[Bug c++/59498] New: Pack expansion error in template alias

[Bug c++/59498] New: Pack expansion error in template alias

[ldionne.2 at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

            Bug ID: 59498
           Summary: Pack expansion error in template alias
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ldionne.2 at gmail dot com

GCC Version
-----------
gcc-4.9 (GCC) 4.9.0 20131201 (experimental)
Installed with Homebrew.


System
------
OS X 10.8.5


Command line to trigger the bug
-------------------------------
gcc-4.9 -std=c++11 -Wall -Wextra -pedantic -o /dev/null -c
template_alias_bug.cpp


Minimal code to reproduce
-------------------------

    template <typename T, typename ...>
    using alias = T;

    template <typename ...T>
    using variadic_alias = alias<T...>;

    using Fail = variadic_alias<int>;

    int main() { }


Error message
-------------
template_alias_bug.cpp:9:34: error: pack expansion argument for non-pack
parameter 'T' of alias template 'template<class T, class ...> using alias = T'
 using variadic_alias = alias<T...>;
                                  ^
template_alias_bug.cpp:5:11: note: declared here
 template <typename T, typename ...>
           ^
template_alias_bug.cpp:11:14: error: expected type-specifier before
'variadic_alias'
 using Fail = variadic_alias<int>;
              ^

Possibly related
----------------
Bug 58856


Comments
--------
Clang compiles the code just fine and I don't see any obvious reason why that
would be invalid C++11 (I could be mistaken), so I think that's a GCC bug.

[Bug c++/59498] Pack expansion error in template alias

[rscrihf at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #1 from Roy Crihfield <rscrihf at gmail dot com> ---
Getting the same behavior on 4.9.0 as stable release.

Rewriting the code to use a struct succeeds:

template <typename T, typename ...>
struct alias { using type = T; };

template <typename ...T>
using variadic_alias = typename alias<T...>::type;

[Bug c++/59498] [4.9/4.10 Regression] Pack expansion error in template alias

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-05-04
      Known to work|                            |4.8.2
            Summary|Pack expansion error in     |[4.9/4.10 Regression] Pack
                   |template alias              |expansion error in template
                   |                            |alias
     Ever confirmed|0                           |1
      Known to fail|                            |4.10.0, 4.9.0

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This is a regression since 4.8  (4.7 gave a "sorry")

[Bug c++/59498] [4.9/4.10 Regression] Pack expansion error in template alias

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |4.9.1

[Bug c++/59498] [4.9/4.10 Regression] Pack expansion error in template alias

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org

--- Comment #3 from Jason Merrill <jason at gcc dot gnu.org> ---
This is due to a change for DR 1430
(http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430), which is still
in drafting.  It seems that my implementation went too far.

[Bug c++/59498] [DR 1430][4.9/4.10 Regression] Pack expansion error in template alias

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
And the current development version of clang also rejects this testcase:

wa.C:5:34: error: pack expansion used as argument for non-pack parameter of
      alias template
    using variadic_alias = alias<T...>;
                                 ^~~~
wa.C:1:24: note: template parameter is declared here
    template <typename T, typename ...>
                       ^

[Bug c++/59498] [DR 1430][4.9/4.10 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.9.1                       |4.9.2

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 4.9.1 has been released.

[Bug c++/59498] [DR 1430][4.9/4.10 Regression] Pack expansion error in template alias

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #7 from Louis Dionne <ldionne.2 at gmail dot com> ---
I ran into this problem in another context, and I think it justifies some
thinking about how this issue is going to be handled by the language. I
_really_ think the following should be valid C++. This is a C++11
implementation of the `quote` utility of the Boost.MPL. Unfortunately,
it won't compile when given an alias:

    #include <type_traits>

    template <template <typename ...> class f>
    struct quote {
        template <typename ...x>
        using apply = f<x...>;
    };

    using test = quote<std::add_pointer_t>::apply<int>;

It is very important for this kind of code to be valid to enable a generic
handling of aliases in metaprograms. Without this, I see two solutions:
    1. Use metafunctions instead of aliases, i.e.
       `quote<std::add_pointer>::apply<int>::type`.
    2. Do not use variadic templates and have numbered forms for `quote` for
       each number of arguments up to some number. This is basically returning
       to C++03.

Tested on gcc 4.9.1 and clang 3.5.0 (trunk).

[Bug c++/59498] [DR 1430][4.9/4.10 Regression] Pack expansion error in template alias

[ldionne.2 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #8 from Louis Dionne <ldionne.2 at gmail dot com> ---
I made further experiments to try and workaround this (I need it badly!) and
I found some things that might be of interest to you. The following test case
compiles on GCC 4.9.1 but not on Clang 3.5.0 (trunk):

    template <template <typename ...> class f>
    struct make {
        template <typename ...x>
        static constexpr auto apply(x...)
            -> decltype(f<x...>{}) // <- compiles on both when this is
commented
        {
            return f<x...>{};
        }
    };

    template <typename> struct f { }; // <- fails on GCC when this is variadic
    template <typename x> using alias = f<x>;


    int main() {
        make<alias>::apply(1);
    }


Now, if I change the definition of f to:

    template <typename...> struct f { };

then it fails to compile on both compilers. Finally, if I remove the
decltype(f<x...>{}) part of the apply function above, then it compiles
on both compilers regardless of f's definition.

[Bug c++/59498] [DR 1430][4.9/5 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.9.2                       |4.9.3

--- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 4.9.2 has been released.

[Bug c++/59498] [DR 1430][4.9/5 Regression] Pack expansion error in template alias

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P5
      Known to fail|4.10.0                      |

[Bug c++/59498] [DR 1430][4.9/5/6 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #10 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 4.9.3 has been released.

[Bug c++/59498] [DR 1430][4.9/5/6 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.9.3                       |4.9.4

[Bug c++/59498] [DR 1430][9/10/11/12 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|8.5                         |9.4

--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 8 branch is being closed.

[Bug c++/59498] [DR 1430][9/10/11/12 Regression] Pack expansion error in template alias

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.4                         |9.5

--- Comment #19 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 9.4 is being released, retargeting bugs to GCC 9.5.

[Bug c++/59498] [DR 1430][9/10/11/12/13 Regression] Pack expansion error in template alias

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|9.5                         |---

[Bug c++/59498] [DR 1430][10/11/12/13 Regression] Pack expansion error in template alias

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|---                         |10.4
            Summary|[DR 1430][9/10/11/12/13     |[DR 1430][10/11/12/13
                   |Regression] Pack expansion  |Regression] Pack expansion
                   |error in template alias     |error in template alias

[Bug c++/59498] [DR 1430][10/11/12/13 Regression] Pack expansion error in template alias

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.4                        |10.5

--- Comment #20 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
GCC 10.4 is being released, retargeting bugs to GCC 10.5.

[Bug c++/59498] [DR 1430][10/11/12/13 Regression] Pack expansion error in template alias

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ncm at cantrip dot org

--- Comment #21 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 109291 has been marked as a duplicate of this bug. ***

[Bug c++/59498] [DR 1430][10/11/12/13 Regression] Pack expansion error in template alias

[ncm at cantrip dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

--- Comment #22 from ncm at cantrip dot org ---
CWG 1430 seems to be about disallowing a construct that requires capturing an
alias declaration into a name mangling. This bug and at least some of those
referred to it do not ask for any such action.

[Bug c++/59498] [DR 1430][11/12/13/14 Regression] Pack expansion error in template alias

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59498

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|10.5                        |11.5

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 10 branch is being closed.