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

[d25fe0be at outlook dot com]

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

--- Comment #4 from d25fe0be@ <d25fe0be at outlook dot com> ---
'_t::_call' is a dependent name, adding 'template' before '_call' fixes the
compilation error.