Address review comments

Author: frederick-vs-ja

libcxx/include/__format/format_arg_store.h

@@ -198,12 +198,14 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
   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_bounded_array_of<_Dp, __context_char_type>) {
-      if (const auto __pzero = char_traits<__context_char_type>::find(__value, extent_v<_Dp>, __context_char_type{}))
+      const auto __pbegin = std::begin(__value);
+      if (const _Dp* __pzero = char_traits<__context_char_type>::find(__pbegin, extent_v<_Dp>, __context_char_type{})) {
         return basic_format_arg<_Context>{
-            __arg, basic_string_view<__context_char_type>{__value, static_cast<size_t>(__pzero - __value)}};
-      else
-        // The behavior is undefined in this case.
-        return basic_format_arg<_Context>{__arg, basic_string_view<__context_char_type>{__value, extent_v<_Dp>}};
+            __arg, basic_string_view<__context_char_type>{__pbegin, static_cast<size_t>(__pzero - __pbegin)}};
+      } else {
+        // Per [format.arg]/5, the behavior is undefined because the array is not null-terminated.
+        return basic_format_arg<_Context>{__arg, basic_string_view<__context_char_type>{__pbegin, extent_v<_Dp>}};
+      }
     } else
       // When the _Traits or _Allocator are different an implicit conversion will fail.
       return basic_format_arg<_Context>{__arg, basic_string_view<__context_char_type>{__value.data(), __value.size()}};