Unexpected! That jump to NULL happens on a suspiciously early offset; I wonder if this this turns out to be some sort of undiagnosed availability issue. (If so, I'm surprised it didn't show up in our back deployment tests.)

It's too early to tell if the fix will require an SDK update, but if you're urgently blocked on this, a quick-and-dirty workaround would be to define something like this in affected modules:

#if swift(>=5.6) && swift(<5.6.99)
extension Set {
  func subtracting<S: Sequence>(_ other: S) -> Set<Element>
  where S.Element == Element {
    var result = self
    for item in other { result.remove(item) }
    return result
  }
}
#endif

(This is considerably slower than the actual implementation, but it will be most likely free of this issue.)

@swift-ci create

Oh, interesting. I haven't seen a crash yet from iOS 15.4, but I've seen a lot from OS versions prior to that. (Particularly iOS 15.3.1). That could just be indicative that we don't have many users on 15.4 yet, though.

Erik's fix is up: #41954 It did turn out to be an availability issue, caused by a misfiring optimization in the compiler.

fix for 5.6: #41954
for main: #41026

Comment by Spencer Transier (JIRA)

@eeckstein Do you know if the fix for this bug was included in Xcode 13.3.1?

It looks like 13.3.1 has the same version of Swift as 13.3. But there has been a 5.6.1 tagged, so we can be hopeful that there'll be a 13.3.2…

(Signal also ran into this bug, generating many crashes a day.)