[Bug c++/85049] Internal compiler error with __integer_pack

[abigail.buccaneer at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85049

--- Comment #1 from AbigailBuccaneer <abigail.buccaneer at gmail dot com> ---
If `index_sequence_for` is replaced with `std::index_sequence_for` (and
`#include <utility>` is added), this code compiles fine under g++-7, but not
g++-8 (where std::index_sequence_for is implemented in terms of __index_pack,
similar to how it is in the code snippet above).