User types, type variables and eliom sides

[Drup]

In current eliom, we have this definition:

val%server Eliom_parameters.user_type :
  ?client_to_and_of : 'a to_and_of Eliom_client_value.t ->
  of_string : (string -> 'a) ->
  to_string : ('a -> string) ->
  string ->
  ('a, [ `WithoutSuffix ], [ `One of 'a ] param_name) params_type

Here, 'a is used both on the client and the server. This is ill-typed, in particular in the new typechecker. I don't intend to ever support that kind of type constraint.

This bug report is here to discuss potential solutions to this. With converters, the solution would naturally be to expect a converter to have a translation. We can't really do that here. Another, unsafe, proposition is just to make it a 'b.

Any ideas ?