Doesn't help with the "temp" variables not being passed over, but allows flatMap to work like Swift 3.

temp-fix.swift

extension CollectionType {
    private func flatMap<S : SequenceType>(transform: (Self.Generator.Element) throws -> S) rethrows -> Set<S.Generator.Element> {
        return
            Set(
                try self
                    .map(transform)
                    .filter { $0 != nil }
                    .map { $0! }
                    .flatten()
        )
    }
}