I think I speak for the entire Swift community when I say that Swift's enums, 
together with the ability to switch over them exhaustively and having 
compile-time guarantees, is one of the best features in Swift. I'm regularly 
annoyed by this when writing other languages like Java and JS (default: throw 
new IllegalArgumentException();)

Now, that's not to say I don't understand why this proposal is necessary. I 
totally get it, and the existing decisions make a lot of sense to me. But I'd 
like us to introduce this while maintaining the ability to guarantee exhaustive 
switch statements, no matter how the enum was defined.

Example: imagine a theoretical SampleKit defines:
public enum E {
case A
case B

It's implicitly non-exhaustive, possibly because the author was not aware of 
the default (which would likely happen often), or possibly because they made a 
conscious decision, as they expect to expand the cases in the future.

In my app, I use SampleKit and decide that I want to make sure I handle all 

switch e {
case A: break
case B: break
default: break // This becomes necessary

As the proposal stands right now, I'm forced to handle any future cases. That's 
fine. What's not fine in my opinion, is that in doing so I lose the ability to 
keep this exhaustiveness moving forward. If I update SampleKit to v2.0, I want 
to know at compile-time if there are new cases I need to be aware of (instead 
of going to some generic handling path). Instead, I’m left in the same place I 
would in other languages like Java or JS:

// No error :(
switch e {
case A: break
case B: break
default: break

Proposed Solution

What I’m proposing is that we introduce a new keyword, unknown (or a better 
name), that serves as a way to handle cases that aren’t yet known, but not 
those that are.

// Error: missing case C
switch e {
case A: break
case B: break
unknown: break // Would handle future cases

With this, you shouldn’t be able to use default AND unknown at the same time, 
as default implicitly includes unknown.

Thanks for reading, and I hope you can consider this change (or some variation 
of it).

Nacho Soto
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