fixes#22850
The `is` operator checks the type of the left hand side, and if it's
generic or if it's a `typedesc` type with no base type, it leaves it to
be evaluated later. But `typedesc` types with no base type precisely
describe the default typeclass `type`/`typeclass`, so this condition is
removed. Maybe at some point this represented an unresolved generic
type?
(cherry picked from commit 995081b56a)