moved from ::iterator to decltype

tests/test_sink.h

@@ -47,7 +47,7 @@ protected:
         base_sink<Mutex>::formatter_->format(msg, formatted);
         // save the line without the eol
         auto eol_len = strlen(details::os::default_eol);
-        using diff_t = typename std::iterator_traits<typename memory_buf_t::iterator>::difference_type; 
+        using diff_t = typename std::iterator_traits<decltype(formatted.end())>::difference_type; 
         if (lines_.size() < lines_to_save) {
             lines_.emplace_back(formatted.begin(), formatted.end() - static_cast<diff_t>(eol_len));
         }