[Bug c++/57384] New: can't expand a parameter pack into a list of function types or function pointer types

[Bug c++/57384] New: can't expand a parameter pack into a list of function types or function pointer types

[eric.niebler at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57384

            Bug ID: 57384
           Summary: can't expand a parameter pack into a list of function
                    types or function pointer types
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eric.niebler at gmail dot com

I believe all of the following 4 test cases should pass:

  template<typename ...Ts>
  struct list
  {};

  template<typename ...Ts>
  struct S
  {
      using type1 = void(int...(Ts));    // fails
      using type2 = list<int...(Ts)>;    // fails
      using type3 = void(int(*...)(Ts)); // fails
      using type4 = list<int(*...)(Ts)>; // works with 4.7, fails with 4.8
  };

Currently (i.e. with gcc-4.8), 1-4 all fail. With gcc-4.7, (4) worked. I'm
compiling with -std=gnu++11

[Bug c++/57384] can't expand a parameter pack into a list of function types or function pointer types

[redi at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57384

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Eric Niebler from comment #0)
> I believe all of the following 4 test cases should pass:

I'll leave it to someone else to rule on that, they make my head hurt ;)

I will note that this works:

      using type2 = list<int(Ts)...>;

and this works:

      using type4 = list<int(*)(Ts)...>;

And of course all these work:

      template<class X>                                                         
        using F = int(X);                                                       
      using type1 = void(F<Ts>...);                                             
      using type2 = list<F<Ts>...>;                                            
      template<class X>                                                         
        using Fp = int(*)(X);                                                   
      using type3 = void(Fp<Ts>...);                                           
      using type4 = list<Fp<Ts>...>;

[Bug c++/57384] can't expand a parameter pack into a list of function types or function pointer types

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57384

--- Comment #2 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
I have the impression that this *could* be related to

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1488

This is unchecked yet, because I'm leaving my place here.
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From: "paolo.carlini at oracle dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/57388] [C++11] ICE when function types with ref-qualifiers meet other function types
Date: Thu, 23 May 2013 17:04:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?idW388

--- Comment #1 from Paolo Carlini <paolo.carlini at oracle dot com> ---
Let's add Jason in CC.

[Bug c++/57384] can't expand a parameter pack into a list of function types or function pointer types

[eric.niebler at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57384

--- Comment #3 from Eric Niebler <eric.niebler at gmail dot com> ---
Interesting. I filed a similar bug against clang
(http://llvm.org/bugs/show_bug.cgi?id=16118), where Richard Smith seems to feel
the test cases should be:

  template<typename ...Ts>
  struct list
  {};

  template<typename ...Ts>
  struct S
  {
      using type1 = void(int...(Ts));    // (1) fails
      using type2 = list<int(Ts)...>;    // (2) works
      using type3 = void(int(*...)(Ts)); // (3) fails
      using type4 = list<int(*)(Ts)...>; // (4) works
  };

This strikes me as ludicrously inconsistent. I think we need guidance from the
committee here. I was basing my bug report(s) on the example in 8.3.5/13 (which
shows:

   template<typename... T> void f(T (* ...t)(int, int));

The suggestion that the pack expansion syntax differs depending on the context
in which the expansion occurs is, um, unsatisfactory.