From the Go language spec:

"For signed integers, the operations +, -, *, and << may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. No exception is raised as a result of overflow. A compiler may not optimize code under the assumption that overflow does not occur. For instance, it may not assume that x < x + 1 is always true."

  func main() {
    var foo int8 = 127
    fmt.Println(foo + 1)
  }

prints -128