[Bug libstdc++/60422] New: headers should give a diagnostic when included directly

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60422

            Bug ID: 60422
           Summary: <bits/xxx.h> headers should give a diagnostic when
                    included directly
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org

Some users do dumb things like:

#include <algorithm>
#include <ext/concurrence.h>
#include <bits/unique_ptr.h>
#include <bits/shared_ptr.h>

and expect to be able to use std::shared_ptr.  This happens to work with GCC
4.8, but not with 4.9 because we re-arranged some headers to allow the library
to use shared_ptr internally.

In order to stop this nonsense we should add preprocessor checks to the
headers:

#ifndef _MEMORY
#error Do not include this header directly, use <memory> instead
#else
...
#endif