[Bug c++/88337] Implement P1002R1, P1327R1, P1330R0, C++20 relaxations of constexpr restrictions.

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88337

--- Comment #7 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Sidecast now works too:

struct A {
  virtual void afn () {}
};

struct B {
  virtual void bfn () {}
};

struct D : A, B { };

constexpr bool
fn ()
{
  bool ok = true;
  D d;
  A *a = &d;
  if (B *bp = dynamic_cast<B*>(a))
    /*OK*/;
  else
    ok = false;

  A &ar = d;
  B &br = dynamic_cast<B&>(ar);

  return ok;
}

static_assert(fn ());