FB15126597: AlignmentGuide crash when animating a Shape property. If an alignmentGuide affects the layout of a Shape with an animated property, changes to that property cause the app to crash. This crash occurs in Xcode 16 RC1 only when running in Swift 6 language mode.

AlignmentGuideForEachCrash.swift

struct CrashingView: View {
    @State private var items: [String] = Array(1...10).map { String("\($0)") }
    
    var body: some View {
        VStack {
            HStack {
                ForEach(items, id: \.self) { item in
                    Text(item)
                        /// Adding @Sendable to the closure fixes the crash
                        .alignmentGuide(HorizontalAlignment.center) { @Sendable d in
                            d[HorizontalAlignment.center]
                        }
                }
            }
            /// Comment out the animation and removing an item will not crash the app.
            .animation(.default, value: items)
            
            HStack {
                /// Tap this button to remove an item and crash the app
                /// It will also log the following
                /// `void * _Nullable NSMapGet(NSMapTable * _Nonnull, const void * _Nullable): map table argument is NULL`
                Button("Remove Item") {
                    removeLastItem()
                }
                .padding()
            }
            .padding()
        }
    }
    
    func removeLastItem() {
        if !items.isEmpty {
            items.removeLast()
        }
    }
}

AlignmentGuideShapeCrash.swift

import SwiftUI

struct CrashingView2: View {
    @State private var radius: CGFloat = 10
    
    var body: some View {
        VStack {
            Color.blue
                /// Remove @Sendable from this closure to see the crash
                .alignmentGuide(.bottom) { @Sendable d in
                    0
                }
                .background(alignment: .bottom) {
                    RoundedRectangle(cornerRadius: radius)
                }
            
            /// Change the radius and the view will crash
            Stepper("Radius", value: $radius, in: 0...100, step: 5)
        }
        /// Comment out the animation and changes to the radius will not crash the app.
        .animation(.default, value: radius)
        .padding()
    }
}

#Preview {
    CrashingView2()
}

Update: Add @.Sendable to the alignmentGuide closures to prevent the crash! Thanks to @mattmassicotte for this fix.