test(vm): add generator regression tests for return_value reset

Add tests ensuring that resetting `return_value` in `push_frame()` does
not break generator yield/return behavior, as requested in the PR #4713
review.

Author: tkshsbcue

core/engine/src/tests/function.rs

@@ -100,6 +100,45 @@ fn implicit_constructor_return_does_not_leak_completion_value_from_same_eval() {
     ]);
 }
 
+/// Regression test for issue #4485.
+/// Generator functions must still correctly yield and return values
+/// after the `return_value` reset in `push_frame`.
+#[test]
+fn generator_yield_and_return_not_affected_by_return_value_reset() {
+    run_test_actions([TestAction::assert(indoc! {r#"
+        function* gen() {
+            yield 1;
+            yield 2;
+            return 3;
+        }
+        var g = gen();
+        var a = g.next();
+        var b = g.next();
+        var c = g.next();
+        a.value === 1 && !a.done &&
+        b.value === 2 && !b.done &&
+        c.value === 3 && c.done;
+    "#})]);
+}
+
+/// Regression test for issue #4485.
+/// Generator functions without an explicit return should yield
+/// `undefined` as the final value, not a stale caller expression.
+#[test]
+fn generator_implicit_return_undefined() {
+    run_test_actions([TestAction::assert(indoc! {r#"
+        function* gen() {
+            yield 1;
+        }
+        99;
+        var g = gen();
+        var a = g.next();
+        var b = g.next();
+        a.value === 1 && !a.done &&
+        b.value === undefined && b.done;
+    "#})]);
+}
+
 #[test]
 fn property_accessor_member_expression_dot_notation_on_function() {
     run_test_actions([TestAction::assert_eq(