squash! more

index.bs

@@ -857,7 +857,7 @@ prototype, are created with the internal slots described in the following table:
 <h4 id="rs-asynciterator-prototype-next" method for="ReadableStreamAsyncIteratorPrototype">next()</h4>
 
 <emu-alg>
-  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, <a>a promise rejected with</a> *TypeError* exception.
+  1. If ! IsReadableStreamAsyncIterator(*this*) is *false*, return <a>a promise rejected with</a> *TypeError* exception.
   1. Let _reader_ be *this*.[[asyncIteratorReader]].
   1. If _reader_.[[ownerReadableStream]] is *undefined*, return <a>a promise rejected with</a> a *TypeError* exception.
   1. Return ! ReadableStreamDefaultReaderRead(_reader_, *true*).

reference-implementation/lib/readable-stream.js

@@ -175,36 +175,38 @@ const AsyncIteratorPrototype = Object.getPrototypeOf(Object.getPrototypeOf(async
 const ReadableStreamAsyncIteratorPrototype = Object.setPrototypeOf({
   next() {
     if (IsReadableStreamAsyncIterator(this) === false) {
-      return Promise.reject(new TypeError());
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
     }
     const reader = this._asyncIteratorReader;
     if (reader._ownerReadableStream === undefined) {
-      return Promise.reject(new TypeError());
+      return Promise.reject(readerLockException('next'));
     }
     return ReadableStreamDefaultReaderRead(reader, true);
   },
 
   return(value) {
     if (IsReadableStreamAsyncIterator(this) === false) {
-      return Promise.reject(new TypeError());
+      return Promise.reject(streamAsyncIteratorBrandCheckException('next'));
     }
     const reader = this._asyncIteratorReader;
     if (this.preventCancel === false) {
       if (reader._ownerReadableStream === undefined) {
-        return Promise.reject(new TypeError());
+        return Promise.reject(readerLockException('next'));
       }
       if (reader._readRequests.length > 0) {
-        return Promise.reject(new TypeError());
+        return Promise.reject(new TypeError(
+          'Tried to release a reader lock when that reader has pending read() calls un-settled'));
       }
       const result = ReadableStreamReaderGenericCancel(reader, value);
       ReadableStreamReaderGenericRelease(reader);
       return result.then(() => ReadableStreamCreateReadResult(value, true, true));
     }
     if (reader._ownerReadableStream === undefined) {
-      return Promise.reject(new TypeError());
+      return Promise.reject(readerLockException('next'));
     }
     if (reader._readRequests.length > 0) {
-      return Promise.reject(new TypeError());
+      return Promise.reject(new TypeError(
+        'Tried to release a reader lock when that reader has pending read() calls un-settled'));
     }
     ReadableStreamReaderGenericRelease(reader);
     return Promise.resolve(ReadableStreamCreateReadResult(value, true, true));
@@ -2128,6 +2130,10 @@ function streamBrandCheckException(name) {
   return new TypeError(`ReadableStream.prototype.${name} can only be used on a ReadableStream`);
 }
 
+function streamAsyncIteratorBrandCheckException(name) {
+  return new TypeError(`ReadableStreamAsyncIterator.${name} can only be used on a ReadableSteamAsyncIterator`);
+}
+
 // Helper functions for the readers.
 
 function readerLockException(name) {