View.onChange(of:) cannot accept an Optional argument

Calling onChange(of: T, perform: { oldValue, newValue }) only works for non-optional Equatable values.

One workaround is to fall back to a sentinel value when checking, as so:

struct MyView {
    @State var someState: String = "ABC"
    @State var someOptionalState: String? = nil

    var body: some View {
        Text("ABC")
            // this works fine
            .onChange(of: someState) { oldValue, newValue in … }
            #if !SKIP
            // this fails to compile in Skip
            .onChange(of: someOptionalState) { oldValue, newValue in … }
            #else
            // this is the workaround for Skip
            .onChange(of: someOptionalState ?? "SOME_SENTINEL") { oldValue, newValue in … }
            #endif
        }
}

Start by reproducing the optional-value failure described for View.onChange(of:), comparing it with the working non-optional call and the sentinel workaround. The change is complete when an Optional Equatable argument compiles and its old and new values are available to the callback without requiring a sentinel.