Strange error with KeyPath on optional value

Question

Consider the following snippet of code

class A {
    
    var value: Int?
    
}

let a: A? = A()
let kp = \A.value
a?[keyPath: kp] = 10
print(a?.value)

This works perfectly and Optional(10) is printed as expected. In my actual app the field I'm trying to set this way is declared as Date? and it causes some strange errors. The MWE of my actual app is this:

class A {
    
    var value: Date?
    
}

let a: A! = A()
let kp = \A.value
a?[keyPath: kp] = Date() // Line with error
print(a?.value)

However the compiler complains about the highlighted line and says:

Value of optional type 'Date?' must be unwrapped to a value of type 'Date'

Fix: Coalesce using '??' to provide a default when the optional value contains 'nil'

Fix: Force-unwrap using '!' to abort execution if the optional value contains 'nil'

Is this a bug of the compiler we can expect to be fixed before the final release or there's something I'm not understanding about key-paths?

I'm using Xcode 11 beta 3 but I've encountered the same problem also in beta 2. If it can be useful the actual code is here.

Answer 1

Writing this without ! and ? works

let a = A()
let kp = \A.value
a[keyPath: kp] = Date() 
print(a.value)

or only as optional

let a: A? = A()
let kp = \A.value
a?[keyPath: kp] = Date() 
print(a?.value)