[libc++][format] Decay character arrays in formatting

Currently, built-in `char`/`wchar_t` arrays are assumed to be
null-terminated sequence with the terminator being the last element in
formatting. This doesn't conform to [format.arg]/6.9.

> otherwise, if `decay_t<TD>` is `char_type*` or `const char_type*`,
> initializes value with `static_cast<const char_type*>(v)`;

The standard wording specifies that character arrays are decayed to
pointers. When the null terminator is not the last element or there's no
null terminator (the latter case is UB), libc++ currently produces
different results.

Author: frederick-vs-ja

libcxx/include/__format/format_arg_store.h

@@ -101,20 +101,14 @@ consteval __arg_t __determine_arg_t() {
   return __arg_t::__long_double;
 }
 
-// Char pointer
+// Char pointer or array
 template <class _Context, class _Tp>
-  requires(same_as<typename _Context::char_type*, _Tp> || same_as<const typename _Context::char_type*, _Tp>)
+  requires(same_as<typename _Context::char_type*, _Tp> || same_as<const typename _Context::char_type*, _Tp>) ||
+          (is_array_v<_Tp> && same_as<_Tp, typename _Context::char_type[extent_v<_Tp>]>)
 consteval __arg_t __determine_arg_t() {
   return __arg_t::__const_char_type_ptr;
 }
 
-// Char array
-template <class _Context, class _Tp>
-  requires(is_array_v<_Tp> && same_as<_Tp, typename _Context::char_type[extent_v<_Tp>]>)
-consteval __arg_t __determine_arg_t() {
-  return __arg_t::__string_view;
-}
-
 // String view
 template <class _Context, class _Tp>
   requires(same_as<typename _Context::char_type, typename _Tp::value_type> &&
@@ -188,15 +182,8 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
   else if constexpr (__arg == __arg_t::__unsigned_long_long)
     return basic_format_arg<_Context>{__arg, static_cast<unsigned long long>(__value)};
   else if constexpr (__arg == __arg_t::__string_view)
-    // Using std::size on a character array will add the NUL-terminator to the size.
-    if constexpr (is_array_v<_Dp>)
-      return basic_format_arg<_Context>{
-          __arg, basic_string_view<typename _Context::char_type>{__value, extent_v<_Dp> - 1}};
-    else
-      // When the _Traits or _Allocator are different an implicit conversion will
-      // fail.
-      return basic_format_arg<_Context>{
-          __arg, basic_string_view<typename _Context::char_type>{__value.data(), __value.size()}};
+    return basic_format_arg<_Context>{
+        __arg, basic_string_view<typename _Context::char_type>{__value.data(), __value.size()}};
   else if constexpr (__arg == __arg_t::__ptr)
     return basic_format_arg<_Context>{__arg, static_cast<const void*>(__value)};
   else if constexpr (__arg == __arg_t::__handle)

libcxx/test/std/utilities/format/format.functions/format_tests.h

@@ -3189,6 +3189,10 @@ void format_tests(TestFunction check, ExceptionTest check_exception) {
     const CharT* data = buffer;
     check(SV("hello 09azAZ!"), SV("hello {}"), data);
   }
+  {
+    CharT buffer[] = {CharT('a'), CharT('b'), CharT('c'), 0, CharT('d'), CharT('e'), CharT('f'), 0};
+    check(SV("hello abc"), SV("hello {}"), buffer);
+  }
   {
     std::basic_string<CharT> data = STR("world");
     check(SV("hello world"), SV("hello {}"), data);