Don't delete leading and trailing space from CJK word (bug#25685)

* lisp/textmodes/fill.el (fill-delete-newlines):
Don't delete leading and trailing space from CJK word.
(fill-separate-heterogeneous-words-with-space):
New user option that controls it (bug#25685).

1 files changed, 14 insertions, 2 deletions

diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index 2957bc62d97..ee523ed5f5c 100644
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -49,6 +49,15 @@ A value of nil means that any change in indentation starts a new paragraph."
   :group 'fill)
 (put 'colon-double-space 'safe-local-variable 'booleanp)
 
+(defcustom fill-separate-heterogeneous-words-with-space nil
+  "Non-nil means that use a space to separate words of different kind.
+This will be done with a word in the end of a line and a word in the
+beginning of the next line when concatenating them for filling those
+lines.  Whether to use a space is up to how the words are categorized."
+  :type 'boolean
+  :group 'fill
+  :version "26.1")
+
 (defvar fill-paragraph-function nil
   "Mode-specific function to fill a paragraph, or nil if there is none.
 If the function returns nil, then `fill-paragraph' does its normal work.
@@ -494,8 +503,11 @@ Point is moved to just past the fill prefix on the first line."
             (replace-match (get-text-property (match-beginning 0) 'fill-space))
           (let ((prev (char-before (match-beginning 0)))
                 (next (following-char)))
-            (if (and (or (aref (char-category-set next) ?|)
-                         (aref (char-category-set prev) ?|))
+            (if (and (if fill-separate-heterogeneous-words-with-space
+                         (and (aref (char-category-set next) ?|)
+                              (aref (char-category-set prev) ?|))
+                       (or (aref (char-category-set next) ?|)
+                           (aref (char-category-set prev) ?|)))
                      (or (aref fill-nospace-between-words-table next)
                          (aref fill-nospace-between-words-table prev)))
                 (delete-char -1))))))