct>, throw_construct const&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_construct>, expect<int>>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_construct>, expect<int>&&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_construct>, expect<int>&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_construct>, expect<int> const&>()));
+    EXPECT_TRUE(std::is_nothrow_copy_constructible<expect<throw_construct>>());
+    EXPECT_TRUE(std::is_nothrow_move_constructible<expect<throw_construct>>());
+    EXPECT_TRUE(std::is_nothrow_copy_assignable<expect<throw_construct>>());
+    EXPECT_TRUE(std::is_nothrow_move_assignable<expect<throw_construct>>());
+    EXPECT_TRUE(std::is_nothrow_destructible<expect<throw_construct>>());
+
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, std::error_code>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, std::error_code&&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, std::error_code&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, std::error_code const&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, throw_copies>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_copies>, throw_copies&&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_copies>, throw_copies&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_copies>, throw_copies const&>()));
+    EXPECT_TRUE(!std::is_nothrow_copy_constructible<expect<throw_copies>>());
+    EXPECT_TRUE(std::is_nothrow_move_constructible<expect<throw_copies>>());
+    EXPECT_TRUE(!std::is_nothrow_copy_assignable<expect<throw_copies>>());
+    EXPECT_TRUE(std::is_nothrow_move_assignable<expect<throw_copies>>());
+    EXPECT_TRUE(std::is_nothrow_destructible<expect<throw_copies>>());
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>().equal(std::declval<expect<throw_copies>>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>().equal(std::declval<throw_copies>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() == std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() == std::declval<throw_copies>()));
+    EXPECT_TRUE(noexcept(std::declval<throw_copies>() == std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() != std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() != std::declval<throw_copies>()));
+    EXPECT_TRUE(noexcept(std::declval<throw_copies>() != std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>().equal(std::declval<expect<throw_moves>>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>().equal(std::declval<throw_moves>())));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() == std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() == std::declval<throw_moves>()));
+    EXPECT_TRUE(noexcept(std::declval<throw_moves>() == std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() != std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(noexcept(std::declval<expect<throw_copies>>() != std::declval<throw_moves>()));
+    EXPECT_TRUE(noexcept(std::declval<throw_moves>() != std::declval<expect<throw_copies>>()));
+
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_moves>, std::error_code>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_moves>, std::error_code&&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_moves>, std::error_code&>()));
+    EXPECT_TRUE((std::is_nothrow_constructible<expect<throw_moves>, std::error_code const&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_moves>, throw_moves>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_moves>, throw_moves&&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_moves>, throw_moves&>()));
+    EXPECT_TRUE(!(std::is_nothrow_constructible<expect<throw_moves>, throw_moves const&>()));
