How to declare View with ViewBuilder in SwiftUI

struct ChartView {
    var body: some View {
        computation
    }

    @ViewBuilder
    var computation: some View {
        let left = 10
        let right = 20
        if left + right == 30 {
            Text("Correct")
        }
    }
}

struct ChartView: View {
    @State private var showsOverlay: Bool = false
    @State private var overlayKind: OverlayKind?

    enum OverlayKind {
        case weather(String)
    }

    var body: some View {
        VStack {
            EmptyView()
        }
        .overlay(
            overlayView
        )
    }

    @ViewBuilder
    private var overlayView: some View {
        if showsOverlay, let overlayKind = overlayKind {
            switch overlayKind {
            case let .weather(city):
                WeatherView(city: city)
            }
        }
    }
}

struct ChartView: View {
    @State private var showsLegend: Bool = false

    var body: some View {
        if showsLegend {
            LegendView()
        }
    }
}

struct ChartView: View {
    var body: some View {
        Text("Chart View")
        footer()
    }

    @ViewBuilder
    func footer() -> some View {

    }
}

struct ChartView: View {
    let decorations: [String: AnyView]

    var body: some View {
        border
        borderColor
        decoration
    }

    var borderColor: Color? {
        nil
    }

    var border: Rectangle? {
        nil
    }

    @ViewBuilder
    var decoration: some View {
        decorations["N/A"]
    }
}