void in C++17 and earlier, but
size_t in C++20: source. Even if you get your program to compile now, it will likely start to break soon.
Second, it is generally not portable to take an address of a member function of a standard library class:
[member.functions]/2 For a non-virtual member function described in the C++ standard library, an implementation may declare a different set of member function signatures, provided that any call to the member function that would select an overload from the set of declarations described in this document behaves as if that overload were selected. [Note: For instance, an implementation may add parameters with default values, or replace a member function with default arguments with two or more member functions with equivalent behavior, or add additional signatures for a member function name. —end note]
So your whole design relies on implementation details.
What is the original problem you are trying to solve, for which you want to use pointers-to-member? There may be other, portable ways to solve it.