diff --git a/lib/src/dsl_util.rs b/lib/src/dsl_util.rs
index c92f01cef3..ac6615e0db 100644
--- a/lib/src/dsl_util.rs
+++ b/lib/src/dsl_util.rs
@@ -458,16 +458,20 @@ impl<P, V> AliasesMap<P, V> {
                 self.symbol_aliases.insert(name, defn.into());
             }
             AliasDeclaration::Function(name, params) => {
-                let overloads = self.function_aliases.entry(name).or_default();
-                match overloads.binary_search_by_key(&params.len(), |(params, _)| params.len()) {
-                    Ok(i) => overloads[i] = (params, defn.into()),
-                    Err(i) => overloads.insert(i, (params, defn.into())),
-                }
+                self.insert_function(name, params, defn.into());
             }
         }
         Ok(())
     }
 
+    fn insert_function(&mut self, name: String, params: Vec<String>, defn: V) {
+        let overloads = self.function_aliases.entry(name).or_default();
+        match overloads.binary_search_by_key(&params.len(), |(params, _)| params.len()) {
+            Ok(i) => overloads[i] = (params, defn),
+            Err(i) => overloads.insert(i, (params, defn)),
+        }
+    }
+
     /// Iterates symbol names in arbitrary order.
     pub fn symbol_names(&self) -> impl Iterator<Item = &str> {
         self.symbol_aliases.keys().map(|n| n.as_ref())
@@ -492,6 +496,18 @@ impl<P, V> AliasesMap<P, V> {
         overloads.find_by_arity(arity)
     }
 
+    /// Looks up a function alias by name and arity, assuming that the function
+    /// can take extra parameters (eg. *args in python).
+    /// Returns identifier, list of parameter names, and definition text.
+    pub fn get_function_with_leftovers(
+        &self,
+        name: &str,
+        arity: usize,
+    ) -> Option<(AliasId<'_>, &[String], &V)> {
+        let overloads = self.get_function_overloads(name)?;
+        overloads.find_by_arity_with_leftovers(arity)
+    }
+
     /// Looks up function aliases by name.
     fn get_function_overloads(&self, name: &str) -> Option<AliasFunctionOverloads<'_, V>> {
         let (name, overloads) = self.function_aliases.get_key_value(name)?;
@@ -518,16 +534,39 @@ impl<'a, V> AliasFunctionOverloads<'a, V> {
         self.arities().next_back().unwrap()
     }
 
-    fn find_by_arity(&self, arity: usize) -> Option<(AliasId<'a>, &'a [String], &'a V)> {
-        let index = self
-            .overloads
-            .binary_search_by_key(&arity, |(params, _)| params.len())
-            .ok()?;
+    fn get_overload(&self, index: usize) -> (AliasId<'a>, &'a [String], &'a V) {
         let (params, defn) = &self.overloads[index];
         // Exact parameter names aren't needed to identify a function, but they
         // provide a better error indication. (e.g. "foo(x, y)" is easier to
         // follow than "foo/2".)
-        Some((AliasId::Function(self.name, params), params, defn))
+        (AliasId::Function(self.name, params), params, defn)
+    }
+
+    fn find_by_arity(&self, arity: usize) -> Option<(AliasId<'a>, &'a [String], &'a V)> {
+        Some(
+            self.get_overload(
+                self.overloads
+                    .binary_search_by_key(&arity, |(params, _)| params.len())
+                    .ok()?,
+            ),
+        )
+    }
+
+    // `find_arity(3)` is equivalent to finding the correct overload for `fn(a, b, c, *args)`.
+    // We need to find the longest match that is still less than arity.
+    fn find_by_arity_with_leftovers(
+        &self,
+        arity: usize,
+    ) -> Option<(AliasId<'a>, &'a [String], &'a V)> {
+        if arity < self.min_arity() {
+            // This is like calling `fn(a, b)` when the definition is `fn(a, b, c ,*args)`
+            None
+        } else {
+            let first_invalid = self
+                .overloads
+                .partition_point(|(args, _)| arity >= args.len());
+            Some(self.get_overload(first_invalid - 1))
+        }
     }
 }
 
@@ -849,4 +888,65 @@ mod tests {
         let f = function("foo", [], [keyword("a", 0), keyword("a", 1)]);
         assert!(f.expect_named_arguments::<1, 1>(&["", "a"]).is_err());
     }
+
+    #[test]
+    fn test_aliases_map_arity() {
+        let mut aliases_map = AliasesMap::<(), i32>::default();
+        aliases_map.insert_function("single".to_string(), vec![], 0);
+        aliases_map.insert_function("overload".to_string(), vec!["first".to_string()], 1);
+        aliases_map.insert_function(
+            "overload".to_string(),
+            vec![
+                "first".to_string(),
+                "second".to_string(),
+                "third".to_string(),
+            ],
+            3,
+        );
+
+        let get_alias = |name, arity| {
+            let (_, params, defn) = aliases_map.get_function_with_leftovers(name, arity)?;
+            Some((params, *defn))
+        };
+
+        assert_eq!(get_alias("nonexistent", 1), None);
+        assert_eq!(get_alias("single", 3), Some(([].as_slice(), 0)));
+        assert_eq!(get_alias("overload", 0), None);
+
+        assert_eq!(
+            get_alias("overload", 1),
+            Some((["first".to_string()].as_slice(), 1))
+        );
+
+        assert_eq!(
+            get_alias("overload", 2),
+            Some((["first".to_string()].as_slice(), 1))
+        );
+
+        assert_eq!(
+            get_alias("overload", 3),
+            Some((
+                [
+                    "first".to_string(),
+                    "second".to_string(),
+                    "third".to_string()
+                ]
+                .as_slice(),
+                3
+            ))
+        );
+
+        assert_eq!(
+            get_alias("overload", 4),
+            Some((
+                [
+                    "first".to_string(),
+                    "second".to_string(),
+                    "third".to_string()
+                ]
+                .as_slice(),
+                3
+            ))
+        );
+    }
 }