>Swift 3 -> 4 code is compatible

Is this really true? Looking through 4.0 release notes I see some incompatible changes like:

  SE-0164 Remove final support in protocol extensions
  SE-0160 Limiting @objc inference

If your code compiled with the old version and doesn't with the new one, then those versions are incompatible.

The compiler has a -swift-version flag which gates the source-incompatible changes. There were a number of changes between -swift-version 3 and -swift-version 4, including the two you highlighted. The delta between -swift-version 4 and 4.2 released this year was much smaller.

The real issue I encounter isn't Swift itself, it's the constant Swift-specific SDK changes... When the `NSNotification.Name` Swift APIs change every year, it means migration is still painful.

It's not the Great Renaming, and it might not be fair to the Swift project itself, but unless I can compile a project using the 10.13 and 10.14 SDKs without source changes (which you can in Objective-C), the impression is that Swift doesn't have source compatibility.

As I'm sure you're aware, these are changes in AppKit, not Swift. AFAIK the Swift project has no control over how this API is exposed.

The Swift compiler does a lot of automatic "rewrites" of (Objective) C headers to be more Swifty, so it does have some measure of control. That said, the largest changes are guided by annotations associated with the headers.

I'm not disputing that. What I am saying is that, until the Swift representations of the Apple platform SDKs are stable between releases (like it is in Objective-C), then the majority of Swift codebases will not be source compatible the way Objective-C codebases are.

They're both ran by Apple, so that's a distinction without a difference.

Unfortunately, this isn't how Apple works. Both teams are more or less separate in this case. Sure, the Swift team can certainly send information over to the AppKit team, but they don't have the ability to make any decisions on naming conventions.

That is compiler compatibility. I am talking about language-level compatibility.

Yes, you can continue compiling v3 code with a v4 compiler with a flag, but if you compile your v3 code without a flag, your code may fail to compile. So while there may not be a compiler breakage, there is a language breakage between v3 and v4, and apparently there will be another one between v4 and v5.

That is right, but as I wrote in another reply here the changes from 4 to 5 are miniscule, mostly just fixing corner cases where the old behavior was not correct.

it is worth noting that swift 3 support is being removed from swift 5.

See https://docs.swift.org/swift-book/GuidedTour/Compatibility.h...

> When the Swift 4.2 compiler is working with Swift 3 code, it identifies its language version as 3.4. As a result, you can use conditional compilation blocks like #if swift(>=3.4) to write code that’s compatible with multiple versions of the Swift compiler.