+    EXPECT_TRUE(std::is_nothrow_copy_constructible<expect<throw_moves>>());
+    EXPECT_TRUE(!std::is_nothrow_move_constructible<expect<throw_moves>>());
+    EXPECT_TRUE(std::is_nothrow_copy_assignable<expect<throw_moves>>());
+    EXPECT_TRUE(!std::is_nothrow_move_assignable<expect<throw_moves>>());
+    EXPECT_TRUE(std::is_nothrow_destructible<expect<throw_copies>>());
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>().equal(std::declval<expect<throw_moves>>())));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>().equal(std::declval<throw_moves>())));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() == std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() == std::declval<throw_moves>()));
+    EXPECT_TRUE(!noexcept(std::declval<throw_moves>() == std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() != std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() != std::declval<throw_moves>()));
+    EXPECT_TRUE(!noexcept(std::declval<throw_moves>() != std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>().equal(std::declval<expect<throw_copies>>())));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>().equal(std::declval<throw_copies>())));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() == std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() == std::declval<throw_copies>()));
+    EXPECT_TRUE(!noexcept(std::declval<throw_copies>() == std::declval<expect<throw_moves>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() != std::declval<expect<throw_copies>>()));
+    EXPECT_TRUE(!noexcept(std::declval<expect<throw_moves>>() != std::declval<throw_copies>()));
+    EXPECT_TRUE(!noexcept(std::declval<throw_copies>() != std::declval<expect<throw_moves>>()));
+}
+
+TEST(Expect, Trivial)
+{
+    EXPECT_TRUE(std::is_trivially_copy_constructible<expect<void>>());
+    EXPECT_TRUE(std::is_trivially_move_constructible<expect<void>>());
+    EXPECT_TRUE(std::is_trivially_destructible<expect<void>>());
+}
+
+TEST(Expect, Assignment)
+{ 
+    expect<std::string> val1{std::string{}};
+    expect<std::string> val2{"foobar"};
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(bool(val1));
+    EXPECT_TRUE(bool(val2));
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_EQ(val1.value(), std::string{});
+    EXPECT_TRUE(*val1 == std::string{});
+    EXPECT_TRUE(boost::equals(val1->c_str(), ""));
+    EXPECT_TRUE(val2.value() == "foobar");
+    EXPECT_TRUE(*val2 == "foobar");
+    EXPECT_TRUE(boost::equals(val2->c_str(), "foobar"));
+    EXPECT_EQ(val1.error(), std::error_code{});
+    EXPECT_EQ(val2.error(), std::error_code{});
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val1 = std::move(val2);
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(bool(val1));
+    EXPECT_TRUE(bool(val2));
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_EQ(val1.value(), "foobar");
+    EXPECT_TRUE(*val1 == "foobar");
+    EXPECT_TRUE(boost::equals(val1->c_str(), "foobar"));
+    EXPECT_EQ(val2.value(), std::string{});
+    EXPECT_TRUE(*val2 == std::string{});
+    EXPECT_TRUE(boost::equals(val2->c_str(), ""));
+    EXPECT_EQ(val1.error(), std::error_code{});
+    EXPECT_EQ(val2.error(), std::error_code{});
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val2 = val1;
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(bool(val1));
+    EXPECT_TRUE(bool(val2));
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_EQ(val1.value(), "foobar");
+    EXPECT_TRUE(*val1 == "foobar");
+    EXPECT_TRUE(boost::equals(val1->c_str(), "foobar"));
+    EXPECT_EQ(val2.value(), "foobar");
+    EXPECT_TRUE(*val2 == "foobar");
+    EXPECT_TRUE(boost::equals(val2->c_str(), "foobar"));
+    EXPECT_EQ(val1.error(), std::error_code{});
