fix: pass previousRealm to AsyncGeneratorCompleteStep in AsyncGeneratorYield

core/engine/src/builtins/generator/mod.rs

@@ -101,10 +101,23 @@ impl GeneratorContext {
         resume_kind: GeneratorResumeKind,
         context: &mut Context,
     ) -> CompletionRecord {
+        // Capture the caller's realm before the stack swap.
+        // `native_function_call` stores the pre-swap realm in `native_caller_realm`,
+        // which is the actual caller's realm (not the function's realm).
+        // AsyncGeneratorYield uses this as `previousRealm` (spec §27.6.3.8, step 8).
+        //
+        // Only set on the initial resume (from `next()`); subsequent resumes
+        // (e.g., await continuations from `run_jobs()`) must preserve the
+        // previously captured caller realm.
+        let caller_realm = context.vm.native_caller_realm.clone();
+
         std::mem::swap(&mut context.vm.stack, &mut self.stack);
-        let Some(frame) = self.call_frame.take() else {
+        let Some(mut frame) = self.call_frame.take() else {
             return CompletionRecord::Throw(PanicError::new("should have a call frame").into());
         };
+        if frame.caller_realm.is_none() {
+            frame.caller_realm = caller_realm;
+        }
         let fp = frame.fp;
         let rp = frame.rp;
         context.vm.push_frame(frame);

core/engine/src/native_function/mod.rs

@@ -360,6 +360,8 @@ pub(crate) fn native_function_call(
 
     context.swap_realm(&mut realm);
     context.vm.native_active_function = Some(this_function_object);
+    let previous_native_caller_realm = context.vm.native_caller_realm.take();
+    context.vm.native_caller_realm = Some(realm.clone());
 
     let result = if constructor.is_some() {
         function.call(&JsValue::undefined(), &args, context)
@@ -369,6 +371,7 @@ pub(crate) fn native_function_call(
     .map_err(|err| err.inject_realm(context.realm().clone()));
 
     context.vm.native_active_function = None;
+    context.vm.native_caller_realm = previous_native_caller_realm;
     context.swap_realm(&mut realm);
 
     context.vm.shadow_stack.pop();

core/engine/src/tests/async_generator.rs

@@ -1,5 +1,5 @@
 use crate::{
-    Context, JsValue, TestAction, builtins::promise::PromiseState, object::JsPromise,
+    Context, JsValue, Source, TestAction, builtins::promise::PromiseState, object::JsPromise,
     run_test_actions,
 };
 use boa_macros::js_str;
@@ -120,3 +120,71 @@ fn return_on_then_queue() {
         TestAction::assert_eq("count", JsValue::from(2)),
     ]);
 }
+
+#[test]
+fn cross_realm_async_generator_yield() {
+    // Exercises AsyncGeneratorYield spec steps 6-8 (previousRealm handling)
+    // by creating a generator in one realm and consuming it from another.
+    // Also verifies that the iter result objects are created in the caller's
+    // realm by checking their prototype against old_realm's Object.prototype.
+    let mut context = Context::default();
+
+    let generator_realm = context.create_realm().unwrap();
+
+    let old_realm = context.enter_realm(generator_realm);
+    let generator = context
+        .eval(Source::from_bytes(
+            b"(async function* g() { yield 42; yield 99; })()",
+        ))
+        .unwrap();
+    context.enter_realm(old_realm.clone());
+
+    // Grab Object.prototype from the caller's realm (old_realm).
+    let caller_object_proto = old_realm.intrinsics().constructors().object().prototype();
+
+    let next_fn = generator
+        .as_object()
+        .unwrap()
+        .get(js_str!("next"), &mut context)
+        .unwrap();
+
+    let call_next = |ctx: &mut Context| -> JsValue {
+        let result = next_fn
+            .as_callable()
+            .unwrap()
+            .call(&generator, &[], ctx)
+            .unwrap();
+        ctx.run_jobs().unwrap();
+        result
+    };
+
+    // First yield: value 42
+    let first = call_next(&mut context);
+    assert_promise_iter_value(&first, &JsValue::from(42), false, &mut context);
+
+    // Verify the iter result was created in old_realm.
+    let first_promise = JsPromise::from_object(first.as_object().unwrap().clone()).unwrap();
+    let PromiseState::Fulfilled(first_result) = first_promise.state() else {
+        panic!("promise was not fulfilled");
+    };
+    assert_eq!(
+        first_result.as_object().unwrap().prototype(),
+        Some(caller_object_proto.clone()),
+        "iter result prototype should be old_realm's Object.prototype"
+    );
+
+    // Second yield: value 99
+    let second = call_next(&mut context);
+    assert_promise_iter_value(&second, &JsValue::from(99), false, &mut context);
+
+    // Verify the iter result was created in old_realm.
+    let second_promise = JsPromise::from_object(second.as_object().unwrap().clone()).unwrap();
+    let PromiseState::Fulfilled(second_result) = second_promise.state() else {
+        panic!("promise was not fulfilled");
+    };
+    assert_eq!(
+        second_result.as_object().unwrap().prototype(),
+        Some(caller_object_proto),
+        "iter result prototype should be old_realm's Object.prototype"
+    );
+}

