call qualified operator () of stateless global functor constructed on demand

global_functor_on_demand.cpp

#include <iostream>
#include <type_traits>
#include <utility>
 
#include <cstdlib>
 
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wglobal-constructors"
template< typename F >
F callee = {};
template< typename F >
auto call = [] (auto &&... args) -> decltype(auto) { return std::forward< F >(callee< std::decay_t< F > >)(std::forward< decltype(args) >(args)...); };
#pragma clang diagnostic pop
 
#define PP { std::cout << ++i << ' ' << __PRETTY_FUNCTION__ << std::endl; }
 
struct L
{
    mutable int i = 0;
    void operator () () const & { PP }
    void operator () () & { PP }
    void operator () () const && { PP }
    void operator () () && { PP }
    void operator () () volatile const & { PP }
    void operator () () volatile & { PP }
    void operator () () volatile const && { PP }
    void operator () () volatile && { PP }
};
 
struct M
{
    mutable int i = 0;
    void operator () () const { PP }
    void operator () () { PP }
    void operator () () volatile const { PP }
    void operator () () volatile { PP }
};
 
int
main()
{
    { auto const & c = call< L >; c(); }
    { auto const & c = call< L & >; c(); }
    { auto const & c = call< L && >; c(); }
    { auto const & c = call< L const >; c(); }
    { auto const & c = call< L const & >; c(); }
    { auto const & c = call< L const && >; c(); }
    { auto const & c = call< volatile L const >; c(); }
    { auto const & c = call< volatile L const & >; c(); }
    { auto const & c = call< volatile L const && >; c(); }
    std::cout << std::endl;
    { auto const & c = call< M >; c(); }
    { auto const & c = call< M & >; c(); }
    { auto const & c = call< M && >; c(); }
    { auto const & c = call< M const >; c(); }
    { auto const & c = call< M const & >; c(); }
    { auto const & c = call< M const && >; c(); }
    { auto const & c = call< volatile M const >; c(); }
    { auto const & c = call< volatile M const & >; c(); }
    { auto const & c = call< volatile M const && >; c(); }
    return EXIT_SUCCESS;
}