Lifting the "Self or associated type" constraint on existentials

Evolution Pitches
70

Lifting the constraint wouldn’t indeed help to avoid creating type erased boxes, but it could somewhat simplify it when working with members that don’t use Self/associated types.
e.g. (primary for being able to mock things) I often write such wrappers:

protocol Foo {
  associatedtype Bar

  var x: Int { get }
  var y: Int { get }
}

struct AnyFoo<T>: Foo {
  typealias Bar = T
  private let _getX: () -> Int
  private let _getY: () -> Int

  var x: Int { return _getX() }
  var y: Int { return _getY() }

  init<K: Foo>(_ val: K) where K.Bar == T {
    self._getX = { return val.x }
    self._getY = { return val.y }
  }
}

Being able to use simple type members would decrease memory footprint of such wrapper and simplify the code.