#include <utility>
#include <memory>
#include <cstdio>

template<auto F> struct caller {
    template<class... Ts>
    auto operator()(Ts&&... ts) const
    noexcept(noexcept(F(std::forward<Ts>(ts)...)))
    {
        return F(std::forward<Ts>(ts)...);
    }
};

int main()
{
    using fileptr = std::unique_ptr<std::FILE, caller<std::fclose>>;
    static_assert(sizeof(fileptr) == sizeof(FILE*));
    fileptr ptr(std::fopen("xyz.txt", "w"));
}