Which version of Xcode are you using?

I can reproduce using the Xcode 8 beta 6 REPL and in a Playground using the 8/25 toolchain. The same issue also arises with `symmetricDifference`.

In the Playground, evaluation proceeds initially without any problem, but if the code is grayed out and re-evaluated, either automatically or manually by pressing the play button, then the problem described above manifests itself.

It seems to me there is a bad interaction among REPL/Playground code, the internal boxing facility recently introduced (`_MutableBoxing` and `_MutableHandle`), `isKnownUniquelyReferenced`, and the fact that `subtract[ing]` and `symmetricDifference` use a nested series of calls to `intersection` and `union` with rvalues that aren't assigned to temporary variables.

[Edit: To clarify, the current implementations of `intersection` and `union` in `CharacterSet` use `_MutableBoxing` facilities to avoid extraneous copies if the underlying Foundation reference type is known to be uniquely referenced, but it seems that something there goes awry when the result of `intersection` is used as an argument for a call to `union` or vice versa, as happens in `subtract[ing]` and `symmetricDifference`, when these methods are evaluated in the REPL.]

This issue persists in Xcode GM.

The underlying issue turns out to be pretty straightforward. `CharacterSet` has no implementation of `subtracting`. The default implementation of `subtracting` in `SetAlgebra` calls `symmetricDifference`, but `CharacterSet.symmetricDifference` calls `subtracting`!

PR #5201 has been created which implements `subtracting` and `subtract`.

Sweet. Thank you @xwu!