[GHC] #6051: Parsing type constraints in GHC 7.4.1

[GHC]

#6051: Parsing type constraints in GHC 7.4.1
-----------------------------------------------+----------------------------
 Reporter:  leepike                            |          Owner:                  
     Type:  bug                                |         Status:  new             
 Priority:  normal                             |      Component:  Compiler        
  Version:  7.4.1                              |       Keywords:                  
       Os:  Linux                              |   Architecture:  Unknown/Multiple
  Failure:  Incorrect warning at compile-time  |       Testcase:                  
Blockedby:                                     |       Blocking:                  
  Related:                                     |  
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 This appears to be a parsing bug, or at least a bad error message, for a
 *malformed* type constraint:

 Given the following malformed program, I get the following output loading
 it with ghci (GHC 7.4.1):

 ------------------------------------
 {{{

 module Test where

 data Foo a = Foo a

 -- Malformed!
 instance Ord a => Eq a => Show (Foo a) where
   show (Foo a) = undefined
 }}}
 -------------------------------------

 {{{
 $ ghci Test.hs
 GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
 Loading package ghc-prim ... linking ... done.
 Loading package integer-gmp ... linking ... done.
 Loading package base ... linking ... done.
 [1 of 1] Compiling Test             ( Test.hs, interpreted )
 *** Exception: compiler/rename/RnSource.lhs:429:14-81: Irrefutable pattern
 failed for pattern Data.Maybe.Just (inst_tyvars,
                                                         _,
                                                         SrcLoc.L _ cls,
                                                         _)

 >
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6051>
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