[Bug c++/95514] New: constexpr dynamic memory allocation compile error with inheritance and the this pointer

[pkeir at outlook dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95514

            Bug ID: 95514
           Summary: constexpr dynamic memory allocation compile error with
                    inheritance and the this pointer
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pkeir at outlook dot com
  Target Milestone: ---

Created attachment 48671
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48671&action=edit
The code described in the bug report.

I believe the C++20 code below should compile, but using `g++ -std=c++2a`
instead produces an error; indicating that there was deallocation of storage
that was not previously allocated.

If the call to the `point_to_me` method is replaced with the assignment on the
previous line, there is no error. Functionally the two forms seem the same to
me. Removing inheritance from the situation removes the error, which makes me
suspect the `static_cast`, but the this pointer usage also catches my eye.
Using an `std::allocator` instead of the `new` expression changes nothing
(included in the attached file as test2).

struct FooBase
{
  constexpr void point_to_me(FooBase*& p) { p = this; }

  FooBase* m_p = nullptr;
};

struct Foo : FooBase {};

constexpr bool test1()
{
  FooBase fb;
  Foo* pf = new Foo;

//  fb.m_p = pf;             // this works
  pf->point_to_me(fb.m_p); // this doesn't

  delete static_cast<Foo*>(fb.m_p);
  return true;
}