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From: "janezz55 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/58109] New: alignas() fails to compile with constant expression
Date: Fri, 09 Aug 2013 12:37:00 -0000
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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];