Invalid_argument("List.iter2") for ill-aritied types in recursive modules.

[ccasin]

On this code:

module rec A : sig
  val c : 'a B.t -> unit
end = struct
  let c _ = ()
end

and B : sig
  type 'a t
end = struct
  type t
end

The compiler currently hits an exception:

Fatal error: exception Invalid_argument("List.iter2")
Raised at Stdlib.invalid_arg in file "stdlib.ml", line 30, characters 20-45
Called from Ctype.occur_univar.occur_desc in file "typing/ctype.ml", lines 1995-2006, characters 12-33
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Ctype.occur_univar_for in file "typing/ctype.ml", line 2024, characters 4-23
Called from Ctype.moregen_occur in file "typing/ctype.ml", line 3725, characters 2-33
Called from Ctype.moregen in file "typing/ctype.ml", line 3739, characters 8-43
Called from Ctype.moregen in file "typing/ctype.ml", line 3759, characters 14-54
Called from Ctype.moregen in file "typing/ctype.ml" (inlined), line 3950, characters 2-46
Called from Ctype.moregeneral.(fun) in file "typing/ctype.ml", line 3978, characters 9-64
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Includecore.value_descriptions in file "typing/includecore.ml", line 82, characters 8-60
Called from Includemod.value_descriptions in file "typing/includemod.ml", line 165, characters 7-72
Called from Includemod.signature_components in file "typing/includemod.ml", line 734, characters 14-75
Called from Includemod.signatures.pair_components in file "typing/includemod.ml", lines 660-662, characters 10-29
Called from Includemod.try_modtypes in file "typing/includemod.ml", line 489, characters 8-67
Called from Includemod.modtypes in file "typing/includemod.ml", line 426, characters 8-64
Called from Includemod.modtypes_with_shape in file "typing/includemod.ml", lines 1199-1200, characters 8-40
Called from Typemod.check_recmodule_inclusion.check_incl.check_inclusion in file "typing/typemod.ml", lines 1982-1984, characters 12-39
Called from Stdlib__List.map in file "list.ml", line 86, characters 15-19
Called from Typemod.type_structure.type_str_item in file "typing/typemod.ml", line 2695, characters 10-52
Called from Typemod.type_structure.type_struct in file "typing/typemod.ml", line 2822, characters 43-75
Called from Typemod.type_structure.run in file "typing/typemod.ml", line 2834, characters 6-42
Called from Builtin_attributes.warning_scope in file "parsing/builtin_attributes.ml", line 343, characters 14-18
Re-raised at Builtin_attributes.warning_scope in file "parsing/builtin_attributes.ml", line 348, characters 4-13
Called from Typemod.type_implementation.(fun) in file "typing/typemod.ml", line 3036, characters 8-38
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Compile_common.typecheck_impl in file "driver/compile_common.ml", lines 95-97, characters 2-48
Called from Compile_common.implementation.(fun) in file "driver/compile_common.ml", line 116, characters 18-44
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Misc.try_finally in file "utils/misc.ml", line 31, characters 8-15
Re-raised at Misc.try_finally in file "utils/misc.ml", line 45, characters 10-56
Called from Compenv.process_action.impl in file "driver/compenv.ml", line 607, characters 4-69
Called from Stdlib__List.iter in file "list.ml", line 112, characters 12-15
Called from Compenv.process_deferred_actions in file "driver/compenv.ml", line 698, characters 2-61
Called from Optmaindriver.main in file "driver/optmaindriver.ml", lines 53-58, characters 6-17
Re-raised at Location.report_exception.loop in file "parsing/location.ml", line 979, characters 14-25
Called from Optmaindriver.main in file "driver/optmaindriver.ml", line 136, characters 4-35
Called from Optmain in file "driver/optmain.ml", line 2, characters 7-57

This looks very similar to #12061, fixed in #12063. I have not taken a close look at that fix to see if it's easy to do something similar here.

[kit-ty-kate]

What version of OCaml are you using? (ocaml --version)

[ccasin]

What version of OCaml are you using? (ocaml --version)

5.3.0+dev0-2023-12-22 (compiled from commit a36a53377d983e06f6b2cfd52b30f429d19c1cae)

Does this not reproduce for you? I suppose it's possible I messed something up locally, but I think this is just a fresh check out of the compiler source.

[Octachron]

As far as I can see, the error has been introduced in OCaml 3.11.0 and it is still present in trunk.

[Octachron]

Fixed in #13055 by adding a consistency check step before checking for recursive module inclusion.