Enable result<void> &= fr. Refs #117.

include/boost/system/result.hpp

@@ -1209,6 +1209,22 @@ result<T, E>& operator&=( result<T, E>& r, F&& f )
     return r;
 }
 
+template<class E, class F,
+    class U = decltype( std::declval<F>()() ),
+    class En1 = typename std::enable_if<detail::is_result<U>::value>::type,
+    class En2 = typename std::enable_if<std::is_void<typename U::value_type>::value>::type,
+    class En3 = typename std::enable_if<std::is_convertible<typename U::error_type, E>::value>::type
+>
+result<void, E>& operator&=( result<void, E>& r, F&& f )
+{
+    if( r )
+    {
+        r = std::forward<F>( f )();
+    }
+
+    return r;
+}
+
 } // namespace system
 } // namespace boost