Common: Add a Result class

This adds a lightweight, easy to use std::variant wrapper intended to
be used as a return type for functions that can return either a result
or an error code.

Source/Core/Common/Result.h

@@ -0,0 +1,30 @@
+// Copyright 2018 Dolphin Emulator Project
+// Licensed under GPLv2+
+// Refer to the license.txt file included.
+
+#pragma once
+
+#include <variant>
+
+namespace Common
+{
+template <typename ResultCode, typename T>
+class Result final
+{
+public:
+  Result(ResultCode code) : m_variant{code} {}
+  Result(const T& t) : m_variant{t} {}
+  Result(T&& t) : m_variant{std::move(t)} {}
+  explicit operator bool() const { return Succeeded(); }
+  bool Succeeded() const { return std::holds_alternative<T>(m_variant); }
+  // Must only be called when Succeeded() returns false.
+  ResultCode Error() const { return std::get<ResultCode>(m_variant); }
+  // Must only be called when Succeeded() returns true.
+  const T& operator*() const { return std::get<T>(m_variant); }
+  const T* operator->() const { return &std::get<T>(m_variant); }
+  T& operator*() { return std::get<T>(m_variant); }
+  T* operator->() { return &std::get<T>(m_variant); }
+private:
+  std::variant<ResultCode, T> m_variant;
+};
+}  // namespace Common