patch 9.0.1798: The 'syntax' option has no completion.

Problem:  The 'syntax' option has no completion.
Solution: Add syntax option completion.

closes: #12900

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Doug Kearns <dougkearns@gmail.com>

src/option.c

@@ -7374,6 +7374,14 @@ set_context_in_set_cmd(
 
     xp->xp_pattern = p + 1;
 
+#ifdef FEAT_SYN_HL
+    if (options[opt_idx].var == (char_u *)&p_syn)
+    {
+	xp->xp_context = EXPAND_OWNSYNTAX;
+	return;
+    }
+#endif
+
     if (flags & P_EXPAND)
     {
 	p = options[opt_idx].var;

src/testdir/test_options.vim

@@ -374,6 +374,12 @@ func Test_set_completion()
   call assert_equal('"set filetype=sshdconfig', @:)
   call feedkeys(":set filetype=a\<C-A>\<C-B>\"\<CR>", 'xt')
   call assert_equal('"set filetype=' .. getcompletion('a*', 'filetype')->join(), @:)
+
+  " Expand values for 'syntax'
+  call feedkeys(":set syntax=sshdconfi\<Tab>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"set syntax=sshdconfig', @:)
+  call feedkeys(":set syntax=a\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal('"set syntax=' .. getcompletion('a*', 'syntax')->join(), @:)
 endfunc
 
 func Test_set_option_errors()

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1798,
 /**/
     1797,
 /**/