https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90548
--- Comment #3 from Marek Polacek <mpolacek at gcc dot gnu.org> --- We ended up with an empty call_args vec because the pack expanded here: 18842 /* Expand the pack expansion and push each entry onto 18843 CALL_ARGS. */ 18844 arg = tsubst_pack_expansion (arg, args, complain, in_decl); expanded to an empty vector. So nothing got pushed onto call_args.