core/engine/src/vm/call_frame/mod.rs

@@ -77,6 +77,10 @@ pub struct CallFrame {
     /// \[\[Realm\]\]
     pub(crate) realm: Realm,
 
+    /// The caller's realm, captured before a generator stack swap.
+    /// Used by `AsyncGeneratorYield` to pass `previousRealm` to `complete_step`.
+    pub(crate) caller_realm: Option<Realm>,
+
     // SAFETY: Nothing in `CallFrameFlags` requires tracing, so this is safe.
     #[unsafe_ignore_trace]
     pub(crate) flags: CallFrameFlags,
@@ -154,6 +158,7 @@ impl CallFrame {
             active_runnable,
             environments,
             realm,
+            caller_realm: None,
             flags: CallFrameFlags::empty(),
         }
     }

core/engine/src/vm/mod.rs

@@ -89,6 +89,11 @@ pub struct Vm {
     /// because we don't push a frame for them.
     pub(crate) native_active_function: Option<JsObject>,
 
+    /// The caller's realm before a native function's `swap_realm`.
+    /// Used by `GeneratorContext::resume()` to capture `previousRealm`
+    /// for `AsyncGeneratorYield`.
+    pub(crate) native_caller_realm: Option<Realm>,
+
     /// Number of nested host calls that re-enter the VM via `Context::run()`.
     ///
     /// This is incremented by high-level host entry points such as
@@ -347,6 +352,7 @@ impl Vm {
             pending_exception: None,
             runtime_limits: RuntimeLimits::default(),
             native_active_function: None,
+            native_caller_realm: None,
             host_call_depth: 0,
             shadow_stack: ShadowStack::default(),
             #[cfg(feature = "trace")]

core/engine/src/vm/opcode/generator/yield_stm.rs

@@ -66,13 +66,22 @@ impl AsyncGeneratorYield {
         let value = context.vm.get_register(value.into());
         let completion = Ok(value.clone());
 
-        // TODO: 6. Assert: The execution context stack has at least two elements.
-        // TODO: 7. Let previousContext be the second to top element of the execution context stack.
-        // TODO: 8. Let previousRealm be previousContext's Realm.
+        // 6. Assert: The execution context stack has at least two elements.
+        // 7. Let previousContext be the second to top element of the execution context stack.
+        // 8. Let previousRealm be previousContext's Realm.
+        let previous_realm = context.vm.frame().caller_realm.clone();
+
+        // Clear caller_realm so the next `next()` call can set a fresh one.
+        context.vm.frame_mut().caller_realm = None;
+
         // 9. Perform AsyncGeneratorCompleteStep(generator, completion, false, previousRealm).
-        if let Err(err) =
-            AsyncGenerator::complete_step(&async_generator_object, completion, false, None, context)
-        {
+        if let Err(err) = AsyncGenerator::complete_step(
+            &async_generator_object,
+            completion,
+            false,
+            previous_realm,
+            context,
+        ) {
             return context.handle_error(err);
         }
 
@@ -114,8 +123,9 @@ impl AsyncGeneratorYield {
         //     a. Set generator.[[AsyncGeneratorState]] to suspended-yield.
         r#gen.data_mut().state = AsyncGeneratorState::SuspendedYield;
 
-        //     TODO: b. Remove genContext from the execution context stack and restore the execution context that is at the top of the execution context stack as the running execution context.
-        //     TODO: c. Let callerContext be the running execution context.
+        //     b. Remove genContext from the execution context stack and restore the execution context
+        //        that is at the top of the execution context stack as the running execution context.
+        //     c. Let callerContext be the running execution context.
         //     d. Resume callerContext passing undefined. If genContext is ever resumed again, let resumptionValue be the Completion Record with which it is resumed.
         //     e. Assert: If control reaches here, then genContext is the running execution context again.
         //     f. Return ? AsyncGeneratorUnwrapYieldResumption(resumptionValue).