[Bug c++/81279] variadic template regression : compiles without error under 5.4.0 , 6.3.0 with error under 7.1.0

[jason.vas.dias at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81279

--- Comment #1 from Jason Vas Dias <jason.vas.dias at gmail dot com> ---
Obviously, G++ 5.4.0 and 6.3.0 are able to expand the 
text '_HeadList...' here into the list of types:
Line 184:
   _t._call<  _HeadList...

But G++ 7.1.0 is not able to do so, and gives no clue
as to why not. I guess I must consider variadic template
arguments simply "broken" in this release of GCC ?