Unable to set non-optional value for `WritableKeyPath` to optional `Value`

Let's say I have a struct with a property containing optional value:

struct SomeStruct {
    var someVal: Int?

I can create a WritableKeyPath to someVal field and assign non-optional value via the keyPath without any error:

var obj = SomeStruct()
let keyPath = \SomeStruct.someVal
obj[keyPath: keyPath] = 5

But when I wrap this assigning logic inside a generic type or a protocol with associated type I am getting compiler error to force-unwrap or provide default value, i.e for following GenericStruct implementation:

struct GenericStruct<T> {
    let keyPath: WritableKeyPath<SomeStruct, T?>

    func assign(to obj: inout SomeStruct, data: T) {
        obj[keyPath: keyPath] = data

I am getting the following error in assign method:

Value of optional type 'T?' must be unwrapped to a value of type 'T'

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

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

I am confused why this error is given by compiler, shouldn't a non-optional value be assigned to an optional value for same type?

It's only a bug in Playgrounds, yeah?

Yes, just checked, working fine in Swift package, bug is only for playground.