Add hash_is_avalanching_test.cpp

test/Jamfile.v2

@@ -139,6 +139,8 @@ build_foa erase_if ;
 build_foa scary_tests ;
 build_foa init_type_insert_tests ;
 
+run unordered/hash_is_avalanching_test.cpp : : : $(CPP11) ;
+
 run exception/constructor_exception_tests.cpp : : : $(CPP11) <define>BOOST_UNORDERED_FOA_TESTS : foa_constructor_exception_tests ;
 run exception/copy_exception_tests.cpp        : : : $(CPP11) <define>BOOST_UNORDERED_FOA_TESTS : foa_copy_exception_tests ;
 run exception/assign_exception_tests.cpp      : : : $(CPP11) <define>BOOST_UNORDERED_FOA_TESTS : foa_assign_exception_tests ;

test/unordered/hash_is_avalanching_test.cpp

@@ -0,0 +1,25 @@
+// Copyright 2022 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/unordered/hash_traits.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+struct X1
+{
+};
+
+struct X2
+{
+    typedef void is_avalanching;
+};
+
+int main()
+{
+    using boost::unordered::hash_is_avalanching;
+
+    BOOST_TEST_TRAIT_FALSE((hash_is_avalanching<X1>));
+    BOOST_TEST_TRAIT_TRUE((hash_is_avalanching<X2>));
+
+    return boost::report_errors();
+}