Method double_double

test/format-impl-test.cc:284–285

  double b;

  constexpr explicit double_double(double a_val = 0, double b_val = 0)
      : a(a_val), b(b_val) {}

  operator double() const { return a + b; }
  auto operator-() const -> double_double { return double_double(-a, -b); }