I'm not a fan of annotating everything with @MainActor. So yes, in the case of Sendable, the newValue must also conform to Sendable. I’ve written Objective-C and pre-Swift 6 code for decades that just assumed that it was. How many weird crashes has that code caused over the years? Yeah, I totally get the value of having the compiler catch these things, it's a solid direction. Personally, I can't say I've run into that many crashes from threading issues in practice, at least judging by crash metrics, things have been pretty stable. I just find it hard to get excited about spending too much time fixing compiler errors :)

Hello, Thanks for your reply! Yeah, I’ve already figured that part out, but there’s one more issue: the value also needs to be sendable. I picked the simplest example, but this new concurrency model adds even more boilerplate code. Honestly, I’m finding this more challenging than working with ObjC back in 2009 🙂

I've prepared a demo to reproduce the crash. I don't know how to attach it directly, so here's a dropbox link: https://www.dropbox.com/scl/fi/s4x49bykhjrnh7yxybajn/swiftui_toolbar_crash_example.zip?rlkey=d6iquywpvb8y3j0sshj6kjvh3&dl=0. You should press: "Click Me", then again "Click me" and then "Reset".