golang multiple case in type switch

when I run the code snippet bellow, it raise a error

> a.test undefined (type interface {} is interface with no methods)

It seem the type switch does not take effect.

package main

import (
	"fmt"
)

type A struct {
	a int
}

func(this *A) test(){
	fmt.Println(this)
}

type B struct {
	A
}

func main() {
	var foo interface{}
	foo = A{}
	switch a := foo.(type){
		case B, A:
			a.test()
	}
}

If I change it to

    switch a := foo.(type){
		case A:
			a.test()
	}

it's ok now.

This is normal behaviour that is defined by the spec (emphasis mine):

> The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the beginning of the implicit block in each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard.

So, in fact, the type switch does take effect, but the variable a keeps the type interface{}.

One way you could get around this is to assert that foo has the method test(), which would look something like this:

package main

import (
	"fmt"
)

type A struct {
	a int
}

func (this *A) test() {
	fmt.Println(this)
}

type B struct {
	A
}

type tester interface {
	test()
}

func main() {
	var foo interface{}
	foo = &B{}
	if a, ok := foo.(tester); ok {
		fmt.Println("foo has test() method")
		a.test()
	}
}