Typeable imposes seemingly redundant constraints on polykinded instances

See also #21822 (closed), #16627.

To derive Data for Const, we need

deriving instance (Typeable k, Data a, Typeable (b :: k)) => Data (Const a b)

Where's that Typeable k constraint come from? It turns out that for reasons I haven't looked into, we can only obtain Typeable (Const a (b :: k)) if we have Typeable k; (Typeable a, Typeable b) is insufficient. Is there a reason for that?

Annoyingly, we can actually get that:

weGotThat :: forall k (a :: k). Typeable a :- Typeable k
weGotThat = Sub $ withTypeable (typeRepKind (typeRep :: TypeRep a)) Dict

But of course that doesn't help us.

Could we use UndecidableSuperClasses to work around this problem? I think we likely can, although I'm concerned about the performance implications:

class (Typeable a, Typeable' k) => Typeable' (a :: k)
instance (Typeable a, Typeable' k) => Typeable' (a :: k)

withTypeable' :: forall k (a :: k) r. TypeRep a -> (Typeable' a => r) -> r
withTypeable' tr f = withTypeable tr $ withTypeable (typeRepKind (typeRep @a)) f

Trac metadata

Trac field	Value
Version	8.2.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	bgamari
Operating system
Architecture