gistfile1.txt

package main

import (
	"fmt"
	"math/rand"
	"reflect"
	"time"
	"unsafe"
)

type I1 interface {
	I1()
}

type I2 interface {
	I1()
	I2()
}

type I3 interface {
	I1()
	I3()
}

type si2 struct {
}

func (s si2) I1() {
	//TODO implement me
	panic("implement me")
}

func (s si2) I2() {
	//TODO implement me
	panic("implement me")
}

type si3 struct {
}

func (s si3) I1() {
	//TODO implement me
	panic("implement me")
}

func (s si3) I3() {
	//TODO implement me
	panic("implement me")
}

func i2() I2 {
	return &si2{}
}
func i3() *si3 {
	return nil
}
func i33() I3 {
	return nil
}

func m(a int) I1 {
	switch a {
	case 0:
		return i2()
	case 1:
		return i3()
	case 2:
		return i33()
	}
	return i2()
}

type iface struct {
	typ int
	ptr int
}

func main() {
	rand.Seed(time.Now().Unix())
	x := m(0)
	fmt.Println(x == nil)
	switch x.(type) {
	case I2:
		fmt.Println("I2", x)
	case I3:
		fmt.Println("I3", x)
	case I1:
		fmt.Println("I1", x)
	default:
		fmt.Println("NO")
	}

	x2 := m(1)

	fmt.Println(x2 == nil)
	switch x2.(type) {
	case I2:
		fmt.Println("I2", x2)
	case I3:
		fmt.Println("I3", x2)
		hack := (*iface)(unsafe.Pointer(&x2))
		fmt.Printf("PTR %08x\n", hack.ptr)
		fmt.Printf("TYPE  %08x\n", hack.typ)
	case I1:
		fmt.Println("I1", x2)
	default:
		fmt.Println("NO", reflect.TypeOf(x2))
	}

	x2 = m(2)

	fmt.Println(x2 == nil)
	switch x2.(type) {
	case I2:
		fmt.Println("I2", x2)
	case I3:
		fmt.Println("I3", x2)
		hack := (*iface)(unsafe.Pointer(&x2))
		fmt.Printf("PTR %08x\n", hack.ptr)
		fmt.Printf("TYPE  %08x\n", hack.typ)
	case I1:
		fmt.Println("I1", x2)
	default:
		fmt.Println("NO", reflect.TypeOf(x2))
	}
}

gistfile2.txt

false
I2 &{}
false
I3 <nil>
PTR 00000000
TYPE  004bb5f8
true
NO <nil>