Method exp_upper

include/fmt/format.h:2494–2498

// Numbers with exponents greater or equal to the returned value will use
// the exponential notation.
template <typename T> FMT_CONSTEVAL auto exp_upper() -> int {
  return std::numeric_limits<T>::digits10 != 0
             ? min_of(16, std::numeric_limits<T>::digits10 + 1)
             : 16;
}

// Use the fixed notation if the exponent is in [-4, exp_upper),
// e.g. 0.0001 instead of 1e-04. Otherwise use the exponent notation.