[Bug libstdc++/69608] New: strsteambuf copy ctor and assignment inaccessible

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69608

            Bug ID: 69608
           Summary: strsteambuf copy ctor and assignment inaccessible
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

As far as I can see, the C++ standard doesn't prevent the copying of
strstreambuf objects.  With the resolution of issue 421 these copies should
have (almost) well-defined semantics.  But the following program fails to
compile with the latest libstdc++.

That said, I'm not sure it's a good idea for the standard to require
implementations to accept the program.  If it did, it seems that the two copies
would share the same underlying character buffer (courtesy of issue 421). 
Accessing the buffer in the copy would be undefined after the copied
strstreambuf object was destroyed.  If this hasn't been discussed in LWG yet it
might be worth bringing it up.  strstreambuf may be deprecated but the same
concern applies to similar user-defined classes that publicly derive from
basic_streambuf and expose an accsible copy ctor or assignment operator of
their own.

$ cat t.cpp && /home/msebor/build/gcc-trunk-bootstrap/gcc/xg++
-B/home/msebor/build/gcc-trunk-bootstrap/gcc -S ... t.cpp
#include <strstream>

int main ()
{
    std::strstreambuf a;
    a = std::strstreambuf (32);
}

In file included from
/src/gcc/trunk/libstdc++-v3/include/backward/strstream:50:0,
                 from t.cpp:1:
/src/gcc/trunk/libstdc++-v3/include/backward/backward_warning.h:32:2: warning:
#warning This file includes at least one deprecated or antiquated header which
may be removed without further notice at a future date. Please use a
non-deprecated interface with equivalent functionality instead. For a listing
of replacement headers and interfaces, consult the file backward_warning.h. To
disable this warning use -Wno-deprecated. [-Wcpp]
t.cpp: In function ‘int main()’:
t.cpp:6:30: error: ‘std::strstreambuf& std::strstreambuf::operator=(const
std::strstreambuf&)’ is private within this context
In file included from t.cpp:1:0:
/src/gcc/trunk/libstdc++-v3/include/backward/strstream:102:5: note: declared
private here