False positive if err.(type) switch checks Unwrap interfaces

[Brian-Williams]

Bug

The lint "type switch on error will fail on wrapped errors" triggers even when you check the Unwrap interfaces.

Error message

Error: example.go:27:14: type switch on error will fail on wrapped errors. Use errors.As to check for specific errors (errorlint)
switch x := err.(type) {

Short example

func ParseAllErr(err error) {
	switch x := err.(type) {
	case interface{ Unwrap() error }:
		fmt.Printf("single unwrap err: %s\n", err)
	case interface{ Unwrap() []error }:
		for _, e := range x.Unwrap() {
			if e != nil {
				fmt.Printf("multi-error found: %s\n", e)
				ParseAllErr(e)
			}
		}
	default:
		// Specific handling for non wrapping errors
		fmt.Printf("error without Unwrap: %s\n", err)
	}
}

func main() {
	err := errors.Join(
		SingleError{Err: errors.New("one")},
		MultiError{Err: errors.New("two"), Err2: errors.New("three")},
	)
	ParseAllErr(err)
}

Runnable example

More fleshed out example in playground: https://go.dev/play/p/X7xE6VILmY9

package main

import (
	"errors"
	"fmt"
)

type SingleError struct {
	Err error
}
type MultiError struct {
	Err  error
	Err2 error
}

func (s SingleError) Error() string {
	return s.Err.Error()
}
func (s SingleError) Unwrap() error {
	return s.Err
}
func (m MultiError) Error() string {
	return fmt.Sprintf("%v, %v", m.Err, m.Err2)
}
func (m MultiError) Unwrap() []error {
	return []error{m.Err, m.Err2}
}

func ParseAllErr(err error) {
	switch x := err.(type) {
	case interface{ Unwrap() error }:
		fmt.Printf("single unwrap err: %s\n", err)
	case interface{ Unwrap() []error }:
		for _, e := range x.Unwrap() {
			if e != nil {
				fmt.Printf("multi-error found: %s\n", e)
				ParseAllErr(e)
			}
		}
	default:
		// Specific handling for non wrapping errors
		fmt.Printf("error without Unwrap: %s\n", err)
	}
}

func main() {
	err := errors.Join(
		SingleError{Err: errors.New("one")},
		MultiError{Err: errors.New("two"), Err2: errors.New("three")},
		MultiError{
			Err: errors.New("four"),
			Err2: MultiError{
				Err:  errors.New("five"),
				Err2: errors.New("six"),
			},
		},
	)
	ParseAllErr(err)

}

Workaround

You can use errors.As with the interfaces to get around the linter in some cases.

	var unwrappable interface{ Unwrap() error }
	if errors.As(err, &unwrappable) { // handling }

	var multiUnwrappable interface{ Unwrap() []error }
	if errors.As(err, &multiUnwrappable) { //handling }

If we were to create a breadth first version of errors.As then we would need to write code like the examples that trigger the lint.