GHCi command listing possible type class instances

This is a reminder ticket,

I want to be able to list possible instances,

>>> :insts Product
(Functor     f, Functor     g) => Functor     (Product f g)
(Applicative f, Applicative g) => Applicative (Product f g)
...

>>> :insts Product Identity
Functor     g => Functor     (Product Identity g)
Applicative g => Applicative (Product Identity g)
Monad       g => Monad       (Product Identity g)
MonadFix    g => MonadFix    (Product Identity g)
MonadZip    g => MonadZip    (Product Identity g)
...

>>> :insts Product Identity Identity
Functor     (Product Identity Identity)
Applicative (Product Identity Identity)
Monad       (Product Identity Identity)
MonadFix    (Product Identity Identity)
MonadZip    (Product Identity Identity)

Would be so useful for GeneralizedNewtypeDeriving

newtype Pair a = P (Product Identity Identity a)
  deriving (Functor, Applicative, Monad, MonadFix, MonadZip, ...)

-- pattern (:#) :: a -> a -> Pair a 
-- pattern a1 :# a2 = P (Identity a1 `Pair` Identity a2)

Trac metadata

Trac field	Value
Version	8.0.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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