[Bug c++/107165] New: Downcasting in constexpr, error: "accessing value of `...Derived::<anonymous>’ through a ‘Derived’ glvalue in a constant expression"

["petr.azmanov at wartsila dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107165

            Bug ID: 107165
           Summary: Downcasting in constexpr, error: "accessing value of
                    `...Derived::<anonymous>’ through a ‘Derived’ glvalue
                    in a constant expression"
           Product: gcc
           Version: 11.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: petr.azmanov at wartsila dot com
  Target Milestone: ---

Created attachment 53670
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53670&action=edit
copy of the test case from the comment

Downcasting in constexpr context is broken if derived class is wrapped in
another structure.

Tested locally with g++ (Ubuntu 11.2.0-19ubuntu1) 11.2.0
Reproduced with g++ 11.3.0 in Godbolt (https://godbolt.org/z/fvoWhrh41)

Isn't reproduced with 12.1 or 12.2.


```
struct Base
{
   int b = 0;
};

struct Derived : Base
{
   int d = 2;

   constexpr bool foo(int x)
   {
      return x > d;
   }
};

int main()
{
   constexpr auto test = []()
   {
      struct Dummy
      {
         Derived derived;
      };

      Dummy d;
      Base & base = d.derived;
      static_cast<Derived &>(base).foo(1);

      // Derived d;
      // Base & base = d;
      // static_cast<Derived &>(base).foo(1);

      return true;
   }();

   return test;
}

```