[review] revisit apply_member_pointer

> apply_member_pointer seems to be trying to do too much. I can get a
pointer to member data of any type, unless that data member is itself
a pointer to member or a pointer to function, in which case I get
something else entirely. Additionally, the description doesn't mention
that PMDs also get special handling.

from http://lists.boost.org/Archives/boost/2017/04/234173.php

Start by locating apply_member_pointer and read its implementation and documentation alongside the linked mailing-list discussion. Compare the reported results for ordinary member data, member pointers, function pointers, and PMDs, then define the intended scope and document or test the corrected behavior.

Written by the indexing model from the issue text.