[Bug c++/112580] [14 Regression]: g++.dg/modules/xtreme-header-4_b.C et al; ICE tree check: expected class 'type', have 'declaration'

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112580

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Reduced testcase for the xtreme-header-{4,5,6} error:

$ cat 112580_a.H
template <typename _Tp>
struct _Formatting_scanner {
  long long _M_unpacked_size;
  struct {
    int _M_values;
  };
  virtual int _M_format_arg() { return _M_values; }
};

_Formatting_scanner<int> s;

$ cat 112580_b.C
import "112580_a.H";

$ g++ -fmodules-ts 112580_a.H 112580_b.C
In module 112580_a.H, imported at 112580_b.C:1:
112580_a.H: In instantiation of ‘int _Formatting_scanner<_Tp>::_M_format_arg()
[with _Tp = int]’:
112580_a.H:7:15:   required from here
    7 |   virtual int _M_format_arg() { return _M_values; }
      |               ^~~~~~~~~~~~~
112580_a.H:7:40: error: invalid use of non-static data member
‘_Formatting_scanner<int>::__as_base ::<anonymous>’
    7 |   virtual int _M_format_arg() { return _M_values; }
      |                                        ^~~~~~~~~