[Bug libstdc++/83598] New: std::basic_regex C++14 conformance issue: resulting flags != passed flags

[andrey.y.guskov at intel dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83598

            Bug ID: 83598
           Summary: std::basic_regex C++14 conformance issue: resulting
                    flags != passed flags
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrey.y.guskov at intel dot com
  Target Milestone: ---

C++14 standard (page 1114, see here:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf#1128) says:

explicit basic_regex(const charT* p, flag_type f =
regex_constants::ECMAScript);
Requires: p shall not be a null pointer.
Throws: regex_error if p is not a valid regular expression.
Effects: Constructs an object of class basic_regex; the object’s internal
finite state machine is constructed from the regular expression contained in
the array of charT of length char_traits<charT>::length(p) whose first element
is designated by p, and interpreted according to the flags f.
Postconditions: flags() returns f. mark_count() returns the number of marked
sub-expressions within the expression.
...

As of now, std::basic_regex implementation disagrees with the standard in that
the flags set upon construction aren`t equal to the flags passed to the
constructor.

Reproducer:

#include <regex>
int main() {
    std::basic_regex<char> ca("(a)(b)", std::regex_constants::icase);
    std::basic_regex<char> cb("(a)(b)", std::regex_constants::nosubs);
    std::basic_regex<wchar_t> wa(L"(a)(b)", std::regex_constants::icase);
    std::basic_regex<wchar_t> wb(L"(a)(b)", std::regex_constants::nosubs);
    printf(((ca.flags() == std::regex_constants::icase)
         || (cb.flags() == std::regex_constants::nosubs)
         || (wa.flags() == std::regex_constants::icase)
         || (wb.flags() == std::regex_constants::nosubs))? "PASS\n" :
"FAIL\n");
    return 0;
}