[GHC] #8044: "Inaccessible code" error reported in wrong place

[GHC]

#8044: "Inaccessible code" error reported in wrong place
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       Reporter:  goldfire                 |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.7
       Keywords:  GADTs                    |  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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 Here is my file `Bug.hs`:

 {{{
 {-# LANGUAGE GADTs, TypeFamilies #-}

 module Bug where

 data X a where
   XInt :: X Int
   XBool :: X Bool
   XChar :: X Char

 type family Frob a where
   Frob Int = Int
   Frob x   = Char

 frob :: X a -> X (Frob a)
 frob XInt = XInt
 frob _    = XChar
 }}}

 Compiling this file produces the error

 {{{
 Bug.hs:15:6:
     Couldn't match type ‛Int’ with ‛Char’
     Inaccessible code in
       a pattern with constructor XInt :: X Int, in an equation for ‛frob’
     In the pattern: XInt
     In an equation for ‛frob’: frob XInt = XInt
 }}}

 The line/column number single out the pattern `XInt` in the first clause
 of the function `frob`. But, the real problem (as I see it), is the right-
 hand side of the ''second'' clause of `frob`. Indeed, when I comment out
 the second line of the function, the error goes away, even though it was
 reported on the first line.

 I do not believe that this error is caused by closed type families, per
 se, because I have run across it without them, just in code that was too
 hard to pare down enough to make a bug report out of.

 This was tested on 7.7.20130702.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8044>
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