[Draft Proposal for pre discussion]NS_ENUM's should be considered Open

Can I get some thoughts about this idea to remove undefined behaviour from switch statements when an enum imported from Objective-C has a raw value outside the expected set?

The proposal is on my fork and below: swift-evolution/0000-consider_ns_enums_open.md at master · josephlord/swift-evolution · GitHub



# NS_ENUMs should be considered Open

* Proposal: [SE-NNNN](https://github.com/josephlord/swift-evolution/blob/master/proposals/0000-consider_ns_enums_open.md\)
* Author(s): [Joseph Lord](https://github.com/josephlord\)
* Status: [Early Draft]
* Review manager: TBD

## Introduction

Enums imported from C/Objective C could (as a result of library versioning or bugs) actually have raw values outside of the defined values. A default case should be required on switch statements involving them and the current undefined behaviour should be removed.

Swift-evolution thread: [To be filled in after discussion]

## Motivation

In cases where an enum value is outside of the expected range and there is no default in a switch the behaviour is currently undefined (it seems to take the first case in practice) potentially leading to runtime crashes. Undefined behaviour and crashes at runtime due to library changes outside the developer's control are highly undesirable and Swift should be helping us to make safe code to prevent them. At the moment if you are handling all cases it is not even possible to add a default case.

I encountered the issue when testing on iOS 8 and hitting [an issue with CoreData](swift - iOS 9 - "attempt to delete and reload the same index path" - Stack Overflow) where it was passing a zero value the NSFetchedResultsChangeType to my code. That thread clearly shows that the issue is affecting people and while it is clearly a bug in the unsafe code of Core Data that is something which Swift should be defending against. There is a simple test example showing the issue in [SR-1258](https://bugs.swift.org/browse/SR-1258\)

## Proposed solution

Redefine NS_ENUMs from being a closed type to an open one. This would then require a default case on all switch statements over the type. With the default case explicit situation must be handled by the developer. They can choose to raise errors call assertionFailure or preconditionFailure as the wish or handle the case safely.

## Detailed design

  - Help required here

## Impact on existing code

This will break all switch statements over an enum imported from Objective-C or C where there is no default case. There should not be any automatic migration because it is appropriate for the developer to decide on the course of action in each case.

## Alternatives considered

1) No change

This leaves the potential for undefined behaviour in all build modes if cases of bugs in code outside of Swift OR in the event that new cases are added to enums in future library versions.

2) Debug assertion

This would have the benefit that developers might discover errors more quickly but could still leave risks where libraries (including OS frameworks) are updated and introduce bugs after application release or in untested configurations.