`@Observable` Macro causes compilation error when in an `extension` of a view

[ngimelliUW]

Description

The @Observable macro only works on view models that are not within extensions. For example this works:

struct MyView: View {
  ...
}

extension MyView {
  final class MyViewModel: ObservableObject {
    ...
  }
} 

but this does not:

extension MyView {
  @Observable final class MyViewModel {
    ...
  }
} 

Steps to reproduce

1. Create a simple view with a basic view model using the @Observable macro.
2. Wrap the view model in an extension of the view; this should cause the error.

Expected behavior
Successful compilation.

Environment

• Swift compiler version info: swift-driver version: 1.75.2 Apple Swift version 5.8.1 (swiftlang-5.8.0.124.5 clang-1403.0.22.11.100)
  Target: arm64-apple-macosx13.0
• Xcode 15.0 Beta
• Deployment target: iOS 17.0 Beta