https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104242
--- Comment #1 from Fedor Chelnokov <fchelnokov at gmail dot com> --- The error message is invalid use of incomplete type 'std::__conditional_t<true, std::is_copy_constructible<A>, std::__not_<std::integral_constant<bool, false> > >' {aka 'struct std::is_copy_constructible<A>'} ...