[Bug c++/106411] New: Wdangling-pointer for a class that cleans up on destruction

[Bug c++/106411] New: Wdangling-pointer for a class that cleans up on destruction

[ed at catmur dot uk]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106411

            Bug ID: 106411
           Summary: Wdangling-pointer for a class that cleans up on
                    destruction
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ed at catmur dot uk
  Target Milestone: ---

Reduced testcase:

#include <utility>
void g(auto f1) {
    auto f2 = std::move(f1);
}
struct F;
struct E {
    F* f = nullptr;
};
struct F {
    E* e;
    explicit F(E* a) : e(a) { e->f = this; }
    ~F() {
        if (e)
            e->f = nullptr;
    }
    F(F&& rhs) : e(std::exchange(rhs.e, nullptr)) {
        e->f = this;
    }
};
int main() {
    E e;
    F f(&e);
    g(std::move(f));
}

At -O -Wall in 12.1 and current trunk 13.0.0:

In constructor 'F::F(F&&)',
    inlined from 'void g(auto:1) [with auto:1 = F]' at <source>:3:10,
    inlined from 'int main()' at <source>:23:6:
<source>:17:14: warning: storing the address of local variable 'f2' in
'*__old_val.E::f' [-Wdangling-pointer=]
   17 |         e->f = this;
      |         ~~~~~^~~~~~
<source>: In function 'int main()':
<source>:3:10: note: 'f2' declared here
    3 |     auto f2 = std::move(f1);
      |          ^~
<source>:3:10: note: '<unnamed>.F::e' declared here

It would be helpful if Wdangling-pointer could realize that the stored address
will be cleared when the local variable is destroyed.

Here `g` is reduced from `boost::asio::detail::executor_function::complete` and
`F` is reduced from our own callback type.  The above reduced testcase only
repros at -O but our unreduced code repros at -O3.