As with fmt.Stringer, the fmt package looks for the error interface when printing values. In case if a type satisfies both interfaces, fmt only on look for error not Stringer. Why? How I could implement both Stringer and error on a type?
Not really a universal solution, but in certain situations one can use pointer vs non-pointer receivers to do the trick: https://play.golang.org/p/DY4Cq4efsT