Unpacking parameter packs in template aliases

Question

I run into a problem with unpacking variadic templates into a template alias.

The following code works with Clang 3.4 and GCC 4.8 but fails with GCC 4.9:

Answer 1

It was reported in gcc4.9 bugzilla:

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

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() { }

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From the explanation from gcc folks - it is not so obvious this is a real bug. This is still discussion held in gcc bugzilla and in DR 1430 (http://open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1430) - summary in now in the question above.