up vote 10 down vote favorite

For the following code

#include <array>

template<unsigned MaxP, typename type>
struct kernel
{
  static constexpr unsigned max_pole(unsigned P)
  { return P>MaxP? MaxP:P; }

  template<unsigned P>
  using array = std::array<type,max_pole(P)>;          // wrong?

  template<unsigned P>
  static void do_something(array<P> const&, array<P>&);
};

gcc 4.7.0 (g++ -c -std=c++11) gives

error: ‘max_pole’ was not declared in this scope

Is this correct (behaviour of the compiler)? Note that if I resolve max_pole by replacing it with kernel::max_pole on the line indicated, it compiles fine.

EDIT Reported to bugzilla, accepted as bug c++/55992, see http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55992. Also occurs with gcc 4.7.x and 4.8.0.

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79% accept rate
Just tested: same for g++ 4.7.2 Someone has a newer version like 4.8? Maybe this is a bug which was fixed... – leemes Jan 15 at 13:54
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1 Answer

up vote 8 down vote accepted

Your template compiles fine with Clang 3.2. I strongly believe it is a GCC bug (which is present in GCC 4.7.2 as well, btw). Change notes for GCC 4.8.0 do not seem to mention any such bugfix.

Also notice, that the compilation error disappears if you remove the declaration of do_something<>, which should not make any difference.

One more hint: while this template does not compile on GCC 4.7.2:

template<unsigned MaxP, typename type>
struct kernel
{
    static constexpr unsigned max_pole(unsigned P)
    { return P>MaxP? MaxP:P; }

     template<typename T>
     using array2 = int[max_pole(3)]; // ERROR!

     static void do_something(array2<int> const&, array2<int>&);
};

This template does compile:

template<unsigned MaxP, typename type>
struct kernel
{
    static constexpr unsigned max_pole(unsigned P)
    { return P>MaxP? MaxP:P; }

     using array2 = int[max_pole(3)]; // OK

     static void do_something(array2 const&, array2&);
};

Since max_pole is in both cases an unqualified independent name, the lookup strategy should be the same in both cases, and it is not. To me, this qualifies it as a bug.

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So, according to your research, this situation happens specifically when you mix two new C++11 features (constexpr + alias templates) under certain circumstances. The perfect scenario then for a compiler bug. – Gorpik Jan 15 at 14:50
@Gorpik: yes, I have the same feeling – Andy Prowl Jan 15 at 14:53
1  
+1 thanks. I had the same feeling. reported at bugzilla. – Walter Jan 15 at 16:03
@Walter : Mind posting a link? :-] – ildjarn Jan 16 at 22:26
@ildjarn see edited question. – Walter Jan 17 at 12:39
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