Explicit construction of quantities using units that are not defined in system compiles but mathematically fails

[tsaubergine]

If I define a unit in a system that doesn't have this base dimension, e.g. angular_velocity in the degrees system:

quantity<unit<angular_velocity_dimension,degree::system>> angular_vel_deg(30*degree::degrees/si::seconds);

This compiles fine, and explicit conversions also compile fine:

quantity<unit<angular_velocity_dimension, si::system>> angular_vel_si(angular_vel_deg);

However when I go to run it, I get angular_vel_si as 30 rad/s rather than the expected 0.523 rad/s

The fact that this compiles seems to be a bug to me, but most importantly I'd like to be able to detect this situation (ideally with a static_assert) so that I don't have code that compiles but performs incorrect conversions.

With a few hours digging around I couldn't find a satisfactory way to do this, though I think it might be related to #36 but the proposed fix (#37) didn't fix my issue.

Here's my full minimal test case. Thanks in advance for any help you can provide:

#include "units/include/boost/units/systems/si.hpp"
#include "units/include/boost/units/systems/angle/degrees.hpp"
#include <cassert>
#include <iostream>
#include <cmath>


int main()
{
    double eps = std::numeric_limits<double>::epsilon();

    
    using boost::units::unit;
    using boost::units::quantity;
    namespace si = boost::units::si;
    namespace degree = boost::units::degree;

    // angle, works as expected
    {
        using boost::units::plane_angle_dimension;
        quantity<unit<plane_angle_dimension,degree::system>> angle_deg(30*degree::degrees);
        quantity<unit<plane_angle_dimension,si::system>> angle_si(angle_deg);

        std::cout << "Degrees: " << angle_deg.value() << ", radians: " << angle_si.value() << std::endl;

        assert(std::abs(angle_deg.value()-angle_si.value()) > eps);
    }
    
    // angular velocity: fails but more importantly, it compiles (when I feel that it shouldn't)
    {
        using boost::units::angular_velocity_dimension;

        quantity<unit<angular_velocity_dimension,degree::system>> angular_vel_deg(30*degree::degrees/si::seconds);
        quantity<unit<angular_velocity_dimension, si::system>> angular_vel_si(angular_vel_deg);

        std::cout << "Degrees/sec: " << angular_vel_deg.value() << ", radians/sec: " << angular_vel_si.value() << std::endl;
        
        assert(std::abs(angular_vel_deg.value()-angular_vel_si.value()) > eps);
    }
    
}

Output:

Degrees: 30, radians: 0.523599
Degrees/sec: 30, radians/sec: 30
a.out: test2.cpp:38: int main(): Assertion `std::abs(angular_vel_deg.value()-angular_vel_si.value()) > eps' failed.
zsh: IOT instruction (core dumped)  ./a.out

Expected (or compile failure would also be acceptable):

Degrees: 30, radians: 0.523599
Degrees/sec: 30, radians/sec: 0.523599