[GHC] #5691: Crash: strange interaction of ScopedTypeVariables extension with type class name resolution

[GHC]

#5691: Crash: strange interaction of ScopedTypeVariables extension with type class
name resolution
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 Reporter:  dominiquedevriese   |          Owner:                                   
     Type:  bug                 |         Status:  new                              
 Priority:  normal              |      Component:  Compiler (Type checker)          
  Version:  7.0.4               |       Keywords:  ScopedTypeVariables, type classes
       Os:  Unknown/Multiple    |   Architecture:  Unknown/Multiple                 
  Failure:  Compile-time crash  |       Testcase:                                   
Blockedby:                      |       Blocking:                                   
  Related:                      |  
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 I saw a compiler crash ("the 'impossible' happened..."), seemingly caused
 by a strange interaction of the ScopedTypeVariables GHC extension with
 type class name resolution. I am attaching a cut-down example showing the
 crash. I hope this helps to find the bug. I have  only tested with GHC
 7.0.4; I am not sure if it is still present in later versions of the
 compiler. The crash occurs when I load the file in GHCi and evaluate the
 code

   parseRD (pure_ 'a') "a"

 or when I simply try to compile the file from the command line.

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Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5691>
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