[Bug c++/92735] unused member variable causes code to compile for member to function for undefined function

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92735

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Related to http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1396