Remove use of mpl::identity from implicit_cast.hpp.

include/boost/implicit_cast.hpp

@@ -5,17 +5,24 @@
 #ifndef IMPLICIT_CAST_DWA200356_HPP
 # define IMPLICIT_CAST_DWA200356_HPP
 
-# include <boost/mpl/identity.hpp>
-
 namespace boost {
 
+namespace detail {
+
+template<class T> struct icast_identity
+{
+    typedef T type;
+};
+
+} // namespace detail
+
 // implementation originally suggested by C. Green in
 // http://lists.boost.org/MailArchives/boost/msg00886.php
 
 // The use of identity creates a non-deduced form, so that the
 // explicit template argument must be supplied
 template <typename T>
-inline T implicit_cast (typename mpl::identity<T>::type x) {
+inline T implicit_cast (typename boost::detail::icast_identity<T>::type x) {
     return x;
 }