comment-indent: Handle BOL already within a multiline comment

* lisp/newcomment.el (comment-indent): Newly handle the case
that BOL is already within a multiline comment (bug#78003).
Thanks to Stefan Monnier for review and reworking the control
flow.

1 files changed, 64 insertions, 55 deletions

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 23fcbc05372..8ccedb0ef0f 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -713,61 +713,70 @@ Point is expected to be at the start of the comment."
 If CONTINUE is non-nil, use the `comment-continue' markers if any."
   (interactive "*")
   (comment-normalize-vars)
-  (let ((starter (or (and continue comment-continue)
-                     comment-start))
-        (ender (or (and continue comment-continue "")
-                   comment-end)))
-    (unless starter (error "No comment syntax defined"))
-    (beginning-of-line)
-    (let* ((eolpos (line-end-position))
-           (begpos (comment-search-forward eolpos t))
-           cpos indent)
-      (if (and comment-insert-comment-function (not begpos))
-          ;; If no comment and c-i-c-f is set, let it do everything.
-          (funcall comment-insert-comment-function)
-        ;; An existing comment?
-        (if begpos
-            (progn
-              (if (and (not (looking-at "[\t\n ]"))
-                       (looking-at comment-end-skip))
-                  ;; The comment is empty and we have skipped all its space
-                  ;; and landed right before the comment-ender:
-                  ;; Go back to the middle of the space.
-                  (forward-char (/ (skip-chars-backward " \t") -2)))
-              (setq cpos (point-marker)))
-          ;; If none, insert one.
-          (save-excursion
-            ;; Some `comment-indent-function's insist on not moving
-            ;; comments that are in column 0, so we first go to the
-            ;; likely target column.
-            (indent-to comment-column)
-            ;; Ensure there's a space before the comment for things
-            ;; like sh where it matters (as well as being neater).
-            (unless (memq (char-before) '(nil ?\n ?\t ?\s))
-              (insert ?\s))
-            (setq begpos (point))
-            (insert starter)
-            (setq cpos (point-marker))
-            (insert ender)))
-        (goto-char begpos)
-        ;; Compute desired indent.
-        (setq indent (save-excursion (funcall comment-indent-function)))
-        ;; If `indent' is nil and there's code before the comment, we can't
-        ;; use `indent-according-to-mode', so we default to comment-column.
-        (unless (or indent (save-excursion (skip-chars-backward " \t") (bolp)))
-          (setq indent comment-column))
-        (if (not indent)
-            ;; comment-indent-function refuses: delegate to line-indent.
-            (indent-according-to-mode)
-          ;; If the comment is at the right of code, adjust the indentation.
-          (unless (save-excursion (skip-chars-backward " \t") (bolp))
-            (setq indent (comment-choose-indent indent)))
-          ;; If that's different from comment's current position, change it.
-          (unless (= (current-column) indent)
-            (delete-region (point) (progn (skip-chars-backward " \t") (point)))
-            (indent-to indent)))
-        (goto-char cpos)
-        (set-marker cpos nil)))))
+  (beginning-of-line)
+  (let* ((starter (or (and continue comment-continue)
+                      comment-start
+                      (error "No comment syntax defined")))
+         (ender (or (and continue comment-continue "")
+                    comment-end))
+         (begpos (comment-search-forward (line-end-position) t))
+         cpos indent)
+    (cond
+     ;; If we couldn't find a comment *starting* on this line, see if we
+     ;; are already within a multiline comment at BOL (bug#78003).
+     ((and (not begpos) (not continue)
+           comment-use-syntax comment-use-global-state
+           (nth 4 (syntax-ppss (line-beginning-position))))
+      ;; We don't know anything about the nature of the multiline
+      ;; construct, so immediately delegate to the mode.
+      (indent-according-to-mode))
+     ((and (not begpos) comment-insert-comment-function)
+      ;; If no comment and c-i-c-f is set, let it do everything.
+      (funcall comment-insert-comment-function))
+     (t
+      ;; An existing comment?
+      (if begpos
+          (progn
+            (if (and (not (looking-at "[\t\n ]"))
+                     (looking-at comment-end-skip))
+                ;; The comment is empty and we have skipped all its space
+                ;; and landed right before the comment-ender:
+                ;; Go back to the middle of the space.
+                (forward-char (/ (skip-chars-backward " \t") -2)))
+            (setq cpos (point-marker)))
+        ;; If none, insert one.
+        (save-excursion
+          ;; Some `comment-indent-function's insist on not moving
+          ;; comments that are in column 0, so we first go to the
+          ;; likely target column.
+          (indent-to comment-column)
+          ;; Ensure there's a space before the comment for things
+          ;; like sh where it matters (as well as being neater).
+          (unless (memq (char-before) '(nil ?\n ?\t ?\s))
+            (insert ?\s))
+          (setq begpos (point))
+          (insert starter)
+          (setq cpos (point-marker))
+          (insert ender)))
+      (goto-char begpos)
+      ;; Compute desired indent.
+      (setq indent (save-excursion (funcall comment-indent-function)))
+      ;; If `indent' is nil and there's code before the comment, we can't
+      ;; use `indent-according-to-mode', so we default to comment-column.
+      (unless (or indent (save-excursion (skip-chars-backward " \t") (bolp)))
+        (setq indent comment-column))
+      (if (not indent)
+          ;; comment-indent-function refuses: delegate to line-indent.
+          (indent-according-to-mode)
+        ;; If the comment is at the right of code, adjust the indentation.
+        (unless (save-excursion (skip-chars-backward " \t") (bolp))
+          (setq indent (comment-choose-indent indent)))
+        ;; If that's different from comment's current position, change it.
+        (unless (= (current-column) indent)
+          (delete-region (point) (progn (skip-chars-backward " \t") (point)))
+          (indent-to indent)))
+      (goto-char cpos)
+      (set-marker cpos nil)))))
 
 ;;;###autoload
 (defun comment-set-column (arg)