+    EXPECT_EQ(val2.error(), std::error_code{});
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val1 = make_error_code(common_error::kInvalidArgument);
+
+    ASSERT_TRUE(val1.has_error());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(!val1);
+    EXPECT_TRUE(bool(val2));
+    EXPECT_TRUE(!val1.has_value());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_EQ(val1.error(), common_error::kInvalidArgument);
+    EXPECT_TRUE(val1 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val1);
+    EXPECT_STREQ(val2.value().c_str(), "foobar");
+    EXPECT_TRUE(*val2 == "foobar");
+    EXPECT_TRUE(boost::equals(val2->c_str(), "foobar"));
+    EXPECT_NE(val1.error(), std::error_code{});
+    EXPECT_EQ(val2.error(), std::error_code{});
+    EXPECT_TRUE(val1.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(val1.matches(std::errc::invalid_argument));
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val2 = val1;
+
+    ASSERT_TRUE(val1.has_error());
+    ASSERT_TRUE(val2.has_error());
+    EXPECT_TRUE(!val1);
+    EXPECT_TRUE(!val2);
+    EXPECT_TRUE(!val1.has_value());
+    EXPECT_TRUE(!val2.has_value());
+    EXPECT_EQ(val1.error(), common_error::kInvalidArgument);
+    EXPECT_TRUE(val1 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val1);
+    EXPECT_EQ(val2.error(), common_error::kInvalidArgument);
+    EXPECT_TRUE(val2 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val2);
+    EXPECT_NE(val1.error(), std::error_code{});
+    EXPECT_NE(val2.error(), std::error_code{});
+    EXPECT_TRUE(val1.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(val2.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(val1.matches(std::errc::invalid_argument));
+    EXPECT_TRUE(val2.matches(std::errc::invalid_argument));
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val1 = std::string{"barfoo"};
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_error());
+    EXPECT_TRUE(bool(val1));
+    EXPECT_TRUE(!val2);
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_value());
+    EXPECT_STREQ(val1.value().c_str(), "barfoo");
+    EXPECT_TRUE(*val1 == "barfoo");
+    EXPECT_TRUE(boost::equals(val1->c_str(), "barfoo"));
+    EXPECT_EQ(val2.error(), common_error::kInvalidArgument);
+    EXPECT_TRUE(val2 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val2);
+    EXPECT_EQ(val1.error(), std::error_code{});
+    EXPECT_NE(val2.error(), std::error_code{});
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(val2.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(val2.matches(std::errc::invalid_argument));
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val2 = val1;
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(bool(val1));
+    EXPECT_TRUE(bool(val2));
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_EQ(val1.value(), "barfoo");
+    EXPECT_TRUE(*val1 == "barfoo");
+    EXPECT_TRUE(boost::equals(val1->c_str(), "barfoo"));
+    EXPECT_EQ(val2.value(), "barfoo");
+    EXPECT_TRUE(*val2 == "barfoo");
+    EXPECT_TRUE(boost::equals(val2->c_str(), "barfoo"));
+    EXPECT_EQ(val1.error(), std::error_code{});
+    EXPECT_EQ(val2.error(), std::error_code{});
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+}
+
+TEST(Expect, AssignmentThrowsOnMove)
+{
+    struct construct_error {};
+    struct assignment_error {};
+
+    struct throw_on_move {
+        std::string msg;
+
+        throw_on_move(const char* msg) : msg(msg) {}
+        throw_on_move(throw_on_move&&) {
+            throw construct_error{};
+        }
+        throw_on_move(throw_on_move const&) = default;
+        ~throw_on_move() = default;
+        throw_on_move& operator=(throw_on_move&&) {
+            throw assignment_error{};
