Could you please try moving your functions outside the boost namespace? Whether those are found depends on exactly where the caller is (I don't think calls from boost::concept will find things in boost:: by default), while having them in the same namespace as Gr would ensure that they are found by ADL.
Thanks very much for this response. Indeed, it compiles if I move my overloads into the same namespace as Gr. I'm not sure why I didn't think of trying that! Many thanks for the fix. I still find it a bit surprising... I guess I need to re-read the ADL rules. :-) I'm pretty sure I've seen some examples in the online documentation which don't follow this pattern (e.g. the LEDA graph adaptation tutorial) but no doubt there is some other intricacy of name lookup that makes them work.... Thanks again! Stephen