20.8.2 [func.require] p1 says:

1 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 t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T;
• ((*t1).*f)(t2, ..., tN) when f is a pointer to a member function of a class T and t1 is not one of the types described in the previous item;
• t1.*f when f is a pointer to member data of a class T and t1 is an object of type T or a reference to an object of type T or a reference to an object of a type derived from T;
• (*t1).*f when f is a pointer to member data of a class T and t1 is not one of the types described in the previous item;
• f(t1, t2, ..., tN) in all other cases.

The question is: What happens in the 3^rd and 4^th bullets when N > 1?

Does the presence of t2, ..., tN get ignored, or does it make the INVOKE ill formed?

Here is sample code which presents the problem in a concrete example:

#include <functional>
#include <cassert>

struct S {
   char data;
};

typedef char S::*PMD;

int main()
{
   S s;
   PMD pmd = &S::data;
   std::reference_wrapper<PMD> r(pmd);
   r(s, 3.0) = 'a';  // well formed?
   assert(s.data == 'a');
}

Without the "3.0" the example is well formed.

[Note: Daniel provided wording to make it explicit that the above example is ill-formed. — end note ]