GHCi :info uses an outdated mechanism to show type synonym arity

Current GHCi behavior:

ghci> type T = Maybe
ghci> :i T
type T :: * -> *
type T = Maybe :: * -> *
  	-- Defined at <interactive>:24:1

I expect to see type T = Maybe without the * -> * annotation, which is redundant: there's a standalone kind signature on the preceding line.

Historical remark

Such annotations were not wholly redundant in the past, as they could influence arity inference. Compare, for example

ghci> type T0 :: forall k. k -> Type; type T0 = Proxy :: forall k. k -> Type
ghci> type T1 :: forall k. k -> Type; type T1 = Proxy

GHC 9.6 assigns different arities to T0 and T1:

ghci> :i T0
type T0 :: forall k. k -> *
type T0 = Proxy :: forall k. k -> *
  	-- Defined at <interactive>:4:33
ghci> :i T1
type T1 :: forall k. k -> *
type T1 = Proxy :: k -> *
  	-- Defined at <interactive>:5:33

However, this arcane mechanism has been removed in e89aa072. Here's what GHC HEAD has to say about these declarations:

ghci> :i T0
type T0 :: forall k. k -> *
type T0 = Proxy :: forall k. k -> *
  	-- Defined at <interactive>:4:33
ghci> :i T1
type T1 :: forall k. k -> *
type T1 = Proxy :: forall k. k -> *
  	-- Defined at <interactive>:5:33

That is, they are equivalent. The modern mechanism for controlling arity is the use of @-binders.

Current status

The pretty-printer has not been updated to use @-binders. Consider this GHCi session (GHC HEAD)

ghci> :set -XTypeAbstractions 
ghci> type T2 :: forall k. k -> Type; type T2 @k = Proxy
ghci> :i T2
type T2 :: forall k. k -> *
type T2 = Proxy :: k -> *
  	-- Defined at <interactive>:5:33

The arity of the printed declaration is not equivalent to the user-written declaration. It should use @k to include the kind variable in the arity: type T2 @k = Proxy.

(A further complication is that the printed example triggers a panic, see #24470 (closed)).

Proposed fix

1. In the pretty-printer, omit the redundant kind annotation on the RHS of type synonyms
2. In the pretty-printer, use @-binders to control the arity

Also, I've only checked type synonyms. There might be similar issues affecting type families.