In the Swift 4 Keypaths proposal  there seem to be several types that are unimplemented (or not working as I expect them to), and I'm not sure whether these are simply unfinished in Beta 1, or just ideas that weren't accepted.
For instance, according to the proposal AnyKeyPath is supposed to be fully type-erased and access properties as optionals at run-time, but using it I only get the compile-time error "Type "FooClass" has no subscript members", even when applied to objects that still have the original type of the KeyPath's root.
Even if I use PartialKeyPath<Root> I get the same error:
var tester = TestClass(name: "steve")
let tester_path = \TestClass.name
let partial_path:PartialKeyPath<TestClass> = tester_path
let name = tester[keyPath: partial_path] /// no subscript members error here
Only when I either use the original path variable, or cast it to KeyPath<TestClass, String>, am I able to access the property without an error. I am wanting to use KeyPaths in a more generic way, so the AnyKeyPath and PartialKeyPath would be helpful. Does anyone have insight into the status of these KeyPath types?