[Pitch #5] SE-0293: Extend property wrappers to function and closure parameters

[hborla (Holly Borla)]

@DevAndArtist

If we say that a wrapper on a parameter always desugars to a local property wrapper which shadows the parameter, there is clearly an opportunity to allow mutation.

Yes, it's feasible, but again, I believe it's unexpected when it appears that you're able to modify a parameter and that change isn't reflected when the caller resumes. This was also a point brought up in the first pitch of this feature:

@Douglas_Gregor

Ever since SE-0003 , parameters have been immutable. I feel like we shouldn’t let property wrappers change that, even when the wrappedValue is nonmutating.

The only reason we chose to synthesize nonmutating setters is that this is typically what's used for property wrappers that provide an abstracted reference to a value, and the ergonomics of using such property wrappers as parameters would be poor otherwise.

@DevAndArtist

That is incorrect. In Swift 5.4 the following code

I understand how local property wrappers work. What's in the proposal is deliberately different. I can make it more clear in the proposal that it's deliberately different.

@DevAndArtist

As I wrote this example I realized that one example in the proposal seems to be incomplete.

You're right - thanks for pointing this out, it was an inadvertent change I made as I updated the proposal for this revision. It's fixed now.