Swift @ MainActor + Combine不一致的编译器错误

我有以下视图模型:

@MainActor
final class ProfileViewModel: ObservableObject {
  let authService: AuthService
  
  @Published var userHandle: String = "Unknown"
  
  nonisolated init(authService: AuthService, appState: AppState) {
    self.authService = authService
    appState.$userData
      .receive(on: DispatchQueue.main)
      .map {
        $0.user?.displayName ?? "Unknown"
      }
      .assign(to: &$userHandle)
  }
}

这会导致错误:"主参与者隔离属性'$userHandle'cannot used 'inout'from a non—isolated context"，位于". assign(to:)"行.

这在某种程度上是意料之中的.据我所知，Combine的动态线程切换并不满足Swift并发模型的编译时保证.

下面是让我困惑的地方.如果我从类中删除@ MainActor注释，并将其应用于userHandle属性，如下所示:

final class ProfileViewModel: ObservableObject {
  let authService: AuthService
  
  @MainActor @Published var userHandle: String = "Unknown"
  
  nonisolated init(authService: AuthService, appState: AppState) {
    self.authService = authService
    appState.$userData
      .receive(on: DispatchQueue.main)
      .map {
        $0.user?.displayName ?? "Unknown"
      }
      .assign(to: &$userHandle)
  }
}

代码编译时没有问题.

从逻辑上讲，这应该引发相同的错误，因为userHandle在两种情况下都是Main actor隔离的.我想知道这是不是编译器的怪癖，还是有一个我不明白的更深层次的语义差异.