Note [Generics and unlifted types]

Normally, all constants are marked with K1/Rec0. The exception to this rule is
when a data constructor has an unlifted argument (e.g., Int#, Char#, etc.). In
that case, we must use a data family instance of URec (from GHC.Generics) to
mark it. As a result, before we can generate K1 or unK1, we must first check
to see if the type is actually one of the unlifted types for which URec has a
data family instance; if so, we generate that instead.

See wiki:commentary/compiler/generic-deriving#handling-unlifted-types for more
details on why URec is implemented the way it is.