Each of these templates shall be a
Cpp17UnaryTypeTrait
with a base characteristic of
true_type if the corresponding condition is true, otherwise
false_type.
Let ELEMS-OF(T) be the parameter pack
get<N>(declval<T>()), where N is the pack of
size_t template arguments of the specialization of
index_sequence denoted by
make_index_sequence<tuple_size_v<remove_reference_t<T>>>.