Why not just always use the second one?

You can have Error return just “permission not allowed” if the array is empty. It would print the same as the first.

@lyse@lyse.isobeef.org do you need to have an explicit Is function? I believe errors.Is has reflect lite and can do the type infer for you. The Is is only really needed if you have a dynamic type. Or are matching a set of types as a single error maybe? The only required one would be Unwrap if your error contained some other base type so that Is/As can reach them in the stack.

As is perfect for your array type because it asserts the matching type out the wrap stack and populates the type for evaluating its contents.

So you would have:

type ErrPermissionNotAllowed []Permission
func (perms ErrPermissionNotAllowed) Is(permission Permission) bool {
    for _, p := range perms {
        if p == permission { return true }
    }
    return false
}
var err error = errPermissionNotAllowed{"is-noob"}

if errors.Is(err, ErrPermissionNotAllowed{}) { ... } // user is not allowed

var e ErrPermissionNotAllowed
if errors.As(err, e) && e.Is("a-noob") { ... } // user is not allowed because they are a noob. 

I suppose to lesson confusion I would rename Is to Because