is_described_class should be false for unions

test/Jamfile.v2

@@ -107,3 +107,5 @@ run is_described_class_test.cpp
   : : : <warnings>extra ;
 run is_described_class_test2.cpp
   : : : <warnings>extra ;
+run is_described_class_test3.cpp
+  : : : <warnings>extra ;

test/is_described_class_test3.cpp

@@ -0,0 +1,31 @@
+// Copyright 2022 Peter Dimov.
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/is_described_class.hpp>
+#include <boost/describe/class.hpp>
+#include <boost/core/lightweight_test_trait.hpp>
+
+union Y1
+{
+};
+
+BOOST_DESCRIBE_STRUCT( Y1, (), () )
+
+union Y2
+{
+    int m1;
+    float m2;
+};
+
+BOOST_DESCRIBE_STRUCT( Y2, (), (m1, m2) )
+
+int main()
+{
+    using boost::container_hash::is_described_class;
+
+    BOOST_TEST_TRAIT_FALSE((is_described_class<Y1>));
+    BOOST_TEST_TRAIT_FALSE((is_described_class<Y2>));
+
+    return boost::report_errors();
+}