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

[redi at gcc dot gnu.org]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-09-24
     Ever confirmed|0                           |1

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, it can only warn when there are no member functions that aren't defined in
the current TU. Even then, it might need additional work to check at the end of
a TU if there are non-inline function definitions:

struct A {
  A();
  void f();
private:
  int i = 0;
};

A::A() { }

void A::f() { }

Here we'd need to do the check at the end of the TU, after everything has been
defined. Clang does warn for this example.

Clang suppresses the warning if the member declaration has attribute unused,
which we would need to do too.