swift234

SE-0020: Swift Language Version Build Configuration

Introduce a new "swift" build configuration that guards declarations
and statements with a language version - if the current language version
of the compiler is at least that version, the block will parse as normal.
For inactive blocks, the code will not be parsed an no diagnostics will
be emitted there.

Example:

    #if swift(>=2.2)
      print("Active")
    #else
      this code will not parse or emit diagnostics
    #endif

https://github.com/apple/swift-evolution/blob/master/proposals/0020-if-swift-version.md
rdar://problem/19823607