Hello.

To the best of my knowledge, there is subtle interaction 
between typing of recursive modules and unfolding of recursive structural types,
i.e, classes and polymorphic variants. 
I remember Jacques has hit a similar problem:
http://caml.inria.fr/pub/ml-archives/caml-list/2008/02/de4c1ddaa8bf3c0ef9350d8fb4f1e8f7.en.html

My understanding is that with recursive modules you may define two types 
that are equivalent but do not join together by simple unfolding 
of abbreviations; this seems to confuse the type checker. 
I believe this is a known bug of the current implementation
of the type checker. 

With best regards,
Keiko