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In the code

{-# Language TypeFamilyDependencies #-}
{-# Language RankNTypes             #-}
{-# Language KindSignatures         #-}
{-# Language DataKinds              #-}
{-# Language TypeInType             #-}
{-# Language GADTs                  #-}

import Data.Kind (Type)

data Code = I

type family
  Interp (a :: Code) = (res :: Type) | res -> a where
  Interp I = Bool

data T :: forall a. Interp a -> Type where
  MkNat :: T False

instance Show (T a) where show _ = "MkNat"

main = do
  print MkNat

but add {-# Options_GHC -dcore-lint #-} and we get the attached log from running runghc /tmp/tPb2.hs > /tmp/tPb2.log.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related	#12102 (closed)
Blocking
CC
Operating system
Architecture