Worked around an MSVC 6 bug (Markus Schoepflin)

[SVN r13430]

include/boost/weak_ptr.hpp

@@ -51,6 +51,8 @@ public:
     {
     }
 
+#if !defined(BOOST_MSVC) || (BOOST_MSVC > 1200)
+
     template<typename Y>
     weak_ptr & operator=(weak_ptr<Y> const & r) // never throws
     {
@@ -67,6 +69,8 @@ public:
         return *this;
     }
 
+#endif
+
     void reset()
     {
         this_type().swap(*this);