set-nth correctly recognizes arglists

src/builtin/functions/list.rs

@@ -81,6 +81,11 @@ pub(crate) fn set_nth(mut args: CallArgs, parser: &mut Parser<'_>) -> SassResult
     args.max_args(3)?;
     let (mut list, sep, brackets) = match args.get_err(0, "list")? {
         Value::List(v, sep, b) => (v, sep, b),
+        Value::ArgList(v) => (
+            v.into_iter().map(|val| val.node).collect(),
+            ListSeparator::Comma,
+            Brackets::None,
+        ),
         Value::Map(m) => (m.as_list(), ListSeparator::Comma, Brackets::None),
         v => (vec![v], ListSeparator::Space, Brackets::None),
     };

tests/list.rs

@@ -91,6 +91,17 @@ test!(
     "a {\n  color: set-nth((c: d, e: f, g: h), 2, i);\n}\n",
     "a {\n  color: c d, i, g h;\n}\n"
 );
+test!(
+    set_nth_arglist,
+    "@mixin foo($args...) {
+        color: set-nth($args, 2, 0);
+    }
+
+    a {
+        @include foo(1, 2, 3, 4);
+    }",
+    "a {\n  color: 1, 0, 3, 4;\n}\n"
+);
 test!(
     append_space_separated,
     "a {\n  color: append(a b, c);\n}\n",