A label label that identifies its outer type as a member of an identity.
Examples:
TEST_CASE("should allow its outer type to satisfy the appropriate generic type tag")
{
static_assert(nkr::generic::tag::identity_tr<nkr::constant_tg>);
static_assert(nkr::generic::tag::identity_tr<const nkr::constant_tg>);
static_assert(nkr::generic::tag::identity_tr<volatile nkr::constant_tg>);
static_assert(nkr::generic::tag::identity_tr<const volatile nkr::constant_tg>);
}