LWG-4037 Static data members of `ctype_base` are not yet required to be usable in constant expressions

[StephanTLavavej]

LWG-4037 Static data members of ctype_base are not yet required to be usable in constant expressions

See also:

• (Status: New) LWG-3998 Constants in std::regex_constants should be allowed to be enumerators
• <regex>: Constants in std​::​regex_constants are not constexpr variables #4100