kludge to convert a cpp-style closure into a c pointer, retains data in an external structure

clos.cpp

namespace {

template <typename F, int I, typename L, typename R, typename ...A>
inline F convert(L&& l, R (*)(A...) noexcept(noexcept(::std::declval<F>()(::std::declval<A>()...)))) {
    static thread_local L l_(::std::forward<L>(l));
    static thread_local bool full;
    if (full) {
        l_.~L();
        new (static_cast<void*>(&l_)) L(::std::forward<L>(l));
    } else
        full = true;

    struct S {
        static R f(A... args) noexcept(noexcept(::std::declval<F>()(::std::forward<A>(args)...))) {
            return l_(::std::forward<A>(args)...);
        }
    };
    
    return &S::f;
}

}

template <typename F, int I = 0, typename L>
F convert(L&& l) {
    return convert<F, I>(::std::forward<L>(l), F());
}