Handle sync errors from calling return()

index.bs

@@ -819,10 +819,14 @@ option. If {{UnderlyingSource/type}} is set to undefined (including via omission
     1. Perform ! [$ReadableStreamDefaultControllerEnqueue$](stream.[=ReadableStream/[[controller]]=],
        |value|).
  1. Let |cancelAlgorithm| be the following steps:
-  1. Let |returnMethod| be ? [$GetMethod$](|iteratorRecord|.\[[Iterator]], "`return`").
-  1. If |returnMethod| is undefined, return [=a promise resolved with=] undefined.
-  1. Let |returnResult| be ? [$Call$](|returnMethod|, |iteratorRecord|.\[[Iterator]]).
-  1. Return [=a promise resolved with=] |returnResult|.
+  1. Let |returnMethod| be [$GetMethod$](|iteratorRecord|.\[[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]]).
+  1. If |returnResult| is an abrupt completion, return [=a promise rejected with=]
+     |returnResult|.\[[Value]].
+  1. Return [=a promise resolved with=] |returnResult|.\[[Value]].
  1. Set |stream| to ! [$CreateReadableStream$](|startAlgorithm|, |pullAlgorithm|, |cancelAlgorithm|).
  1. Return |stream|.
 </div>

reference-implementation/lib/ReadableStream-impl.js

@@ -192,11 +192,21 @@ exports.implementation = class ReadableStreamImpl {
     }
 
     function cancelAlgorithm() {
-      const returnMethod = GetMethod(iteratorRecord.iterator, 'return');
+      let returnMethod;
+      try {
+        returnMethod = GetMethod(iteratorRecord.iterator, 'return');
+      } catch (e) {
+        return promiseRejectedWith(e);
+      }
       if (returnMethod === undefined) {
         return promiseResolvedWith(undefined);
       }
-      const returnResult = Call(returnMethod, iteratorRecord.iterator);
+      let returnResult;
+      try {
+        returnResult = Call(returnMethod, iteratorRecord.iterator);
+      } catch (e) {
+        return promiseRejectedWith(e);
+      }
       return promiseResolvedWith(returnResult);
     }