[js_interop] Compile-time error if type argument of `isA` is a extension type on JS primitive

[sgrekhov]

It is a compile-time error if a type argument of isA is an extension type on JS primitive type. Is this expected?

extension type MyAny(JSAny _) implements JSAny {}

extension type MyNumber(JSNumber _) implements JSNumber {}

extension type MyString(JSString _) implements JSString {}

extension type MyBoolean(JSBoolean _) implements JSBoolean {}

extension type MyBigInt(JSBigInt _) implements JSBigInt {}

extension type MySymbol(JSSymbol _) implements JSSymbol {}

extension type MyObject(JSObject _) implements JSObject {}

test(JSAny v) {
  v.isA<MyAny>(); // Ok
  v.isA<MyObject>(); // Ok
  v.isA<JSNumber>(); // Ok
  v.isA<MyNumber>(); // Error: Type argument 'MyNumber' wraps primitive JS type 'JSNumber', which is specially handled using 'typeof'.
...
}

The same error occurs for all other extensions on JS primitive types (MyString, MyBoolean etc). Is this expected? Please note that there is no error for JSNumber but there is for MyNumber.

cc @srujzs

Dart SDK version: 3.10.0-edge.fc16a52f2d5b2a60cb6b98da6ce21b8e8ea1ae40 (main) (Thu Jul 31 18:03:22 2025 -0700) on "linux_x64"

[srujzs]

Yes. Per the documentation:

Using this method with a T that wraps a primitive JS type will result in an error telling you to use the primitive JS type instead.

https://api.dart.dev/dart-js_interop/JSAnyUtilityExtension/isA.html

The reasoning is that it can lead to confusion as we don't use the user interop type for these types. Unlike types that wrap JSObject (or its subtypes), such user interop types don't have a parallel in JS.

[sgrekhov]

Thank you!