title: golang overflow int64 with operating directly but not with assigned a value beforehand?
body: <pre><code>func main() {
     var a = math.MaxInt64
     fmt.Println(a + 1)             //-9223372036854775808
     fmt.Println(math.MaxInt64 + 1) //constant 9223372036854775808 overflows int
  }
  
  why the two ways perform differently?
  </code></pre>