As I said, errors.Is works with ErrValue and errors.As works with ErrType. I guess the word "Is" is doing too much work here, because I wouldn't expect ErrType{Code:1} and ErrType{Code:0} to match under errors.Is, though ErrType{Code:1} and ErrType{Code:1} would, but yes you could implement an Is method to override that default behavior.