Associated data families don't use superclasses when deriving instances

class Functor (TotalMap key) => Key key where
  data TotalMap key :: Type -> Type
  (¡) :: TotalMap key val -> key -> val

a constraint Key key entails a Functor (TotalMap key) constraint, using the notation of constraints

prf1 :: Key key :- Functor (TotalMap key)
prf1 = Sub Dict

prf2 :: (Key key1, Key key2) :- (Functor (TotalMap key1), Functor (TotalMap key2))
prf2 = prf1 *** prf1

Yet these proofs are not used in

-- tzLp.hs:28:89-95: error: …
--     • No instance for (Functor (TotalMap key1))
--         arising from the first field of ‘PairMap’
--           (type ‘TotalMap key1 (TotalMap key2 val)’)
--       Possible fix:
--         use a standalone 'deriving instance' declaration,
--           so you can specify the instance context yourself
--     • When deriving the instance for (Functor (TotalMap (key1, key2)))
-- Compilation failed.

instance (Key key1, Key key2) => Key (key1, key2) where
  data TotalMap (key1, key2) val = PairMap (TotalMap key1 (TotalMap key2 val)) 
    deriving Functor

  (¡) :: TotalMap (key1, key2) val -> ((key1, key2) -> val)
  PairMap keys ¡ (k1, k2) = keys ¡ k1 ¡ k2

I would expect it to work like

deriving instance (Key key1, Key key2) => Functor (TotalMap (key1, key2))

Same problem occurs with only a single constraint

-- tzLp.hs:33:14-20: error: …
--     • No instance for (Functor (TotalMap key))
--         arising from the first field of ‘Id’ (type ‘TotalMap key val’)
--       Possible fix:
--         use a standalone 'deriving instance' declaration,
--           so you can specify the instance context yourself
--     • When deriving the instance for (Functor
--                                         (TotalMap (Identity key)))
-- Compilation failed.

instance Key key => Key (Identity key) where
  data TotalMap (Identity key) val = Id (TotalMap key val)
    deriving Functor

which should work like

deriving instance Key key                => Functor (TotalMap (Identity key))
-- or
deriving instance Functor (TotalMap key) => Functor (TotalMap (Identity key))

Trac metadata

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