False detection of self cycle error with an explicit module type

[cannorin]

Version: 9.1.4

Assume we have the following code of name b.res:

module Types : {
  module B : {
    type t = private int
  }
} = {
  module B = {
    type t = private int
  }
}

open Types

module B = {
  open B
  let t : t = Obj.magic(42)
}

This fails with a FAILED: B has a self cycle error despite that this does not contain any dependency to itself.
The open B actually refers to Types.B, but the compiler falsely detects it as a self-reference.

Interestingly, when you remove the explicit module type of the Types module, it compiles correctly:

module Types = {
  module B = {
    type t = private int
  }
}

open Types

module B = {
  open B
  let t : t = Obj.magic(42)
}

[cannorin]

I believe this is a remnant of an old bug of ocamldep: ocaml/ocaml#4618

This seems to be considered by the community as one of the many limitations of ocamldep, as this and many other ocamldep bugs are marked either stale, suspended, or not fixable.

I think self-cycles should not be considered as fatal error, as the result reported from jscomp/ml/Depend.ml is unreliable. For example, dune just ignores any self-reference reported from ocamldep: https://github.com/ocaml/dune/blob/574f0f7de27c2391d660394e1dc72e9d70b0531e/boot/duneboot.ml#L875-L876

[cannorin]

Can I open a PR to disable self-reference errors? I believe it's not very good to reject codes which should be valid. If a source file does indeed contain a self-reference, it would fail to typecheck & compile anyway.

[cristianoc]

Would you know where to look to check where this behaviour originates.
Notice the compiler has its own build system, so presumably it would be somewhere in there.

[cannorin]

@cristianoc

• https://github.com/rescript-lang/rescript-compiler/blob/e60482c6f6a69994907b9bd56e58ce87052e3659/jscomp/bsb_helper/bsb_helper_depfile_gen.ml#L137 is where the FAILED: B has a self cycle error is raised.
• jscomp/ml/depend.ml is where the dependencies are checked, which is used to create dep_raw_string and inherits the bug ocamldep generates circular dependencies which don't exist ocaml/ocaml#4618.

[cristianoc]

Great!
If you know where the fix goes go ahead.

[cannorin]

PR opened: #5393