Method set and nil interface

methodset.go

// https://stackoverflow.com/questions/41922181/go-why-implicit-non-pointer-methods-not-satisfy-interface
// https://golang.org/doc/faq#different_method_sets
// https://golang.org/doc/faq#nil_error
package main

import "fmt"

type user struct {
	id   int
	name string
}

func (u *user) String() string {
	return fmt.Sprint("ID: ", u.id, ", Name: ", u.name)
}

func main() {
	var x = user{name: "John", id: 1}
	var y = &user{name: "Jim", id: 2}

	//var xs fmt.Stringer = x
	var ys fmt.Stringer = y
	var zs fmt.Stringer
	var ws fmt.Stringer = (*user)(nil)

	if ws != nil {
		fmt.Println(x)          // {1 John}
		fmt.Println(x.String()) // ID: 1 / Name: John
		fmt.Println(y)          // ID: 2 / Name: Jim
		fmt.Println(ys)         // ID: 2 / Name: Jim
		fmt.Println(zs)         // <nil>
		fmt.Println(ws)         // <nil>
		_ = zs.String()         // panic
		_ = ws.String()         // panic in String()
	}
}