WritableStream can't be constructed with a defined type

Author: anonrig

src/workerd/api/streams/standard.c++

@@ -3490,6 +3490,16 @@ void WritableStreamJsController::setup(jsg::Lock& js,
     jsg::Optional<StreamQueuingStrategy> maybeQueuingStrategy) {
   auto underlyingSink = kj::mv(maybeUnderlyingSink).orDefault({});
   auto queuingStrategy = kj::mv(maybeQueuingStrategy).orDefault({});
+
+  if (FeatureFlags::get(js).getPedanticWpt()) {
+    // Per the spec, the type property for WritableStream's underlying sink must be undefined.
+    // If it's anything else, throw a RangeError.
+    KJ_IF_SOME(type, underlyingSink.type) {
+      JSG_REQUIRE(
+          type.size() == 0, RangeError, "Invalid type '", type, "'. Only undefined is valid.");
+    }
+  }
+
   // We account for the memory usage of the WritableStreamDefaultController and AbortSignal together
   // because their lifetimes are identical and memory accounting itself has a memory overhead.
   state = js.allocAccounted<WritableStreamDefaultController>(

src/wpt/streams-test.ts

@@ -747,12 +747,10 @@ export default {
         ? [
             'controller argument should be passed to start method',
             'WritableStream should be writable and ready should fulfill immediately if the strategy does not apply backpressure',
-            "WritableStream can't be constructed with a defined type",
           ]
         : [
             'controller argument should be passed to start method',
             'WritableStream should be writable and ready should fulfill immediately if the strategy does not apply backpressure',
-            "WritableStream can't be constructed with a defined type",
             'underlyingSink argument should be converted after queuingStrategy argument',
           ],
   },