diff --git a/include/boost/range/adaptor/ref_unwrapped.hpp b/include/boost/range/adaptor/ref_unwrapped.hpp
index 0369cc3..7ac5115 100644
--- a/include/boost/range/adaptor/ref_unwrapped.hpp
+++ b/include/boost/range/adaptor/ref_unwrapped.hpp
@@ -15,7 +15,7 @@
 #include <boost/range/adaptor/transformed.hpp>
 #include <boost/range/reference.hpp>
 #include <boost/range/concepts.hpp>
-
+#include <boost/type_traits/declval.hpp>
 #include <utility>
 
 #if !defined(BOOST_NO_CXX11_DECLTYPE)
@@ -32,7 +32,7 @@ namespace boost
             typedef BOOST_DEDUCED_TYPENAME
                           range_reference<SinglePassRange>::type argument_type;
 
-            typedef decltype(std::declval<argument_type>().get() ) result_type;
+            typedef decltype( boost::declval<argument_type>().get() ) result_type;
 
             result_type operator()( argument_type &&r ) const
             {