[Bug c++/53648] New: nested empty tuple

[Bug c++/53648] New: nested empty tuple

[chesstr at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

             Bug #: 53648
           Summary: nested empty tuple
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: chesstr@hotmail.com


#include <tuple>

int main(){
    auto b = std::tuple<std::tuple<std::tuple<>>>{};
}

gives : 

error: 'std::_Tuple_impl<1ul>' is an ambiguous base of 'std::_Tuple_impl<0ul,
std::tuple<std::tuple<> > >'

[Bug c++/53648] nested empty tuples

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-06-12
     Ever Confirmed|0                           |1

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-12 16:42:37 UTC ---
I have a fix for this already, IIRC it's simply a case of not using the EBO for
a tuple that contains std::tuple<>

[Bug libstdc++/53648] [C++11] nested empty tuples

[chesstr at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

--- Comment #2 from chesstr at hotmail dot com 2012-06-12 17:13:33 UTC ---
(In reply to comment #1)
> I have a fix for this already, IIRC it's simply a case of not using the EBO for
> a tuple that contains std::tuple<>

Yes, an easy fix in tuple implementation by modifying __empty_not_final as
below compiles :


template<typename _Tp>
    using __empty_not_final
      = typename conditional<__is_final(_Tp)||is_same<_Tp,tuple<>>::value,
false_type, is_empty<_Tp>>::type;

instead of 

template<typename _Tp>
    using __empty_not_final
      = typename conditional<__is_final(_Tp), false_type, is_empty<_Tp>>::type;

[Bug libstdc++/53648] [C++11] nested empty tuples

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-12 17:27:50 UTC ---
There are other cases involving non-empty tuples that will still result in an
ambiguity e.g.

struct A { };
auto d = tuple<tuple<tuple<A, A>, A>, A>{};

My solution avoids using the EBO for some condition I don't remember (the
code's on another machine) but it handles all the cases I tested.

I also preserve the property that sizeof(tuple<char, tuple<>>)==1, which I
think your suggestion loses.

[Bug libstdc++/53648] [C++11] nested empty tuples

[chesstr at hotmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

--- Comment #4 from chesstr at hotmail dot com 2012-06-12 18:03:56 UTC ---
(In reply to comment #3)
> There are other cases involving non-empty tuples that will still result in an
> ambiguity e.g.
> 
> struct A { };
> auto d = tuple<tuple<tuple<A, A>, A>, A>{};
> 
> My solution avoids using the EBO for some condition I don't remember (the
> code's on another machine) but it handles all the cases I tested.
> 
> I also preserve the property that sizeof(tuple<char, tuple<>>)==1, which I
> think your suggestion loses.

You are right, the suggestion does not solve the real problem at all. Nice
example.

[Bug libstdc++/53648] [C++11] nested empty tuples

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-14 22:07:37 UTC ---
Author: redi
Date: Thu Jun 14 22:07:33 2012
New Revision: 188636

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=188636
Log:
    PR libstdc++/53648
    * include/std/tuple (__empty_not_final): Do not use EBO for tuples.
    * testsuite/20_util/tuple/53648.cc: New.
    * testsuite/20_util/uses_allocator/cons_neg.cc: Adjust dg-error line
    number.

Added:
    trunk/libstdc++-v3/testsuite/20_util/tuple/53648.cc
      - copied, changed from r188635,
trunk/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc
Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/std/tuple
    trunk/libstdc++-v3/testsuite/20_util/uses_allocator/cons_neg.cc

[Bug libstdc++/53648] [C++11] nested empty tuples

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53648

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.8.0
      Known to fail|                            |4.7.1

--- Comment #6 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-14 22:12:13 UTC ---
Fixed