SE-0376 (second review): Function back deployment

[michelf (Michel Fortin)]

@tshortli

As evidenced by the review and pitch threads, folks understandably have a bit of trouble grokking what it means for an API to be back-deployed "up to" a release, instead of "back to". Many expressed that they wanted to see the role of @available and the new attribute reversed, where @available would indicate the release of ABI introduction and the new attribute would indicate how far back the the API can be used.

I agree it's hard to reason about. I think the problem is backDeployed is expressing a counter measure for the absence of an ABI rather than just declaring its presence. It highlights what is not there rather than what is there. I think if the attribute was describing things going forward rather than backward, it'd be easier to read correctly:

@available(macOS 11, *)
@abi(macOS 13, *)
func doSomething() {...}