add tests for member functions

[JeffGarland]

This should work -- we should have unit tests for this.

https://godbolt.org/z/dfEfeWWe4

#include <functional>
#include <iostream>
 
struct Foo
{
    Foo(int num) : num_(num) {}
    void print_add(int i) const { std::cout << num_ + i << '\n'; }
    int num_;
};


int main()
{
  
  // store a call to a member function
    std::copyable_function<void(const Foo&, int)> f_add_display = &Foo::print_add;
    const Foo foo(314159);
    f_add_display(foo, 1);
    f_add_display(314159, 1);
}