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 
using alias = T;

template 
using variadic_alias = alias;

using Fail = variadic_alias;

int main() { }

---

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.