Type family equation foralls allow strange re-quantification of class-bound type variables

The following code typechecks on GHC HEAD (8.7+):

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import Data.Kind

class C1 a where
  type T1 a b
class C2 a where
  type T2 a b

instance C1 (Maybe a) where
  type forall a b. T1 (Maybe a) b = b
instance C2 (Maybe a) where
  type forall   b. T2 (Maybe a) b = b

But ought it to? There is something funny happening in the C1 instance: it's explicitly quantifying a, despite the fact that it had previously been quantified by the class head! Moreover, it appears that you can safely drop the a in the explicit forall, as the C2 (Maybe a) instance witnesses.

What does the documentation have to say on this topic? This is all I could find:

When an explicit forall is present, all //type// variables mentioned must be bound by the forall.

I couldn't find anything on the interaction between this feature and associated type families. We should:

1. Decide which of the two programs above should be accepted, and
2. Update the documentation to reflect this.

Trac metadata

Trac field	Value
Version	8.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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