`@propertyWrapper` + `@MainActor var wrappedValue` which propagates `@MainActor` context to the encapsulating type. https://twitter.com/inamiy/status/1659530309501853696

MainActor-PropertyWrapper.swift

@MainActor
class ViewModel: ObservableObject {}

@propertyWrapper
struct Wrapper<T> {
    var wrappedValue: T
}

@propertyWrapper
struct MainWrapper<T> {
    @MainActor var wrappedValue: T
}

//----------------------------------------

struct Foo1 {
    var viewModel: ViewModel = ViewModel() // ERROR: Call to main actor-isolated initializer 'init()' in a synchronous nonisolated context
}

struct Foo2 {
    @StateObject
    var viewModel: ViewModel = ViewModel() // Compile OK
}

struct Foo3 {
    @Wrapper
    var viewModel: ViewModel = ViewModel() // ERROR: Call to main actor-isolated initializer 'init()' in a synchronous nonisolated context
}

struct Foo4 {
    @MainWrapper
    var viewModel: ViewModel = ViewModel() // Compile OK
}

struct Foo5 {
    @MainWrapper
    var viewModel: ViewModel = ViewModel() // Compile OK

    // Compile OK, even though it looks same as Foo1,
    // because `Foo5.init` will automatically gain `@MainActor` by `@MainWrapper`,
    // and `MainWrapper.init` will automatically gain `@MainActor` by `@MainActor var wrappedValue`.
    var viewModel2: ViewModel = ViewModel()
}

// NOTE: Requires `@MainActor`, although there is no `@MainActor Foo5` or `@MainActor init`.
func testFoo5() {
    _ = Foo5() // ERROR: Call to main actor-isolated initializer 'init()' in a synchronous nonisolated context
}

Another interesting fact when having multiple @globalActor-ed @propertyWrappers: https://twitter.com/kntkymt/status/1659595057929031680