[Bug libstdc++/95545] New: thread:: conflicts with std::thread

[Bug libstdc++/95545] New: thread:: conflicts with std::thread

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

            Bug ID: 95545
           Summary: thread:: conflicts with std::thread
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: dje at gcc dot gnu.org
  Target Milestone: ---

AIX, MSVC, and possibly other systems, implemented their own definition of
thread different from Posix threads.

libstdc++ <thread> header eventually includes <gthr-posix.h>, which includes
the system <pthread.h> header, which, on AIX, eventually includes
<sys/thread.h>

The AIX header nicely does

#ifdef __cplusplus
extern "C" {
#endif

struct  thread {
...
}

which creates an ambiguity when a user references thread:: without std::thread.

LLVM C++ has a pattern that helps to avoid the conflict because of a similar
problem for MSVC:

namespace llvm {
typedef std::thread thread;
}

llvm/include/llvm/Support/thread.h

Would libstdc++ consider a similar solution?

[Bug libstdc++/95545] thread:: conflicts with std::thread

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

David Edelsohn <dje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-06-04

--- Comment #1 from David Edelsohn <dje at gcc dot gnu.org> ---
Confirmed for AIX and MSVC.

[Bug libstdc++/95545] thread:: conflicts with std::thread

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to David Edelsohn from comment #0)
> which creates an ambiguity when a user references thread:: without
> std::thread.

Could you show an example of code that's ambiguous?

> Would libstdc++ consider a similar solution?

Could you clarify what the solution involves?

Basically I don't understand the problem being described, so I don't see how to
solve it.

[Bug libstdc++/95545] thread:: conflicts with std::thread

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

--- Comment #3 from David Edelsohn <dje at gcc dot gnu.org> ---
#include <stdio.h>
#include <thread>
using namespace std;

int main(void)
{
        int maxThreads = thread::hardware_concurrency();
        printf("maxThreads: %d\n", maxThreads);
        return(0);
}

$ g++ -pthread /tmp/nameclash.cpp -o /tmp/nc
/tmp/nameclash.cpp: In function 'int main()':
/tmp/nameclash.cpp:7:19: error: reference to 'thread' is ambiguous
  int maxThreads = thread::hardware_concurrency();
                   ^~~~~~
In file included from /tmp/nameclash.cpp:2:
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/thread:62:9: note:
candidates are: 'class std::thread'
   class thread
         ^~~~~~
In file included from /usr/include/sys/ptrace.h:28,
                 from /usr/include/sys/proc.h:44,
                 from /usr/include/sys/pri.h:43,
                 from /usr/include/sys/sched.h:38,
                 from /usr/include/sched.h:51,
                 from /usr/include/pthread.h:63,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr-posix.h:35,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr-default.h:30,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/powerpc-ibm-aix7.2.0.0/pthread/bits/gthr.h:148,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/ext/atomicity.h:35,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/memory:73,
                 from
/opt/freeware/lib/gcc/powerpc-ibm-aix7.2.0.0/8/include/c++/thread:39,
                 from /tmp/nameclash.cpp:2:
/usr/include/sys/thread.h:106:8: note:                 'struct thread'
 struct thread {
        ^~~~~~

[Bug libstdc++/95545] thread:: conflicts with std::thread

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

David Edelsohn <dje at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #4 from David Edelsohn <dje at gcc dot gnu.org> ---
Impossible.

[Bug libstdc++/95545] thread:: conflicts with std::thread

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem is that 'using namespace std;' introduces an ambiguity. That is in
the user's code, there's nothing we can do in libstdc++ to avoid it.

We could add a different name for std::thread eg.

namespace __gnu_cxx { using thread = std::thread; }

and then users could be careful to always refer to __gnu_cxx::thread instead of
just thread.

But firstly, that is just a different portability problem, because that won't
work with the MSVC library or with libc++. And secondly, if you're going to be
careful and always refer to it with a qualified name, just use std::thread.

Referring to std::thread is portable and always works.

Alternatively, put the 'using namespace std;' at function scope, so that inside
the main() function name lookup finds std::thread, and never looks in the
global scope to find ::thread.

We could do:

namespace std { using __fred = thread; }

which would allow the code to have 'using namespace std;' at global scope, and
refer to __fred::hardware_concurrency() unqualified, but that's still not
portable.

The user code causes the problem, and the user code needs to fix it. There's no
libstdc++ bug and nothing libstdc++ can do that doesn't just shift the problem
somewhere else.

There are similar problems for any name in std:: which also exists at global
scope on some platforms, e.g. std::bind conflicts with POSIX bind(3).

It's well known that 'using namespace std;' causes problems like this. Even if
it works today, it might not work in future because new versions of the C++
standard will add new names to namespace std, so 'using namespace std;' is
basically asking for an unbounded set of names to be dropped into the global
namespace, with potential collisions.

Either refer to 'std::thread' explicitly or introduce the name 'thread' into a
more narrow scope than the one that already has the AIX 'thread'.

[Bug libstdc++/95545] thread:: conflicts with std::thread

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95545

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> ---
See
https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rs-using
in the C++ Core Guidelines.