Why can't existential types be compared?

[ksluder (Kyle Sluder)]

AnyHashable still has its own confusing pitfalls:

@Jumhyn

@xwu

it wouldn’t be very nice if they compare not-equal when boxed in AnyHashable…

I'm not sure this totally justifies things—after all, AnyHashable does not attempt to recover this behavior for arbitrary BinaryInteger types:

struct S: BinaryInteger {
  var wrapped: Int
  // details omitted...
}
0 as Int == 0 as S                           // true
AnyHashable(0 as Int) == AnyHashable(0 as S) // false

Of course, we can restore this by conforming the 'secret' _HasCustomAnyHashableRepresentation protocol:

extension S: _HasCustomAnyHashableRepresentation {
  public func _toCustomAnyHashable() -> AnyHashable? {
      return wrapped._toCustomAnyHashable()
  }
}

AnyHashable(0 as Int) == AnyHashable(0 as S) // true

But based on the implementation of _toCustomAnyHashable() for integer types it seems clear to me that the goal there is to match NSNumber semantics, not to attempt to provide a canonical representation based on BinaryInteger equality.