File "t12bad.ml", lines 10-21, characters 0-7:
10 | module rec M :
11 |     sig
12 |       class ['a] c : 'a -> object
13 |         method map : ('a -> 'b) -> 'b M.c
14 |       end
...
18 |         method map : 'b. ('a -> 'b) -> 'b M.c
19 |           = fun f -> new M.c (f x)
20 |       end
21 |     end..
Error: This recursive type is not regular.
       The type constructor M.c is defined as
         type 'a M.c
       but it is used as
         'b M.c.
       All uses need to match the definition for the recursive type to be regular.