AK: Add Result<void, ErrorType> specialization, cleanup

Add a specialization for a void ValueType. This is useful if a generic
function wants to return a Result<T, E> where the user might not
actually care abut the T, and default it to void. In this case it
basically becomes Unexpected<E> instead of Result, but hey, it works :)

AK/Result.h

@@ -38,14 +38,17 @@ public:
         : m_result(res)
     {
     }
+
     Result(ValueType&& res)
         : m_result(move(res))
     {
     }
+
     Result(const ErrorType& error)
         : m_error(error)
     {
     }
+
     Result(ErrorType&& error)
         : m_error(move(error))
     {
@@ -57,21 +60,9 @@ public:
     {
     }
 
-    Result(Result&& other)
-        : m_result(move(other.m_result))
-        , m_error(move(other.m_error))
-    {
-    }
-
-    Result(Result& other)
-        : m_result(other.m_result)
-        , m_error(other.m_error)
-    {
-    }
-
-    ~Result()
-    {
-    }
+    Result(Result&& other) = default;
+    Result(const Result& other) = default;
+    ~Result() = default;
 
     ValueType& value()
     {
@@ -93,6 +84,39 @@ private:
     Optional<ErrorType> m_error;
 };
 
+// Partial specialization for void value type
+template<typename ErrorType>
+class [[nodiscard]] Result<void, ErrorType> {
+public:
+    Result(const ErrorType& error)
+        : m_error(error)
+    {
+    }
+
+    Result(ErrorType&& error)
+        : m_error(move(error))
+    {
+    }
+
+    Result() = default;
+    Result(Result&& other) = default;
+    Result(const Result& other) = default;
+    ~Result() = default;
+
+    ErrorType& error()
+    {
+        return m_error.value();
+    }
+
+    bool is_error() const
+    {
+        return m_error.has_value();
+    }
+
+private:
+    Optional<ErrorType> m_error;
+};
+
 }
 
 using AK::Result;