为什么可以使用for循环两次声明相同的var？

I was trying Go on http://tour.golang.org/, and I saw that it was possible to declarate two times the same var using := in for loop. The output is the same with the Go compiler.

Here is my test : (see the var i, it was declared two times)

package main

import "fmt"

func main() {
    i := "Hello"
    a := 0
    for a < 2 {
        fmt.Println(i)
        i := "World !"
        fmt.Println(i)
        a++
    }       
}

Output :

Hello

World !

Hello

World !

Can someone explain that ?

The short variable declaration i := ... will overshadow the same variable declared outside of the scope of the for loop block.

Each "if", "for", and "switch" statement is considered to be in its own implicit block

You can see more at "Go gotcha #1: variable shadowing within inner scope due to use of := operator"

It refers to this goNuts discussion.

A short variable declaration can redeclare the same variable within a block, but since i is also declared outside the for block, it keeps its value outside said block (different scope).

The first i has been defined inside the braces ({}) of main function wheras the second i is declared within the scope of the for loop. The name is same but the scope is different.