Proposed resolution:

This wording is relative to the FDIS.

1. Change [re.synopt] as indicated:

   namespace std {
     namespace regex_constants {
       typedef T1 syntax_option_type;
       static constexpr syntax_option_type icase = unspecified ;
       static constexpr syntax_option_type nosubs = unspecified ;
       static constexpr syntax_option_type optimize = unspecified ;
       static constexpr syntax_option_type collate = unspecified ;
       static constexpr syntax_option_type ECMAScript = unspecified ;
       static constexpr syntax_option_type basic = unspecified ;
       static constexpr syntax_option_type extended = unspecified ;
       static constexpr syntax_option_type awk = unspecified ;
       static constexpr syntax_option_type grep = unspecified ;
       static constexpr syntax_option_type egrep = unspecified ;
     }
   }
   

2. Change [re.matchflag] as indicated:

   namespace std {
     namespace regex_constants {
       typedef T2 match_flag_type;
       static constexpr match_flag_type match_default = 0{};
       static constexpr match_flag_type match_not_bol = unspecified ;
       static constexpr match_flag_type match_not_eol = unspecified ;
       static constexpr match_flag_type match_not_bow = unspecified ;
       static constexpr match_flag_type match_not_eow = unspecified ;
       static constexpr match_flag_type match_any = unspecified ;
       static constexpr match_flag_type match_not_null = unspecified ;
       static constexpr match_flag_type match_continuous = unspecified ;
       static constexpr match_flag_type match_prev_avail = unspecified ;
       static constexpr match_flag_type format_default = 0{};
       static constexpr match_flag_type format_sed = unspecified ;
       static constexpr match_flag_type format_no_copy = unspecified ;
       static constexpr match_flag_type format_first_only = unspecified ;
     }
   }
   

3. Change [re.err] as indicated:

   namespace std {
     namespace regex_constants {
       typedef T3 error_type;
       static constexpr error_type error_collate = unspecified ;
       static constexpr error_type error_ctype = unspecified ;
       static constexpr error_type error_escape = unspecified ;
       static constexpr error_type error_backref = unspecified ;
       static constexpr error_type error_brack = unspecified ;
       static constexpr error_type error_paren = unspecified ;
       static constexpr error_type error_brace = unspecified ;
       static constexpr error_type error_badbrace = unspecified ;
       static constexpr error_type error_range = unspecified ;
       static constexpr error_type error_space = unspecified ;
       static constexpr error_type error_badrepeat = unspecified ;
       static constexpr error_type error_complexity = unspecified ;
       static constexpr error_type error_stack = unspecified ;
     }
   }