handle non list separator in join()

src/builtin/list.rs

@@ -147,10 +147,14 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
         "join".to_owned(),
         Box::new(|mut args, _| {
             max_args!(args, 4);
+            let mut is_first_list = true;
             let (mut list1, sep1, brackets) = match arg!(args, 0, "list1") {
                 Value::List(v, sep, brackets) => (v, sep, brackets),
                 Value::Map(m) => (m.entries(), ListSeparator::Comma, Brackets::None),
-                v => (vec![v], ListSeparator::Space, Brackets::None),
+                v => {
+                    is_first_list = false;
+                    (vec![v], ListSeparator::Space, Brackets::None)
+                }
             };
             let (list2, sep2) = match arg!(args, 1, "list2") {
                 Value::List(v, sep, ..) => (v, sep),
@@ -164,7 +168,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             ) {
                 Value::Ident(s, ..) => match s.as_str() {
                     "auto" => {
-                        if list1.is_empty() {
+                        if list1.is_empty() || !is_first_list {
                             sep2
                         } else {
                             sep1

tests/list.rs

@@ -222,6 +222,11 @@ test!(
     "a {\n  color: join((c: d, e: f), (g: h, i: j));\n}\n",
     "a {\n  color: c d, e f, g h, i j;\n}\n"
 );
+test!(
+    join_non_list_first_takes_separator_of_second,
+    "a {\n  color: join(c, (d, e));\n}\n",
+    "a {\n  color: c, d, e;\n}\n"
+);
 test!(bracketed_ident, "a {\n  color: [a];\n}\n");
 test!(bracketed_space_list, "a {\n  color: [a b];\n}\n");
 test!(bracketed_comma_list, "a {\n  color: [a, b];\n}\n");