Type variable names should be retained

[dsheets]

In error messages regarding polymorphic values, it would be nice to have the names of type variables retained.

For instance:

class ['repo, 'pkg] universe :
object ('self)
  constraint 'repo =
  < filter        : ('repo -> 'pkg -> bool) -> 'repo;
    map           : ('repo -> 'pkg -> 'pkg) -> 'repo;
    name          : string;
    opam          : OpamTypes.repository;
    packages      : 'pkg OpamPackage.Map.t;
    priority      : int;
    with_priority : int -> 'repo;
    ..
  >

but then when a type error is printed:

A type parameter has type
         < filter : ('b -> < package : OpamPackage.Set.elt; .. > -> bool) ->
                    'a;
           map : 'c -> 'a; name : OpamRepositoryName.Map.key;
           opam : OpamTypes.repository;
           packages : < conflicts : OpamTypes.formula;
                        depends : OpamTypes.formula;
                        depopts : OpamTypes.formula; .. >
                      OpamPackage.Map.t;
           priority : int; with_priority : int -> 'a; .. >
         as 'a
       but is expected to have type
         < filter : ('d ->
                     (< conflicts : OpamTypes.formula;
                        depends : OpamTypes.formula;
                        depopts : OpamTypes.formula;
                        package : OpamTypes.package; .. >
                      as 'e) ->
                     bool) ->
                    'd;
           map : ('d -> 'e -> 'e) -> 'd; name : string;
           opam : OpamTypes.repository; packages : 'e OpamPackage.Map.t;
           priority : int; with_priority : int -> 'd; .. >
         as 'd
       This instance of 'd is ambiguous:
       it would escape the scope of its equation

This probably requires annotation and a disambiguation pass before error printing.

[dsheets]

The whole error message, which contains ambiguous type names:

File "lib/opamfUniverse.ml", line 1:
Error: The implementation lib/opamfUniverse.ml
       does not match the interface lib/opamfUniverse.cmi:
       Class declarations do not match:
         class ['a, 'b] universe :
           object ('c)
             constraint 'a =
               < filter : ('d ->
                           < package : OpamPackage.Set.elt; .. > -> bool) ->
                          'a;
                 map : 'e -> 'a; name : string; opam : OpamTypes.repository;
                 packages : 'b OpamPackage.Map.t; priority : int;
                 with_priority : int -> 'a; .. >
             constraint 'b =
               < conflicts : OpamTypes.formula; depends : OpamTypes.formula;
                 depopts : OpamTypes.formula; .. >
             val max_versions :
               OpamPackage.Version.Set.elt OpamPackage.Name.Map.t Lazy.t
             val package_set : OpamPackage.Set.t Lazy.t
             val packages : ('a * 'b) OpamPackage.Map.t Lazy.t
             val repos : 'a OpamRepositoryName.Map.t
             val versions :
               OpamPackage.Version.Set.t OpamPackage.Name.Map.t Lazy.t
             method add_repo : 'a -> 'c
             method filter : ?depopts:bool -> ('a -> 'b -> bool) -> 'c
             method filter_repos : ('a -> bool) -> 'c
             method load_repos : 'a OpamRepositoryName.Map.t -> 'c
             method map : 'e -> 'c
             method map_repos : ('a -> 'a) -> 'c
             method max_versions :
               OpamPackage.Version.Set.elt OpamPackage.Name.Map.t
             method package : OpamPackage.Map.key -> 'a * 'b
             method package_set : OpamPackage.Set.t
             method packages : ('a * 'b) OpamPackage.Map.t
             method push_repo : 'a -> 'c
             method repos : 'a OpamRepositoryName.Map.t
             method versions :
               OpamPackage.Version.Set.t OpamPackage.Name.Map.t
           end
       does not match
         class ['a, 'b] universe :
           object ('c)
             constraint 'a =
               < filter : ('a -> 'b -> bool) -> 'a;
                 map : ('a -> 'b -> 'b) -> 'a; name : string;
                 opam : OpamTypes.repository;
                 packages : 'b OpamPackage.Map.t; priority : int;
                 with_priority : int -> 'a; .. >
             constraint 'b =
               < conflicts : OpamTypes.formula; depends : OpamTypes.formula;
                 depopts : OpamTypes.formula; package : OpamTypes.package;
                 .. >
             method add_repo : 'a -> 'c
             method filter : ?depopts:bool -> ('a -> 'b -> bool) -> 'c
             method filter_repos : ('a -> bool) -> 'c
             method load_repos : 'a OpamRepositoryName.Map.t -> 'c
             method map : ('a -> 'b -> 'b) -> 'c
             method map_repos : ('a -> 'a) -> 'c
             method max_versions : OpamTypes.version OpamPackage.Name.Map.t
             method package : OpamTypes.package -> 'a * 'b
             method package_set : OpamPackage.Set.t
             method packages : ('a * 'b) OpamPackage.Map.t
             method push_repo : 'a -> 'c
             method repos : 'a OpamRepositoryName.Map.t
             method versions : OpamTypes.version_set OpamPackage.Name.Map.t
           end
       A type parameter has type
         < filter : ('b -> < package : OpamPackage.Set.elt; .. > -> bool) ->
                    'a;
           map : 'c -> 'a; name : string; opam : OpamTypes.repository;
           packages : < conflicts : OpamTypes.formula;
                        depends : OpamTypes.formula;
                        depopts : OpamTypes.formula; .. >
                      OpamPackage.Map.t;
           priority : int; with_priority : int -> 'a; .. >
         as 'a
       but is expected to have type
         < filter : ('d ->
                     (< conflicts : OpamTypes.formula;
                        depends : OpamTypes.formula;
                        depopts : OpamTypes.formula;
                        package : OpamTypes.package; .. >
                      as 'e) ->
                     bool) ->
                    'd;
           map : ('d -> 'e -> 'e) -> 'd; name : string;
           opam : OpamTypes.repository; packages : 'e OpamPackage.Map.t;
           priority : int; with_priority : int -> 'd; .. >
         as 'd
       This instance of 'd is ambiguous:
       it would escape the scope of its equation