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This code doesn't typecheck due to F2:

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

import Data.Kind

f :: (forall (a :: Type). a -> a) -> Bool
f g = g True

type F1 (g :: forall (a :: Type). a -> a) = g True

type family F2 (g :: forall (a :: Type). a -> a) :: Bool where
  F2 g = g True

$ /opt/ghc/8.2.1/bin/ghci Bug.hs
GHCi, version 8.2.0.20170623: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:14:6: error:
    • Expected kind ‘forall a. a -> a’, but ‘g’ has kind ‘Bool -> Bool’
    • In the first argument of ‘F2’, namely ‘g’
      In the type family declaration for ‘F2’
   |
14 |   F2 g = g True
   |      ^

This is surprising to me, since F2 seems like the type family counterpart to f and F1, both of which typecheck.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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