Using a polymorphic type in a type family in haskell

Question

I am studying the type of family, but it is so confusing. When I define a polymorphic type outside the class definition, it works well.

{-# LANGUAGE Rank2Types #-} type T = Num a => a 

But when a polymorphic type is defined inside a class definition,

 {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE Rank2Types #-} data D = D class A a where type T a :: * instance AD where type TD = Num a => a 

then complier shows an error:

 Illegal polymorphic or qualified type: forall a. Num a => a In the type instance declaration for 'T' In the instance declaration for 'AD' 

Is there a way to make a function in a class whose return type is polymorphic, for example 3 (Num a => a)?