Index: path.cpp
===================================================================
--- path.cpp	(revision 73013)
+++ path.cpp	(working copy)
@@ -225,12 +225,15 @@
     size_type pos(m_pathname.rfind(dot));
     if (pos != string_type::npos && pos >= filename_pos(m_pathname, m_pathname.size()))
       m_pathname.erase(pos);
+    
+    if (!source.empty()) 
+    {
+      // append source extension
+      if (source.m_pathname[0] != dot)
+        m_pathname.push_back(dot);
+      m_pathname.append(source.m_pathname);
+    }
 
-    // append source extension if any
-    pos = source.m_pathname.rfind(dot);
-    if (pos != string_type::npos)
-      m_pathname += source.c_str() + pos;
-
     return *this;
   }