Proposed resolution:

This wording is relative to N4971.

1. Modify [category.ctype.general] as indicated:

   namespace std {
     class ctype_base {
     public:
       using mask = see below;
   
       // numeric values are for exposition only.
       static constexpr mask space  = 1 << 0;
       static constexpr mask print  = 1 << 1;
       static constexpr mask cntrl  = 1 << 2;
       static constexpr mask upper  = 1 << 3;
       static constexpr mask lower  = 1 << 4;
       static constexpr mask alpha  = 1 << 5;
       static constexpr mask digit  = 1 << 6;
       static constexpr mask punct  = 1 << 7;
       static constexpr mask xdigit = 1 << 8;
       static constexpr mask blank  = 1 << 9;
       static constexpr mask alnum  = alpha | digit;
       static constexpr mask graph  = alnum | punct;
     };
   }
   

   -1- The type mask is a bitmask type ([bitmask.types]).

[ Tokyo 2024-03-23; Status changed: Voting → WP. ]

[ 2024-03-12; Reflector poll ]

Set status to Tentatively Ready after five votes in favour during reflector poll.

It may be desired that static data members ctype_base are "real constants", i.e. usable in constant expressions. However, this is not strictly required because mask is only required to be a bitmask type that can be a class type, which makes the plain const potentially insufficient.