lisp/newcomment.el: Uncommenting with whitespace `comment-continue'

* lisp/newcomment.el (uncomment-region-default-1): Make
all-whitespace `comment-continue' work (bug#50226).

Copyright-paperwork-exempt: yes

lisp/newcomment.el

@@ -932,7 +932,8 @@ This function is the default value of `uncomment-region-function'."
   (setq end (copy-marker end))
   (let* ((numarg (prefix-numeric-value arg))
 	 (ccs comment-continue)
-	 (srei (comment-padright ccs 're))
+	 (srei (or (comment-padright ccs 're)
+		   (and (stringp comment-continue) comment-continue)))
 	 (csre (comment-padright comment-start 're))
 	 (sre (and srei (concat "^\\s-*?\\(" srei "\\)")))
 	 spt)

test/lisp/newcomment-tests.el

@@ -0,0 +1,39 @@
+;;; newcomment-tests.el  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2010-2021 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software: you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(require 'ert)
+
+(ert-deftest test-uncomment-space-comment-continue ()
+  (let ((comment-style 'multi-line)
+        (comment-continue "   ")
+        (text "  a\n  b"))
+    (should
+     (equal text
+            (with-temp-buffer
+              (c-mode)
+              (insert text)
+              (comment-region (point-min) (point-max))
+              (uncomment-region (point-min) (point-max))
+              (buffer-string))))))
+
+;;; newcomment-testsuite.el ends here