[GHC] #6065: Suggested type signature causes a type error (even though it appears correct)

[GHC]

#6065: Suggested type signature causes a type error (even though it appears
correct)
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 Reporter:  tvynr                      |          Owner:                                          
     Type:  bug                        |         Status:  new                                     
 Priority:  normal                     |      Component:  Compiler (Type checker)                 
  Version:  7.4.1                      |       Keywords:  type signature typeclass instance forall
       Os:  Linux                      |   Architecture:  x86_64 (amd64)                          
  Failure:  GHC rejects valid program  |       Testcase:                                          
Blockedby:                             |       Blocking:                                          
  Related:                             |  
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 The attached file, MWE.hs, contains an experiment attempting a rudimentary
 encoding of extensible ASTs in Haskell (without using compdata or a
 similar package relying upon OverlappingInstances and so forth).  The
 definition of the upcast function appears to be correct.  Compiling
 without a type signature produces a warning and the suggestion to include
 a type signature.  Including the suggested type signature (which appears
 to be the correct one) causes a type error.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/6065>
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