Update

I'm trying to use the hierarchical List view in SwiftUI (List( tree, children:)). I find that if the first list element has no children, only the top level of the hierarchy is shown. This looks like a bug in SwiftUI to me. I wonder if anyone knows of a good fix.

[This is a better version of my question, using the same struct type and same array elements]

@main
struct bug_reportApp: App {
    var body: some Scene {
        WindowGroup {
            ContentView()
        }
    }
}


struct FileItem: Hashable, Identifiable, CustomStringConvertible {
    var id: Self { self }
    var name: String
    var children: [FileItem]? = nil
    var description: String {
        return name
    }
}


let tree0: [FileItem] = [
  FileItem(name: "users", children:
    [FileItem(name: "user1234", children:
      [FileItem(name: "Photos", children:
        [FileItem(name: "photo001.jpg"),
         FileItem(name: "photo002.jpg")]),
       FileItem(name: "Movies", children:
         [FileItem(name: "movie001.mp4")]),
          FileItem(name: "Documents")
      ]),
     FileItem(name: "newuser", children:
       [FileItem(name: "Documents", children: [])
       ])
    ]),
    FileItem(name: "private", children: nil)
]

let tree1 = [tree0[1], tree0[0]]
let tree2 = [FileItem(name: "private", children: []), tree0[0]]


struct ContentView: View {

    

    var body: some View {
        HStack{
            VStack{
                Text("Works").font(.headline)
                List(tree0, children: \.children) { item in
                    Text(item.description)
                }
            }
            
            VStack{
                Text("First element has empty .children").font(.headline)
                List(tree2, children: \.children) { item in
                    Text(item.description)
                }
            }
        
            VStack{
                Text("First element has nil .children").font(.headline)
                List(tree1, children: \.children) { item in
                    Text(item.description)
                }
            }
          
        }
    }
}

Here an annotated screenshot, highlighting where the missing tree expansion arrow is/should be.

Original question

I'm trying to use the hierarchical List view in SwiftUI (List( tree, children:)).

I can't see why my simple example here displays only the 3 top-level 3 items, with no little arrow to expand the sub-list under list element #2.

I can cut and paste very similar looking examples (into the same project, same file) and they do work... but I can't spot the difference!

My code that doesn't work:

import SwiftUI

struct TreeIndexNode: Hashable, Identifiable, CustomStringConvertible {
    var id: Int
    var children: [TreeIndexNode]? = nil
    var description: String {
        return String("\(id)")
    }
}

struct ContentView: View {
    
    let tree: [TreeIndexNode] = [ TreeIndexNode(id: 1), TreeIndexNode(id: 2, children: [
        TreeIndexNode(id: 3),
        TreeIndexNode(id: 4),
    ]),
    TreeIndexNode(id: 8),
    ]
    
    var body: some View {
        List( tree, children: \.children) { row in
            Text(row.description)
        }
    }
}

but if I switch it to this example from Apple Developer pages, it works as expected:

struct ContentView: View {
    struct FileItem: Hashable, Identifiable, CustomStringConvertible {
        var id: Self { self }
        var name: String
        var children: [FileItem]? = nil
        var description: String {
            switch children {
            case nil:
                return "📄 \(name)"
            case .some(let children):
                return children.isEmpty ? "📂 \(name)" : "📁 \(name)"
            }
        }
    }
    let fileHierarchyData: [FileItem] = [
      FileItem(name: "users", children:
        [FileItem(name: "user1234", children:
          [FileItem(name: "Photos", children:
            [FileItem(name: "photo001.jpg"),
             FileItem(name: "photo002.jpg")]),
           FileItem(name: "Movies", children:
             [FileItem(name: "movie001.mp4")]),
              FileItem(name: "Documents", children: [])
          ]),
         FileItem(name: "newuser", children:
           [FileItem(name: "Documents", children: [])
           ])
        ]),
        FileItem(name: "private", children: nil)
    ]
    var body: some View {
        List(fileHierarchyData, children: \.children) { item in
            Text(item.description)
        }
    }
}

I started from the Xcode 12.4 "Multiplatform" project template. Here's what it looks like on MacOS:

Screen shot of non-working MacOS build of my code above

When I build for iOS, it does work as expected:

my code above in iOS simulator