Unexpected type reported in type error

[SkySkimmer]

module M = struct
  type a
  let f (x:a) = x
end

type b = M.a

let _ = M.f 2

Got:

Error: This expression has type int but an expression was expected of type b

Expected:

Error: This expression has type int but an expression was expected of type M.a

In the original case b is an alias with some semantic meaning (even though as an alias it can't produce a strong guarantee) like https://github.com/coq/coq/blob/506136d60c0dcc4fc2a2ca83ef3b586fbede55a2/kernel/names.mli#L118 (and f was https://github.com/coq/coq/blob/506136d60c0dcc4fc2a2ca83ef3b586fbede55a2/kernel/names.mli#L56)

This behaviour is because of -short-paths (I guess from #1743 ?). I haven't reviewed the history of that commit but the commit message only talks about ocamlc -i, would it be possible to avoid using -short-paths for other commands (and also avoid putting it in .merlin)?

[ghost]

This seems like an OCaml bug, could you report it upstream? -short-paths is in general beneficial for developers, so disabling it in Dune doesn't seem right.

[SkySkimmer]

https://caml.inria.fr/mantis/view.php?id=7911

[SkySkimmer]

Looks like they don't think it's a bug either

[ghost]

Indeed. Looking at the example again, it doesn't look like a bug to me either. In any case, there is not much we can do in Dune.

[SkySkimmer]

Very well then

[rgrinberg]

@SkySkimmer Perhaps you could try making it a private type alias:

type b = private M.a

And then you'll have to do all the coercions explicitly. That shouldn't be so bad, if there's really a semantic meaning to your alias.

In general, you really want -short-paths as without it you will be really disappointed by the error messages, especially in the presence of wrapped libraries.