[Bug c++/119319] [14/15 Regression] incorrect error: invalid use of incomplete type

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119319

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is:
```
  using US0 = vv < std::pair < n0::Xyzzy, typename n1::CL1 < n1::S0, 999 >::ps
> >;
  class CL0 : public Error
  {
  private:
    US0 us0;
  public:
    CL0 (int err, const char *estr, const US0 & vec)
      : Error (err, estr), us0 (vec) { }
  };
  class Xyzzy : public n1::CL2 < x0 > { };

```

Xyzzy is used in CL0's us0 via the `std::vector<std::pair<Xyzzy,...>>`.