[Bug c++/122135] New: [13/14/15/16 Regression] accepts default template argument on inner template class definition

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122135

            Bug ID: 122135
           Summary: [13/14/15/16 Regression] accepts default template
                    argument on inner template class definition
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
struct outer
{
    template <typename T = long>
    struct inner;
};

template <typename T = long>
struct outer::inner{};
```

This is (incorrectly) accepted since GCC 6.1.0