Xcode 8 Objective-C category warning

I'm using Xcode 8 and Swift 3.0. What does this error message mean?

ld: warning: Some object files have incompatible Objective-C category definitions. Some category metadata may be lost. All files containing Objective-C categories should be built using the same compiler.

I also had this issue in a UIColor extension, my app is entirely made with swift except for some frameworks that use Objective-c so I have no problem in declaring the var as @nonobjc:

extension UIColor {
   @nonobjc static var lol: UIColor {
      return UIColor.red
   }
}

From the apple docs:

The nonobjc attribute tells the compiler to make the declaration unavailable in Objective-C code...

Since this code is unavailable to Objective-C the warning disappears.

In my case, the reason was having computed type property in an extension:

extension NSParagraphStyle {
    class var defaultStyle: NSParagraphStyle {
        return ...
    }
}

Not sure what the exact reason behind this is, but to get rid of the warning I had to convert the computed type property (class var) to a type method (class func):

extension NSParagraphStyle {
    class func defaultStyle() -> NSParagraphStyle {
        return ...
    }
}