[boost] [Proto] Computing type in expression

I have an expression wrapper class tied to a domain and a grammar. Everythign works ok except for the fact it seems I can't compute type inside the expression class that depends on the expression type itself. I end up with tons of incomplete type error in proto/fusion.hpp. The example is a bit big, i'm still working on a small repro but basically, what I do is using the expression framework from ym last mail is to have a value_type on expression which represent the value_type of the expression element. Regards