[Bug libstdc++/121765] `std::formatter` specialization for user-defined type inheriting from `std::formatter` fails concept `std::formattable` check unexpectedly

[nishuangcheng at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121765

--- Comment #4 from Shuangcheng Ni <nishuangcheng at gmail dot com> ---
A possible fix:
```cpp
namespace std {
namespace __format {
template <__char _CharT>
struct _Iter_for<_CharT> {
    using type = _Sink_iter<_CharT>;
};
}  // namespace __format
}  // namespace std
```

Test case: https://godbolt.org/z/nP36zq4qM