Instances for ImpredicativeTypes

Now that we have ImpredicativeTypes available in a nice way, it'd be cool if we could use them in type class instances.

Motivation

I'm playing with OverloadedRecordDot and running into the problem that I can't write a polymorphic virtual method. For example:

data User = User { name :: String }

instance HasField "showWithName" User (forall a . Show a => a -> String) where
    getField self a = 
        concat [self.name, " : ", show a]

What's actually worse is that I can't include any type variables, at all, so I can't even write, like,

instance HasField "identity" User (forall a. a -> a) where
    getField _ a = 
        a

More simply,

Actually, the problem is that we can't write an instance for an impredicative type, at all.

{-# Language ImpredicativeTypes, FlexibleInstances #-}

class C a

instance C (forall a . a -> a)

This fails with the error message:

src/Impl/TH.hs:30:10-30: error:
    • Illegal polymorphic type: forall a. a -> a
    • In the instance declaration for ‘C (forall a. a -> a)’
   |
30 | instance C (forall a . a -> a)
   |          ^^^^^^^^^^^^^^^^^^^^^

I don't know if this is properly a bug or a feature request - on the one hand, I am asking for new functionality, and on the other hand, I am asking for the intersection of existing functionality to work.