Swift 2.2: 7.3.1 (7D1014)
Swift 3.0: Xcode 8.0 beta 6 (8S201h), master ae69ffb2
When adding a static stored property to an Objective-C classes in an extension, the compiler fails with the error message A declaration cannot be both 'final' and 'dynamic' which is not associated with any source location.
If TestClass does not inherit from NSObject (and is hence not @objc) the code compiles fine.
If you write class let x = 5 instead of static let x = 5, the compiler fails with the error message Class stored properties not supported in classes; did you mean 'static'? suggesting the static keyword.
If adding static stored properties to @objc classes is not supported in extensions, we should produce a diagnostic that clearly points out the issue (including a source location).