concept_check and B&N trick

I ran into problems using BOOST_CLASS_REQUIRES in conjunction with the base class in the Barton-Nackman trick/Curiously recursive template technique. I want to require functions that take the derived type as an argument. So I declare a variable of the derived at struct level in the concept class, but then g++ (2.95) complains about it being an invalid use of undefined type... forward declaration... . Is there a way to make this work? If so, does anyone have/know of an example I can see how to order things to make it work? Thanks, E