How about the following:

  contract Addable(t T) {
  	t + t
  }

  func Sum.<T Addable>(x []T) T {
	var total T
	for _, v := range x {
		total += v
	}
	return total
  }

It has the nice property that it's somewhat analogous to type conversions, the grammar should be unambiguous, and it's much less visually confusing than re-using parentheses IMO.