How to pass optional @ViewBuilder parameter in SwiftUI?

My team delivers a UI SDK implemented with UIKit. I'm investigating the best practices for how to ship a framework with reusable UI controls implemented in SwiftUI.

One feature I really like in SwiftUI is @ViewBuilder. I could see one path for us is to ship a TableViewCell (well, the replacement) which has our classic API with String and UILabel-like parameters, but also provide a @ViewBuilder parameter that lets the developer do more significant customizations than we support in the control.

I have this working nicely, where the @ViewBuilder property is required. But, it does not support an optional parameter, which I think is necessary for the struct init(...) method to behave as expected. Per the Swift 5.1 fix, I should be able to init a struct with only the un-inited parameters; I should be able to ignore the optional or pre-defined parameters.

Desired behavior:

ObjectView(title: "Hello, World")

Current behavior:

ObjectView(title: "Hello, World", main: { EmptyView() })

Working example where @ViewBuilder parameter is required.

// UIKit View
class ObjectTableViewCell: UITableViewCell {
   var title: String?
   var titleLabel: UILabel
   var contentView: UIView // inherited from table view cell, gives developer ability to customize layout
}

// SwiftUI View
class ObjectView: View {
    var title: String? = nil
    var titleText: Text? = nil
    private let mainItems: () -> MainItems // custom @ViewBuilder property, gives developer ability to supply custom 'subviews'

    init(title: String? = nil, titleText: Text? = nil, @ViewBuilder main mainItems: @escaping () -> MainItems) {
        self.title = title
        self.titleText = titleText
        self.mainItems = mainItems
    }

    var body: some View {
   
        HStack(alignment: .firstTextBaseline) {
            VStack(alignment: .leading, spacing: 3) {
                if titleText != nil {
                    titleText!
                } else if title != nil {
                    Text(title!)
                } else {
                    mainItems()
                }
            } // + adjacent views ...
        }.padding()
    }
}

Unfortunately, if I modify the definition of the mainItems parameter to (() -> MainItems)?, I get the error: "Function builder attribute 'ViewBuilder' can only be applied to a parameter of function type." Fair enough, but it should be able to account for optionality on that closure parameter.