Support fn.contains(f) where f is a function. Fixes #46.

test/contains3_test.cpp

@@ -0,0 +1,33 @@
+// Copyright 2023 Peter Dimov
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/function.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+static int f()
+{
+    return 1;
+}
+
+static int g()
+{
+    return 2;
+}
+
+int main()
+{
+    {
+        boost::function<int()> fn;
+        BOOST_TEST( !fn.contains( f ) );
+        BOOST_TEST( !fn.contains( g ) );
+    }
+
+    {
+        boost::function<int()> fn( f );
+        BOOST_TEST( fn.contains( f ) );
+        BOOST_TEST( !fn.contains( g ) );
+    }
+
+    return boost::report_errors();
+}