diff --git a/boost/yap/operators.hpp b/boost/yap/operators.hpp
index f48ff4d..1c78f41 100644
--- a/boost/yap/operators.hpp
+++ b/boost/yap/operators.hpp
@@ -256,6 +256,12 @@ namespace boost { namespace yap {
 
     namespace function_objects {
 
+        template <typename T>
+        constexpr auto transform_expression (T && t);
+
+        template <typename T>
+        constexpr auto eval_placeholder (T && t);
+
         template <typename T>
         constexpr auto eval_unary_plus (T && t);
         template <typename T>