[Bug libstdc++/64571] New: [5.0 Regression] Link failure with fstream due to new string implementation

["reichelt at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64571

            Bug ID: 64571
           Summary: [5.0 Regression] Link failure with fstream due to new
                    string implementation
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: reichelt at gcc dot gnu.org
                CC: redi at gcc dot gnu.org

The following valid program (compiled with -std=c++11) fails to link using
trunk since 2014-12-19:

=======================================
#include <fstream>

int main()
{
  const std::string s = "MyFile.txt";

  std::fstream fs1(s);
  fs1.close();
  fs1.open(s);
  fs1.close();

  std::ifstream fs2(s);
  fs2.close();
  fs2.open(s);
  fs2.close();

  std::ofstream fs3(s);
  fs3.close();
  fs3.open(s);
  fs3.close();

  return 0;
}
=======================================

/tmp/ccSD2wgR.o: In function `main':
StringBug.cc:(.text+0x62): undefined reference to `std::basic_fstream<char,
std::char_traits<char> >::basic_fstream(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
StringBug.cc:(.text+0x9b): undefined reference to `std::basic_fstream<char,
std::char_traits<char> >::open(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
StringBug.cc:(.text+0xb9): undefined reference to `std::basic_ifstream<char,
std::char_traits<char> >::basic_ifstream(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
StringBug.cc:(.text+0xe6): undefined reference to `std::basic_ifstream<char,
std::char_traits<char> >::open(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
StringBug.cc:(.text+0x110): undefined reference to `std::basic_ofstream<char,
std::char_traits<char> >::basic_ofstream(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
StringBug.cc:(.text+0x149): undefined reference to `std::basic_ofstream<char,
std::char_traits<char> >::open(std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&, std::_Ios_Openmode)'
collect2: error: ld returned 1 exit status

This is related to PR64422 and PR64449.