package main

import (
	"errors"
	"io"
)

type CustomWrapper struct {
	Err error
}

func (c CustomWrapper) Error() string {
	return "error"
}

func (c CustomWrapper) Unwrap() error {
	return c.Err
}

func main() {
	var customWrapper = new(CustomWrapper)
	customWrapper.Err = customWrapper
	if errors.Is(customWrapper, io.EOF) {
		panic("eof")
	}
}

func is(err, target error, targetComparable bool) bool {
	const maxDepth = 1_000_000
	for range maxDepth {
		if targetComparable && err == target {
			return true
		}
		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
			return true
		}
		switch x := err.(type) {
		case interface{ Unwrap() error }:
			err = x.Unwrap()
			if err == nil {
				return false
			}
		case interface{ Unwrap() []error }:
			for _, err := range x.Unwrap() {
				if is(err, target, targetComparable) {
					return true
				}
			}
			return false
		default:
			return false
		}
	}
	return false
}