[Bug c++/56991] New: constexpr std::initializer_list crashes on too complex initialization

["morwenn29 at hotmail dot fr" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56991

             Bug #: 56991
           Summary: constexpr std::initializer_list crashes on too complex
                    initialization
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: morwenn29@hotmail.fr


I found some strange behaviour that, after a discussion on StackOverflow, seems
to be a bug (discussion here:
http://stackoverflow.com/questions/16057690/confusion-about-constant-expressions/16068953?noredirect=1#16068953).

It seems that GCC implements N3471 which means that every function of an
std::initializer_list are constexpr. When trying to pass simple constexpr
things in the initializer_list, it works fine:

#include <array>
#include <initializer_list>

int main()
{
    constexpr std::array<int, 3> a = {{ 1, 2, 3 }};
    constexpr int a0 = a[0];
    constexpr int a1 = a[1];
    constexpr int a2 = a[2];
    constexpr std::initializer_list<int> b = { a0, a1, a2 };

    return 0;
}

However, without the intermediate variables a0, a1 and a2, the example above
crashes:

#include <array>
#include <initializer_list>

int main()
{
    constexpr std::array<int, 3> a = {{ 1, 2, 3 }};
    constexpr std::initializer_list<int> b = { a[0], a[1], a[2] };

    return 0;
}

The error is the following one:

error: 'const std::initializer_list<int>{((const int*)(&<anonymous>)), 3u}' is
not a constant expression

This last example works fine if I remove the constexpr qualifier at the
beginning of the line or if I replace the initializer_list by a std::array. It
seems that the bug is only triggered when using std::initializer_list with
constexpr.