[GHC] #9893: Switching on TypeFamilies extension stops code from typechecking

[GHC]

#9893: Switching on TypeFamilies extension stops code from typechecking
-------------------------------------+-------------------------------------
       Reporter:  phischu            |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  Compiler           |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  x86_64 (amd64)     |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  GHC
     Blocked By:                     |  rejects valid program
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 The following code has the `TypeFamilies` extension enabled but does not
 use it. It does not typecheck with GHC 7.8.3. If we do not enable
 `TypeFamilies` (i.e. delete the first line) it typechecks.

 {{{
 {-# LANGUAGE TypeFamilies #-}
 module ExistentialFamilies where

 import Control.Monad.ST (runST,ST)

 un :: ()
 un = runST f where
     f = return un :: ST s ()
 }}}
 This happens when trying to compile the program as well as when trying to
 load it into ghci.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9893>
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