newtype and existentials

[Ralf Hinze]

Is there a strong reason why

> {-# OPTIONS -fglasgow-exts #-}
> data Type :: (* -> *) where
>   Int   ::  Type Int
> data Dynamic  =  forall a . Dyn (a, Type a)

works, but not

> newtype Dynamic  =  forall a . Dyn (a, Type a)

Cheers, Ralf