For questions about Go, see https://golang.org/wiki/Questions. Or just read the language spec, which goes into detail to explain how this works and why.

For future questions, please show a small self-contained program that demonstrates what you are asking. Go does not have any concept like "sub type pointers" so I'm not sure what you mean by that.

type Interface interface {
  Get()
}

type A struct {
}

func (this *A) Get() {
}

var a Interface
var b = a.(*A)     //this will panic

do you know what i mean?
@iamoryanmoshe

I'm sorry, I do not know what you mean. I agree that with the code you show the initialization of b will panic, because the value in a is not a *A. It is nil. On the other hand, you could write

var a Interface = &A{}
var b = a.(*A)

and then the code would not panic.

@ianlancetaylor
i know how to write some code will not panic. i'm considering, why not allow a nil interface convert to a nil pointer?

Because a nil interface has no type. It is not the same as an interface that holds a typed nil pointer. For example, these lines will also not panic:

var a Interface = (*A)(nil)
var b = a.(*A)

See also https://golang.org/doc/faq#nil_error for a related issue.

@ianlancetaylor
i know a nil interface has no type. but when it could convert to *T, it's just a nil *T, nil can do nothing.

for example, if a nil interface cast to a pointer will not panic:

var a Interface
var b = a.(*A)     //then b is nil

but right now , i must write some code like:

var a Interface 
var b, ok = a.(*A)

switch a.(type) {
case *A:
  b = a.(*A)
default:
  b = nil
}

if a == nil {
  b = nil
} else {
  b = a.(*A)
}

the codes is short and beautiful.

@ianlancetaylor

i wrote this sample.

three ways to cast a nil interface to a nil pointer, none is simple.

As said before, please take this question to the forums. This thread is going in circles and there's no issue here, so I'm going to lock it for now.