Throw if iterator.return() does not fulfill with an object

index.bs

@@ -2574,15 +2574,19 @@ create them does not matter.
        <!-- TODO (future): If we allow changing the queuing strategy, this Enqueue might throw.
              We'll then need to catch the error and close the async iterator. -->
  1. Let |cancelAlgorithm| be the following steps, given |reason|:
-  1. Let |returnMethod| be [$GetMethod$](|iteratorRecord|.\[[Iterator]], "`return`").
+  1. Let |iterator| be |iteratorRecord|.\[[Iterator]].
+  1. Let |returnMethod| be [$GetMethod$](|iterator|, "`return`").
   1. If |returnMethod| is an abrupt completion, return [=a promise rejected with=]
      |returnMethod|.\[[Value]].
   1. If |returnMethod|.\[[Value]] is undefined, return [=a promise resolved with=] undefined.
-  1. Let |returnResult| be [$Call$](|returnMethod|.\[[Value]], |iteratorRecord|.\[[Iterator]],
-     « |reason| »).
+  1. Let |returnResult| be [$Call$](|returnMethod|.\[[Value]], |iterator|, « |reason| »).
   1. If |returnResult| is an abrupt completion, return [=a promise rejected with=]
      |returnResult|.\[[Value]].
-  1. Return [=a promise resolved with=] |returnResult|.\[[Value]].
+  1. Let |returnPromise| be [=a promise resolved with=] |returnResult|.\[[Value]].
+  1. Return the result of [=reacting=] to |returnPromise| with the following fulfillment steps,
+     given |iterResult|:
+   1. If [$Type$](|iterResult|) is not Object, throw a {{TypeError}}.
+   1. Return undefined.
  1. Set |stream| to ! [$CreateReadableStream$](|startAlgorithm|, |pullAlgorithm|, |cancelAlgorithm|,
     0).
  1. Return |stream|.

reference-implementation/lib/abstract-ops/readable-streams.js

@@ -1910,9 +1910,10 @@ function ReadableStreamFromIterable(asyncIterable) {
   }
 
   function cancelAlgorithm(reason) {
+    const iterator = iteratorRecord.iterator;
     let returnMethod;
     try {
-      returnMethod = GetMethod(iteratorRecord.iterator, 'return');
+      returnMethod = GetMethod(iterator, 'return');
     } catch (e) {
       return promiseRejectedWith(e);
     }
@@ -1921,11 +1922,17 @@ function ReadableStreamFromIterable(asyncIterable) {
     }
     let returnResult;
     try {
-      returnResult = Call(returnMethod, iteratorRecord.iterator, [reason]);
+      returnResult = Call(returnMethod, iterator, [reason]);
     } catch (e) {
       return promiseRejectedWith(e);
     }
-    return promiseResolvedWith(returnResult);
+    const returnPromise = promiseResolvedWith(returnResult);
+    return transformPromiseWith(returnPromise, iterResult => {
+      if (!typeIsObject(iterResult)) {
+        throw new TypeError('The promise returned by the iterator.return() method must fulfill with an object');
+      }
+      return undefined;
+    });
   }
 
   stream = CreateReadableStream(startAlgorithm, pullAlgorithm, cancelAlgorithm, 0);