Francois Pottier writes:

   Date: Wed, 4 Aug 1999 15:52:39 +0200
   Subject: Weird behavior with mutually recursive type definitions
   
   [...]

     type 'a t = 'a dummy
      and specialized = int t
   
   O'Caml still accepts the code, but this time, it constrains 'a
   to be equal to int, as if the type constructor t could not be
   used polymorphically within its own declaration:
   
     type 'a t = 'a dummy constraint 'a = int
                          ^^^^^^^^^^^^^^^^^^^
     type specialized = int t
   
   Why is this? Can someone explain, or is it a typechecker bug?
   
A few month ago we run into the same problem. From a private
conversation with Jacques Garrigue I understood, that the above
behavior is actually a feature of the typechecker, because there
is no better solution known at the moment. :-(

We solved the problem by eliminating type abbreviations (by
replacing it with its expansion) until these constraints
vanished. (You can use option ocamlc -i on your code to see which
constraints are generated.) The elimination of the abbreviations
leads to ugly and unreadable code, but it works at least.

Hope this helps,

Hendrik