patch 8.2.3747: cannot remove highlight from an existing sign

Problem:    Cannot remove highlight from an existing sign. (James McCoy)
Solution:   Only reject empty argument for a new sign.

src/sign.c

@@ -1078,13 +1078,28 @@ sign_define_by_name(
 	return FAIL;
 
     if (linehl != NULL)
-	sp->sn_line_hl = syn_check_group(linehl, (int)STRLEN(linehl));
+    {
+	if (*linehl == NUL)
+	    sp->sn_line_hl = 0;
+	else
+	    sp->sn_line_hl = syn_check_group(linehl, (int)STRLEN(linehl));
+    }
 
     if (texthl != NULL)
-	sp->sn_text_hl = syn_check_group(texthl, (int)STRLEN(texthl));
+    {
+	if (*texthl == NUL)
+	    sp->sn_text_hl = 0;
+	else
+	    sp->sn_text_hl = syn_check_group(texthl, (int)STRLEN(texthl));
+    }
 
     if (culhl != NULL)
-	sp->sn_cul_hl = syn_check_group(culhl, (int)STRLEN(culhl));
+    {
+	if (*culhl == NUL)
+	    sp->sn_cul_hl = 0;
+	else
+	    sp->sn_cul_hl = syn_check_group(culhl, (int)STRLEN(culhl));
+    }
 
     return OK;
 }
@@ -1319,7 +1334,11 @@ sign_define_cmd(char_u *sign_name, char_u *cmdline)
     char_u	*linehl = NULL;
     char_u	*texthl = NULL;
     char_u	*culhl = NULL;
-    int failed = FALSE;
+    int		failed = FALSE;
+    sign_T	*sp_prev;
+    int		exists;
+
+    exists = sign_find(sign_name, &sp_prev) != NULL;
 
     // set values for a defined sign.
     for (;;)
@@ -1341,7 +1360,7 @@ sign_define_cmd(char_u *sign_name, char_u *cmdline)
 	else if (STRNCMP(arg, "linehl=", 7) == 0)
 	{
 	    arg += 7;
-	    if (check_empty_group(p - arg, "linehl") == FAIL)
+	    if (!exists && check_empty_group(p - arg, "linehl") == FAIL)
 	    {
 		failed = TRUE;
 		break;
@@ -1351,7 +1370,7 @@ sign_define_cmd(char_u *sign_name, char_u *cmdline)
 	else if (STRNCMP(arg, "texthl=", 7) == 0)
 	{
 	    arg += 7;
-	    if (check_empty_group(p - arg, "texthl") == FAIL)
+	    if (!exists && check_empty_group(p - arg, "texthl") == FAIL)
 	    {
 		failed = TRUE;
 		break;
@@ -1361,7 +1380,7 @@ sign_define_cmd(char_u *sign_name, char_u *cmdline)
 	else if (STRNCMP(arg, "culhl=", 6) == 0)
 	{
 	    arg += 6;
-	    if (check_empty_group(p - arg, "culhl") == FAIL)
+	    if (!exists && check_empty_group(p - arg, "culhl") == FAIL)
 	    {
 		failed = TRUE;
 		break;

src/testdir/test_signs.vim

@@ -126,9 +126,34 @@ func Test_sign()
   call assert_fails("sign define Sign4 text= linehl=Comment", 'E239:')
   call assert_fails("sign define Sign4 text=\\ ab  linehl=Comment", 'E239:')
 
-  call assert_fails("sign define Sign4 linehl=", 'E1249: Group name missing for linehl')
+  " an empty highlight argument for a new sign is an error
-  call assert_fails("sign define Sign4 culhl=", 'E1249: Group name missing for culhl')
+  call assert_fails("sign define SignX linehl=", 'E1249: Group name missing for linehl')
-  call assert_fails("sign define Sign4 texthl=", 'E1249: Group name missing for texthl')
+  call assert_fails("sign define SignX culhl=", 'E1249: Group name missing for culhl')
+  call assert_fails("sign define SignX texthl=", 'E1249: Group name missing for texthl')
+
+  " an empty highlight argument for an existing sign clears it
+  sign define SignY texthl=TextHl culhl=CulHl linehl=LineHl
+  let sl = sign_getdefined('SignY')[0]
+  call assert_equal('TextHl', sl.texthl)
+  call assert_equal('CulHl', sl.culhl)
+  call assert_equal('LineHl', sl.linehl)
+
+  sign define SignY texthl= culhl=CulHl linehl=LineHl
+  let sl = sign_getdefined('SignY')[0]
+  call assert_false(has_key(sl, 'texthl'))
+  call assert_equal('CulHl', sl.culhl)
+  call assert_equal('LineHl', sl.linehl)
+
+  sign define SignY linehl=
+  let sl = sign_getdefined('SignY')[0]
+  call assert_false(has_key(sl, 'linehl'))
+  call assert_equal('CulHl', sl.culhl)
+
+  sign define SignY culhl=
+  let sl = sign_getdefined('SignY')[0]
+  call assert_false(has_key(sl, 'culhl'))
+
+  sign undefine SignY
 
   " define sign with whitespace
   sign define Sign4 text=\ X linehl=Comment

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3747,
 /**/
     3746,
 /**/