Reviving this thread so we can work on the revision!
When I first wrote the proposal I was unaware of what the wildcard was precisely supposed to represent, but the compiler code has several pieces of documentation saying that it represents the minimum deployment target of the platform, as Ted mentioned. This supports the idea of having #unavailable(*) return false which was shown to be important during the review. Here's an updated draft to start with:
Semantics of *
The compiler uses the platform wildcard * to ease porting to new platforms. Because new platforms typically branch from existing platforms, the wildcard allows availability checks to execute the guarded branch on the new platform without requiring a modification to every availability guard in the program.
To achieve this in practice, the wildcard represents the minimum deployment target of the unspecified platform being compiled.
if #available(*) {
// ...
} else {
// Will never be executed
}
When multiple platforms are present in the statement, the wildcard represents only the platforms that were not specified. A check like #available(iOS 13, *) means "if compiling for iOS, iOS 13, otherwise, the minimum deployment target" and not "if compiling for iOS, iOS 13 and iOS's deployment target, otherwise, just the minimum deployment target". The wildcard doesn't include platforms that were explictly added to the statement, which can be visualized by how it's not possible to specify a platform multiple times.
if #available(iOS 12, *)
if #available(iOS 12, iOS 13, *) // Error: Version for 'iOS' already specified
For unavailability, this means that #unavailable(*) and #unavailable(notWhatImCompiling X, *) should return false. Since the minimum deployment target will always be present, the statement can never be true. This behavior also matches how a theoretical !#available(*) would behave if building expressions with #available was possible.
if #unavailable(*) {
// Will never be executed
} else {
// ...
}
The wildcard only represents platforms that were unspecified in the statement. This means that #unavailable(iOS 13, *) doesn't mean "iOS 13 and iOS's minimum deployment target", but "if iOS, iOS 13, otherwise the minimum deployment target"
As an interesting side effect, this means that having multiple unavailability checks in the same statement (#unavailable(iOS 13, *), #unavailable(watchOS 3, *) as opposed to #unavailable(iOS 13, watchOS 3, *)) would cause the statement to always be false.
In these cases, since wildcard checks are eventually optimized to boolean literals, the compiler will already emit a warning indicating that the code will never be executed. Still, we can provide a more descriptive diagnostic that suggests using a single check that considers all platforms.
if #unavailable(iOS 13, *), #unavailable(watchOS 3, *) {
// ...
// Warning: code will never be executed
// Error: unavailability checks are canceling each other, use a single check that treats all platforms
// fix-it: #unavailable(iOS 13, watchOS 3, *)
}