How to fix “Property declares an opaque return type, but has no initializer expression from which to infer an underlying type”

Fully updated for Xcode 11.1

This usually happens because you accidentally created a view without filling in its contents, like this:

struct DetailView: View {
    var body: some View {

    }
}

If you created that view as a placeholder, just give it so temporary content to make the compiler happy, like this:

struct DetailView: View {
    var body: some View {
        Text("Placeholder")
    }
}