Looks like that's too complicated. This is what godbolt.org shows the output at (if you add the missing #include <utility> and pick "trunk" for gcc):

    <source>: In function 'void test(foo*)':
    <source>:16:16: error: 'int foo::m_x' is private within this context
           if (ptr->m_x >= 3)
                    ^~~ 
    <source>:9:11: note: declared private here
           int m_x;
               ^~~
    Compiler returned: 1