+        }
+        throw_on_move& operator=(throw_on_move const&) = default;
+    };
+
+    expect<throw_on_move> val1{expect<const char*>{"foobar"}};
+    expect<throw_on_move> val2{common_error::kInvalidArgument};
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_error());
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_value());
+    EXPECT_STREQ(val1->msg.c_str(), "foobar");
+    EXPECT_EQ(val2.error(), common_error::kInvalidArgument);
+
+    EXPECT_THROW(val2 = std::move(val1), construct_error);
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_error());
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_value());
+    EXPECT_STREQ(val1->msg.c_str(), "foobar");
+    EXPECT_EQ(val2.error(), common_error::kInvalidArgument);
+
+    EXPECT_THROW(val1 = expect<const char*>{"barfoo"}, assignment_error);
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_error());
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_value());
+    EXPECT_STREQ(val1->msg.c_str(), "foobar");
+    EXPECT_EQ(val2.error(), common_error::kInvalidArgument);
+
+    EXPECT_NO_THROW(val2 = val1);
+
+    ASSERT_TRUE(val1.has_value());
+    ASSERT_TRUE(val2.has_value());
+    EXPECT_TRUE(!val1.has_error());
+    EXPECT_TRUE(!val2.has_error());
+    EXPECT_STREQ(val1->msg.c_str(), "foobar");
+    EXPECT_STREQ(val2->msg.c_str(), "foobar");
+}
+
+TEST(Expect, EqualWithStrings)
+{
+    expect<std::string> val1{std::string{}};
+    expect<std::string> val2{"barfoo"};
+    expect<boost::string_ref> val3{boost::string_ref{}};
+
+    EXPECT_TRUE(!val1.equal(val2));
+    EXPECT_TRUE(val1.equal(val3));
+    EXPECT_TRUE(!val2.equal(val1));
+    EXPECT_TRUE(!val2.equal(val3));
+    EXPECT_TRUE(val3.equal(val1));
+    EXPECT_TRUE(!val3.equal(val2));
+    EXPECT_TRUE(!(val1 == val2));
+    EXPECT_TRUE(!(val2 == val1));
+    EXPECT_TRUE(val1 == val3);
+    EXPECT_TRUE(val3 == val1);
+    EXPECT_TRUE(!(val2 == val3));
+    EXPECT_TRUE(!(val3 == val2));
+    EXPECT_TRUE(val1 != val2);
+    EXPECT_TRUE(val2 != val1);
+    EXPECT_TRUE(!(val1 != val3));
+    EXPECT_TRUE(!(val3 != val1));
+    EXPECT_TRUE(val2 != val3);
+    EXPECT_TRUE(val3 != val2);
+
+    EXPECT_TRUE(val1.equal(""));
+    EXPECT_TRUE(val2.equal("barfoo"));
+    EXPECT_TRUE(val3.equal(""));
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!val3.equal(std::error_code{}));
+    EXPECT_TRUE(val1 == "");
+    EXPECT_TRUE("" == val1);
+    EXPECT_TRUE(val2 == "barfoo");
+    EXPECT_TRUE("barfoo" == val2);
+    EXPECT_TRUE(val3 == "");
+    EXPECT_TRUE("" == val3);
+    EXPECT_TRUE(!(val1 != ""));
+    EXPECT_TRUE(!("" != val1));
+    EXPECT_TRUE(!(val2 != "barfoo"));
+    EXPECT_TRUE(!("barfoo" != val2));
+    EXPECT_TRUE(!(val3 != ""));
+    EXPECT_TRUE(!("" != val3));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(!(val3 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val3));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+    EXPECT_TRUE(val3 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val3);
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+    EXPECT_TRUE(!val3.matches(std::error_condition{}));
+
+    val2 = make_error_code(common_error::kInvalidArgument);
+
+    EXPECT_TRUE(!val1.equal(val2));
+    EXPECT_TRUE(val1.equal(val3));
+    EXPECT_TRUE(!val2.equal(val1));
+    EXPECT_TRUE(!val2.equal(val3));
+    EXPECT_TRUE(val3.equal(val1));
+    EXPECT_TRUE(!val3.equal(val2));
+    EXPECT_TRUE(!(val1 == val2));
+    EXPECT_TRUE(!(val2 == val1));
+    EXPECT_TRUE(val1 == val3);
+    EXPECT_TRUE(val3 == val1);
+    EXPECT_TRUE(!(val2 == val3));
+    EXPECT_TRUE(!(val3 == val2));
+    EXPECT_TRUE(val1 != val2);
+    EXPECT_TRUE(val2 != val1);
