[ 2018-06-18 after reflector discussion ]

Priority set to 4

In C++03 strstreambuf was not copyable, because basic_streambuf wasn't copyable. In C++11 we made basic_streambuf copyable by derived classes, and strstreambuf doesn't define any special members, so it (unintentionally?) became copyable, with completely unspecified semantics.

VC++ and libc++ make it movable not copyable, and libstdc++ still follows C++03, so it's neither movable nor copyable. Making it movable seems to be the sane option, and consistent with filebuf and stringbuf.