Proposed resolution:

This wording is relative to N4981.

1. Modify [version.syn] as indicated:

   #define __cpp_lib_ranges_concat 202403L // freestanding, also in <ranges>
   

2. Modify [ranges.syn] as indicated:

   #include <compare>              // see [compare.syn]
   #include <initializer_list>     // see [initializer.list.syn]
   #include <iterator>             // see [iterator.synopsis]
   
   namespace std::ranges {
     […]
     // [range.concat], concat view
     template<input_range... Views>
       requires see below
     class concat_view;                                                                // freestanding
     
     namespace views { inline constexpr unspecified concat = unspecified; }            // freestanding
     […]
   }
   

[ 2024-04-21; Daniel comments ]

The specification of some member functions of concat_view seem to depend on freestanding-deleted get overloads for variant, but so does join_with_view, which is marked as freestanding, so it does not seem to be a good reason to accept join_with_view but not concat_view as freestanding.

concat_view can be freestanding, but this never seems to come up in the discussion, which seems to be an oversight.