Error with overloaded operators in the Quantity class

[markusrunkel]

When using a specification of the Quantity class, the wrong operator is used for multiplication and division of two such specifications.
Instead of the operator which calculates two quantities, the operator which uses only one quantity and one template parameter is used.

Instead of

template <typename DX, typename DY, typename X, typename Y>
friend constexpr detail::Product<DX, DY, X, Y>
operator*( quantity<DX, X> const & lhs, quantity< DY, Y > const & rhs );

template <typename DX, typename DY, typename X, typename Y>
friend constexpr detail::Quotient<DX, DY, X, Y>
operator/( quantity<DX, X> const & x, quantity< DY, Y > const & y );

the following operators are used, e.g.:

template <typename D, typename X, typename Y>
friend constexpr quantity<D, detail::PromoteMul<X,Y>>
operator*( quantity<D, X> const & x, const Y & y );

template <typename D, typename X, typename Y>
friend constexpr quantity<D, detail::PromoteMul<X,Y>>
operator/( quantity<D, X> const & x, const Y & y );

Proposed sloution

In the overloaded binary and assignment operators, which are handling a typename and a quantity, a fourth template parameter was added.
The fourth template parameter enables the function only if the typename is convertible to the underlying type of the quantity.

For example:

template< typename D, typename X, typename Y, typename = std::enable_if_t<std::is_convertible<Y,X>::value>>
constexpr quantity<D, X> &
operator/=( quantity<D, X> & x, const Y & y )

[martinmoene]

@markusrunkel thanks for your report. I hope to look at it further in a few days.

[martinmoene]

@markusrunkel can you give a minimal compilable example that shows what you describe?

[markusrunkel]

@martinmoene thank you for answer.

#include <phys/units/quantity.hpp>

template<typename T>
struct Specification: public T 
{
  Specification(const T& aValue): T(aValue) {}; 
};

using Length = Specification<phys::units::quantity<phys::units::length_d>>;

int main () {
  auto rectangleLength = Length{2 * phys::units::meter};
  auto rectangleWidth = Length{3 * phys::units::meter};

  auto area = rectangleLength * rectangleWidth;
}

The problem I have is in the multiplication in the main function.

[martinmoene]

@markusrunkel ok, thanks.