[Bug c++/104850] New: Instantiating a destructor for a template class too early, before the calling destructor is seen - rejects valid code

["kirshamir at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104850

            Bug ID: 104850
           Summary: Instantiating a destructor for a template class too
                    early, before the calling destructor is seen - rejects
                    valid code
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kirshamir at gmail dot com
  Target Milestone: ---

The following code is rejected as trying to use incomplete type, while when the
actual use would actually appear the type would be complete:

template<typename T>
struct uptr {
    uptr(nullptr_t) {}
    ~uptr() {
        delete (new T);
    }
};

class A
{
public:
  A();
  ~A();
private:
  class B;
  uptr<B> m_b = nullptr; // the dtor of uptr tries to be instantiated here
};

If we change the initialization of:
uptr<B> m_b = nullptr; 
To:
uptr<B> m_b {nullptr}; 

The destructor instantiation is delayed, till ~A() is seen, and the code can be
compiled.

See:
https://stackoverflow.com/questions/71397495/why-does-default-member-initializer-request-instantiation-of-unique-ptr-destru