Issue Description:

The Problem:

I got this error today in Xcode 11.4. This seems like a compiler bug:

'P.KP' cannot be a subclass of both 'PartialKeyPath<Property.Root>' and 'WritableKeyPath<Property.Root, Property.Value>'

This seems wrong, because WritableKeyPath inherits from KeyPath, which inherits from PartialKeyPath. By definition then, any subclass of WritableKeyPath is a subclass of PartialKeyPath when they share the same root, so what am I missing here?

This also happens on Xcode 11.3 and 10.3.

Here is the underlying code:

protocol AnyPropertyProtocol {
    associatedtype Root = Any
    associatedtype Value = Any
    associatedtype KP: AnyKeyPath
    var key: KP { get }
    var value: Value { get }
}

protocol PartialPropertyProtocol: AnyPropertyProtocol
where KP: PartialKeyPath<Root> {
}

protocol PropertyProtocol: PartialPropertyProtocol
where KP: WritableKeyPath<Root, Value> {
}

extension Dictionary where Value: AnyPropertyProtocol {
    subscript<R, V, P>(key: Key, path: WritableKeyPath<R, V>) -> P? where P: PropertyProtocol, P.Root == R, P.Value == V {
        return self[key] as? P
    }
}

The faulty compiler error is thrown on the subscript declaration.

Current Workaround

Changing the subscript declaration to the below code compiles fine, even though it seems to say the same thing (unless I'm missing something):

subscript<P: PropertyProtocol>(key: Key, path: P.KP) -> P?