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

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

[morwenn29 at hotmail dot fr]

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.

[Bug c++/56991] constexpr std::initializer_list rejects too complex initialization

[paolo.carlini at oracle dot com]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|constexpr                   |constexpr
                   |std::initializer_list       |std::initializer_list
                   |crashes on too complex      |rejects too complex
                   |initialization              |initialization

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> 2013-04-18 07:50:36 UTC ---
I don't see the compiler *crashing*, I see it *rejecting* the snippet.

[Bug c++/56991] constexpr std::initializer_list rejects too complex initialization

[mizvekov at gmail dot com]

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

--- Comment #2 from Matheus Izvekov <mizvekov at gmail dot com> 2013-04-18 08:10:19 UTC ---
I get also a similar bug:

#include <initializer_list>

//is accepted by gcc
constexpr std::initializer_list<int> good1 = { 1, 2, 3 };

struct foo { int a, b; };

//still ok
constexpr foo good2 = { 1, 2 };

//gcc rejects this
constexpr std::initializer_list<foo> bad = { { 1, 2 }, { 3, 4} };

Compiling this gives a similar error:

test.cc:12:64: error: ‘const std::initializer_list<foo>{((const foo*)(&
_ZGRL3bad0)), 2u}’ is not a constant expression
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From: "morwenn29 at hotmail dot fr" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/56991] constexpr std::initializer_list rejects too complex initialization
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idV991

--- Comment #3 from Morwenn <morwenn29 at hotmail dot fr> 2013-04-18 08:12:44 UTC ---
(In reply to comment #1)
> I don't see the compiler *crashing*, I see it *rejecting* the snippet.

Indeed, sorry for the poor formulation.

[Bug c++/56991] constexpr std::initializer_list rejects too complex initialization

[paolo.carlini at oracle dot com]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2013-05-24
             Blocks|                            |54367
     Ever confirmed|0                           |1

[Bug c++/56991] constexpr std::initializer_list rejects too complex initialization

[paolo.carlini at oracle dot com]

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

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |4.9.1

--- Comment #5 from Paolo Carlini <paolo.carlini at oracle dot com> ---
This is fixed in 4.9.1 and mainline. I'm adding a reduced testcase and closing
the bug.

[Bug c++/56991] constexpr std::initializer_list rejects too complex initialization

[paolo at gcc dot gnu.org]

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

--- Comment #6 from paolo at gcc dot gnu.org <paolo at gcc dot gnu.org> ---
Author: paolo
Date: Fri Aug 29 13:12:02 2014
New Revision: 214729

URL: https://gcc.gnu.org/viewcvs?rev=214729&root=gcc&view=rev
Log:
2014-08-29  Paolo Carlini  <paolo.carlini@oracle.com>

    PR c++/56991
    * g++.dg/cpp0x/constexpr-56991.C: New.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-56991.C
Modified:
    trunk/gcc/testsuite/ChangeLog