Hi,

I have two (or more) default (or user defined) system units. For certain operations I assume that one quantity in one system can be converted into another quantity in some other system with the same dimension. What conversions are allowed depend on the included headers at best.

What is the best way to check at compile time that one quantity can be converted into another?

For example, I have this function that sums two quantities with the same dimension and returns the result of the left term. The function assumes that qright is convertible to qleft. If it is not convertible I would like to catch the error early with static_assert (or even with some enable_if to ignore the function)

    template<class Dimension, class LeftSystem, class RightSystem>
    quantity< unit<Dimension, LeftSystem> > sum_to_left(
        quantity< unit<Dimension, LeftSystem> > const& qleft,
        quantity< unit<Dimension, RightSystem> > const& qright
    ){
        return boost::units::operator + (qleft, quantity< unit<Dimension, LeftSystem> >(qright));
    }

Thank you,
Alfredo