diff --git a/hash/test/Jamfile.v2 b/hash/test/Jamfile.v2
index c91691f..0f43439 100644
--- a/hash/test/Jamfile.v2
+++ b/hash/test/Jamfile.v2
@@ -36,6 +36,7 @@ test-suite functional/hash
         [ run hash_complex_test.cpp ]
         [ run link_test.cpp link_test_2.cpp ]
         [ run link_ext_test.cpp link_no_ext_test.cpp ]
+        [ run extensions_hpp_test.cpp ]
         [ run container_fwd_test.cpp ]
         [ compile-fail hash_no_ext_fail_test.cpp ]
         [ compile-fail namespace_fail_test.cpp ]
diff --git a/hash/test/extensions_hpp_test.cpp b/hash/test/extensions_hpp_test.cpp
new file mode 100644
index 0000000..182ec01
--- /dev/null
+++ b/hash/test/extensions_hpp_test.cpp
@@ -0,0 +1,17 @@
+
+// Copyright 2009 Daniel James.
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+// Check that boost/functional/hash/extensions.hpp works okay.
+//
+// It probably should be in boost/functional/hash/detail, but since it isn't it
+// should work.
+
+#include <boost/functional/hash/extensions.hpp>
+
+int main() {
+    int x[2] = { 2, 3 };
+    boost::hash<int[2]> hf;
+    hf(x);
+}
diff --git a/include/boost/functional/hash/extensions.hpp b/include/boost/functional/hash/extensions.hpp
index c289056..7d416f1 100644
--- a/include/boost/functional/hash/extensions.hpp
+++ b/include/boost/functional/hash/extensions.hpp
@@ -7,6 +7,9 @@
 //  http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2005/n1756.pdf
 //  issue 6.18. 
 
+// This implements the extensions to the standard.
+// It's undocumented, so you shouldn't use it....
+
 #if !defined(BOOST_FUNCTIONAL_HASH_EXTENSIONS_HPP)
 #define BOOST_FUNCTIONAL_HASH_EXTENSIONS_HPP
 
@@ -14,6 +17,8 @@
 # pragma once
 #endif
 
+#include <boost/functional/hash/hash.hpp>
+
 #if defined(BOOST_NO_FUNCTION_TEMPLATE_ORDERING)
 #include <boost/type_traits/is_array.hpp>
 #endif