[Bug libstdc++/65543] New: rvalue stream insertion and extraction operators incorrectly implemented

[rs2740 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65543

            Bug ID: 65543
           Summary: rvalue stream insertion and extraction operators
                    incorrectly implemented
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rs2740 at gmail dot com

Repro:

#include <iostream>
#include <sstream>

struct A{};

void operator<<(std::ostream &, A const &){ }
void operator>>(std::istream &, A &){ }

int main()
{
    A a;

    std::ostringstream() << a;
    std::istringstream() >> a;

    return 0;
}

The standard says that operator<< for rvalue streams performs `os << x` and
returns `os`. The libstdc++ implementation instead does 

return (__os << __x);

which breaks if the operator<< called does not return a reference stream. A
similar issue affects operator>> for rvalue streams.