Update hash_is_avalanching_test

test/unordered/hash_is_avalanching_test.cpp

@@ -2,6 +2,28 @@
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
+// an imitation of a third-party header specializing hash_is_avalanching
+// (boost/container_hash/hash.hpp is an example doing that)
+
+#include <boost/type_traits/integral_constant.hpp>
+
+struct X3
+{
+};
+
+namespace boost
+{
+namespace unordered
+{
+
+    template<class T> struct hash_is_avalanching;
+    template<> struct hash_is_avalanching< ::X3 >: boost::true_type {};
+
+} // namespace unordered
+} // namespace boost
+
+//
+
 #include <boost/unordered/hash_traits.hpp>
 #include <boost/core/lightweight_test_trait.hpp>
 
@@ -20,6 +42,7 @@ int main()
 
     BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X1>));
     BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X2>));
+    BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X3>));
 
     return boost::report_errors();
 }