Why a type assertion works in one case, not in another?

Here is the problematic source code:

http://play.golang.org/p/lcN4Osdkgs

package main
import(
	"net/url"
	"io"
	"strings"
) 

func main(){
	v := url.Values{"key": {"Value"}, "id": {"123"}}
	body := strings.NewReader(v.Encode())
	_ = proxy(body)
	// this work 

	//invalid type assertion: body.(io.ReadCloser) (non-interface type *strings.Reader on left)
	_, _ = body.(io.ReadCloser)

}

func proxy( body io.Reader) error{
	_, _ = body.(io.ReadCloser)
	return  nil
}

Can someone tell me why this code wont work ?

The error occur here:

body := strings.NewReader(v.Encode())

rc, ok := body.(io.ReadCloser)

// invalid type assertion: body.(io.ReadCloser) (non-interface type *strings.Reader on left)

However proxy(body io.Reader) do the same thing but have no error. Why?

http://play.golang.org/p/CWd-zMlrAZ

You are dealing with two different Reader:

• strings.NewReader() returns a strings.Reader, which isn't an interface.
  But since it implements io.Reader, you can pass it to proxy().
• proxy() takes an io.Reader, which is an interface.

For non-interface types, the dynamic type is always the static type.
A type assertion works for interfaces only, which can have arbitrary underlying type.
(see my answer on interface, and "Go: Named type assertions and conversions")