Fix for-await-of inside async generators permanently suspending

[Copilot]

for await inside async function* bodies never resumed after the iterator's .next() Promise resolved, leaving the outer .next() Promise permanently pending.

async function* gen() {
  for await ([,] of [[]]) console.log("iteration"); // never ran
}
gen().next().then(() => console.log("OK")); // never ran

Root cause

SuspendForAsyncIteration registered onFulfilled/onRejected handlers that called AsyncGeneratorResumeNext(). That method exits early when _asyncGeneratorQueue is empty — which it always is at this point, because the current request was already dequeued before execution started. The generator body was left permanently suspended.

Changes

• AsyncGeneratorInstance: Add AsyncGeneratorContinueForAwait(PromiseCapability) — resumes the current request's execution directly (_asyncGeneratorState = Executing, _isResuming = true, ResumeExecution(capability)) without requiring a queued request.

• SuspendData: Add RejectedValue to ForAwaitSuspendData so rejected iterator Promises can be propagated as throws on resume.

• JintForInForOfStatement.SuspendForAsyncIteration (async generator branch):

  • Capture _currentPromiseCapability before suspension
  • Wrap handlers in engine.AddToEventLoop (matching async function pattern for correct microtask ordering)
  • Call AsyncGeneratorContinueForAwait instead of AsyncGeneratorResumeNext
  • Store RejectedValue in the rejection handler

• JintForInForOfStatement.ExecuteInternal: On for-await-of resumption inside an async generator, check ForAwaitSuspendData.RejectedValue and throw it into the generator context.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Async test failures in REPL</issue_title>
<issue_description>Version/platform/runtime used

.NET 10.0.201 Linux aarch64

Describe the bug

I tried compared results from Jint.Tests.Test262 vs my own test262 harness that runs Jint.Repl and noticed that quite a lot of async tests are failing when running against Jint.Repl binary. Especially most of test/language/statements/for-await-of/ tests.

To Reproduce

Couple representative examples, stripped down to bare minimum:

$ cat min1.js
// test/language/statements/for-await-of/async-gen-decl-dstr-array-elision-val-array.js
async function* gen() {
  for await ([,] of [[]]) console.log("iteration");
}
gen().next().then(() => console.log("OK"));
$ node min1.js
iteration
OK
$ ./Jint.Repl min1.js
[object Promise]

$ cat min2.js 
// test/language/statements/async-generator/yield-star-async-next.js
const obj = {
  [Symbol.asyncIterator]() {
    return {
      get next() {
        console.log("get next");
        return function () {
          console.log("call next");
          return {
            then(resolve) {
              resolve({
                get value() {
                  console.log("get value");
                  return "OK";
                },
                get done() {
                  console.log("get done");
                  return false;
                }
              });
            }
          };
        };
      }
    };
  }
};
async function* gen() {
  yield* obj;
}
gen().next().then(x => console.log(x.value));
$ node min2.js
get next
call next
get done
get value
OK
$ ./Jint.Repl min2.js
get next
get next
call next
null
[object Promise]

Additional context

I attached full list of errors I'm getting: jint-diffs.json

This is a subset of tests that Jint.Tests.Test262 is configured to run and passes, extracted from an instrumented Jint.Tests.Test262 binary, and for which I can't reproduce success with Jint.Repl. Most of them are async related.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Async test failures in REPL #2352

---

⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.

[Copilot]

Pull request overview

Fixes a deadlock where for await...of inside async function* would suspend the generator body and never resume, leaving the outer .next() promise permanently pending.

Changes:

• Add an async-generator-specific continuation path (AsyncGeneratorContinueForAwait) to resume the current dequeued request after a for-await-of suspension.
• Track for-await-of iterator .next() rejections via ForAwaitSuspendData.RejectedValue and rethrow on resume within async generators.
• Add regression tests covering for await inside async generators (including destructuring and multiple suspensions).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
Jint/Runtime/SuspendData.cs	Extends ForAwaitSuspendData with RejectedValue to propagate iterator .next() promise rejections on resume.
Jint/Runtime/Interpreter/Statements/JintForInForOfStatement.cs	Switches async-generator for-await-of resumption to AsyncGeneratorContinueForAwait, queues resumption via event loop, and throws stored rejection on resume.
Jint/Native/AsyncGenerator/AsyncGeneratorInstance.cs	Introduces AsyncGeneratorContinueForAwait to resume execution without relying on the request queue.
Jint.Tests/Runtime/AsyncTests.cs	Adds regression tests for for await inside async function* bodies (including destructuring and repeated suspensions).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.