meh_narrowing.hpp

#include <type_traits>
#include <assert>

// debug = gsl::narrow, release = gsl::narrow_cast

namespace util
{

template <class T, class U>
constexpr T narrowcast(U&& u) noexcept
{
    static_assert(std::is_integral<T>::value, "T must be an integral type");
    static_assert(!std::is_same<T, bool>::value, "T must not be a bool");

    static_assert(std::is_integral<U>::value, "U must be an integral type");
    static_assert(!std::is_same<U, bool>::value, "U must not be a bool");

    assert(static_cast<U>(static_cast<T>(u)) != u);
    assert((!details::is_same_signedness<T, U>::value && ((static_cast<U>(static_cast<T>(u)) < T{}) != (u < U{}))));

    return static_cast<T>(std::forward(u));
}

}