This sample project is meant to demonstrate a linker issue we're seeing on Alamofire for the URLSessionTaskMetrics APIs on watchOS 3.0+.
Compile the Alamofire watchOS scheme in the sample app to produce the linker error.
- watchOS 4.2
 - Xcode 9.2 (9C40b)
 
Unsure on this one. By simply defining a global property with an @available check, the linker succeeds.
This issue has been filed as rdar://36082113 on 2017-12-15 and can be found on Open Radar.
Currently still open.
