swift234

Disallow use of associated types on existentials.

  protocol BaseProto {
    typealias AssocTy
  }
  var a: BaseProto.AssocTy // error!

While this could be interpreted to mean "a is an existential bounded by
whatever AssocTy is bounded by", we don't actually support this in any
other contexts; if a protocol has a function that returns an associated type,
it can only be used in a concrete or generic context, not an existential one.

<rdar://problem/15850024>

Swift SVN r12600