[GHC] #11348: Local open type families instances ignored during type checking

4 Jan 2016

      #11348: Local open type families instances ignored during type checking
-------------------------------------+-------------------------------------
           Reporter:  alexvieth      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 {{{#!hs
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeInType #-}

 import Data.Kind
 import Data.Proxy

 type family TrivialFamily t :: Type
 type instance TrivialFamily (t :: Type) = Bool

 data R where
     R :: Proxy Bool -> R

 type ProblemType t = 'R ('Proxy :: Proxy (TrivialFamily t))
 }}}

 Compiling this program as-is, GHC rejects it!

 {{{#!sh
 error:
     • Expected kind ‘Proxy Bool’,
         but ‘'Proxy’ has kind ‘Proxy (TrivialFamily t)’
     • In the first argument of ‘R’, namely
         ‘(Proxy :: Proxy (TrivialFamily t))’
       In the type ‘R (Proxy :: Proxy (TrivialFamily t))’
       In the type declaration for ‘ProblemType’
 }}}

 But if we move `TrivialFamily` to another module and import it, GHC
 discovers that `TrivialFamily t = Bool` and the program is accepted.

 When compiling the rejected program (with the local family instance) I
 observe that the instance environments given by `FamInst.tcGetFamInstEnvs`
 contain no instances! The renamer processes the local instance, but no
 `FamInst` is created for it, and nothing enters the `TcGblEnv`'s family
 instance record.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11348
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