Proposed resolution:

This wording is relative to N4830.

1. Modify [re.regex], class template basic_regex synopsis, as indicated:

   […]
   // [re.regex.assign], assign
   […]
   basic_regex& assign(const charT* ptr, flag_type f = regex_constants::ECMAScript);
   basic_regex& assign(const charT* p, size_t len, flag_type f = regex_constants::ECMAScript);
   template<class string_traits, class A>
     basic_regex& assign(const basic_string<charT, string_traits, A>& s,
                         flag_type f = regex_constants::ECMAScript);
   template<class InputIterator>
     basic_regex& assign(InputIterator first, InputIterator last,
                         flag_type f = regex_constants::ECMAScript);
   basic_regex& assign(initializer_list<charT>,
                       flag_type = regex_constants::ECMAScript);
   […]
   

[ 2019-10-31 Issue Prioritization ]

Status to Tentatively Ready and priority to 0 after six positive votes on the reflector.

The declaration of the overload of basic_regex<>::assign(const charT* p, size_t len, flag_type f) has an inconsistent default argument for the flag_type f parameter.

[re.regex] p3:

basic_regex& assign(const charT* p, size_t len, flag_type f);

[re.regex.assign] before p12:

basic_regex& assign(const charT* ptr, size_t len, flag_type f = regex_constants::ECMAScript);

Since all other overloads have a default argument in both [re.regex] and [re.regex.assign] I propose to add a default argument for this overload in the declaration in [re.regex].

It should be pointed out that there exists implementation divergence due to the current wording state: libc++ and libstdc++ do not implement the default argument. The MS STL library does have the default argument.