Original bug ID: 5965
Reporter: @gasche
Assigned to: @alainfrisch
Status: closed (set by @xavierleroy on 2015-12-11T18:18:48Z)
Resolution: fixed
Priority: normal
Severity: minor
Target version: 4.00.2+dev
Fixed in version: 4.01.0+dev
Category: typing
Monitored by: @jberdine

Bug description

Using "module type of X" in the definition of the recursive module X is in general rejected by the type-checker:

module rec X : sig
  module type S = module type of X (*Error on this occurence*)
end = X
Error: Unbound module X

It happens to somehow work when there is a name conflict with a functor parameter:

module Conflict (X : sig type foo end) = struct
  module rec X : sig
    module type S = module type of X (* suspect occurrence *)
  end = X
end

The code above type-check correctly, but the output of -i is strange, and not consistent with the output of -dtypedtree (the latter being only available in the trunk).

ocamlc -i returns

module Conflict :
  functor (X : sig  end) ->
    sig
      module rec X :
        sig
          module type S = sig
            module type S = sig
              module type S = sig type foo end
            end
          end
        end
    end

Note the triple wrapping module type S = sig module ... which is strange.

(Reproduced on 3.12.1, 4.00.1 and trunk)

Finally, the -dtypedtree output appears to indicate instead that the suspect occurrence of X is understood as a self-reference. Here is the output, curated to remove some noise:

Pstr_module "Conflict/1034"
  Pmod_functor "X/1009" (Pmty_signature [ Psig_type [ foo/1008 .. ] ])
    Pmod_structure [
      Pstr_recmodule [
        X/1011
        (Pmty_signature [
          Psig_modtype "S/1013"
            Pmodtype_manifest (Pmty_typeof (Pmod_ident "X/1011"))
        ])
        (Pmod_constraint (Pmod_ident "X/1011")
           (Pmty_signature [
                  Psig_modtype "S/1013"
                  Pmodtype_manifest (Pmty_typeof (Pmod_ident "X/1011"))
              ]
            )
         )
      ]
    ]

Note that the arguments to Ptmy_typeof make reference not to the functor argument X/1009, but to the recursively bound identifier X/1011.

This bug was discovered and reported by Arthur Wendling.