[Bug middle-end/100861] False positive -Wmismatched-new-delete with destroying operator delete

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100861

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
          Component|c++                         |middle-end
   Last reconfirmed|                            |2021-06-01
                 CC|                            |msebor at gcc dot gnu.org
     Ever confirmed|0                           |1
             Blocks|                            |100406

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning doesn't do anything special with destroying operator delete or any
other kinds of the operators (other than scalar vs array).  It triggers for
this test case because it sees the result of ::operator new() being passed to
Widget::operator delete (Widget*).  If Widget::operator delete() is inlined
(e.g., declared with attribute always_inline) the warning goes away just as
long as the operator doesn't pass the pointer to the wrong overload of delete. 
Alternatively, if Widget defines a non-inline member operator new() that also
prevents the warning because calls to both operators match.

With that, I'm not sure that suppressing the warning for a destroying operator
delete() would be a good solution.  It seems to me that the right fix is to
solve the broader problem where one of the operators is inlined and the other
isn't (similar to pr100485, except with the definitions of both operators
available in the same translation unit).

Until/unless a solution is developed I would suggest to either define the
destroying operator delete inline and have it call an out-of-line function to
do the work (as shown below) or to force the inlining of the destroying delete.

struct Widget {
  const WidgetKind Kind : 4;
  unsigned OtherThings : 28;

  Widget(WidgetKind k) : Kind(k) {}

  void operator delete(Widget *widget, std::destroying_delete_t) {
    destroy_delete (widget);
  }

  static __attribute__ ((noinline)) void destroy_delete (Widget *);
};


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100406
[Bug 100406] bogus/missing -Wmismatched-new-delete