[GHC] #13643: Core lint error with TypeInType and TypeFamilyDependencies

3 May 2017

      #13643: Core lint error with TypeInType and TypeFamilyDependencies
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  InjectiveFamilies, TypeInType      |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  12102
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 In the code

 {{{#!hs
 {-# 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`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13643
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