Write test for empty_value private inheritance

test/Jamfile.v2

@@ -226,6 +226,7 @@ run empty_value_test.cpp ;
 run empty_value_size_test.cpp ;
 run empty_value_final_test.cpp ;
 run empty_value_constexpr_test.cpp ;
+compile-fail empty_value_compile_fail_casting.cpp ;
 
 run quick_exit_test.cpp ;
 run-fail quick_exit_fail.cpp ;

test/empty_value_compile_fail_casting.cpp

@@ -0,0 +1,20 @@
+// Copyright 2024 Braden Ganetsky
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/core/empty_value.hpp>
+
+struct empty {};
+
+// This test ensures private inheritance of `boost::empty_value<T>` for empty `T`.
+// With public inheritance, `boost::empty_value<empty>*` could cast to `empty*`.
+void test_empty_not_convertible_to_base()
+{
+    const boost::empty_value<empty> x(boost::empty_init);
+    const empty* x2 = static_cast<const empty*>(&x);
+    (void)x2;
+}
+
+int main()
+{
+}