Crash when substituting for the same name at different arities

[lukemaurer]

The MWE is highly suggestive of what's going on:

module type S1 = sig
  type t0
  type 'a t := unit

  val x : t0 t
end

module type S2 = sig
  type t (* must be the same name as [S1.t] *)

  include S1 with type t0 := t
end

module type S3 = sig
  type t1

  include S2 with type t := t1
end

The result is an Invalid_argument out of List.fold_left2, caused by a length mismatch between the parameter list and the argument list.

$ ocamlc -c test.mli -bin-annot
$ _build/install/default/bin/odoc compile test.cmti
  odoc: internal error, uncaught exception:
      Invalid_argument("List.fold_left2")
      Raised at Stdlib.invalid_arg in file "stdlib.ml", line 33, characters 20-45
      Called from Odoc_xref2__Subst.type_expr in file "subst.ml", line 598, characters 21-59
      Called from Odoc_xref2__Subst.value in file "subst.ml" (inlined), line 842, characters 19-38
      Called from Odoc_xref2__Subst.apply_sig_map.inner.(fun) in file "subst.ml", line 1089, characters 19-52
      Called from Odoc_xref2__Component.Delayed.get in file "component.ml" (inlined), line 55, characters 16-22
      Called from Odoc_xref2__Lang_of.signature_items.inner in file "lang_of.ml", line 438, characters 16-39
      Called from Odoc_xref2__Lang_of.signature in file "lang_of.ml" (inlined), line 466, characters 12-43
      Called from Odoc_xref2__Lang_of.include_ in file "lang_of.ml", line 641, characters 18-69
      ...

[gpetiot]

I cannot reproduce this with the master version, do you still observe this bug with the last odoc build?

[lukemaurer]

I cannot reproduce this with the master version, do you still observe this bug with the last odoc build?

I do. Notably, I only get it when going through a .cmti, though. I'll update with more specific instructions.

[panglesd]

I made a repro at https://github.com/panglesd/odoc/tree/fix-930.

I think .cmigives odoc module type whose values are "already computed" by the compiler, hence the fact that there is no issue.

[lukemaurer]

Seeing this again with a slightly different MWE:

module type A = sig
  type 'a t
  type 'a t := 'a t (*_ must be the same name; can also be unit t *)

  val x : _ t
end

module type B = sig
  type t

  include A with type _ t := t (*_ must be the same name *)
end

module type C = B with type t := unit

Notably, this now works through either a .cmt or a .cmti.