handle non-list to nth()

src/builtin/list.rs

@@ -25,7 +25,7 @@ pub(crate) fn register(f: &mut HashMap<String, Builtin>) {
             max_args!(args, 2);
             let list = match arg!(args, 0, "list") {
                 Value::List(v, _) => v,
-                _ => return Err("Missing argument $list.".into()),
+                v => vec![v],
             };
             let n = match arg!(args, 1, "n") {
                 Value::Dimension(num, _) => num,

tests/list.rs

@@ -33,6 +33,11 @@ test!(
     "a {\n  color: nth((a, b, c), 3);\n}\n",
     "a {\n  color: c;\n}\n"
 );
+test!(
+    nth_non_list,
+    "a {\n  color: nth(foo, 1);\n}\n",
+    "a {\n  color: foo;\n}\n"
+);
 test!(
     list_separator_space_separated,
     "a {\n  color: list-separator(a b c);\n}\n",