Adding an extension type with a typedef representation type makes pkg/kernel/test/dart_scope_calculator_test throw during serialization

[srujzs]

The issue was originally found in https://dart-review.googlesource.com/c/sdk/+/370663, with logs here. The simplest repro I can find is this patch:

Patch

diff --git a/sdk/lib/_internal/js_shared/lib/js_types.dart b/sdk/lib/_internal/js_shared/lib/js_types.dart
index cbc9acffcb0..6c01759b2da 100644
--- a/sdk/lib/_internal/js_shared/lib/js_types.dart
+++ b/sdk/lib/_internal/js_shared/lib/js_types.dart
@@ -68,5 +68,7 @@ typedef JSBigIntRepType = interceptors.JavaScriptBigInt;
 // to create a new shared library.
 typedef ExternalDartReferenceRepType = Object;
 
+typedef TD<T> = T;
+
 // JSVoid is just a typedef for void.
 typedef JSVoidRepType = void;
diff --git a/sdk/lib/_internal/wasm/lib/js_types.dart b/sdk/lib/_internal/wasm/lib/js_types.dart
index 1730dbc5213..35ed6fdcb3a 100644
--- a/sdk/lib/_internal/wasm/lib/js_types.dart
+++ b/sdk/lib/_internal/wasm/lib/js_types.dart
@@ -80,6 +80,8 @@ typedef JSBigIntRepType = js.JSValue;
 // to create a new shared library.
 typedef ExternalDartReferenceRepType = js.JSValue;
 
+typedef TD<T> = js.JSValue;
+
 // JSVoid is just a typedef for void. While we could just use JSUndefined, in
 // the future we may be able to use this to elide `return`s in JS trampolines.
 typedef JSVoidRepType = void;
diff --git a/sdk/lib/js_interop/js_interop.dart b/sdk/lib/js_interop/js_interop.dart
index dc572970e72..a0174047f27 100644
--- a/sdk/lib/js_interop/js_interop.dart
+++ b/sdk/lib/js_interop/js_interop.dart
@@ -549,6 +549,8 @@ extension ObjectToExternalDartReference on Object {
   external ExternalDartReference get toExternalReference;
 }
 
+extension type E<T>._(TD<T> _) {}
+
 /// Conversions from [JSPromise] to [Future].
 extension JSPromiseToFuture<T extends JSAny?> on JSPromise<T> {
   /// A [Future] that either completes with the result of the resolved

It essentially adds the extension type to dart:js_interop, and defines the typedef representation type in dart:_js_types. To repro, just patch that onto the SDK and run dart_scope_calculator_test.

Interestingly, if we define typedef TD<T> = T in the dart2wasm patch instead, this doesn't repro.

[srujzs]

@johnniwinther - I want to get the above CL in this week before the 3.5 cutoff, and so I believe this will be blocking. Is there anything I should do here to avoid this?

[jensjoha]

Sorry, I forgot about this one. Looking at it now.

[srujzs]

No worries, thank you!