Issue Description:

The following code fails to compile in a playground:

struct Identified<A>: Identifiable {
  var value: A
  var id: AnyHashable  init<B: Hashable>(_ value: A, by hashable: (A) -> B) {
    self.value = value
    self.id = hashable(value)
  }

//  init<B: Hashable>(_ value: A, by hashable: KeyPath<A, B>) {
//    self.value = value
//    self.id = value[keyPath: hashable]
//  }
}

Identified(1, by: \.self)

cannot convert value of type 'WritableKeyPath<_, _>' to expected argument type '(Int) -> B'

The commented-out overload seems to be required. When commented-in, the code behaves as expected.

In general, the "self" key path doesn't seem to work in expected situations. For example:

[Optional(1), 2, 3].compactMap(\.self)

error: type of expression is ambiguous without more context