Unable to match instance signatures

Richard said GHC should be able to infer this

{-# Language PolyKinds         #-}
{-# Language RankNTypes        #-}
{-# Language TypeFamilies      #-}
{-# Language FlexibleInstances #-}
{-# Language InstanceSigs      #-}

import Data.Kind

class Ríki ob where
  type Arr :: ob -> ob -> Type

  ið  :: forall (a :: ob). Arr a a

instance Ríki Type where
  type Arr = (->)

  ið :: Arr a a
  ið a = a

$ ~/code/unmodifiedghc/inplace/bin/ghc-stage2 --interactive -ignore-dot-ghci ~/hs/693_bug.hs
GHCi, version 8.7.20181029: http://www.haskell.org/ghc/  :? for help
[1 of 1] Compiling Main             ( /home/baldur/hs/693_bug.hs, interpreted )

/home/baldur/hs/693_bug.hs:18:3: error:
    • Couldn't match kind ‘ob’ with ‘*’
      ‘ob’ is a rigid type variable bound by
        the type signature for:
          ið :: forall ob (a :: ob). Arr a a
        at /home/baldur/hs/693_bug.hs:17:9-15
      When matching types
        p0 :: *
        a :: ob
      Expected type: Arr a a
        Actual type: p0 -> p0
    • The equation(s) for ‘ið’ have one argument,
      but its type ‘Arr a a’ has none
      In the instance declaration for ‘Ríki *’
    • Relevant bindings include
        ið :: Arr a a (bound at /home/baldur/hs/693_bug.hs:18:3)
   |
18 |   ið a = a
   |   ^^^^^^^^
Failed, no modules loaded.

It works if we specify the kind of a or use (->)

  ið :: forall (a :: Type). Arr a a 

  ið :: a -> a 

Trac metadata

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