Add a test for valueless std::variant values

test/Jamfile.v2

@@ -135,3 +135,5 @@ run hash_tuple_like_test2.cpp
 
 run is_range_test4.cpp ;
 run hash_container_test2.cpp ;
+
+run hash_variant_test2.cpp ;

test/hash_variant_test2.cpp

@@ -0,0 +1,50 @@
+// Copyright 2026 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/container_hash/hash.hpp>
+#include <boost/config.hpp>
+#include <boost/config/pragma_message.hpp>
+
+#if defined(BOOST_NO_CXX17_HDR_VARIANT)
+
+BOOST_PRAGMA_MESSAGE( "Skipping test because BOOST_NO_CXX17_HDR_VARIANT is defined" )
+int main() {}
+
+#else
+
+#include <boost/core/lightweight_test.hpp>
+#include <variant>
+
+struct X
+{
+    operator int() const { throw 5; }
+};
+
+std::variant<int, float> make_valueless_variant()
+{
+    std::variant<int, float> v;
+
+    try
+    {
+        v.emplace<int>( X() );
+    }
+    catch( int )
+    {
+    }
+
+    BOOST_TEST( v.valueless_by_exception() );
+
+    return v;
+}
+
+int main()
+{
+    std::variant<int, float> v1, v2 = make_valueless_variant();
+
+    BOOST_TEST_NE( (boost::hash<std::variant<int, float>>()( v1 )), (boost::hash<std::variant<int, float>>()( v2 )) );
+
+    return boost::report_errors();
+}
+
+#endif