The whole naming issue seems to be caused by the .union(_ function. The Swift Guidelines say that mutating functions should use a verb, and non-mutating forms should use a noun, but in this case, the word union itself is a verb and a noun.
Have we considered this, then:
_ = a.union(b) //non-mutating
There is no ambiguity in most situations, and the fact the Swift compiler can't disambiguate this at the moment is a bug I'd like to see fixed in the Swift 3 timeframe. I think this wouldn't be such a bad compromise, and other functions could still use the standard -ed/-ing system alongside this without the API looking inconsistent, unlike with the form- prefix.
Admittedly, there is merit to the idea that functional methods should make non-mutating forms the primary form, but I feel like we should figure out what our stance is on this methodology in general. A mention in the Guidelines one way or the other would be nice, since the current rules seem to support this.
From James F