[Bug libstdc++/53657] [C++11] pair(pair&&) move constructor is non-trivial

["paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53657

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |4.5.1, 4.6.0
   Target Milestone|---                         |4.7.2
            Summary|[C++11] pair(pair&&) move   |[4.7/4.8 Regression][C++11]
                   |constructor is non-trivial  |pair(pair&&) move
                   |                            |constructor is non-trivial
      Known to fail|                            |4.5.0

Paolo Carlini <paolo.carlini at oracle dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|4.5.1, 4.6.0                |
   Target Milestone|4.7.2                       |---
            Summary|[4.7/4.8 Regression][C++11] |[C++11] pair(pair&&) move
                   |pair(pair&&) move           |constructor is non-trivial
                   |constructor is non-trivial  |
      Known to fail|4.5.0                       |

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-06-14 09:39:58 UTC ---
Thanks.

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-06-14 09:40:44 UTC ---
The issue is related to LWG2005, and can be summarized with the following
testcase, which should not trigger the static_assert.

In short, if we want std::pair' move constructor defaulted, we have to use
std::is_constructible, not std::is_convertible, to constrain container::insert.

If people agree with my assessment - I don't see what else we could possibly do
on the library side - I can try again moving the latter to use
std::is_constructible, should mostly work.

/////////////////

#include <type_traits>
#include <utility>

struct move_only
{
  move_only(const move_only&) = delete;
  move_only(move_only&&) = default;
};

template<typename _T1, typename _T2>
 struct pair
 {
   pair(pair&&) = default;

   template<class _U1, class _U2>
     pair(pair<_U1, _U2>&& __p)
     : first(std::forward<_U1>(__p.first)),
       second(std::forward<_U2>(__p.second)) { }

   _T1 first;
   _T2 second;
 };

typedef pair<move_only, move_only> Pair;
typedef pair<const move_only, move_only> CPair;

static_assert(std::is_convertible<Pair, CPair>::value, "Error");
//static_assert(std::is_constructible<CPair, Pair&&>::value, "Error");