[libc++][format] Fixes formatting code units as integers. (#73396)

This paper was voted in as a DR, so it's retroactively enabled back to
C++20; the C++ version that introduced std::format.

Implements:
- P2909R4 Fix formatting of code units as integers (Dude, where’s my
``char``?)

2 files changed, 14 insertions, 12 deletions

diff --git a/libcxx/include/__format/format_arg_store.h b/libcxx/include/__format/format_arg_store.h
index 15ec8eb..2962962 100644
--- a/libcxx/include/__format/format_arg_store.h
+++ b/libcxx/include/__format/format_arg_store.h
@@ -172,9 +172,13 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
   // __basic_format_arg_value.  First handle all types needing adjustment, the
   // final else requires no adjustment.
   if constexpr (__arg == __arg_t::__char_type)
-    // On some platforms initializing a wchar_t from a char is a narrowing
-    // conversion.
-    return basic_format_arg<_Context>{__arg, static_cast<typename _Context::char_type>(__value)};
+
+#  ifndef _LIBCPP_HAS_NO_WIDE_CHARACTERS
+    if constexpr (same_as<typename _Context::char_type, wchar_t> && same_as<_Dp, char>)
+      return basic_format_arg<_Context>{__arg, static_cast<wchar_t>(static_cast<unsigned char>(__value))};
+    else
+#  endif
+      return basic_format_arg<_Context>{__arg, __value};
   else if constexpr (__arg == __arg_t::__int)
     return basic_format_arg<_Context>{__arg, static_cast<int>(__value)};
   else if constexpr (__arg == __arg_t::__long_long)
diff --git a/libcxx/include/__format/formatter_char.h b/libcxx/include/__format/formatter_char.h
index d6e61e8..3358d42 100644
--- a/libcxx/include/__format/formatter_char.h
+++ b/libcxx/include/__format/formatter_char.h
@@ -21,7 +21,7 @@
 #include <__format/parser_std_format_spec.h>
 #include <__format/write_escaped.h>
 #include <__type_traits/conditional.h>
-#include <__type_traits/is_signed.h>
+#include <__type_traits/make_unsigned.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
@@ -51,22 +51,21 @@ public:
       return __formatter::__format_escaped_char(__value, __ctx.out(), __parser_.__get_parsed_std_specifications(__ctx));
 #  endif
 
-    if constexpr (sizeof(_CharT) <= sizeof(int))
-      // Promotes _CharT to an integral type. This reduces the number of
-      // instantiations of __format_integer reducing code size.
+    if constexpr (sizeof(_CharT) <= sizeof(unsigned))
       return __formatter::__format_integer(
-          static_cast<conditional_t<is_signed_v<_CharT>, int, unsigned>>(__value),
+          static_cast<unsigned>(static_cast<make_unsigned_t<_CharT>>(__value)),
           __ctx,
           __parser_.__get_parsed_std_specifications(__ctx));
     else
-      return __formatter::__format_integer(__value, __ctx, __parser_.__get_parsed_std_specifications(__ctx));
+      return __formatter::__format_integer(
+          static_cast<make_unsigned_t<_CharT>>(__value), __ctx, __parser_.__get_parsed_std_specifications(__ctx));
   }
 
   template <class _FormatContext>
   _LIBCPP_HIDE_FROM_ABI typename _FormatContext::iterator format(char __value, _FormatContext& __ctx) const
     requires(same_as<_CharT, wchar_t>)
   {
-    return format(static_cast<wchar_t>(__value), __ctx);
+    return format(static_cast<wchar_t>(static_cast<unsigned char>(__value)), __ctx);
   }
 
 #  if _LIBCPP_STD_VER >= 23
@@ -84,8 +83,7 @@ template <>
 struct _LIBCPP_TEMPLATE_VIS formatter<char, wchar_t> : public __formatter_char<wchar_t> {};
 
 template <>
-struct _LIBCPP_TEMPLATE_VIS formatter<wchar_t, wchar_t> : public __formatter_char<wchar_t> {
-};
+struct _LIBCPP_TEMPLATE_VIS formatter<wchar_t, wchar_t> : public __formatter_char<wchar_t> {};
 
 #  endif // _LIBCPP_HAS_NO_WIDE_CHARACTERS