module with type constraint crashes merlin

[EduardoRFS]

I'm actually using ocaml-lsp but this seems to be caused by the code on the editor, so probably it's merlin fault. The following code crashes the ocaml-lsp, by removing the constraint part it stops crashing, but of course it doesn't do the same thing.

module type Monad = sig
  type 'a t
end
module type Monad_option = Monad with type 'a t = 'a option constraint 'a = int

[voodoos]

I copy-pasted your example on my editor using OCaml-LSP, tried to hover on a few parts of it, but could not produce a crash. Can you give more precise instructions on how you trigger the crash ?

[EduardoRFS]

You need to build first, if it doesn't have a config it works. Build it using dune, then hover it.

Repro repo below

Editor: VSCode
OS: Arch Linux
Package Manager: esy or opam

https://github.com/EduardoRFS/repro-crashes-lsp

[voodoos]

Thank you I was able to reproduce the crash.
Calling Merlin directly does not causes a crash but a failure:

❯ ocamlmerlin single errors -position 6:18 -filename hover_this.ml < hover_this.ml | jq
{
  "class": "failure",
  "value": "Graph.add: type already defined",
  "notifications": [],
  "timing": {
   ...
  }
}

This comes from ocaml/typing/short_paths_graph.ml.

I guess it is bad that Merlin reports this failure instead of the expected error which is:

File "hover_this.ml", lines 5-7, characters 2-23:
5 | ..Monad
6 |     with type 'a t = 'a option
7 |     constraint 'a = int
Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained signature:
       Type declarations do not match:
         type 'a t = 'a option constraint 'a = int
       is not included in
         type 'a t
       Their constraints differ.
       File "hover_this.ml", line 2, characters 2-11: Expected declaration
       File "hover_this.ml", lines 6-7, characters 9-23: Actual declaration

@trefis do you have an idea of what is happening here ?

[voodoos]

Surprisingly it only happens after building:

$ dune clean
$ ocamlmerlin single errors -filename hover_this.ml < hover_this.ml| jq
{
  "class": "return",
  "value": [
    {
      "type": "config",
      "sub": [],
      "valid": true,
      "message": "No config found for file \"hover_this.ml\" in \".\". Try calling `dune build`."
    },
    {
      "start": {
        "line": 5,
        "col": 2
      },
      "end": {
        "line": 7,
        "col": 23
      },
      "type": "typer",
      "sub": [],
      "valid": true,
      "message": "In this `with' constraint, the new definition of t\ndoes not match its original definition in the constrained signature:\nType declarations do not match:\n  type 'a t = 'a option constraint 'a = int\nis not included in\n  type 'a t\nTheir constraints differ.\nFile \"hover_this.ml\", line 2, characters 2-11: Expected declaration\nFile \"hover_this.ml\", line 6, characters 9-54: Actual declaration"
    }
  ],
  "notifications": [],
  "timing": {
    "clock": 19,
    "cpu": 4,
    "query": 1,
    "pp": 0,
    "reader": 1,
    "ppx": 0,
    "typer": 2,
    "error": 0
  }
}


$ dune build
File "hover_this.ml", lines 5-7, characters 2-23:
5 | ..Monad
6 |     with type 'a t = 'a option
7 |     constraint 'a = int
Error: In this `with' constraint, the new definition of t
       does not match its original definition in the constrained signature:
       Type declarations do not match:
         type 'a t = 'a option constraint 'a = int
       is not included in
         type 'a t
       Their constraints differ.
       File "hover_this.ml", line 2, characters 2-11: Expected declaration
       File "hover_this.ml", lines 6-7, characters 9-23: Actual declaration
$ ocamlmerlin single errors -filename hover_this.ml < hover_this.ml| jq
{
  "class": "failure",
  "value": "Graph.add: type already defined",
  "notifications": [],
  "timing": {
    "clock": 25,
    "cpu": 7,
    "query": 3,
    "pp": 0,
    "reader": 1,
    "ppx": 0,
    "typer": 2,
    "error": 0
  }
}

[trefis]

-short-paths falls into @lpw25's area of expertise, so I'll defer to him.

[rgrinberg]

Surprisingly it only happens after building:

Probably because short paths is only turned on after dune prints the merlin config that enables short-paths.

I fixed a similar issue recently: ocaml/ocaml-lsp#429 the pretty printing code was causing a similar exceptions.

[lpw25]

Cherry-picking the change in ocaml/ocaml#10382 fixes this issue.