module M = struct
  module type S = sig type a val v : a end
  type 'a s = (module S with type a = 'a)
end

module B = struct
  class type a = object method a : 'a. 'a M.s -> 'a end
end

module M' = M
module B' = B

class b : B.a = object
  method a : 'a. 'a M.s -> 'a = fun (type a) ((module X) : (module M.S with type a = a)) -> X.v
end

class b' : B.a = object
  method a : 'a. 'a M'.s -> 'a = fun (type a) ((module X) : (module M'.S with type a = a)) -> X.v
end

Error: The class type object method a : 'a M'.s -> 'a end
       is not matched by the class type B.a
       The class type object method a : 'a M'.s -> 'a end
       is not matched by the class type object method a : 'a M.s -> 'a end
       The method a has type 'a. 'a M'.s -> 'a but is expected to have type
         'a. 'a M.s -> 'a
       Type 'a M'.s = (module M'.S with type a = 'a)
       is not compatible with type 'a0 M.s = (module M.S with type a = 'a0)