Add implicit conversion between compatible reference wrappers (refs #83)

test/ref_implicit_fail4.cpp

@@ -0,0 +1,21 @@
+//
+// Incompatible reference_wrappers must not implicitly convert to each other
+//
+// Copyright 2020 Peter Dimov
+//
+// 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
+//
+
+#include <boost/ref.hpp>
+
+void f( boost::reference_wrapper< int const > )
+{
+}
+
+int main()
+{
+    long y = 0;
+    f( boost::ref(y) ); // should fail
+}