[iterator-helpers] Check undefined instead of call

This CL changes Symbol.iterator fallback from callable check to
undefined/null check. This is based on
tc39/proposal-iterator-helpers#272 and
discussion on May 2023 TC39 meeting.

Bug: v8:13558
Change-Id: If06bf77df383b06d1a8f6875f1760568089c08ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4545750
Commit-Queue: Rezvan Mahdavi Hezaveh <rezvan@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#87758}

src/builtins/iterator-from.tq

@@ -15,85 +15,32 @@ macro NewJSValidIteratorWrapper(implicit context: Context)(
 }
 
 // https://tc39.es/proposal-iterator-helpers/#sec-getiteratorflattenable
-//
-// Currently never used with the async hint, so only the sync path is
-// implemented.
 transitioning macro GetIteratorFlattenable(implicit context: Context)(
     obj: JSReceiver): IteratorRecord {
-  try {
-    // 1. If obj is not an Object, throw a TypeError exception.
-    // (Done by caller.)
-
-    // 2. Let alreadyAsync be false.
-    //
-    // (Unimplemented because the async path is unused.)
-
-    // 3. Let method be undefined.
-    //
-    // (Done below.)
-
-    // 4. If hint is async, then
-    //   a. Set method to ? Get(obj, @@asyncIterator).
-    //   b. Set alreadyAsync to true.
-    //
-    // (Unimplemented because unused.)
-
-    // 5. If IsCallable(method) is false, then
-    //   a. Set method to ? Get(obj, @@iterator).
-    const method = GetProperty(obj, IteratorSymbolConstant());
-
-    // b. Set alreadyAsync to false.
-    //
-    // (Unimplemented because unused.)
-
-    let iterator: JSAny;
-
-    // 6. If IsCallable(method) is false, then
-    if (!Is<Callable>(method)) {
-      // a. Let iterator be obj.
-      iterator = obj;
-
-      // b. Set alreadyAsync to true.
-      //
-      // (Unimplemented because unused.)
-    } else {
-      // 7. Else,
-      // a. Let iterator be ? Call(method, obj).
-      iterator = Call(context, UnsafeCast<Callable>(method), obj);
-    }
+  // 1. If obj is not an Object, throw a TypeError exception.
+  // (Done by caller.)
 
-    // 8. If iterator is not an Object, throw a TypeError exception.
-    const iteratorObj = Cast<JSReceiver>(iterator)
-        otherwise goto IteratorNotObject(obj, method);
-
-    // 9. Let nextMethod be ? Get(iterator, "next").
-    const nextMethod = GetProperty(iteratorObj, kNextString);
+  let iterator: JSAny;
+  try {
+    // 2. Let method be ? GetMethod(obj, @@iterator).
+    const method = GetMethod(obj, IteratorSymbolConstant())
+        otherwise IfNullOrUndefined;
+
+    // 4. Else (method is not undefined),
+    //  a. Let iterator be ? Call(method, obj).
+    iterator = Call(context, method, obj);
+  } label IfNullOrUndefined {
+    // 3. If method is undefined, then
+    //  a. Let iterator be obj.
+    iterator = obj;
+  }
 
-    // 10. If IsCallable(nextMethod) is false, throw a TypeError exception.
-    if (!Is<Callable>(nextMethod)) {
-      ThrowTypeError(
-          MessageTemplate::kPropertyNotFunction, nextMethod, kNextString, obj);
-    }
+  // 5. If iterator is not an Object, throw a TypeError exception.
+  const iteratorObj = Cast<JSReceiver>(iterator)
+      otherwise ThrowTypeError(MessageTemplate::kNotIterable, obj);
 
-    // 11. Let iteratorRecord be the Iterator Record { [[Iterator]]: iterator,
-    //     [[NextMethod]]: nextMethod, [[Done]]: false }.
-    const iteratorRecord =
-    IteratorRecord{object: iteratorObj, next: nextMethod};
-
-    // 12. If hint is async and alreadyAsync is false, then
-    //   a. Return CreateAsyncFromSyncIterator(iteratorRecord).
-    //
-    // (Unimplemented because unused.)
-
-    // 13. Return iteratorRecord.
-    return iteratorRecord;
-  } label IteratorNotObject(obj: JSAny, method: JSAny) deferred {
-    if (Is<Callable>(method)) {
-      ThrowTypeError(MessageTemplate::kSymbolIteratorInvalid);
-    } else {
-      ThrowTypeError(MessageTemplate::kNotIterable, obj);
-    }
-  }
+  // 6. Return ? GetIteratorDirect(iterator).
+  return GetIteratorDirect(iteratorObj);
 }
 
 // https://tc39.es/proposal-iterator-helpers/#sec-iterator.from

test/mjsunit/harmony/iterator-from.js

@@ -72,7 +72,10 @@ assertThrows(() => {
 }, TypeError);
 
 assertThrows(() => {
-  Iterator.from({[Symbol.iterator]() {
-    return { next: "not callable" };
-  }});
+  Iterator.from({
+    [Symbol.iterator]: 0,
+    next() {
+      return 42;
+    },
+  });
 }, TypeError);