[GHC] #8214: 'Untouchable' error in list comprehension

[GHC]

#8214: 'Untouchable' error in list comprehension
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       Reporter:  MartijnVanSteenbergen    |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.6.3
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 Hi,

 I think this program should compile but it doesn't:

 {{{
 {-# LANGUAGE GADTs #-}

 data X a where X :: X Int

 foo :: Bool
 foo = null [ () | X <- [] ]
 }}}

 It fails with:

 {{{
     Couldn't match expected type `a0' with actual type `()'
       `a0' is untouchable
            inside the constraints (t_g ~ Int)
            bound at a pattern with constructor
                       X :: X Int,
                     in a pattern binding in
                          list comprehension
     In the pattern: X
     In a stmt of a list comprehension: X <- []
     In the first argument of `null', namely `[() | X <- []]'
 }}}

 If I remove the type declaration GHC compiles without errors and happily
 infers {{{foo :: Bool}}}.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8214>
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