Assert that reader is a default reader in finalize

Author: MattiasBuelens

index.bs

@@ -2233,10 +2233,9 @@ The following abstract operations operate on {{ReadableStream}} instances at a h
    1. [=Finalize=], passing along |error| if it was given.
   * <dfn id="rs-pipeTo-finalize"><i>Finalize</i></dfn>: both forms of shutdown will eventually ask
     to finalize, optionally with an error |error|, which means to perform the following steps:
+   1. Assert: |reader| [=implements=] {{ReadableStreamDefaultReader}}.
    1. Perform ! [$WritableStreamDefaultWriterRelease$](|writer|).
-   1. If |reader| [=implements=] {{ReadableStreamBYOBReader}}, perform
-      ! [$ReadableStreamBYOBReaderRelease$](|reader|).
-   1. Otherwise, perform ! [$ReadableStreamDefaultReaderRelease$](|reader|).
+   1. Perform ! [$ReadableStreamDefaultReaderRelease$](|reader|).
    1. If |signal| is not undefined, [=AbortSignal/remove=] |abortAlgorithm| from |signal|.
    1. If |error| was given, [=reject=] |promise| with |error|.
    1. Otherwise, [=resolve=] |promise| with undefined.

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

@@ -339,6 +339,7 @@ function ReadableStreamPipeTo(source, dest, preventClose, preventAbort, preventC
     }
 
     function finalize(isError, error) {
+      assert(ReadableStreamDefaultReader.isImpl(reader));
       WritableStreamDefaultWriterRelease(writer);
       ReadableStreamDefaultReaderRelease(reader);