Reduced by using local variables:

import Foundation

enum MyError: Error {
  case someThing
}

func test() {
  let error = MyError.someThing
  
  let foundationError = error as NSError
  
  if let urlError = foundationError as? URLError {
      print(urlError)
  }
}

test()

@mikeash, this sounds familiar, but I can't find a dup, and it's still reproducing for me on pretty-recent-master.

I'm not aware of something tracking this. I think it's an ARC bug. The generated code is calling swift_dynamicCast on the error with TakeOnSuccess and DestroyOnFailure flags, but then it also emits a call to objc_release the error.

Compiler bug, then. Thanks!

@swift-ci create

Actually a runtime bug after all. The two releases are correct, as the value is at +2 at that point. The problem is that swift_dynamicCast passes all flags through to tryDynamicCastNSErrorObjectToValue. When DestroyOnFailure is set and the cast fails, tryDynamicCastNSErrorObjectToValue obediently destroys the value. Then swift_dynamicCast reacts to that failure by also destroying the value.

#28804