Bogus "No instance" error when type families appear in kinds

The following code typechecks:

{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where

import Data.Kind
import Data.Proxy

class C a where
  c :: Proxy a

type family F

data D :: F -> Type

instance C (D :: F -> Type) where
  c = undefined

However, if we try to actually //use// that C D instance, like so:

c' :: Proxy (D :: F -> Type)
c' = c @(D :: F -> Type)

Then GHC gives a rather flummoxing error message:

$ /opt/ghc/8.4.3/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:20:6: error:
    • No instance for (C D) arising from a use of ‘c’
    • In the expression: c @(D :: F -> Type)
      In an equation for ‘c'’: c' = c @(D :: F -> Type)
   |
20 | c' = c @(D :: F -> Type)
   |      ^^^^^^^^^^^^^^^^^^^

But that error is clearly misleading, as we defined such a C D instance directly above it!

The use of the type family F in the kind of D appears to play an important role in this bug. If I change F to be a data type (e.g., data F), then c' is accepted.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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