patch 8.2.2289: Vim9: 'cpo' can become empty

Problem:    Vim9: 'cpo' can become empty.
Solution:   Use empty_option instead of an empty string.  Update quickfix
            buffer after restoring 'cpo'.  (closes #7608)

src/eval.c

@@ -1940,7 +1940,7 @@ pattern_match(char_u *pat, char_u *text, int ic)
 
     // avoid 'l' flag in 'cpoptions'
     save_cpo = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
     regmatch.regprog = vim_regcomp(pat, RE_MAGIC + RE_STRING);
     if (regmatch.regprog != NULL)
     {
@@ -6200,8 +6200,14 @@ do_string_sub(
     if (p_cpo == empty_option)
 	p_cpo = save_cpo;
     else
+    {
 	// Darn, evaluating {sub} expression or {expr} changed the value.
+	// If it's still empty it was changed and restored, need to restore in
+	// the complicated way.
+	if (*p_cpo == NUL)
+	    set_option_value((char_u *)"cpo", 0L, save_cpo, 0);
 	free_string_option(save_cpo);
+    }
 
     return ret;
 }

src/evalbuffer.c

@@ -68,7 +68,7 @@ buflist_find_by_name(char_u *name, int curtab_only)
     save_magic = p_magic;
     p_magic = TRUE;
     save_cpo = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
 
     buf = buflist_findnr(buflist_findpat(name, name + STRLEN(name),
 						    TRUE, FALSE, curtab_only));

src/evalfunc.c

@@ -6316,7 +6316,7 @@ find_some_match(typval_T *argvars, typval_T *rettv, matchtype_T type)
 
     // Make 'cpoptions' empty, the 'l' flag should not be used here.
     save_cpo = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
 
     rettv->vval.v_number = -1;
     if (type == MATCH_LIST || type == MATCH_POS)
@@ -8024,8 +8024,14 @@ theend:
     if (p_cpo == empty_option)
 	p_cpo = save_cpo;
     else
+    {
 	// Darn, evaluating the {skip} expression changed the value.
+	// If it's still empty it was changed and restored, need to restore in
+	// the complicated way.
+	if (*p_cpo == NUL)
+	    set_option_value((char_u *)"cpo", 0L, save_cpo, 0);
 	free_string_option(save_cpo);
+    }
 
     return retval;
 }
@@ -8723,7 +8729,7 @@ f_split(typval_T *argvars, typval_T *rettv)
 
     // Make 'cpoptions' empty, the 'l' flag should not be used here.
     save_cpo = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
 
     str = tv_get_string(&argvars[0]);
     if (argvars[1].v_type != VAR_UNKNOWN)

src/ex_eval.c

@@ -1695,7 +1695,7 @@ ex_catch(exarg_T *eap)
 		    *end = NUL;
 		}
 		save_cpo  = p_cpo;
-		p_cpo = (char_u *)"";
+		p_cpo = empty_option;
 		// Disable error messages, it will make current_exception
 		// invalid.
 		++emsg_off;

src/gui_motif.c

@@ -3525,7 +3525,7 @@ find_replace_callback(
 	char_u	*save_cpo = p_cpo;
 
 	// No need to be Vi compatible here.
-	p_cpo = (char_u *)"";
+	p_cpo = empty_options;
 	u_undo(1);
 	p_cpo = save_cpo;
 	gui_update_screen();

src/map.c

@@ -2475,7 +2475,7 @@ add_map(char_u *map, int mode)
     char_u	*s;
     char_u	*cpo_save = p_cpo;
 
-    p_cpo = (char_u *)"";	// Allow <> notation
+    p_cpo = empty_option;	// Allow <> notation
     s = vim_strsave(map);
     if (s != NULL)
     {

src/quickfix.c

@@ -8099,6 +8099,7 @@ ex_helpgrep(exarg_T *eap)
     int		new_qi = FALSE;
     char_u	*au_name =  NULL;
     char_u	*lang = NULL;
+    int		updated = FALSE;
 
     switch (eap->cmdidx)
     {
@@ -8150,14 +8151,24 @@ ex_helpgrep(exarg_T *eap)
 	qfl->qf_ptr = qfl->qf_start;
 	qfl->qf_index = 1;
 	qf_list_changed(qfl);
-	qf_update_buffer(qi, NULL);
+	updated = TRUE;
     }
 
     if (p_cpo == empty_option)
 	p_cpo = save_cpo;
     else
-	// Darn, some plugin changed the value.
+    {
+	// Darn, some plugin changed the value.  If it's still empty it was
+	// changed and restored, need to restore in the complicated way.
+	if (*p_cpo == NUL)
+	    set_option_value((char_u *)"cpo", 0L, save_cpo, 0);
 	free_string_option(save_cpo);
+    }
+
+    if (updated)
+	// This may open a window and source scripts, do this after 'cpo' was
+	// restored.
+	qf_update_buffer(qi, NULL);
 
     if (au_name != NULL)
     {

src/syntax.c

@@ -5670,7 +5670,7 @@ get_syn_pattern(char_u *arg, synpat_T *ci)
 
     // Make 'cpoptions' empty, to avoid the 'l' flag
     cpo_save = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
     ci->sp_prog = vim_regcomp(ci->sp_pattern, RE_MAGIC);
     p_cpo = cpo_save;
 
@@ -5858,7 +5858,7 @@ syn_cmd_sync(exarg_T *eap, int syncing UNUSED)
 
 		// Make 'cpoptions' empty, to avoid the 'l' flag
 		cpo_save = p_cpo;
-		p_cpo = (char_u *)"";
+		p_cpo = empty_option;
 		curwin->w_s->b_syn_linecont_prog =
 		       vim_regcomp(curwin->w_s->b_syn_linecont_pat, RE_MAGIC);
 		p_cpo = cpo_save;

src/testdir/test_quickfix.vim

@@ -702,6 +702,26 @@ func Test_helpgrep()
   call s:test_xhelpgrep('l')
 endfunc
 
+def Test_helpgrep_vim9_restore_cpo()
+  assert_equal('aABceFs', &cpo)
+
+  var rtp_save = &rtp
+  var dir = 'Xruntime/after'
+  &rtp ..= ',' .. dir
+  mkdir(dir .. '/ftplugin', 'p')
+  writefile(['vim9'], dir .. '/ftplugin/qf.vim')
+  filetype plugin on
+  silent helpgrep grail
+  cwindow
+  silent helpgrep grail
+
+  assert_equal('aABceFs', &cpo)
+  delete(dir, 'rf')
+  &rtp = rtp_save
+  cclose
+  helpclose
+enddef
+
 func Test_errortitle()
   augroup QfBufWinEnter
     au!

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2289,
 /**/
     2288,
 /**/