(I already sent this to swift-evolution yesterday but cross-posting.)

One of the Stage 1 goals of Swift 4 is source compatibility with Swift 3, but 
what does that precisely mean and how will we determine if the goal is 
achieved?  This is the current proposal of how this goal ties back to the Swift 
3.x and Swift 4 releases.  Is there anything else that’s missing or should be 

The source compatibility goal for Swift 4 means that sources that previously 
compiled with the Swift 3 compiler will continue to compile with no source 
modifications with the Swift 3.x and Swift 4 compilers.  

Swift 3.x releases have the goal of source compatibility with Swift 3.  This is 
the default mode and there is no compatibility flag.  
This means that Swift 3 code should be able to build with the Swift 3.x 
compiler without modification as much as practically possible. The same applies 
to SwiftPM packages with Swift 3 source code.  
Swift 3.x code will not necessarily compile with the Swift 3 compiler.  For 
example, new language features could be added.

The Swift 4 compiler will include a language compatibility flag -swift-version 
that accepts 3 or 4 as arguments and controls the compatibility mode (SR- 2582 
The -swift-version 3 compatibility mode has the goal of source compatibility as 
much as practically possible with Swift 3.x (including 3, 3.0.x, 3.x). 
The -swift-version 4 compatibility allows approved source code breakages from 
Swift 3.x (including 3, 3.0.x, 3.x).  Proposed source breaking changes should 
be discussed on swift-evolution and highlighted in evolution proposals.
All the sources in a module (e.g,  SwiftPM packages, targets in Xcode) will be 
built in the same compatibility mode, but modules built with different 
compatibility modes can be linked into a single binary and will interoperate. 
Thus a module can be migrated from Swift 3 to 3.x to 4 independently of its 
The Swift Package Manager will allow package authors to specify a list of 
compatibility modes.
The Swift compiler already provides a mechanism for conditional compilation 
based on the compiler version with the #if swift syntax. SE-0020 
  Swift 4 will also provide mechanisms to conditionalize code based on the 
compatibility version. 
The @available(...) attribute has been expanded to support the swift language 
version with SE-0141 
 For example, @available(swift 3).

Maintaining source compatibility with Swift 3 in practice while continuing 
development is not always clear-cut.  It often comes down to tradeoffs.  Let’s 
review a few cases.
New features may become available if they do not take up syntactic space that 
might be commonly used by existing Swift code. 
If a bug fix breaks source in a fringe case as determined by a scan of existing 
source, the benefit might outweigh the cost and it could be allowed.
Changes to existing APIs have high bar to clear as they should provide a 
significant benefit to clients and have minimal impact on existing, real-life 
code.  These changes should be considered for inclusion in the -swift-version 4 
compatibility mode of the Swift 4 compiler instead.
New warnings are allowed because they do not require source code changes.
To summarize, any proposed source breakage from Swift 3 for the Swift 3.x 
compilers or the -swift-version 3 compatibility mode of the Swift 4 compiler, 
should be small in scope of code affected and weighed against the goals of 
source compatibility described above. It should be approved on swift-evolution. 

For Apple Platforms, source compatibility can also be affected by changes to 
the Apple SDKs.  Last year, Swift 2.3 required a migration from Swift 2.2 to 
account for changes in the SDKs even though no language changes were 
introduced.  This year, source compatibility is defined more strictly to 
require that the Apple SDKs only add source compatible changes when used with 
the Swift 3.x compilers or when used with the Swift 3 compatibility mode 
(-swift-version 3) in the Swift 4 compiler.  The  ‘API notes’ feature has been 
augmented and can now express how an API is imported for a specific Swift 
version.  This feature allows the view of the evolved Apple SDKs to stay the 
same as with Swift 3 when Swift 3 source compatibility is expected.

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