libstdc++: Disable std::formatter specializations (LWG 3944)

This was just approved in Tokyo as a DR for C++23. It doesn't affect us
yet, because we don't implement the __cpp_lib_format_ranges features. We
can add the disabled specializations and add a testcase now though.

libstdc++-v3/ChangeLog:

	* include/std/format (formatter): Disable specializations that
	would allow sequences of narrow characters to be formatted as
	wchar_t without conversion, as per LWG 3944.
	* testsuite/std/format/formatter/lwg3944.cc: New test.

1 files changed, 23 insertions, 0 deletions

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index 613016d..22dcb5f 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -2478,6 +2478,29 @@ namespace __format
     };
   /// @}
 
+#if defined _GLIBCXX_USE_WCHAR_T && __cpp_lib_format_ranges
+  // _GLIBCXX_RESOLVE_LIB_DEFECTS
+  // 3944. Formatters converting sequences of char to sequences of wchar_t
+
+  namespace __format { struct __disabled; }
+
+  // std::formatter<__disabled, C> uses the primary template, which is disabled.
+  template<>
+    struct formatter<char*, wchar_t>
+    : private formatter<__format::__disabled, wchar_t> { };
+  template<>
+    struct formatter<const char*, wchar_t>
+    : private formatter<__format::__disabled, wchar_t> { };
+  template<size_t _Nm>
+    struct formatter<char[_Nm], wchar_t>
+    : private formatter<__format::__disabled, wchar_t> { };
+  template<class _Traits, class _Allocator>
+    struct formatter<basic_string<char, _Traits, _Allocator>, wchar_t>
+    : private formatter<__format::__disabled, wchar_t> { };
+  template<class _Traits>
+    struct formatter<basic_string_view<char, _Traits>, wchar_t>
+    : private formatter<__format::__disabled, wchar_t> { };
+#endif
 
 /// @cond undocumented
 namespace __format