package main

import (
	"fmt"

	"golang.org/x/xerrors"
)

type myErr struct {
	foo []string // uncomparable
}

func (myErr) Error() string {
	return "error"
}

func (myErr) Is(target error) bool {
	return true
}

func main() {
	if xerrors.Is(myErr{}, myErr{}) {
		fmt.Println("foo")
	}
	fmt.Println("bar")
}

panic: runtime error: comparing uncomparable type main.myErr

goroutine 1 [running]:
golang.org/x/xerrors.Is(0x4d1200, 0xc00000c080, 0x4d1200, 0xc00000c0a0, 0xc000032728)
	~/go/pkg/mod/golang.org/x/xerrors@v0.0.0-20190410155217-1f06c39b4373/wrap.go:55 +0x141
main.main()
	/tmp/mod/main.go:22 +0xb7