2019-07-15 Generic parameter subject could not be inferred

In a condensed form, I had the following SwiftUI code:

    struct Input {
        var someString = "OH HAI"
    }

    struct ContentView : View {
        @State private var input = Input()
        
        var body: some View {
            TextField("Enter string", text: $input.someString)
                .textFieldStyle(.roundedBorder)
                .padding()
        }
    }

Then I changed someString to a computed property:

    struct Input {
        var someString: String {
            return "OH HAI"
        }
    }

This results in the following error: "Generic parameter 'Subject' could not be inferred". It took me ten minutes to realize what was the problem: I forgot to add a setter.