C++ type trait to see if `static_cast<uint32_t>(k)` can be called on any variable of type `K`

Question

I'm trying to write a type trait for detecting if, given a type K, I can call static_cast on variables of this type K (or reference to K) to uint32_t or not.

This is what I'm arriving at, but can't seem to make it work.

template <typename K, typename Whatever = void> struct ConvertibleToUint32 {
    static constexpr bool value = false;
};

template <typename K>
struct ConvertibleToUint32<K, decltype(static_cast<uint32_t>(std::declval<K>()))> {
    static constexpr bool value = true;
};

I have also tried using std::is_convertible, but doesn't seem to work here.

Here's a paste with a run - https://wandbox.org/permlink/iuP99PNAVIYhh208

The specialization doesn't seem to get hit.

Here's the full test program -

#include <iostream>
#include <stdint.h>

    template <typename K, typename Whatever = void> struct ConvertibleToUint32 {
        static constexpr bool value = false;
    };

    template <typename K>
    struct ConvertibleToUint32<K, decltype(static_cast<uint32_t>(std::declval<K>()))> {
        static constexpr bool value = true;
    };


struct A {
    explicit operator uint32_t() const { return 1; }
};

struct B {
};

int main()
{
    std::cout << ConvertibleToUint32<A>::value << "\n";
    std::cout << ConvertibleToUint32<B>::value << "\n";
    std::cout << std::is_convertible<A, uint32_t>::value << "\n";
}

Answer 1

It doesen't get hit because decltype(static_cast<uint32_t>(std::declval<K>())) is not void, so it's not a more specialized version of the primary template for the argument Whatever=void.

The fix is fairly easy

template <typename K>
struct ConvertibleToUint32<K, decltype(static_cast<uint32_t>(std::declval<K>()), void())> {
    static constexpr bool value = true;
};

The second argument matches the default, and will therefore be picked up in partial ordering. Alternatively, you can just specify a default of Whatever=uint32_t and get the same effect. void is just what the general idiom employs.

template <typename K, typename Whatever = uint32_t>
struct ConvertibleToUint32 {
    static constexpr bool value = false;
};

---

^{_{As an aside, I suggest you start favoring the cstdint header and its std::uint32_t alias. Same net effect usually, but the use of the C++ version is generally preferable.}}