When basic_stringbuf::setbuf has been called we need to copy the
contents of the buffer into _M_string first, before returning that.
libstdc++-v3/ChangeLog:
PR libstdc++/123100
* include/std/sstream (basic_stringbuf::str()&&): Handle the
case where _M_string is not being used for the buffer.
* testsuite/27_io/basic_stringbuf/str/char/123100.cc: New test.
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
a94bd31fd8