[Bug libstdc++/103630] New: [9/10/11/12 Regression] std::make_exception_ptr<T&>(t) is ill-formed

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103630

            Bug ID: 103630
           Summary: [9/10/11/12 Regression] std::make_exception_ptr<T&>(t)
                    is ill-formed
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid, wrong-code
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

I'm not 100% this is valid, but it used to work before the changes for PR
68297:

#include <exception>
int i = 0;
auto p = std::make_exception_ptr<int&>(i);

It fails now because the new implementation uses the template argument as
`_Ex*` and as `new _Ex` and those aren't valid for a reference type.

And if we fix that, there's another problem which is that it uses typeid(__ex)
which means we create an exception using the static type of the argument, but
which matches a handler of the dynamic type:

#include <exception>
#include <stdlib.h>

struct B { virtual ~B() = default; };
struct D : B { };

int main()
{
  try {
    D d;
    auto p = std::make_exception_ptr<B&>(d);
    std::rethrow_exception(p);
  } catch (const D&) {
    abort();
  } catch (const B&) {
  }
}

This throws a B but catches a D.