[Bug c++/62274] New: [C++11] Variadic templates expansion into non-variadic class template

[Bug c++/62274] New: [C++11] Variadic templates expansion into non-variadic class template

[juchem at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

            Bug ID: 62274
           Summary: [C++11] Variadic templates expansion into non-variadic
                    class template
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juchem at gmail dot com

The code below won't compile for gcc 4.9.0, 4.8.1 and 4.7.3. Tested in Debian
sid (aug/2014), http://gcc.godbolt.org/ and http://ideone.com.

#include <type_traits>
using namespace std;

struct bar { typedef double foo; };

template <typename T>
using get_foo = typename T::foo;

template <template <typename...> class T>
struct baz {
  template <typename... UArgs>
  using type = T<UArgs...>;
};

int main() {
  return !std::is_same<double, baz<get_foo>::type<bar>>::value;
}

error output:
prog.cpp: In substitution of ‘template<class T> using get_foo = typename T::foo
[with T = UArgs ...]’:
prog.cpp:13:26:   required from ‘struct baz<get_foo>’
prog.cpp:17:49:   required from here
prog.cpp:8:32: error: ‘UArgs ...’ is not a class, struct, or union type
 using get_foo = typename T::foo;
                                ^
prog.cpp: In function ‘int main()’:
prog.cpp:17:59: error: template argument 2 is invalid
  static_assert(std::is_same<double, baz<get_foo>::type<bar>>::value,
"mismatch");
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Subject: [Bug target/62262] aarch64 gcc generates invalid assembler
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idb262

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Carrot from comment #5)
> Actually the original source code is guarded by assert, and the parameter
> passed to CLZ can be guaranteed not 0, so "value <<= zeros" is well defined
> in our original source code.

The issue is the second CLZ (not the first one).  Though if I read the code
correctly value<<1 can never be zero but GCC does not optimize away that check.

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |ville.voutilainen at gmail dot com

--- Comment #2 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
Clang doesn't accept the code either. The technique looks reasonable, the
validity of this should probably be queried from CWG.

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[juchem at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

--- Comment #3 from juchem at gmail dot com ---
Clang does accept this code: http://goo.gl/YKBt2l

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

--- Comment #4 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
(In reply to juchem from comment #3)
> Clang does accept this code: http://goo.gl/YKBt2l

Clang 3.3 and 3.4 do, yes. 3.5, 3.6 and their trunk don't.

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> ---
EDG accepts it.

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC, clang and MSVC all reject it with a similar error message.
ICC accepts it.

[Bug c++/62274] [C++11] Variadic templates expansion into non-variadic class template

[ville.voutilainen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62274

Ville Voutilainen <ville.voutilainen at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at redhat dot com

--- Comment #7 from Ville Voutilainen <ville.voutilainen at gmail dot com> ---
I think this may be subject to various Core discussions. Jason?