I have created a custom locale facet by specializing std::ctype for int32_t, to accommodate my std::basic_string<int32_t>.

namespace std
{
    template <>
    class ctype<std::int32_t> : public std::ctype_base
    {
    public:
        static std::locale::id id;
        // other members
    };
}

However, I find that this facet gets ignored completely by std::locale::ctype when reusing facets from another locale:

using my_ctype = std::ctype<int32_t>;
std::locale custom_loc(std::locale::classic(), new my_ctype());
bool a = std::has_facet<my_ctype>(custom_loc); // true

std::locale with_original_ctype(std::locale(""), custom_loc, std::locale::ctype);
bool b = std::has_facet<my_ctype>(with_original_ctype); // false

std::locale everything_replaced(custom_loc, std::locale(""), std::locale::all);
bool c = std::has_facet<my_ctype>(everything_replaced); // true

At least on MSVC, even though with_original_ctype should reuse all std::locale::ctype facets from custom_loc, it is missing my_ctype. Worse even, everything_replaced, despite being constructed with std::locale::all taken from std::locale(""), still retains the custom facet.

How do I make the facet be correctly interpreted as being in the category std::locale::ctype?