Move functor to its own namespace to block bringing boost::core to ADL.

test/Jamfile.v2

@@ -195,6 +195,9 @@ compile-fail fclose_deleter_compile_fail_adl.cpp
   : <target-os>windows:<define>_CRT_SECURE_NO_WARNINGS <target-os>windows:<define>_CRT_SECURE_NO_DEPRECATE $(warnings-as-errors-off) ;
 
 run functor_test.cpp ;
+compile-fail functor_compile_fail_adl.cpp
+  : $(warnings-as-errors-off) ;
+
 
 run pointer_traits_pointer_test.cpp ;
 run pointer_traits_element_type_test.cpp ;

test/functor_compile_fail_adl.cpp

@@ -0,0 +1,32 @@
+/*
+ *             Copyright Andrey Semashev 2024.
+ * Distributed under the Boost Software License, Version 1.0.
+ *    (See accompanying file LICENSE_1_0.txt or copy at
+ *          http://www.boost.org/LICENSE_1_0.txt)
+ */
+/*!
+ * \file   functor_compile_fail_adl.cpp
+ * \author Andrey Semashev
+ * \date   02.02.2024
+ *
+ * This file tests that \c boost::core::functor doesn't bring namespace
+ * boost::core into ADL.
+ */
+
+#include <boost/core/functor.hpp>
+
+void func() {}
+
+namespace boost::core {
+
+void check_adl(functor< ::func > const&)
+{
+}
+
+} // namespace boost::core
+
+int main()
+{
+    // Must not find boost::check_adl
+    check_adl(boost::core::functor< ::func >());
+}