The problem is that the first-level error in most Go functions is fmt.Errorf(some error message, some args). The idea of pre-allocating an exported global error

The problem is that the first-level error in most Go functions is fmt.Errorf(some error message, some args). The idea of pre-allocating an exported global error object (like io.EOF) is very rarely useful, and error structs are very rarely used either, because they require much more ceremony than just returning a fmt.Errorf() (or even an errors.New()) on the spot. But errors.Is is only useful if you take the first option, and errors.As is only useful if you take the second option. So, in practice, neither errors.Is nor errors.As are terribly useful, and %w just gives you a false sense of usefulness.