WhatTheBinding.swift

import SwiftUI

struct ContentView: View {
    @State var a: Bool = false
    
    var body: some View {
        A(a: $a)
    }
}

struct A : View {
    @Binding var a: Bool
    
    var body: some View {
            // Fails with "Cannot convert value of type 'Binding<Bool>' to expected argument type 'Binding<Bool?>'"
            // ^~~~
            //B(a: $a)
            B(a: Binding.constant(self.a))
    }
}

struct B : View {
    @Binding var a: Bool?
    
    var body: some View {
        
        if a == nil {
            return Text("a is nil")
        } else {
            return Text("a is \(a! ? "true" : "false")")
        }
    }
}

Just lurking, that's because it's expecting an optional Bool.