Here is what I am working on:

1. there are 2 kind of Structs, TypeA and TypeB, both conform to a TestProtocol
2. there are 2 views, viewA and viewB, each of them has a Binding var of TypeA and TypeB
3. in the mainView, there is an array of [TestProtocol]
4. I want to use ForEach to present that array.

here is my code:

protocol TestProtocol {
    var testId: String {get set}
}
struct TypeA: TestProtocol{
    var testId: String
    var testString1: String
}
struct TypeB: TestProtocol{
    var testId: String
    var testString2: String
}
struct viewA: View{
    @Binding var a: TypeA
    var body: some View{
        TextField("test", text: $a.testString1)
    }
}

struct viewB: View{
    @Binding var b: TypeB
    var body: some View{
        TextField("test", text: $b.testString2)
    }
}

struct SwiftUIViewForBindingTest: View {
    @State var testArray = [TestProtocol]()

    var body: some View {
        VStack{
            if !testArray.isEmpty{
                ForEach($testArray, id:\.testId){ $item in
                    if item is TypeA{
                        viewA(a: $item as! Binding<TypeA>)
                    }else if item is TypeB{
                        viewB(b: $item as! Binding<TypeB>)
                    }
                }
            }

            Button("add Data"){
                testArray = [TypeA(testId: "a", testString1: "aaaa"), TypeB(testId: "b", testString2: "bbbb")]
            }
        }
    }
}

Xcode gave me the error Cast from 'Binding<any TestProtocol>' to unrelated type 'Binding<TypeA>' always fails on the line viewA(a: $item as! Binding<TypeA>)

I think I shouldn't Type Cast the Binding in this way. How could I convert Binding <any TestProtocal> to Binding<TypeA>, or it's impossible to do that?