[Bug c++/53609] New: Wrong variadic template pack expansion

[Bug c++/53609] New: Wrong variadic template pack expansion

[Gaetano.Checinski at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

             Bug #: 53609
           Summary: Wrong variadic template pack expansion
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: Gaetano.Checinski@googlemail.com


g++-4.7 missinterperts folowing code :


template<class...I> struct List{};
template<int I> struct Z{ static constexpr int value=I;};
template<int...I> using NumList = List<  Z<I>... >;

template<class L>struct Mover;
template<template<class...>class C, class...I>
struct Mover<C<I...>>
{
    using NL = NumList< I::value... > ;  // aka :  List<  Z<I>... >
    // missinterpreted as : List< Z<I...> >
};

using L=List< Z<1>,Z<2>,Z<3> >; // Works fine
using L2= NumList< 1,2,3 >; // Works also fine

Mover< L >::NL A; // dont work : should be 1 not 3 args in 'Z'

[Bug c++/53609] Wrong variadic template pack expansion in alias template

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-06-08
                 CC|                            |dodji at gcc dot gnu.org
            Summary|Wrong variadic template     |Wrong variadic template
                   |pack expansion              |pack expansion in alias
                   |                            |template
     Ever Confirmed|0                           |1
      Known to fail|                            |4.7.0, 4.8.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-06-08 09:49:33 UTC ---
Confirmed.

It works fine if you rewrite it without alias templates:

template<int...I> struct NumList { typedef List<  Z<I...> > type; };

then use NumList::type instead of NumList, so the problem is with the alias
template substitution.

G++ seems to be treating Z<I>... as Z<I...> in the type-id of the
alias-declaration.

[Bug c++/53609] Wrong variadic template pack expansion in alias template

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |dodji at gcc dot gnu.org
                   |gnu.org                     |

[Bug c++/53609] Wrong variadic template pack expansion in alias template

[dodji at seketeli dot org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

--- Comment #2 from dodji at seketeli dot org <dodji at seketeli dot org> 2012-09-20 08:48:09 UTC ---
A candidate patch for this has been sent to
http://gcc.gnu.org/ml/gcc-patches/2012-09/msg01436.html

[Bug c++/53609] Wrong variadic template pack expansion in alias template

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

--- Comment #3 from Dodji Seketeli <dodji at gcc dot gnu.org> 2013-01-22 10:05:13 UTC ---
Author: dodji
Date: Tue Jan 22 10:05:05 2013
New Revision: 195367

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=195367
Log:
PR c++/53609 - Wrong variadic template pack expansion in alias template

Consider this example:

     1    template<class...I> struct List {};
     2    template<int T> struct Z {static const int value = T;};
     3    template<int...T> using LZ = List<Z<T>...>;
     4
     5    template<class...U>
     6    struct F
     7    {
     8      using N = LZ<U::value...>; //#1 This should amount to
List<Z<U::value>...>
     9    }
    10
    11    F<Z<1>, Z<2> >::N A; //#2

which G++ fails to compile, with this error message:

test-PR53609-3.cc: In instantiation of 'struct F<Z<1>, Z<2> >':
test-PR53609-3.cc:11:15:   required from here
test-PR53609-3.cc:3:43: error: wrong number of template arguments (2, should be
1)
 template<int...T> using LZ = List<Z<T>...>;
                                           ^
test-PR53609-3.cc:2:24: error: provided for 'template<int T> struct Z'
 template<int T> struct Z {static const int value = T;};

I think this is because in #1, when we substitute the argument pack
{U::value...} into the pack expansion Z<T>..., tsubst_pack_expansion
yields Z<U::value...>, instead of Z<U::value>..., so the instantiation
of LZ amounts to List<Z<U::value...> >, instead of
List<Z<U::value>...>.

The idea of this patch is to make tsubst_pack_expansion support
substituting an argument pack (into a pack expansion) where one of the
arguments (let's call it the Ith argument) is itself a pack expansion
P.  In that case, the Ith element resulting from the substituting
should be a pack expansion P'.

The pattern of P' is then the pattern of P into which the pattern of
the Ith argument of the argument pack has been substituted.

Tested on x86_64-unknown-linux-gnu against trunk.

gcc/cp/

    * pt.c (argument_pack_element_is_expansion_p)
    (make_argument_pack_select, use_pack_expansion_extra_args_p)
    (gen_elem_of_pack_expansion_instantiation): New static functions.
    (tsubst): When looking through an ARGUMENT_PACK_SELECT tree node,
    look through the possibly resulting pack expansion as well.
    (tsubst_pack_expansion): Use use_pack_expansion_extra_p to
    generalize when to use the PACK_EXPANSION_EXTRA_ARGS mechanism.
    Use gen_elem_of_pack_expansion_instantiation to build the
    instantiation piece-wise.  Don't use arg_from_parm_pack_p anymore,
    as gen_elem_of_pack_expansion_instantiation and the change in
    tsubst above generalize this particular case.
    (arg_from_parm_pack_p): Remove this for it's not used by
    tsubst_pack_expansion anymore.

gcc/testsuite/

    * g++.dg/cpp0x/variadic139.C: New test.
    * g++.dg/cpp0x/variadic140.C: Likewise.
    * g++.dg/cpp0x/variadic141.C: Likewise.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/variadic139.C
    trunk/gcc/testsuite/g++.dg/cpp0x/variadic140.C
    trunk/gcc/testsuite/g++.dg/cpp0x/variadic141.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/pt.c
    trunk/gcc/testsuite/ChangeLog

[Bug c++/53609] Wrong variadic template pack expansion in alias template

[dodji at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53609

Dodji Seketeli <dodji at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED

--- Comment #4 from Dodji Seketeli <dodji at gcc dot gnu.org> 2013-01-30 11:44:49 UTC ---
Fixed in trunk (4.8)