[Bug c++/81338] New: stringstream remains empty after being moved into multiple times

["zxy19980101 at sina dot cn" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81338

            Bug ID: 81338
           Summary: stringstream remains empty after being moved into
                    multiple times
           Product: gcc
           Version: 6.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zxy19980101 at sina dot cn
  Target Milestone: ---

Given the code below:

#include <sstream>
#include <iostream>

int main() {
        std::stringstream ss;
        while (true) {
                for (int i = rand() % 10 + 10; i > 0; --i) {
                        ss << static_cast<char>(rand() % 26 + 'a');
                }
                std::cout << ss.str() << "\n";
                ss = std::stringstream();
        }
        return 0;
}

The output become blank lines after the first few. The code works on visual
studio 2017.

My g++ version is:

g++.exe (x86_64-posix-seh-rev2, Built by MinGW-W64 project) 6.3.0
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.