+    EXPECT_TRUE(!(val1 != val3));
+    EXPECT_TRUE(!(val3 != val1));
+    EXPECT_TRUE(val2 != val3);
+    EXPECT_TRUE(val3 != val2);
+
+    EXPECT_TRUE(!val1.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(val2.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!val3.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(val2 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val2);
+    EXPECT_TRUE(!(val2 != common_error::kInvalidArgument));
+    EXPECT_TRUE(!(common_error::kInvalidArgument != val2));
+    EXPECT_TRUE(val2.matches(std::errc::invalid_argument));
+    EXPECT_TRUE(!val2.matches(std::error_condition{}));
+
+    val1 = expect<std::string>{"barfoo"};
+
+    EXPECT_TRUE(!val1.equal(val2));
+    EXPECT_TRUE(!val1.equal(val3));
+    EXPECT_TRUE(!val2.equal(val1));
+    EXPECT_TRUE(!val2.equal(val3));
+    EXPECT_TRUE(!val3.equal(val1));
+    EXPECT_TRUE(!val3.equal(val2));
+    EXPECT_TRUE(!(val1 == val2));
+    EXPECT_TRUE(!(val2 == val1));
+    EXPECT_TRUE(!(val1 == val3));
+    EXPECT_TRUE(!(val3 == val1));
+    EXPECT_TRUE(!(val2 == val3));
+    EXPECT_TRUE(!(val3 == val2));
+    EXPECT_TRUE(val1 != val2);
+    EXPECT_TRUE(val2 != val1);
+    EXPECT_TRUE(val1 != val3);
+    EXPECT_TRUE(val3 != val1);
+    EXPECT_TRUE(val2 != val3);
+    EXPECT_TRUE(val3 != val2);
+
+    EXPECT_TRUE(val1.equal("barfoo"));
+    EXPECT_TRUE(val1 == "barfoo");
+    EXPECT_TRUE("barfoo" == val1);
+    EXPECT_TRUE(!(val1 != "barfoo"));
+    EXPECT_TRUE(!("barfoo" != val1));
+    EXPECT_TRUE(!val1.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!(val1 == common_error::kInvalidArgument));
+    EXPECT_TRUE(!(common_error::kInvalidArgument == val1));
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!val1.matches(std::error_condition{}));
+    EXPECT_TRUE(!val1.matches(std::errc::invalid_argument));
+}
+
+TEST(Expect, EqualWithVoid)
+{
+    const expect<void> val1;
+    expect<void> val2;
+
+    EXPECT_TRUE(val1.equal(val2));
+    EXPECT_TRUE(val2.equal(val1));
+    EXPECT_TRUE(!val1.equal(std::error_code{}));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(val1 == val2);
+    EXPECT_TRUE(val2 == val1);
+    EXPECT_TRUE(!(val1 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val1));
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(!(val1 != val2));
+    EXPECT_TRUE(!(val2 != val1));
+    EXPECT_TRUE(val1 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val1);
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+
+    val2 = make_error_code(common_error::kInvalidArgument);
+
+    EXPECT_TRUE(!val1.equal(val2));
+    EXPECT_TRUE(!val2.equal(val1));
+    EXPECT_TRUE(!val1.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(val2.equal(common_error::kInvalidArgument));
+    EXPECT_TRUE(!val2.equal(std::error_code{}));
+    EXPECT_TRUE(!(val1 == val2));
+    EXPECT_TRUE(!(val2 == val1));
+    EXPECT_TRUE(val2 == common_error::kInvalidArgument);
+    EXPECT_TRUE(common_error::kInvalidArgument == val2);
+    EXPECT_TRUE(!(val2 == std::error_code{}));
+    EXPECT_TRUE(!(std::error_code{} == val2));
+    EXPECT_TRUE(val1 != val2);
+    EXPECT_TRUE(val2 != val1);
+    EXPECT_TRUE(!(val2 != common_error::kInvalidArgument));
+    EXPECT_TRUE(!(common_error::kInvalidArgument != val2));
+    EXPECT_TRUE(val2 != std::error_code{});
+    EXPECT_TRUE(std::error_code{} != val2);
+}
+
+TEST(Expect, EqualNoCopies)
+{
+    struct copy_error {};
+
+    struct throw_on_copy {
+        throw_on_copy() = default;
+        throw_on_copy(int) noexcept {}
+        throw_on_copy(throw_on_copy const&) {
+            throw copy_error{};
+        }
+        ~throw_on_copy() = default;
+        throw_on_copy& operator=(throw_on_copy const&) {
