Data type return kinds don't obey the forall-or-nothing rule

Originally noticed here. GHC accepts this:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}

import Data.Kind

data Foo :: forall a. a -> b -> Type where
  MkFoo :: a -> Foo a b

Despite the fact that Foo's return kind is headed by an explicit forall which does not quantify b.

The users' guide doesn't explicitly indicate that the forall-or-nothing rule should apply to data type return kinds, but goldfirere believes that not doing so is an inconsistent design, so I'm opening this ticket to track this.

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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