Confusing error message when signatures of functor arguments differ

[bclement-ocp]

Consider the following files:

• bug.ml:

  type 'a f
  
  type 'a g
  
  module Make(X : sig
    val f : int f
  
    val g : int g
  end) = struct end

• bug.mli:

  type 'a f
  
  type 'a g
  
  module Make(X : sig
    val f : int f
  
    val g : int
  end) : sig end

The implementation does not match the interface: there is val g : int g in the .ml file, but val g : int in the .mli file. The OCaml compiler refuses this file (the following is obtained with trunk at 1fd6eae but OCaml 5.3 and OCaml 4.14 also give a similar reason), but seems to be confused about the justification:

$ ocamlc.opt -nopervasives -nostdlib bug.mli && ocamlopt.opt -nopervasives -nostdlib -I . bug.ml
File "bug.ml", line 1:
Error: The implementation bug.ml does not match the interface bug.mli:
       In module Make:
       Modules do not match:
         (X : $S1) -> ...
       is not included in
         (X : $T1) -> ...
       Module types do not match:
         $S1 = sig val f : int f val g : int g end
       does not include
         $T1 = sig val f : int f val g : int end
       Values do not match: val f : int f is not included in val f : int f
       The type int f is not compatible with the type int f
       Type f is abstract because no corresponding cmi file was found
       in path.
       File "bug.ml", line 6, characters 2-15: Expected declaration
       File "bug.mli", line 6, characters 2-15: Actual declaration

[Octachron]

This was the result of the mechanism for higher-level error messages for functors missing some equalities between paired items. Thus inside the functor argument, the equality t(implementation)=t(interface) was missing. Moreover, since the typing environment during the inclusion check only contains the implementation-side identifiers, the error message concluded that if t(interface) was absent of the environment this was surely due to a missing cmi.

`This is fixed in #14221.

[Octachron]

This bug will be fixed in the upcoming 5.4 release, thanks for the report !