Type '()' cannot conform to View (except it definitely is a View, no shenanigans this time)

Shenaniganically, you are trying to use ViewBuilder syntax in the trailing closure, but you didn't adorn content with the @ViewBuilder annotation. So Swift infers that the trailing closure returns () (also called Void).

Change the init declaration to mention @ViewBuilder:

struct CompatibilityPicker<blah blah blah>: View where blah blah blah {

    init(
        _ label : Label,
        selection : SelectionValue,
        @ViewBuilder content : @escaping () -> Content
     // ^^^^^^^^^^^^
    ) {
        blah blah blah

UPDATE

    @Binding private var _selection : SelectionValue

    blah blah blah

    init(_ label : Label, selection : SelectionValue, content : @escaping () -> Content) {
        self.label = label
        self._selection = selection
        self.content = content
    }

The _selection variable is wrapped by the Binding wrapper, which means that it is really a computed property. The stored property is named __selection (note the two underscores) and has type Binding<SelectionValue>. Because _selection is a computed property, init cannot mention it until all stored properties are initialized. Probably you should change init to take a Binding<SelectionValue> argument instead of a SelectionValue argument:

    init(
        _ label : Label,
        selection : Binding<SelectionValue>,
        @ViewBuilder content : @escaping () -> Content
     // ^^^^^^^^^^^^
    ) {
        self.label = label
        self.content = content
        __selection = selection
    }

UPDATE 2

I looked at your other question and your code here and I think I know what you mean by “it doesn't work with anything but Int”.

The problem as that, when you say Text("Easy").tag(0), Swift treats 0 as an Int. If your Picker's SelectionValue is, say, Int16, then indeed the Picker will not be able to use the 0 tag because the types don't match.

You can make your tag work with Picker by giving 0 the correct type. For example: Text("Easy").tag(0 as Int16).

However, my recommendation is that you stop mucking about with CompatibilityPicker. It is a symptom of primitive obsession. The idiomatic solution is to use an enum for your tags:

enum Difficulty: Hashable {
    case easy
    case medium
    case hard
}

struct Demo1: View {
    @State var difficulty: Difficulty = .easy

    var body: some View {
        Picker("Difficulty", selection: $difficulty) {
            Text("Easy").tag(Difficulty.easy)
            Text("Medium").tag(Difficulty.medium)
            Text("Hard").tag(Difficulty.hard)
        }
    }
}

You could go even further and do this:

extension Difficulty: CaseIterable { }

extension Difficulty {
    var stringKey: LocalizedStringKey {
        switch self {
        case .easy: return "Easy"
        case .medium: return "Medium"
        case .hard: return "Hard"
        }
    }
}

struct Demo2: View {
    @State var difficulty: Difficulty = .easy

    var body: some View {
        Picker("Difficulty", selection: $difficulty) {
            ForEach(Difficulty.allCases, id: \.self) {
                Text($0.stringKey).tag($0)
            }
        }
    }
}

Here is a working variant based on my previous answer. Tested with Xcode 14 / iOS 16

There are several errors in code in question: selection value w/o binding, content w/o view builder, conflict of hidden binding autogenerated properties, etc., so I just updated a bit my previous solution.

struct CompatibilityPicker<Label, SelectionValue, Content>: View where Label : StringProtocol, SelectionValue : BinaryInteger, Content : View {

    var label : Label
    @Binding var selection : SelectionValue
    var content : () -> Content

    init(_ label : Label, selection : Binding<SelectionValue>, @ViewBuilder content : @escaping () -> Content) {
        self.label = label
        self._selection = selection
        self.content = content
    }

    private var value: Binding<Int> { Binding<Int>(
        get: {
            Int(selection)
        },
        set: {
            self.selection = SelectionValue($0)
        })
    }

    var body: some View {
        Picker(label, selection: value, content: content)
    }
}

struct CompatibilityPicker_Previews: PreviewProvider {

    struct TestView: View {
        @State var selection : UInt8 = 1
        var body: some View {
            CompatibilityPicker("Difficulty", selection: $selection) {
                Text("Easy").tag(0)
                Text("Normal").tag(1)
                Text("Hard").tag(2)
            }
        }
    }
    static var previews: some View {
        TestView()
    }
}