1 
import SwiftUI


struct PlainGroupBoxStyle: GroupBoxStyle {
    func makeBody(configuration: Configuration) -> some View {
        VStack(alignment: .leading) {
            configuration.label
            configuration.content
        }
    }
}

extension GroupBoxStyle where Self == PlainGroupBoxStyle {
    static var plainGroupBoxStyle: PlainGroupBoxStyle { .init() }
}

struct CardGroupBoxStyle: GroupBoxStyle {
    func makeBody(configuration: Configuration) -> some View {
        VStack(alignment: .leading) {
            configuration.label
            configuration.content
        }
        .padding()
        .background(Color(.systemGroupedBackground))
        .clipShape(RoundedRectangle(cornerRadius: 28, style: .continuous))
    }
}

extension GroupBoxStyle where Self == CardGroupBoxStyle {
    static var plainGroupBoxStyle: CardGroupBoxStyle { .init() }     // `plainGroupBoxStyle` again here, why no error?
}

Autocomplete show both:

Screen Shot 2021-08-10 at 11.13.19 AM

The whole thing 
import SwiftUI


struct PlainGroupBoxStyle: GroupBoxStyle {
    func makeBody(configuration: Configuration) -> some View {
        VStack(alignment: .leading) {
            configuration.label
            configuration.content
        }
    }
}

extension GroupBoxStyle where Self == PlainGroupBoxStyle {
    static var plainGroupBoxStyle: PlainGroupBoxStyle { .init() }
}

struct CardGroupBoxStyle: GroupBoxStyle {
    func makeBody(configuration: Configuration) -> some View {
        VStack(alignment: .leading) {
            configuration.label
            configuration.content
        }
        .padding()
        .background(Color(.systemGroupedBackground))
        .clipShape(RoundedRectangle(cornerRadius: 28, style: .continuous))
    }
}

extension GroupBoxStyle where Self == CardGroupBoxStyle {
    static var cardGroupBoxStyle: CardGroupBoxStyle { .init() }     // `plainGroupBoxStyle` again here, why no error?
}

struct GroupBoxDemo: View {
    var body: some View {
        ScrollView {
            LazyVGrid(columns: [.init(), .init()]) {
                ForEach(0..<10) { _ in
                    GroupBox(
                        label: Label("Heart Rate", systemImage: "heart.fill")
                            .foregroundColor(.red)
                    ) {
                        Text("Your hear rate is 90 BPM.")
                    }
                    .groupBoxStyle(.plainGroupBoxStyle)
                }
            }.padding()
        }
    }
}

struct GroupBoxDemo_Previews: PreviewProvider {
    static var previews: some View {
        GroupBoxDemo()
    }
}
2

There’s no situation where these extensions apply to the same thing, right? Why do you expect there’s a “collision?”

3

They are static var, I don't think you can have two duplicated in one protocol?

4

What type would be a GroupBoxStyle where Self is both PlainGroupBoxStyle and CardGroupBoxStyle.

5

Ok, I see now. It's probably bad practice because it prevents implicit static member lookup. But they are two different concrete types and they can be disambiguated with fully qualifying.