swift234

Value-to-optional conversions are only subtypes for class references.

  let x: Int? = 4 as Int // okay
  let f: (Int) -> Void = { (x: Int?) -> Void in println(x) } // error!

As part of this, remove an invalid "protection" of value-to-optional
conversions that involve T? -> T??. Unfortunately, this results in an
ambiguity in the following code:

  var z: Int?
  z = z ?? 42

Both of our overloads for ?? here require one value-to-optional conversion,
and the overload preference isn't conclusive. (Turns out (T?, T) and
(U?, U?) can be unified as T=U or as T=U?.) The best thing to do would be
to take our solved T=Int binding into account, but the type checker isn't
set up to do that at the moment.

After talking to JoeP, I ended up just hardcoding a preference against
the (T?, T?) -> T? overload of ??. This is only acceptable because both
overloads have the same result, and I'll be filing another Radar to
remove this hack in the future.

Part of rdar://problem/19600325

Swift SVN r25045