Is this a conforming implementation of duration_cast?

Sadly VS2012's duration_cast is broken, and I actually need the functionality which is broken. So, I wrote my own:

template
ToUnit duration_cast_2(const std::chrono::duration& right)
{
    typedef std::ratio_divide::type ratio;
    typedef std::common_type::type, intmax_t>::type common_type;
    return ToUnit(static_cast(static_cast(right.count()) * static_cast(ratio::num) / static_cast(ratio::den)));
}

But I'm not entirely confident that it's correct.

I can't say for sure about the internal logic but I can point some problems or things that could be improved in your piece of code:

-
First of all, typename is missing before the names, making your code non-working with some compilers. I would also use using instead of typedef, but that's a mere matter of taste. Updated version:

using ratio = typename std::ratio_divide::type;
using common_type = std::common_type::type, intmax_t>::type;

-
std::common_type is variadic. You don't have to bother with nested std::common_types:

using common_type = typename std::common_type::type;

-
I know that MSVC has not a full support for constexpr, but duration_cast is supposed to be constexpr (it should work with the November 2013 CTP though):

template
constexpr ToUnit duration_cast_2(const std::chrono::duration& right)
{
    // ...
}

StackExchange Code Review Q#22836, answer score: 5