[Bug c++/87409] Implement -Wunused-private-field

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87409

--- Comment #1 from Martin Liška <marxin at gcc dot gnu.org> ---
Example:

private field 'm_next' is not used: -Wunused-private-field