[Bug c++/70810] std::function template variadic template arguments do not unpack in function template

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70810

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Similar problems described at:
http://stackoverflow.com/q/11103384/981959
http://stackoverflow.com/q/34051435/981959