Merge pull request #23979 from neovim/backport-23975-to-release-0.9

[Backport release-0.9] fix(helptags): make multibyte help tags work properly

src/nvim/help.c

@@ -457,7 +457,7 @@ int find_help_tags(const char *arg, int *num_matches, char ***matches, bool keep
         // Replace "^x" by "CTRL-X". Don't do this for "^_" to make
         // ":help i_^_CTRL-D" work.
         // Insert '-' before and after "CTRL-X" when applicable.
-        if (*s < ' '
+        if ((uint8_t)(*s) < ' '
             || (*s == '^' && s[1]
                 && (ASCII_ISALPHA(s[1]) || vim_strchr("?@[\\]^", (uint8_t)s[1]) != NULL))) {
           if (d > IObuff && d[-1] != '_' && d[-1] != '\\') {

src/nvim/tag.c

@@ -1751,7 +1751,7 @@ static tagmatch_status_T findtags_parse_line(findtags_state_T *st, tagptrs_T *ta
     if (st->state == TS_BINARY) {
       int tagcmp;
       // Simplistic check for unsorted tags file.
-      int i = (int)tagpp->tagname[0];
+      int i = (uint8_t)tagpp->tagname[0];
       if (margs->sortic) {
         i = TOUPPER_ASC(tagpp->tagname[0]);
       }

test/functional/ex_cmds/help_spec.lua

@@ -4,6 +4,10 @@ local clear = helpers.clear
 local command = helpers.command
 local eq = helpers.eq
 local funcs = helpers.funcs
+local meths = helpers.meths
+local mkdir = helpers.mkdir
+local rmdir = helpers.rmdir
+local write_file = helpers.write_file
 
 describe(':help', function()
   before_each(clear)
@@ -25,4 +29,17 @@ describe(':help', function()
     -- Before #9773, Nvim would crash on quitting the help window.
     eq(1002, funcs.win_getid())
   end)
+
+  it('multibyte help tags work #23978', function()
+    mkdir('Xhelptags')
+    finally(function()
+      rmdir('Xhelptags')
+    end)
+    mkdir('Xhelptags/doc')
+    write_file('Xhelptags/doc/Xhelptags.txt', '*…*')
+    command('helptags Xhelptags/doc')
+    command('set rtp+=Xhelptags')
+    command('help …')
+    eq('*…*', meths.get_current_line())
+  end)
 end)