fix: unique wrapper param names for nested currying

prqlc/prqlc/src/semantic/resolver/functions.rs

@@ -123,10 +123,13 @@ impl Resolver<'_> {
             // Get the missing params (params that don't have args yet)
             let missing = inner_closure.params[inner_closure.args.len()..].to_vec();
 
-            // Create wrapper params and add references to them as args to the inner closure
+            // Create wrapper params and add references to them as args to the inner closure.
+            // The param names must be globally unique: nested partial applications share the
+            // NS_PARAM namespace, so a per-materialization index would collide and resolve to
+            // the wrong binding (issue #5978).
             let mut wrapper_params = Vec::with_capacity(missing.len());
-            for (i, param) in missing.iter().enumerate() {
-                let param_name = format!("_partial_{i}");
+            for param in missing.iter() {
+                let param_name = format!("_partial_{}", self.id.gen());
                 let substitute_arg = Expr::new(Ident::from_path(vec![
                     NS_PARAM.to_string(),
                     param_name.clone(),

prqlc/prqlc/tests/integration/sql.rs

@@ -7385,6 +7385,33 @@ fn test_partial_application_of_transform() {
     ");
 }
 
+/// Regression test for issue #5978: currying a function through more than two
+/// `let`-bound wrappers produced the wrong result, because the wrapper params
+/// synthesized during partial-application materialization were named by a
+/// per-materialization index (`_partial_0`, `_partial_1`, …) and collided
+/// across nested partial applications sharing the `NS_PARAM` namespace.
+#[test]
+fn test_nested_currying() {
+    // `y` (curried one arg at a time) must match `z` (curried directly).
+    assert_snapshot!(compile(r#"
+    let f1 = func a b c -> a + b + c
+    let f2 = func a b -> f1 a b
+    let f3 = func a -> f2 a
+
+    from t | derive {
+      y = (((f3 100) 20) 3),
+      z = ((f2 100 20) 3),
+    }
+    "#).unwrap(), @"
+    SELECT
+      *,
+      100 + 20 + 3 AS y,
+      100 + 20 + 3 AS z
+    FROM
+      t
+    ");
+}
+
 #[test]
 fn test_tuple_map() {
     assert_snapshot!(compile(r###"