C++ Kahan summation utility class

KahanSum.h

// TODO concepts/constraints/SFINAE to keep T as FP
// TODO make this more robust than the very basics

template <class T>
class KahanSum {
 public:
  KahanSum() : sum_{}, err_{} {}
  KahanSum(T init) : sum_{init}, err_{} {}

  constexpr KahanSum& Add(T val) {
    T y = val - err_;
    T t = sum_ + y;
    err_ = (t - sum_) - y;
    sum_ = t;
    return *this;
  }

  constexpr KahanSum& Sub(T val) {
    return Add(-val);
  }

  constexpr T Get() const {
    return sum_;
  }

  constexpr KahanSum& operator=(T val) {
    sum_ = val;
    err_ = 0;
    return *this;
  }

  constexpr KahanSum& operator+=(T val) {
    return Add(val);
  }

  constexpr KahanSum& operator-=(T val) {
    return Sub(val);
  }

  constexpr operator T() const {
    return Get();
  }

 private:
  T sum_;
  T err_;
};