[Bug c++/58109] New: alignas() fails to compile with constant expression

[Bug c++/58109] New: alignas() fails to compile with constant expression

[janezz55 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

            Bug ID: 58109
           Summary: alignas() fails to compile with constant expression
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: janezz55 at gmail dot com

Created attachment 30626
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30626&action=edit
file demonstrating bug

The following example compiles without issue with clang++ 3.3:

#include <cassert>

#include <string>

#include <type_traits>

#include <typeinfo>

#include <utility>

namespace detail
{

template <typename A, typename ...B>
struct max_align
  : std::integral_constant<std::size_t,
      (alignof(A) > max_align<B...>{}) ? alignof(A) : max_align<B...>{}>
{
};

template <typename A, typename B>
struct max_align<A, B>
  : std::integral_constant<std::size_t,
      (alignof(A) > alignof(B)) ? alignof(A) : alignof(B)>
{
};

template <typename A>
struct max_align<A>
  : std::integral_constant<std::size_t, alignof(A)>
{
};

};

template <typename ...T>
struct test
{
  alignas(::detail::max_align<T...>::value) char store_[10];
};

int main()
{
  test<std::string, int> a;

  return 0;
}

But even with g++-4.9, it fails with:

t.cpp:39:59: error: requested alignment is not an integer constant
   alignas(::detail::max_align<T...>::value) char store_[10];

[Bug c++/58109] alignas() fails to compile with constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-08-09
                 CC|                            |dodji at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Let's simplify this:

template <typename T>
struct max_align
{
  static const int value = alignof(T);
};

template <typename T>
struct test
{
  alignas(max_align<T>::value) char store_;
};

template class test<double>;

[Bug c++/58109] alignas() fails to compile with constant expression

[janezz55 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

--- Comment #2 from Janez Zemva <janezz55 at gmail dot com> ---
Please don't neglect std::integral_constant, as it is has a constexpr
conversion operator and so one can use its "instance" as a template parameter.

[Bug c++/58109] alignas() fails to compile with constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Paolo Carlini <paolo.carlini at oracle dot com> ---
I don't think it's necessary to demonstrate the bug. Whoever will fix it, for
sure will double check with the original testcase too.

Adding Jason too in CC, this could be a re-opened c++/56859.

[Bug c++/58109] alignas() fails to compile with constant expression

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |redi at gcc dot gnu.org

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> ---
*** Bug 59013 has been marked as a duplicate of this bug. ***

[Bug c++/58109] alignas() fails to compile with constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58109

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |6.0