<format>: `charT` for `std::formatter<basic_string_view>` is not supposed to be template parameter

https://eel.is/c++draft/format.formatter.spec#2 says

Let charT be either char or wchar_t. ...
...

• For each charT, the debug-enabled string type specializations
  ...

  template<class traits>
  struct formatter<basic_string_view<charT, traits>, charT>;
  

meaning the two specializations should be written separately

template<class traits>
struct formatter<basic_string_view<char, traits>, char>;

template<class traits>
struct formatter<basic_string_view<wchar_t, traits>, wchar_t>;

but they are implemented as https://github.com/microsoft/STL/blob/d43d49a3fdfa9d4fdbafd82dc517250b93269c94/stl/inc/__msvc_formatter.hpp#L131-L132 https://github.com/microsoft/STL/blob/d43d49a3fdfa9d4fdbafd82dc517250b93269c94/stl/inc/__msvc_formatter.hpp#L276-L284

This may cause trouble if users add templated specializations but basic_string_view is not excluded from its constraint, but only supporting char (Godbolt link):

#include <format>
#include <string_view>
#include <concepts>

struct X {};

template<class T>
concept C = std::same_as<T, X> || std::same_as<T, std::string_view>;

template<C T>
struct std::formatter<T>;

std::formatter<std::string_view> f{};

Then the specialization is ambiguous

<source>(13): error C2752: 'std::formatter<std::string_view,char>': more than one partial specialization matches the template argument list
Z:/compilers/msvc/14.41.33923-14.41.33923.0/include\__msvc_formatter.hpp(261): note: could be 'std::formatter<std::basic_string_view<_CharT,_Traits>,_CharT>'
<source>(11): note: or       'std::formatter<T,char>'

Start with the formatter specializations in stl/inc/__msvc_formatter.hpp at lines 131-132 and 276-284, then compare them with the format.formatter.spec wording linked in the issue. Reproduce the ambiguity using the supplied Godbolt example. Done means the basic_string_view specializations match the standard and the example no longer reports competing partial specializations.