Define INVOKE(f, t1, t2, ..., tN) as follows:
(t1.*f)(t2, ..., tN) when f is a pointer to a member function of a class T and is_base_of_v<T, decay_t<decltype(t1)>> is true;
(t1.get().*f)(t2, ..., tN) when f is a pointer to a member function of a class T and decay_t<decltype(t1)> is a specialization of reference_wrapper;
((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 does not satisfy the previous two items;
t1.*f when N == 1 and f is a pointer to data member of a class T and is_base_of_v<T, decay_t<decltype(t1)>> is true;
t1.get().*f when N == 1 and f is a pointer to data member of a class T and decay_t<decltype(t1)> is a specialization of reference_wrapper;
(*t1).*f when N == 1 and f is a pointer to data member of a class T and t1 does not satisfy the previous two items;
f(t1, t2, ..., tN) in all other cases.
Define INVOKE<R>(f, t1, t2, ..., tN) as static_cast<void>(INVOKE(f, t1, t2, ..., tN)) if R is cv void, otherwise INVOKE(f, t1, t2, ..., tN) implicitly converted to R.
Every call wrapper shall be MoveConstructible. A forwarding call wrapper is a call wrapper that can be called with an arbitrary argument list and delivers the arguments to the wrapped callable object as references. This forwarding step shall ensure that rvalue arguments are delivered as rvalue references and lvalue arguments are delivered as lvalue references. A simple call wrapper is a forwarding call wrapper that is CopyConstructible and CopyAssignable and whose copy constructor, move constructor, and assignment operator do not throw exceptions. [ Note: In a typical implementation forwarding call wrappers have an overloaded function call operator of the form
template<class... UnBoundArgs>
R operator()(UnBoundArgs&&... unbound_args) cv-qual;— end note ]