Support Negative Availability Literals

[xwu (Xiaodi Wu)]

@bjhomer

In either case, the * is undesired. Under my interpretation ("... or on any other platforms"), it's the wrong default. Even if you disagree with that interpretation, it's clear that there's not a consistent interpretation that Swift users will understand, so the * should be invalid. It does not have a consistent understandable meaning that is easily explained, so we should not allow it.

I like the interpretation of "...or any other platform"; to me, though, it does mean the same thing as proposed by @rockbruno:

// If macOS 11+ is available, or any other platform is available...
if #available(macOS 11, *) { }

// If macOS 11+ is unavailable, or any other platform is unavailable...
if #unavailable(macOS 11, *) { }

It does not make sense to me how one would arrive at the opposite meaning when using this interpretation. But perhaps Swift users will simply not understand it.

By contrast, to me, I cannot understand how if #unavailable(iOS 13) { } would be skipped by other platforms: it plainly says that the code should be run if iOS 13 is unavailable, and iOS 13 is clearly unavailable if we're on macOS.

So if there is no consensus that can be achieved here which would not be completely interpreted in the opposite way by someone else, then perhaps this is a sign that the feature should not be added at all.