Fix: Unify TypeVars when indexing generic parameters twice

When a generic function indexes the same TypeVar parameter multiple
times,
all resulting TypeVars must share a single element type. This change
ensures
that subsequent indexing operations unify their fresh TypeVar with an
existing
one if the parameter has already been indexed, preventing incorrect type
inference and false negatives.

A new test, `test_let_poly_multi_index_all_results_typed`, is added to
verify this behavior.

src/ast/compiler/type_checker.rs

@@ -1438,10 +1438,20 @@ impl TypeChecker {
                             && matches!(&elems[0], StaticType::Int)
                     );
                     if is_index_call && matches!(ret_ty, StaticType::Any) {
-                        let elem_var = self.fresh_var();
-                        let StaticType::TypeVar(elem_id) = &elem_var else { unreachable!() };
-                        self.index_constraints.borrow_mut().insert(*n, *elem_id);
-                        ret_ty = elem_var;
+                        let existing = self.index_constraints.borrow().get(n).copied();
+                        if let Some(existing_ret_id) = existing {
+                            // Same TypeVar indexed again — all index results on the same
+                            // parameter must share one element TypeVar. Unify the fresh
+                            // var with the existing one so they resolve together.
+                            let elem_var = self.fresh_var();
+                            self.unify(elem_var.clone(), StaticType::TypeVar(existing_ret_id), diag);
+                            ret_ty = Self::apply_subst(elem_var, &self.subst.borrow());
+                        } else {
+                            let elem_var = self.fresh_var();
+                            let StaticType::TypeVar(elem_id) = &elem_var else { unreachable!() };
+                            self.index_constraints.borrow_mut().insert(*n, *elem_id);
+                            ret_ty = elem_var;
+                        }
                     }
                 }