// performance becomes an issue.

// BaseUnitsForType provides a list of typed unit values for each basis in the
// requested type.
// When T is real,
// BaseUnitsForType<T>::values() is just a single-entry vector [1]
// When T is complex,
// BaseUnitsForType<T>::values() is a two-entry vector [1, i] - the unit
// values in each of its two bases.
template <typename T>
struct BaseUnitsForType {};  // Specializations below