Comment-dwim is commenting properly but uncommenting improperly

[mattiasdrp]

Area

Other

Expected behavior

It should be noted that I don't mind working on this if you don't have the time, feel free to ask me to do the PR ;-)

(* let () =  *)
(*   let a = 3 in  *)
(*   Format.eprintf "%d@." a  *)

Uncommenting this selection should give

let () =  
  let a = 3 in  
  Format.eprintf "%d@." a  

Actual behavior

let () =  *)
  let a = 3 in  *)
  Format.eprintf "%d@." a  *)

Steps to reproduce

I created a minimal setup:

;;; init.el --- init file                            -*- lexical-binding: t; -*-

;; Copyright (C) 2026  mattias

;; Author: mattias <mattias@mattias-pc>
;; Keywords: lisp

(defvar elpaca-installer-version 0.12)
(defvar elpaca-directory (expand-file-name "elpaca/" user-emacs-directory))
(defvar elpaca-builds-directory (expand-file-name "builds/" elpaca-directory))
(defvar elpaca-sources-directory (expand-file-name "sources/" elpaca-directory))
(defvar elpaca-order '(elpaca :repo "https://github.com/progfolio/elpaca.git"
                              :ref nil :depth 1 :inherit ignore
                              :files (:defaults "elpaca-test.el" (:exclude "extensions"))
                              :build (:not elpaca-activate)))
(let* ((repo  (expand-file-name "elpaca/" elpaca-sources-directory))
       (build (expand-file-name "elpaca/" elpaca-builds-directory))
       (order (cdr elpaca-order))
       (default-directory repo))
  (add-to-list 'load-path (if (file-exists-p build) build repo))
  (unless (file-exists-p repo)
    (make-directory repo t)
    (when (<= emacs-major-version 28) (require 'subr-x))
    (condition-case-unless-debug err
        (if-let* ((buffer (pop-to-buffer-same-window "*elpaca-bootstrap*"))
                  ((zerop (apply #'call-process `("git" nil ,buffer t "clone"
                                                  ,@(when-let* ((depth (plist-get order :depth)))
                                                      (list (format "--depth=%d" depth) "--no-single-branch"))
                                                  ,(plist-get order :repo) ,repo))))
                  ((zerop (call-process "git" nil buffer t "checkout"
                                        (or (plist-get order :ref) "--"))))
                  (emacs (concat invocation-directory invocation-name))
                  ((zerop (call-process emacs nil buffer nil "-Q" "-L" "." "--batch"
                                        "--eval" "(byte-recompile-directory \".\" 0 'force)")))
                  ((require 'elpaca))
                  ((elpaca-generate-autoloads "elpaca" repo)))
            (progn (message "%s" (buffer-string)) (kill-buffer buffer))
          (error "%s" (with-current-buffer buffer (buffer-string))))
      ((error) (warn "%s" err) (delete-directory repo 'recursive))))
  (unless (require 'elpaca-autoloads nil t)
    (require 'elpaca)
    (elpaca-generate-autoloads "elpaca" repo)
    (let ((load-source-file-function nil)) (load "./elpaca-autoloads"))))
(add-hook 'after-init-hook #'elpaca-process-queues)
(elpaca `(,@elpaca-order))

;; Install use-package support
(elpaca bind-key)

(elpaca elpaca-use-package
  ;; Enable :ensure use-package keyword.
  (elpaca-use-package-mode)
  ;; Assume :ensure t unless otherwise specified.
  (setopt use-package-always-ensure t))

(elpaca-wait)

(use-package neocaml
  :load-path "~/neocaml/"
  :config (message "`neocaml' loaded"))

(provide 'init)

;;; init.el ends here

Write OCaml code, comment it, uncomment it, the leading *) should appear

Environment

Emacs: 31.0.50
System: gnu/linux
neocaml: 0.4.1
Grammars: ocaml (expected: v0.24.2), ocaml-interface (expected: v0.24.2)
Eglot: inactive

[bbatsov]

Thanks for the report! This is a known upstream Emacs 31 bug in treesit-forward-comment -- it overshoots by one position ((1+ (treesit-node-end ...))) which causes uncomment-region to leave *) behind on multi-line regions. It works correctly on Emacs 30.

We already have a skipped test for this: https://github.com/bbatsov/neocaml/blob/main/test/neocaml-fill-test.el#L190-L203

If you'd like to investigate potential workarounds on the neocaml side while we wait for an upstream fix, PRs are welcome! One possible direction would be advising or overriding uncomment-region to clean up the leftover *) suffixes.

[mattiasdrp]

Ah, thanks for the tip, I'll look into it!