class A {
    var a = false
}


class B {
    let a = A()
}


let b: B = B()
let keyPath: WritableKeyPath<A, Bool> = \.a
b.a[keyPath: \.a] = true // This compiles and works as expected
b.a[keyPath: keyPath] = true // Compilation error: Cannot assign through subscript: 'a' is a 'let' constant
var a = b.a
a[keyPath: keyPath] = true // Workaround