[Bug c++/106924] Nested class: virtual function returns wrong pointer of covariant type

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106924

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-09-14

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced to remove unnecessary virtual functions that are never used, and fix
the printf formats, and make it fail at runtime:

extern "C" int printf(const char*, ...);
extern "C" void abort();

struct BaseA
{
  virtual ~BaseA(){}
};

struct BaseB
{
  virtual ~BaseB(){}
};

struct DerivedAB;

struct FabForB
{
  virtual BaseA* getPtr()=0;
  virtual ~FabForB(){}
};

struct DerivedAB : BaseB, BaseA
{
  struct ABFabNested : FabForB
  {
    virtual DerivedAB* getPtr();
  };
};

void print(const DerivedAB* dp, const BaseA* ap, bool nested = false)
{
}

DerivedAB d;

DerivedAB* DerivedAB::ABFabNested::getPtr()
{
  DerivedAB* p = &d;
  printf("TEST fab nested: addr %p base %p\n", p, (BaseA*)p);
  return p;
}

struct ABFab : FabForB
{
  virtual DerivedAB* getPtr()
  {
    DerivedAB* p = &d;
    printf("TEST fab:        addr %p base %p\n", p, (BaseA*)p);
    return p;
  }
};

int main()
{
  ABFab fab;
  DerivedAB::ABFabNested fabNested;

  BaseA* a1 = fab.getPtr();
  BaseA* a2 = static_cast<FabForB&>(fab).getPtr();
  printf("TEST             addr %p %p\n", a1, a2);

  a1 = fabNested.getPtr();
  a2 = static_cast<FabForB&>(fabNested).getPtr();
  printf("TEST nested      addr %p %p\n", a1, a2);

  if (a1 != a2)
    abort();
}


G++ prints:

TEST fab:        addr 0x404060 base 0x404068
TEST fab:        addr 0x404060 base 0x404068
TEST             addr 0x404068 0x404068
TEST fab nested: addr 0x404060 base 0x404068
TEST fab nested: addr 0x404060 base 0x404068
TEST nested      addr 0x404068 0x404060
Aborted (core dumped)

When we call fabNested.getPtr() to get a DerivedAB* and then convert to BaseA*
at the call site, the pointer is correctly adjusted to the BaseA base
subobject.

When we call static_cast<FabForB&>(fabNested).getPtr() to get a BaseA* the
covariant return is not adjusted, and the address of the DerivedAB complete
object is returned, not its BaseA base subobejct..