[Bug c++/122135] [13/14/15/16 Regression] accepts redeclaration of 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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC 5.5.0 produced:
```
<source>:8:15: error: default argument for template parameter for class
enclosing 'struct outer::inner<T>'
 struct outer::inner{};
               ^
```

I suspect when we started to accepting:
```
struct outer
{
    template <typename T>
    struct inner;
};

template <typename T = long>
struct outer::inner{};
```
Is when we started to accepting this one too.  Like there is a missing check if
both had default template arguments.