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

[SainoNamkho]

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

STL/stl/inc/__msvc_formatter.hpp

Lines 131 to 132 in d43d49a

	template <class _CharT>
	concept _Format_supported_charT = _Is_any_of_v<_CharT, char, wchar_t>;

STL/stl/inc/__msvc_formatter.hpp

Lines 276 to 284 in d43d49a

	template <_Format_supported_charT _CharT, class _Traits>
	struct formatter<basic_string_view<_CharT, _Traits>, _CharT>
	: _Formatter_base<basic_string_view<_CharT, _Traits>, _CharT, _Basic_format_arg_type::_String_type> {
	#if _HAS_CXX23
	constexpr void set_debug_format() noexcept {
	this->_Set_debug_format();
	}
	#endif // _HAS_CXX23
	};

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>'

[davidmrdavid]

We discussed this on today's maintainer sync and we think the standard is unclear on the interplay between concepts and WG21-N5001 [namespace.std]/2. We'll return to this issue after some review.

At first glance, we feel the specialization should not be permitted as it stomps on std::formatter<std::string_view>.

[frederick-vs-ja]

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
  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>;

I don't think anything in that paragraph disallows merging specializations for char and wchar_t. libc++ also merges these specializations. libstdc++ merges some but not all.