GHC sometimes not computing open type family application in kind inference

In writing code for type-level lenses, I've run into an issue with kind inference. I've boiled it down to the following oddity:

{-# LANGUAGE DataKinds    #-}
{-# LANGUAGE PolyKinds    #-}
{-# LANGUAGE TypeFamilies #-}

module Bug where 

import Data.Kind(Type)

type family FooKind (a :: Type) :: k
class Foo (a :: Type) where
  type FooType a :: FooKind a

-- OK
type instance FooKind Bool = Type
instance Foo Bool where
  type FooType Bool = Int

-- not OK
data A
type instance FooKind A = Type
instance Foo A where
  type FooType A = Int

This produces the following error:

• Expected kind ‘FooKind A’, but ‘Int’ has kind ‘Type’

• In the type ‘Int’

In the type instance declaration for ‘FooType’

In the instance declaration for ‘Foo A’

I thought it might be related to the type family FooKind being polymorphic in its return kind, but changing the return kind to 'Type' does not affect this. \\ Changing "FooKind" to be a closed type family fixes this problem.

\\ \\ \\ For the context in which this came up, I was writing type-level optics of which one can take the product (with disjointness guaranteed at the type level with type families). Then I needed a type family that chooses which product to use. For instance I have kind annotations:

(     (Index 0 :: Optic (V Int 3) Int)
  :*: (Index 2 :: Optic (V Int 3) Int)
      ) :: Optic (V Int 3) (V Int 2) )

type Fields = '[ '("a", Int), '("b", Bool), '("c", Float) ]
(     (Name "a" :: Optic (Struct Fields) Int
  :*: (Name "c" :: Optic (Struct Fields) Float
      ) :: Optic Fields '[ '("a", Int), '("c", Float) ]

The type family :*: thus needs to figure out which product structure to use... in the first case I need to use V Int :: Nat -> Type, and in the second Struct :: [(Symbol,Type)] -> Type. \\ I wanted to use open type families for extensibility of which cartesian structure to use.

Trac metadata

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