+            throw copy_error{};
+        }
+
+        bool operator==(throw_on_copy const&) const noexcept { return true; }
+    };
+
+    expect<throw_on_copy> val1{expect<int>{0}};
+    expect<throw_on_copy> val2{expect<int>{0}};
+
+    EXPECT_TRUE(val1.equal(val2));
+    EXPECT_TRUE(val2.equal(val1));
+    EXPECT_TRUE(val1 == val2);
+    EXPECT_TRUE(val2 == val1);
+    EXPECT_TRUE(!(val1 != val2));
+    EXPECT_TRUE(!(val2 != val1));
+
+    EXPECT_TRUE(val1.equal(throw_on_copy{}));
+    EXPECT_TRUE(val1 == throw_on_copy{});
+    EXPECT_TRUE(throw_on_copy{} == val1);
+    EXPECT_TRUE(!(val1 != throw_on_copy{}));
+    EXPECT_TRUE(!(throw_on_copy{} != val1));
+
+    throw_on_copy val3;
+
+    EXPECT_TRUE(val1.equal(val3));
+    EXPECT_TRUE(val1 == val3);
+    EXPECT_TRUE(val3 == val1);
+    EXPECT_TRUE(!(val1 != val3));
+    EXPECT_TRUE(!(val3 != val1));
+
+    expect<throw_on_copy> val4{common_error::kInvalidArgument};
+
+    EXPECT_TRUE(!val4.equal(throw_on_copy{}));
+    EXPECT_TRUE(!(val4 == throw_on_copy{}));
+    EXPECT_TRUE(!(throw_on_copy{} == val4));
+    EXPECT_TRUE(val4 != throw_on_copy{});
+    EXPECT_TRUE(throw_on_copy{} != val4);
+    EXPECT_TRUE(!val4.equal(val3));
+    EXPECT_TRUE(!(val4 == val3));
+    EXPECT_TRUE(!(val3 == val4));
+    EXPECT_TRUE(val4 != val3);
+    EXPECT_TRUE(val3 != val4);
+}
+
+TEST(Expect, Macros) {
+    EXPECT_TRUE(
+        [] () -> ::common_error {
+            MONERO_PRECOND(true);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> ::common_error {
+            MONERO_PRECOND(false);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+    EXPECT_TRUE(
+        [] () -> std::error_code {
+            MONERO_PRECOND(true);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> std::error_code {
+            MONERO_PRECOND(false);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+    EXPECT_TRUE(
+        [] () -> expect<void> {
+            MONERO_PRECOND(true);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> expect<void> {
+            MONERO_PRECOND(false);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+    EXPECT_TRUE(
+        [] () -> expect<int> {
+            MONERO_PRECOND(true);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> expect<int> {
+            MONERO_PRECOND(false);
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+
+    EXPECT_TRUE(
+        [] () -> std::error_code {
+            MONERO_CHECK(expect<void>{});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> std::error_code {
+            MONERO_CHECK(expect<void>{common_error::kInvalidArgument});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+    EXPECT_TRUE(
+        [] () -> expect<void> {
+            MONERO_CHECK(expect<void>{});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> expect<void> {
+            MONERO_CHECK(expect<void>{common_error::kInvalidArgument});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+    EXPECT_TRUE(
+        [] () -> expect<int> {
+            MONERO_CHECK(expect<void>{});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidErrorCode
+    );
+    EXPECT_TRUE(
+        [] () -> expect<int> {
+            MONERO_CHECK(expect<void>{common_error::kInvalidArgument});
+            return {common_error::kInvalidErrorCode};
+        } () == common_error::kInvalidArgument
+    );
+
+    EXPECT_NO_THROW(MONERO_UNWRAP(success()));
+    EXPECT_NO_THROW(MONERO_UNWRAP(expect<void>{}));
+    EXPECT_NO_THROW(MONERO_UNWRAP(expect<int>{0}));
+    EXPECT_THROW(
+        MONERO_UNWRAP(expect<void>{common_error::kInvalidArgument}), std::system_error
+    );
+    EXPECT_THROW(
+        MONERO_UNWRAP(expect<int>{common_error::kInvalidArgument}), std::system_error
+    );
+}
+