No skolem info panic

I've been toying with some type-level lenses and running into some issues with kind unification, when I ran into a panic:

bug.hs:30:34: error:ghc.exe: panic! (the 'impossible' happened)
  (GHC version 8.6.2 for x86_64-unknown-mingw32):
        No skolem info:
  [k_a1Hgj]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler\utils\Outputable.hs:1160:37 in ghc:Outputable
        pprPanic, called at compiler\\typecheck\\TcErrors.hs:2891:5 in ghc:TcErrors

Here's a boiled down version (with a bit of extraneous code left in for context, as it's so short):

{-# LANGUAGE DataKinds              #-}
{-# LANGUAGE FlexibleInstances      #-}
{-# LANGUAGE GADTs                  #-}
{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE PolyKinds              #-}
{-# LANGUAGE ScopedTypeVariables    #-}
{-# LANGUAGE TypeFamilies           #-}
{-# LANGUAGE TypeOperators          #-}

data Optic a where
  --Index :: Nat    -> Optic a
  --Name  :: Symbol -> Optic a
  (:.:) :: Optic a -> Optic b -> Optic a -- composition

class Gettable a (optic :: Optic a) where
  type Get a (optic :: Optic a)

{-
some basic instances, e.g.
instance Gettable (a,b) (Index 0) where
  type Get (a,b) (Index 0) = a
...
-}

instance forall a b (g1 :: Optic a) (g2 :: Optic b).
        ( Gettable a g1
        , b ~ Get a g1
        , Gettable b g2
        ) => Gettable a (g1 :.: g2) where
  type Get a (g1 :.: g2) = Get a g2

The program I am actually trying to write has the instance declaration changed to

instance forall a b (g1 :: Optic a) (g2 :: Optic (Get a g1)).
        ( Gettable a g1
        , b ~ Get a g1
        , Gettable b g2
        ) => Gettable a (g1 :.: g2) where
  type Get a (g1 :.: g2) = Get (Get a g1) g2

but GHC complains that it can't match kinds:

    • Expected kind ‘Optic b’, but ‘g2’ has kind ‘Optic (Get a g1)’
    • In the second argument of ‘Gettable’, namely ‘g2’
      In the instance declaration for ‘Gettable a (g1 :.: g2)’
   |
20 |         , Gettable b g2
   |

I don't know if there is a way around that, and I'd be interested to hear any advice.

Trac metadata

Trac field	Value
Version	8.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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