Update: I have made a pull request to make the message more accurate :)
Hello Swift Forums,
currently, as known @IBInspectable & @GKInspectable are only to be used as instance properties of class types, however, when incorrectly used as instance properties in another type, the error only instance properties can be declared @IBInspectable / @GKInspectable is blurted out
I believe the error message should be changed to reflect the attributes only being allowed to be used as class instance properties, say, only class instance properties can be declared @IBInspectable / @GKInspectable