> On Apr 20, 2017, at 4:55 PM, Jordan Rose via swift-dev <swift-dev@swift.org>
> wrote:
>
> TLDR: Should we just always import C/ObjC types under their Swift 4 names,
> and use typealiases in Swift 3 mode?
>
> ---
>
> Hi, swift-dev. As my recent PRs have probably indicated, I've been working on
> the problems that can come up when mixing Swift 3 and Swift 4 code. Most of
> these problems have to do with C/ObjC APIs that might present themselves
> differently in Swift 3 and Swift 4, using the "API notes" feature in our
> downstream branch of Clang, and a good subset of these problems have to do
> with types getting renamed. (This includes being "renamed" into a member,
> such as NSNotificationName becoming (NS)Notification.Name in Swift.)
>
> What's the problem? Well, there are a few. First of all, an API defined in
> terms of the Swift 3 name should still be callable in Swift 4. As an example,
> let's pretend NSNotification.Name was going to be renamed
> NSNotification.Identifier in Swift 4.
>
> // Swift 3 library
> public func postTestNotification(named name: NSNotification.Name) { … }
>
> // Swift 4 app
> let id: Notification.Identifier = …
> postTestNotification(named: id) // should work
>
> This means the reference to "NSNotification.Name" in the library's
> swiftmodule needs to still be resolvable. This isn't too bad if we leave
> behind a typealias for 'NSNotification.Name'. I have a reasonable (but too
> broad) implementation at https://github.com/apple/swift/pull/8737
> <https://github.com/apple/swift/pull/8737>.
>
> That just leads us to another problem, though: because Swift functions can be
> overloaded, the symbol name includes the type, and the type has changed. The
> Swift 3 library exposes a symbol
> '_T03Lib20postTestNotificationySo14NSNotificationC4NameV5named_tF', but the
> Swift 4 client expects
> '_T03Lib20postTestNotificationySo14NSNotificationC10IdentifierV5named_tF'.
>
> My planned approach to combat this was to use the C name of the type in the
> mangling, producing
> '_T03Lib20postTestNotificationySo18NSNotificationNamea5named_tF'. This is
> prototyped in https://github.com/apple/swift/pull/8871
> <https://github.com/apple/swift/pull/8871>.
>
>
> At this point Slava pointed out I was chasing down a lot of issues when
> there's a much simpler solution for Swift 4: when importing types, always use
> the Swift 4 name, and use typealiases to handle Swift 3 compatibility. This
> defines both of the previous issues away, as well as any more that I just
> haven't thought of yet.
>
> There are some downsides:
> - We currently keep people from using Swift 4 names in Swift 3 code, and we
> wouldn't be able to do that, since the actual declaration of the type always
> needs to be available.
I don’t know if this is an important distinction to worry about. That code will
still be able to use features from Swift 4, and perhaps even Swift 4 only types
(e.g. Substring from SE-0163).
> - We'd probably want to tweak the "aka" printing in diagnostics to not look
> through these typealiases. That's not hard, though.
> - We can't keep doing this once we have ABI stability. Hopefully framework
> owners aren't going to continue changing Swift names of types, but we'll
> probably need to implement my "C name in the mangling" plan anyway, just in
> case.
>
Would this fall under the realm of library evolution, wherein name changes
should be versioned? In that case, would we need both symbols whether they came
from C or not?
> What do people think?
>
> Jordan
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