I’m trying to see if an object of type T can be constructed with an argument of type U.

You’d think std::is_constructible would be close enough (aside from the fact that it also checks destructability, which I would’ve ideally liked to avoid), but apparently this fails:

#include <type_traits>
using U = int;
enum T : U { };
static_assert(std::is_constructible<T, U>::value, "error");  // fails
static_assert(std::is_convertible<U, T>::value, "error");  // fails

I guess that makes sense, since T e((U())) fails, but that’s not what I’m looking for because auto v = T(U()) and auto v = static_cast<T>(U()) both succeed.

This also makes me wonder what other edge cases I may have missed… I have no idea.

What’s the right way to test solely constructability via the T(...) syntax, ideally using standard traits, with as few false-positives or false-negatives as possible?