Add experimental forward declaration of hash containers

include/boost/container/container_fwd.hpp

@@ -186,6 +186,35 @@ template <class Key
          ,class Allocator = void >
 class flat_multimap;
 
+//Experimental forward declarations, not implemented yet!
+template <class Key
+         ,class Hash //= hash<Key>,
+         ,class Pred = std::equal_to<Key>
+         ,class Compare  = std::less<Key>
+         ,class Allocator = void
+         ,class Options = void>
+class hash_set;
+
+template <class Key
+         ,class Compare  = std::less<Key>
+         ,class Allocator = void
+         ,class Options = void >
+class hash_multiset;
+
+template <class Key
+         ,class T
+         ,class Compare  = std::less<Key>
+         ,class Allocator = void
+         ,class Options = void >
+class hash_map;
+
+template <class Key
+         ,class T
+         ,class Compare  = std::less<Key>
+         ,class Allocator = void
+         ,class Options = void >
+class hash_multimap;
+
 #ifndef BOOST_NO_CXX11_TEMPLATE_ALIASES
 
 //! Alias templates for small_flat_[multi]{set|map} using small_vector as container