[Bug libstdc++/85098] undefined reference to std::regex::extended

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85098

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Untested fix:

--- a/libstdc++-v3/include/bits/regex.h
+++ b/libstdc++-v3/include/bits/regex.h
@@ -776,6 +776,37 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
       _AutomatonPtr    _M_automaton;
     };

+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::icase;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::nosubs;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::optimize;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::collate;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::ECMAScript;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::basic;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::extended;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::awk;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::grep;
+  template<typename _Ch, typename _Tr>
+    constexpr regex_constants::syntax_option_type
+    basic_regex<_Ch, _Tr>::egrep;
+
 #if __cpp_deduction_guides >= 201606
   template<typename _ForwardIterator>
     basic_regex(_ForwardIterator, _ForwardIterator,