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).
